Upload your image
DSS Images Other Images
Submit a new article
|Hubble's top 10.|
|Fast transition between classical and weak lined T Tauri stars due to external UV dissipation|
The discovery of optical jets immersed in the strong UV radiation fieldof the Rosette Nebula sheds new light on, but meanwhile poses challengesto, the study of externally irradiated jets. The jet systems in theRosette are found to have a high state of ionization and show uniquefeatures. In this paper, we investigate the evolutionary status of thejet-driving sources for young solar-like stars. To our surprise, thesejet sources indicate unexpected near infrared properties with no excessemission. They are bathed in harsh external UV radiation such thatevaporation leads to a fast dissipation of their circumstellar material.This could represent a transient phase of evolution of young solar-likestars between classical and weak lined T Tauri stars. Naked T Tauristars formed in this way have indistinguishable evolutionary ages fromthose of classical T Tauri stars resulting from the same episode of starformation. However, it would be hard for such sources to be identifiedif they are not driving an irradiated jet in a photoionized medium.
|A Survey of Dense Cores in the Orion A Cloud|
We have carried out an H13CO+(J=1-0) core surveyin a large area of 1.5deg×0.5deg, coveringthe whole region of the Orion A molecular cloud, using the Nobeyama 45 mradio telescope with the 25 Beam Array Receiver System (BEARS). Thissurvey is unique in that a large area (~48 pc2) of the cloudwas covered with a high spatial resolution of 21" (0.05 pc) and with adeep integration (1 σ~0.1 K in T*A),resulting in a core mass detection of 1.6 Msolar. Themorphology of the H13CO+(J=1-0) emission is verysimilar to that of the 850 μm continuum emission. We identified 236dense cores from our data with the clumpfind algorithm. The cores areclose to virial equilibrium, independent of whether they are thermal orturbulent. We predict an initial mass function (IMF) from the core massfunction, considering binary formation and confusion along the line ofsight, and find that this IMF agrees well with the Orion Nebula clusterIMF for a star formation efficiency of ~40%. Therefore, we suggest thatthe IMF is determined at the time of the dense core formation.Furthermore, we discovered three cores with large velocity widths,significantly wider than those of the other cores, only toward the M42 HII region, suggesting that the energy input from the H II regionincreases the velocity width. Since the three cores can produce the mostmassive stars, owing to their large mass accretion rates, massive starformation in the next generation in the Orion A cloud is likely to becaused by nearby stellar activity.
|Astronomical Observations of Galactic Radiosources with ESCORT: an small radiotelescope|
The aim of this work was to measure the radio flux density at 3.7 GHz ofthermal (HII regions) and nonthermal emissions (synchrotron radiation)of Galactic radio sources. The observations were made through ESCORT,the 2 meter diameter antenna of the radioastronomy center at Universidadde Oriente (UDO), situated in Ciudad Bolívar, Venezuela. Theantenna temperature and system temperature were calibrated by using thetransit time and the flux density of the sun as reference. In adition,the HPBW and effective aperture of ESCORT were determined.
|New Photometry and Spectra of AB Doradus C: An Accurate Mass Determination of a Young Low-Mass Object with Theoretical Evolutionary Tracks|
We present new photometric and spectroscopic measurements for theunique, young, low-mass evolutionary track calibrator AB Dor C. Whilethe new Ks photometry is similar to that we have previouslypublished, the spectral type is found to be much earlier. Based on new Hand K IFS spectra of AB Dor C from Thatte et al. (Paper I), we adopt aspectral type of M5.5+/-1.0 for AB Dor C. This is considerably earlierthan the M8+/-1 previously estimated by Close et al. and Nielsen et al.yet is consistent with the M6+/-1 independently derived by Luhman &Potter. However, the spectrum presented in Paper I and analyzed here isa significant improvement over any previous spectrum of AB Dor C. Wealso present new astrometry for the system, which further supports a0.090+/-0.005 Msolar mass for the system. Once armed with anaccurate spectrum and Ks flux, we find L=0.0021+/-0.0005Lsolar and Teff=2925+170-145K for AB Dor C. These values are consistent with a ~75 Myr,0.090+/-0.005 Msolar object like AB Dor C according to theDUSTY evolutionary tracks. Hence, masses can be estimated from the H-Rdiagram with the DUSTY tracks for young low-mass objects such as AB DorC. However, we cautiously note that underestimates of the mass from thetracks can occur if one lacks a proper (continuum-preserved) spectrum oris relying on near-infrared fluxes alone.Based on observations made with ESO telescopes at the ParanalObservatories under program 276.C-5013.
|Bias-free Measurement of Giant Molecular Cloud Properties|
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.
|PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC 3603|
We present spatial distribution of polycyclic aromatic hydrocarbons(PAHs) and ionized gas within the Galactic giant H II region NGC 3603.Using the IRS instrument on board the Spitzer Space Telescope, we studyin particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3μm, and the [Ar II] 6.99 μm, [Ne II] 12.81 μm, [Ar III] 8.99μm, and [S IV] 10.51 μm forbidden emission lines. The observationsprobe both ionized regions and photodissociation regions. Silicateemission is detected close to the central cluster while silicateabsorption is seen further away. We find no significant variation of thePAH ionization fraction across the whole region. The emission of verysmall grains (VSGs) lies closer to the central stellar cluster thanemission of PAHs. The PAH/VSG ratio anticorrelates with the hardness ofthe interstellar radiation field suggesting a destruction mechanism ofthe molecules within the ionized gas, as shown for low-metallicitygalaxies by Madden et al.
|The Puzzle of the Metallic Line Stars|
In the puzzle of the metallic line (Am) stars, there still seem to bemissing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak. TheAm stars have no ultraviolet emission lines. They are binaries that,with very few exceptions, have rotational velocities vsini lower than100 km s-1. Of the main-sequence A stars, 20% to 30% are Amstars. Here we rediscuss previous suggestions that tried to explain thepeculiar line strengths in the Am star spectra. In particular, wecompare the well-studied properties of Hyades A and Am stars in order toidentify reasons that can or cannot explain the differences. We findthat accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres.Charge-exchange reactions also seem to be important.
|A Surprising Reversal of Temperatures in the Brown Dwarf Eclipsing Binary 2MASS J05352184-0546085|
The newly discovered brown dwarf eclipsing binary 2MASSJ05352184-0546085 provides a unique laboratory for testing thepredictions of theoretical models of brown dwarf formation andevolution. The finding that the lower mass brown dwarf in this system ishotter than its higher mass companion represents a challenge to browndwarf evolutionary models, none of which predict this behavior. Here wepresent updated determinations of the basic physical properties of2M0535-05, bolstering the surprising reversal of temperatures with massin this system. We compare these measurements with widely used browndwarf evolutionary tracks, and find that the temperature reversal can beexplained by some models if the components of 2M0535-05 are mildlynon-coeval, possibly consistent with dynamical simulations of browndwarf formation. Alternatively, a strong magnetic field on the highermass brown dwarf might explain its anomalously low surface temperature,consistent with emerging evidence that convection is suppressed inmagnetically active, low-mass stars. Finally, we discuss futureobservational and theoretical work needed to further characterize andunderstand this benchmark system.
|OSIRIS: AO-assisted integral-field spectroscopy at the Keck Observatory|
OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a newfacility instrument for the Keck Observatory. After seeing first lightin February 2005, OSIRIS is currently undergoing commissioning. OSIRISprovides the capability of performing three-dimensional spectroscopy inthe near-infrared z, J, H, and K bands at the resolution limit of theKeck II telescope, which is equipped with adaptive optics and a laserguide star. The science case for OSIRIS is summarized, and theinstrument and associated data reduction software are described.
|3He in the Milky Way Interstellar Medium: Ionization Structure|
The cosmic abundance of the 3He isotope has importantimplications for many fields of astrophysics. We are using the 8.665 GHzhyperfine transition of 3He+ to determine the3He/H abundance in Milky Way H II regions and planetarynebulae. This is one in a series of papers in which we discuss issuesinvolved in deriving accurate 3He/H abundance ratios from theavailable measurements. Here we describe the ionization correction weuse to convert the 3He+/H+ abundance,y+3, to the 3He/H abundance,y3. In principle the nebular ionization structure cansignificantly influence the y3 derived for individualsources. We find that in general there is insufficient informationavailable to make a detailed ionization correction. Here we make asimple correction and assess its validity. The correction is based onradio recombination line measurements of H+ and4He+, together with simple core-halo sourcemodels. We use these models to establish criteria that allow us toidentify sources that can be accurately corrected for ionization andthose that cannot. We argue that this effect cannot be very large formost of the sources in our observational sample. For a wide range ofmodels of nebular ionization structure we find that the ionizationcorrection factor varies from 1 to 1.8. Although larger corrections arepossible, there would have to be a conspiracy between the density andionization structure for us to underestimate the ionization correctionby a substantial amount.
|Discovery of two young brown dwarfs in an eclipsing binary system|
Brown dwarfs are considered to be `failed stars' in the sense that theyare born with masses between the least massive stars (0.072 solarmasses, Msolar) and the most massive planets(~0.013Msolar); they therefore serve as a critical link inour understanding of the formation of both stars and planets. Even themost fundamental physical properties of brown dwarfs remain, however,largely unconstrained by direct measurement. Here we report thediscovery of a brown-dwarf eclipsing binary system, in the Orion Nebulastar-forming region, from which we obtain direct measurements of massand radius for these newly formed brown dwarfs. Our mass measurementsestablish both objects as brown dwarfs, with masses of 0.054 +/-0.005Msolar and 0.034 +/- 0.003Msolar. At the sametime, with radii relative to the Sun's of 0.669 +/-0.034Rsolar and 0.511 +/- 0.026Rsolar, these browndwarfs are more akin to low-mass stars in size. Such large radii aregenerally consistent with theoretical predictions for young brown dwarfsin the earliest stages of gravitational contraction. Surprisingly,however, we find that the less-massive brown dwarf is the hotter of thepair; this result is contrary to the predictions of all currenttheoretical models of coeval brown dwarfs.
|Outflow and Infall in a Sample of Massive Star-forming Regions|
We present single-pointing observations of SiO, HCO+, andH13CO+ from the James Clerk Maxwell Telescopetoward 23 massive star-forming regions previously known to containmolecular outflows and ultracompact H II regions. We detected SiO toward14 sources and suggest that the nondetections in the other nine sourcescould be due to those outflows being older and without ongoing shocks toreplenish the SiO. We serendipitously detected SO 2 toward 17sources in the same tuning as HCO+. We detectedHCO+ toward all sources, and suggest that it is tracinginfall in nine cases. For seven infall candidates, we estimate massinfall rates between 1×10-2 and 2×10-5Msolar yr-1. Seven sources show both SiOdetections (young outflows) and HCO+ infall signatures. Wealso find that the abundance of H13 CO+ tends toincrease along with the abundance of SiO in sources for which we coulddetermine abundances. We discuss these results with respect to currenttheories of massive star formation via accretion. From this survey, wesuggest that perhaps both models of ionized accretion and haltedaccretion may be important in describing the evolution of a massiveprotostar (or protostars) beyond the formation of an H II region.
|Treasures of Orion.|
|Infrared Extinction toward Nearby Star-forming Regions|
We present an independent estimate of the interstellar extinction lawfor the Spitzer IRAC bands, as well as a first attempt at extending thelaw to the 24 μm MIPS band. The source data for these measurementsare observations of five nearby star-forming regions: the Orion A cloud,NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excessratios EH-Ks/EKs-[λ]were measured for stars without infrared excess dust emission fromcircumstellar disks/envelopes. For four of these five regions, theextinction laws are similar at all wavelengths and differ systematicallyfrom a previous determination of the extinction law, which was dominatedby the diffuse ISM, derived for the IRAC bands. This difference could bedue to the difference in the dust properties of the dense molecularclouds observed here and those of the diffuse ISM. The extinction law atlonger wavelengths toward the Ophiuchus region lies between that to theother four regions studied here and that for the ISM. In addition, weextended our extinction law determination to 24 μm for Serpens andNGC 2068/2071 using Spitzer MIPS data. We compare these results againstseveral ISO extinction law determinations, although in each case thereare assumptions which make absolute comparison uncertain. However, ourwork confirms a relatively flatter extinction curve from 4 to 8 μmthan the previously assumed standard, as noted by all of these recentstudies. The extinction law at 24 μm is consistent with previousmeasurements and models, although there are relatively largeuncertainties.
|The H+ region contribution to [CII] 158-μm emission|
The [CII] 158-μm line is an important emission-line diagnostic inphotodissociation regions (PDRs), but this emission line can also emergefrom ionized gas. This work calculates the contribution of [CII]emission from ionized gas over a wide range of parameter space byconsidering the simplified case of an H+ region and PDR inpressure equilibrium. Additionally, these calculations also predict thestrong correlation observed between [NII] 205-μm emission and [CII]discussed by previous authors. Overall, the results of thesecalculations have wide-ranging applications to the interpretation of[CII] emission in astrophysical environments.
|A Spitzer Census of the IC 348 Nebula|
Spitzer mid-infrared surveys enable an accurate census of young stellarobjects by sampling large spatial scales, revealing very embeddedprotostars, and detecting low-luminosity objects. Taking advantage ofthese capabilities, we present a Spitzer-based census of the IC 348nebula and embedded star cluster, covering a 2.5 pc region andcomparable in extent to the Orion Nebula. Our Spitzer censussupplemented with ground-based spectra has added 42 Class II T Taurisources to the cluster membership and identified ~20 Class 0/Iprotostars. The population of IC 348 likely exceeds 400 sources afteraccounting statistically for unidentified diskless members. Our Spitzercensus of IC 348 reveals a population of Class I protostars that isanticorrelated spatially with the Class II/III T Tauri members, whichcomprise the centrally condensed cluster around a B star. The protostarsare instead found mostly at the cluster periphery about ~1 pc from the Bstar and spread out along a filamentary ridge. We further find that thestar formation rate in this protostellar ridge is consistent with thatrate which built the older exposed cluster, while the presence of 15cold, starless, millimeter cores intermingled with this protostellarpopulation indicates that the IC 348 nebula has yet to finish formingstars. Moreover, we show that the IC 348 cluster is of order 3-5crossing times old, and, as evidenced by its smooth radial profile andconfirmed mass segregation, is likely relaxed. While it seems apparentthat the current cluster configuration is the result of dynamicalevolution and its primordial structure has been erased, our finding of afilamentary ridge of Class I protostars supports a model in whichembedded clusters are built up from numerous smaller subclusters.Finally, the results of our Spitzer census indicate that the suppositionthat star formation must progress rapidly in a dark cloud should notpreclude these observations that show it can be relatively long lived.
|Post-outburst phase of McNeil's nebula (V1647 Orionis)|
We present a detailed study of the post-outburst phase of McNeil'snebula (V1647 Orionis) using optical B, V, R, I and near-infrared (NIR)J, H, K photometric and low-resolution optical spectroscopicobservations. The observations were carried out with the Himalaya FaintObject Spectrograph Camera (HFOSC), NIR camera (NIRCAM), the TataInstitute of Fundamental Research (TIFR) Near-Infrared Camera (TIRCAM)and NICMOS cameras on the 2-m Himalayan Chandra Telescope (HCT) and1.2-m Physical Research Laboratory (PRL) telescopes during the period2004 February-2005 December. The optical and NIR observations show ageneral decline in the brightness of the exciting source of McNeil'snebula (V1647 Ori). Our recent optical images show that V1647 Ori hasfaded by more than 3 mag since February 2004. McNeil's nebula has alsofaded considerably. The optical/NIR photometric data also show asignificant variation in the magnitudes (ΔV= 0.78, ΔR= 0.44,ΔI= 0.21, ΔJ= 0.24 and ΔH= 0.20 mag) of V1647 Oriwithin a period of one month, which is possibly undergoing a phasesimilar to eruptive variables, like EXors or FUors. The optical spectrashow a few features such as strong Hα emission with blue-shiftedabsorption and the CaII IR triplet (8498, 8542 and 8662 Å) inemission. As compared to the period just after outburst, there is adecrease in the depth and extent of the blue-shifted absorptioncomponent, indicating a weakening in the powerful stellar wind. Thepresence of the CaII IR triplet in emission confirms that V1647 Ori is apre-main-sequence star. The long-term, post-outburst photometricobservations of V1647 Ori suggest an EXor rather than an FUor event. Anoptical/IR comparison of the region surrounding McNeil's nebula showsthat the optical nebula is more widely and predominantly extended to thenorth, whereas the IR nebula is relatively confined (diameter ~60arcsec), but definitely extended, to the south, too. The large colourgradient from north to south and the sudden absence of an optical nebulato the south are suggestive of a large-scale disc-like structure (orenvelope) surrounding the central source that hides the southern nebula.
|Astrophysical lasers and nonlinear optical effects in space|
The present state of the art concerning astrophysical lasers (APL) andnonlinear optical effects under astrophysical conditions is reviewed.The operational conditions of an APL (amplification under non-LTEconditions) and astrophysical predecessors of the laboratory lasers areconsidered in the introduction of the review. The rareness of observedAPL action in the visible range in comparison with astrophysical masers(APM) in the microwave range is explained. Early proposals of APLs withcollisional and optical pumping are discussed. APL/M in the mid-IR andsubmillimeter ranges linking APL and APM are also discussed. APLs in theWeigelt blobs of Eta Carinae operating in FeII and OI with a Bowen typeoptical pumping are considered in detail. General questions (narrowingof APL spectral lines, the possibility of scattering feedback and waysof measuring the true spectral width of an APL) are considered.Nonlinear optical effects in astrophysical conditions andresonance-enhanced two-photon conditions, in particular, are discussedin the conclusion.
|Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*|
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
|Interaction of Supernova Ejecta with Nearby Protoplanetary Disks|
The early solar system contained short-lived radionuclides such as60Fe (t1/2=1.5 Myr) whose most likely source was anearby supernova. Previous models of solar system formation considered asupernova shock that triggered the collapse of the Sun's nascentmolecular cloud. We advocate an alternative hypothesis, that the solarsystem's protoplanetary disk had already formed when a very close (<1pc) supernova injected radioactive material directly into the disk. Weconduct the first numerical simulations designed to answer two questionsrelated to this hypothesis: Will the disk be destroyed by such a closesupernova, and will any of the ejecta be mixed into the disk? Oursimulations demonstrate that the disk does not absorb enough momentumfrom the shock to escape the protostar to which it is bound. Only lowamounts (<1%) of mass loss occur, due to stripping byKelvin-Helmholtz instabilities across the top of the disk, which alsomix into the disk about 1% of the intercepted ejecta. These lowefficiencies of destruction and injection are due to the fact that thehigh disk pressures prevent the ejecta from penetrating far into thedisk before stalling. Injection of gas-phase ejecta is too inefficientto be consistent with the abundances of radionuclides inferred frommeteorites. On the other hand, the radionuclides found in meteoriteswould have condensed into dust grains in the supernova ejecta, and weargue that such grains will be injected directly into the disk withnearly 100% efficiency. The meteoritic abundances of the short-livedradionuclides such as 60Fe therefore are consistent withinjection of grains condensed from the ejecta of a nearby (<1 pc)supernova, into an already formed protoplanetary disk.
|Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy|
High spatial resolution spectroscopy at 8-13 μm with T-ReCS onGemini-S has revealed striking variations in the mid-infrared emissionand absorption in the nucleus of the Circinus galaxy (hereafterCircinus) on subarcsecond scales. The core of Circinus is compact andobscured by a substantial column of cool silicate dust. Weak extendedemission to the east and west coincides with the coronal line region andarises from featureless dust grains which are probably heated by lineemission in the coronal emission zone. The extended emission on the eastside of the nucleus displays a much deeper silicate absorption than thaton the west, indicating significant columns of cool material along theline of sight and corresponding to an additional extinction ofAV~ 25 mag. Emission bands from aromatic hydrocarbons are notsubject to this additional extinction, are relatively weak in the coreand in the coronal line region, and are much more spatially extendedthan the continuum dust emission; they presumably arise in thecircumnuclear star-forming regions. These data are interpreted in termsof an inclined disc-like structure around the nucleus extending overtens of parsecs and possibly related to the inner disc found fromobservations of water masers by Greenhill et al..
|Far-ultraviolet scattering by dust in Orion|
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
|Optically Thick [O I] and [C II] Emission toward NGC 6334A|
This work focuses on [O I] and [C II] emission toward NGC 6334A, anembedded H+ region/PDR only observable at infrared or longerwavelengths. A geometry in which nearly all the emission escapes out theside of the cloud facing the stars, such as Orion, is not applicable tothis region. Instead, we find the geometry to be one in which theH+ region and associated PDR is embedded in the molecularcloud. Constant-density PDR calculations are presented which predictline intensities as a function of AV [or N(H)], hydrogendensity (nH), and incident UV radiation field(G0). We find that a single-component model withAV~650 mag, nH=5×105cm-3, and G0=7×104 reproduces theobserved [O I] and [C II] intensities, and that the low [O I] 63 to 146μm ratio is due to line optical depth effects in the [O I] lines,produced by a large column density of atomic/molecular gas. We find thatthe effects of a density law would increase our derived AV,while the effects of an asymmetric geometry would decreaseAV, with the two effects largely canceling. We conclude thatoptically selected H+ regions adjacent to PDRs, such asOrion, likely have a different viewing angle or geometry than similarregions detected through IR observations. Overall, the theoreticalcalculations presented in this work have utility for any PDR embedded ina molecular cloud.
|A census of the Carina Nebula - I. Cumulative energy input from massive stars|
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
|The Role of Mass and Environment in Multiple-Star Formation: A 2MASS Survey of Wide Multiplicity in Three Young Associations|
We present the results of a search for wide binary systems among 783members of three nearby young associations: Taurus-Auriga, Chamaeleon I,and two subgroups of Upper Scorpius. Near-infrared (JHK) imagery from2MASS was analyzed to search for wide (1"-30" ~150-4500 AU) companionsto known association members, using color-magnitude cuts to rejectlikely background stars. We identify a total of 131 candidate binarycompanions with colors consistent with physical association, of which 39have not been identified previously in the literature. Our resultssuggest that the wide binary frequency is a function of both mass andenvironment, with significantly higher frequencies among high-mass starsthan lower mass stars and in the T associations than in the OBassociation. We discuss the implications for wide binary formation andconclude that the environmental dependence is not a direct result ofstellar density or total association mass, but instead might depend onanother environmental parameter like the gas temperature. The binarypopulations in these associations generally follow the empiricalmass-maximum separation relation observed for field binaries, but wehave found one candidate low-mass system (USco 160611.9-193532Mtot~0.4 Msolar) that has a projected separation(10.8" 1550 AU) much larger than the suggested limit for its mass.Finally, we find that the binary frequency in the USco-B subgroup issignificantly higher than in the USco-A subgroup and is consistent withthe measured values in Taurus and ChamI. This discrepancy, the absenceof high-mass stars in USco-B, and its marginally distinct kinematicssuggest that it might not be directly associated with the OBassociations of Sco-Cen but instead represents an older analog of theyounger ρ Oph or Lupus associations.
|Hydroxyl maser disc and outflow in the Orion-BN/KL region|
Multi Element Radio Linked Interferometer Network (MERLIN) measurementsof 1.6-GHz hydroxyl (OH) masers associated withOrion-Becklin-Neugebauer/Kleinmann-Low (Orion-BN/KL) are presented, andthe data are compared with data on other masers, molecular lines,compact radio continuum sources and infrared sources in the region. OHmasers are detected over an area of 30 arcsec in diameter, with themajority lying along an approximately E-W structure that extends for ~18arcsec, encompassing the infrared sources IRc2, IRc6 and IRc7. Radialvelocities range from -13 to +42kms-1. The system of OHmasers shows a velocity gradient together with non-circular motions. Thekinematics are modelled in terms of an expanding and rotating disc ortorus. The rotation axis is found to be in the same direction as themolecular outflow. There is an inner cavity of radius ~1300 au with noOH masers. The inner cavity, like the H2O `shell' masers andSiO masers, is centred on radio source I. Some of the OH masers occur invelocity-coherent strings or arcs that are longer than 5 arcsec (2250au). One such feature, Stream A, is a linear structure at position angle~45°, lying between IRc2 and BN. We suggest that these masers traceshock fronts, and have appeared, like a vapour trail, 200 yr after thepassage of the runaway star BN. The radio proper motions of BN, source Iand source n project back to a region near the base of Stream A that islargely devoid of OH masers. The 1612-MHz masers are kinematicallydistinct from the other OH masers. They are also more widely distributedand appear to be associated with the outflow as traced by H2Omasers and by the 2.12-μm emission from shocked H2. Themagnetic field traced by the OH masers ranges from 1.8 to 16.3mG, with apossible reversal. No OH masers were found associated with even the mostprominent proplyds within 10 arcsec of θ1 Ori C.
|The Deep X-Ray Radio Blazar Survey. III. Radio Number Counts, Evolutionary Properties, and Luminosity Function of Blazars|
Our knowledge of the blazar surface densities and luminosity functions,which are fundamental parameters, relies still on samples at relativelyhigh flux limits. As a result, our understanding of this rare class ofactive galactic nuclei is mostly based on relatively bright andintrinsically luminous sources. We present the radio number counts,evolutionary properties, and luminosity functions of the faintest blazarsample with basically complete (~95%) identifications. Based on the DeepX-Ray Radio Blazar Survey (DXRBS), it includes 129 flat-spectrum radioquasars (FSRQs) and 24 BL Lac objects down to a 5 GHz flux and power ~50mJy and ~1024 W Hz-1, respectively, an order ofmagnitude improvement as compared to previously published(radio-selected) blazar samples. DXRBS FSRQs are seen to evolvestrongly, up to redshift ~1.5, above which high-power sources show adecline in their comoving space density. DXRBS BL Lac objects, on theother hand, do not evolve. High-energy and low-energy peaked BL Lacobjects (HBLs and LBLs, respectively) share the same lack ofcosmological evolution, which is at variance with some previous results.The observed luminosity functions are in good agreement with thepredictions of unified schemes, with FSRQs getting close to theirexpected minimum power. Despite the fact that the large majority of ourblazars are FSRQs, BL Lac objects are intrinsically ~50 times morenumerous. Finally, the relative numbers of HBLs and LBLs in the radioand X-ray bands are different from those predicted by the so-calledblazar sequence and support a scenario in which HBLs represent a smallminority (~10%) of all BL Lac objects.
|The infrared Hourglass cluster in M8*†|
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.
|25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a|
We report here on the photometric and kinematic properties of awell-defined group of nearly 200 low-mass pre-main-sequence stars,concentrated within ~1° of the early-B star 25 Ori, in the OrionOB1a subassociation. We refer to this stellar aggregate as the 25Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori anda dozen other early-type stars with photometry, parallaxes, and somewith IR excess emission, indicative of group membership. The number ofhigh- and low-mass stars is in agreement with expectations from astandard initial mass function. The velocity distribution for thelow-mass stars shows a narrow peak at 19.7 km s-1, offset~-10 km s-1 from the velocity characterizing the youngerstars of the Ori OB1b subassociation, and -4 km s-1 from thevelocity of widely spread young stars of the Ori OB1a population; thisresult provides new and compelling evidence that the 25 Ori group is adistinct kinematic entity, and that considerable space and velocitystructure is present in the Ori OB1a subassociation. The low-massmembers follow a well-defined band in the color-magnitude diagram,consistent with an isochronal age of ~7-10 Myr. The ~2 time drop in theoverall Li I equivalent widths and accretion fraction between theyounger Ori OB1b and the 25 Ori group is consistent with the latterbeing significantly older. In a simple-minded kinematic evolutionscenario, the 25 Ori group may represent the evolved counterpart of theyounger σ Ori cluster. The 25 Ori stellar aggregate is the mostpopulous ~10 Myr sample yet known within 500 pc, setting it as anexcellent laboratory to study the evolution of solar-like stars andprotoplanetary disks.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona;and the Fred Lawrence Whipple Observatory of the SmithsonianInstitution.
|Circular polarimetry and the line of sight to the Becklin-Neugebauer object|
The 3.1-μm absorption feature of water-ice has been observedspectroscopically in many molecular clouds and, when it has beenobserved spectropolarimetrically, usually a corresponding polarizationfeature is seen. Typically, on these occasions, and particularly for theBecklin-Neugebauer (BN) object, a distinct position angle shift betweenthe feature and continuum is seen, which indicates both a fractionationof the icy material and a changing alignment direction along the line ofsight.Here, the dependence of circular polarimetry on fractionation along theline of sight is investigated and it is shown that the form of itsspectrum, together with the sign of the position angle shift, indicateswhere along the line of sight the icy material lies. More specifically,a coincidence between the sign of the position angle displacement in theice feature, measured north through east, and that of the circularpolarization ice feature means that the icy grains are overlaid by baregrains. Some preliminary circular polarimetry of BN has thischaracteristic, and a similar situation is found in the only two othercases for which relevant observations so far exist.
|On the Formation of Glycolaldehyde (HCOCH2OH) and Methyl Formate (HCOOCH3) in Interstellar Ice Analogs|
Binary mixtures of methanol (CH3OH) and carbon monoxide (CO)ices were irradiated at 10 K with energetic electrons to mimic theenergy transfer processes that occur in the track of the trajectories ofMeV cosmic-ray particles. The formation of glycolaldehyde(HCOCH2OH) was established through the appearance of newbands in the infrared spectrum at 1757, 1700, 1690, 1367, 1267, and 1067cm-1. A second C2H4O2isomer, methyl formate (HCOOCH3), was also identified byabsorptions appearing at 1718, 1159, and 914 cm-1. Massspectrometer signals during the warm-up of the ice sample showedsublimation of both the glycolaldehyde and methyl formate; these specieswere monitored via theC2H4O2+ molecular ion atmass-to-charge ratio, m/e, of 60 originating from both glycolaldehydeand the methyl formate isomer. The latter was distinguishable by thepresence of a second signal at m/e=45, i.e., theHCO2+ ion. Kinetic fits of the column densities ofthe reactants and products suggest the initial step of the formationprocess is the cleavage of a CH bond in the methanol molecule togenerate either the hydroxymethyl (CH2OH) or methoxy(CH3O) radical plus atomic hydrogen. The hydrogen atom holdsexcess kinetic energy, allowing it to overcome entrance barriersrequired; therefore, a hydrogen could add to a CO molecule, generatingthe formyl radical (HCO). This can recombine with the hydroxymethylradical to form glycolaldehyde or with the methoxy radical to yieldmethyl formate. Similar processes are expected to form glycolaldehydeand methyl formate in the interstellar medium on grains and possibly oncometary ices, thus providing alternatives to gas-phase processes forthe generation of complex species whose fractional abundances comparedwith H2 of typically a few times 10-9 cannot beaccounted for solely by gas-phase chemistry.
|The maximum stellar mass, star-cluster formation and composite stellar populations|
We demonstrate that the mass of the most massive star in a clustercorrelates non-trivially with the cluster mass. A simple algorithm,according to which a cluster is filled up with stars that are chosenrandomly from the standard initial mass function (IMF) but sorted withincreasing mass, yields an excellent description of the observationaldata. Algorithms based on random sampling from the IMF without sortedadding are ruled out with a confidence larger than 0.9999. A physicalexplanation of this would be that a cluster forms by more-massive starsbeing consecutively added until the resulting feedback energy sufficesto revert cloud contraction and stops further star formation. This hasimportant implications for composite populations. For example,104 clusters of mass 102Msolar will notproduce the same IMF as one cluster with a mass of106Msolar. It also supports the notion that theintegrated galaxial stellar IMF (IGIMF) should be steeper than thestellar IMF and that it should vary with the star formation rate of agalaxy.
|New pre-main sequence candidates in the Taurus-Auriga star forming region|
Aims.We have studied the X-ray source population of the Taurus MolecularCloud (TMC) to search for new members of the Taurus-Auriga star formingregion. Methods: Candidate members have been selected among the X-raysources detected in 24 fields of the XMM-Newton Extended Survey of theTaurus Molecular Cloud, having an IR counterpart in the 2MASS catalog,based on color-magnitude and color-color diagrams. Their X-ray spectralproperties have been compared with those of known members and otherX-ray sources in the same fields but without a NIR counterpart. A searchfor flare-like variability in the time series of all new candidates andthe analysis of the X-ray spectra of the brightest candidates have beenused to identify sources with a high probability of membership. Results: We have found that 347 of 1909 detected X-ray sources have aninfrared counterpart in the 2MASS catalogue. Among them, we haveselected 57 sources that are consistent with being new pre-main sequencestar candidates at the distance of the Taurus-Auriga star formingregion; the X-ray spectral properties of this sample are, on the whole,similar to the properties of known TMC members and different from thoseof X-ray sources without an IR counterpart, most of which are likely tobe of extragalactic origin. For 12 such candidates, the likelihood ofmembership is very high, based on the relatively high plasmatemperatures derived from their X-ray spectra and/or the observation ofpowerful flares in their light curves.Table [see full textsee full textsee full textsee full textsee fulltext] and Fig. [see full textsee full textsee full textsee full textseefull text] are only available in electronic form at http://www.aanda.org
|Dead zones and extrasolar planetary properties|
Most low-mass protostellar discs evolve in clustered environments wherethey are affected by external radiation fields, while others evolve inmore isolated star-forming regions. Assuming that the magnetorotationalinstability (MRI) is the main source of viscosity, we calculate the sizeof a poorly ionized, MRI inactive and hence low viscosity region - the`dead zone'- in these protostellar discs. We include disc ionization byX-rays, cosmic rays, radioactive elements and thermal collisions,recombination by molecules, metals and grains, as well as the effect ofturbulence stimulation in the dead zone by the active layers lying aboveit. We also calculate the gap-opening masses of planets, which aredetermined by a disc's viscosity and a disc aspect ratio, for discs inthese environments and compare them with each other.We find that the dead zone is a robust feature of the protostellar discsthat is largely independent of their environment, typically stretchingout to ~15 au. We analyse the possible effects of dead zones on planetformation, migration and eccentricity evolution. We show that thegap-opening mass inside the dead zone is expected to be of the order ofterrestrial and ice giant mass planets while that outside the dead zoneis Jovian or super-Jovian mass planets, largely independent of thestar-forming environment. We show that dead zones can significantly slowdown both type I and type II planetary migration due to their lowerviscosity. We also find that the growth of eccentricity of massiveextrasolar planets is particularly favourable through the planet-discinteraction inside the dead zones due to the large gaps expected to beopened by planets.
|Comparison of 13CO line and far-infrared continuum emission as a diagnostic of dust and molecular gas physical conditions - III. Systematic effects and scientific implications|
Far-infrared (far-IR) continuum data from the COBE/DIRBE instrument werecombined with Nagoya 4-m 13COJ = 1 -> 0 spectral line datato infer the multiparsec-scale physical conditions in the OrionA and Bmolecular clouds, using 140 μm/240 μm dust colour temperatures andthe 240 μm/13COJ = 1 -> 0 intensity ratios. In theory,the ratio of far-IR, submillimetre, or millimetre continuum to that of a13CO (or C18O) rotational line can place reliableupper limits on the temperature of the dust and molecular gas onmultiparsec scales; on such scales, both the line and continuum emissionare optically thin, resulting in a continuum-to-line ratio that suffersno loss of temperature sensitivity in the high-temperature limit asoccurs for ratios of CO rotational lines or ratios of continuum emissionin different wavelength bands.Two-component models fit the Orion data best, where one has a fixedtemperature and the other has a spatially varying temperature. Theinferred physical conditions are consistent with those determined frompreviously observed maps of 12COJ = 1 -> 0 and J = 2 ->1 that cover the entire OrionA and B molecular clouds. The modelsrequire that the dust-gas temperature difference is 0 +/- 2K. If thissurprising result is confirmed with independent studies and applies tomuch of the Galactic interstellar medium (ISM), except in unusualregions such as the Galactic Centre, then there are a number ofimplications. These include dust-gas thermal coupling that is commonlyfactors of 5-10 stronger than previously believed, Galactic-scalemolecular gas temperatures closer to 20K than to 10K, an improvedexplanation for the N(H2)/I(CO) conversion factor (a fulldiscussion of this is deferred to a later paper), and ruling out atleast one dust grain alignment mechanism. The simplest interpretation ofthe models suggests that about 40-50 per cent of the Orion clouds are inthe form of cold (i.e. ~3-10K) dust and gas, although alternativeexplanations are not ruled out. These alternatives include thecontribution to the 240-μm continuum by dust associated with atomichydrogen and reduced 13CO abundance towards the clouds'edges. Even considering these alternatives, it is still likely that coldmaterial with temperatures of ~7-10K still exists. If this cold gas anddust are common in the Galaxy, then mass estimates of the Galactic ISMmust be revised upwards by up to 60 per cent.The feasibility of submillimetre or millimetre continuum to13CO line ratios constraining estimates of dust and moleculargas temperatures was tested. The model fits allowed the simulation ofthe necessary millimetre-continuum and 13COJ = 1 -> 0 mapsused in the test. In certain `hot spots' - that have continuum-to-lineratios above some threshold value - the millimetre continuum to13CO ratio can estimate the dust temperature to within afactor of 2 over large ranges of physical conditions. Nevertheless,supplemental observations of the 13COJ = 2 -> 1 line or ofshorter wavelength continuum are advisable in placing lower limits onthe estimated temperature. Even without such supplemental observations,this test shows that the continuum-to-line ratio places reliable upperlimits on the temperature.
|Comparison of 13CO line and far-infrared continuum emission as a diagnostic of dust and molecular gas physical conditions - II. The simulations: testing the method|
The reliability of modelling the far-infrared continuum to13COJ = 1 -> 0 spectral line ratios applied to the Orionclouds (see previous paper in the series) on the scales of severalparsecs (i.e. ~7 pc) is tested by applying the models to simulated data.The two-component models are found to give the dust-gas temperaturedifference, ΔT, to within 1 or 2 K. However, other parameters likethe column density per velocity interval and the gas density can bewrong by an order of magnitude or more. In particular, the density canbe systematically underestimated by an order of magnitude or more. Theoverall mass of the clouds is estimated correctly to within a few percent.These results may permit us to reliably constrain estimates of the Orionclouds' physical parameters, based on the real observations of thefar-infrared continuum and 13COJ = 1 -> 0 spectral line.Nevertheless, other systematics must be treated first. These include theeffects of background/foreground subtraction, effects of the HIcomponent of the interstellar medium, and others. These will bediscussed in a future paper.
|Radio Recombination Lines in Galactic H II Regions|
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.
|Hydrodynamics of Cometary Compact H II Regions|
We present numerical radiation-hydrodynamic simulations of cometary H IIregions for a number of champagne flow and bow shock models. For thechampagne flow models we study smooth density distributions with bothsteep and shallow gradients. We also consider cases in which theionizing star has a strong stellar wind and cases in which the staradditionally has a proper motion within the ambient density gradient. Wefind that our champagne flow plus stellar wind models havelimb-brightened morphologies and kinematics that can see theline-of-sight velocities change sign twice between the head and tail ofthe cometary H II region, with respect to the rest frame velocity. Ourbow shock models show that pressure gradients across and within theshell are very important for the dynamics and that simple analyticmodels assuming thin shells in ram pressure balance are whollyinadequate for describing the shape and kinematics of these objects atearly times in their evolution. The dynamics of the gas behind the shockin the neutral material ahead of the ionization front in both champagneflow and bow shock type cometary H II regions is also discussed. Wepresent simulated emission-measure maps and long-slit spectra of ourresults. Our numerical models are not tailored to any particular object,but comparison with observations from the literature shows that, inparticular, the models combining density gradients and stellar winds areable to account for both the morphology and general radial velocitybehavior of several observed cometary H II regions, such as thewell-studied object G29.96-0.02.
|Nonblind Catalog of Extragalactic Point Sources from the Wilkinson Microwave Anisotropy Probe (WMAP) First 3 Year Survey Data|
We have used the MHW2 filter to obtain estimates of the flux densitiesat the WMAP frequencies of a complete sample of 2491 sources, mostlybrighter than 500 mJy at 5 GHz, distributed over the whole sky excludinga strip around the Galactic equator (|b|<=5deg). Afterhaving detected 933 sources at the >=3 σ level in the MHW2filtered maps-our New Extragalactic WMAP Point Source Catalogue(NEWPS3σ)-we are left with 381 sources at >=5σ in at least one WMAP channel, 369 of which constitute ourNEWPS5σ catalog. It is remarkable to note that 98(i.e., 26%) sources detected at >=5 σ are ``new,'' they are notpresent in the WMAP catalog. Source fluxes have been corrected for theEddington bias. Our flux density estimates before such correction aregenerally in good agreement with the WMAP ones at 23 GHz. At higherfrequencies WMAP fluxes tend to be slightly higher than ours, probablybecause WMAP estimates neglect the deviations of the point spreadfunction from a Gaussian shape. On the whole, above the estimatedcompleteness limit of 1.1 Jy at 23 GHz we detected 43 sources missed bythe blind method adopted by the WMAP team. On the other hand, ourlow-frequency selection threshold left out 25 WMAP sources, only 12 ofwhich, however, are >=5 σ detections and only three haveS23GHz>=1.1 Jy. Thus, our approach proved to becompetitive with and complementary to the WMAP one.
|Warm HCN, C2H2, and CO in the Disk of GV Tau|
We present the first high-resolution, ground-based observations of HCNand C2H2 toward the T Tauri binary star system GVTau. We detected strong absorption due to HCN ν3 and weakC2H2 [ν3 andν2+(ν4+ν5)0+]absorption toward the primary (GV Tau S) but not the infrared companion.We also report CO column densities and rotational temperatures, andpresent abundances relative to CO of HCN/CO~0.6% andC2H/CO~1.2% and an upper limit for CH4/CO<0.37%toward GV Tau S. Neither HCN nor C2H2 weredetected toward the infrared companion, and results suggest thatabundances may differ between the two sources.
|Understanding the Kinetics and Dynamics of Radiation-induced Reaction Pathways in Carbon Monoxide Ice at 10 K|
Carbon monoxide is the second most abundant molecule on icy grains inthe interstellar medium. It also exists on Pluto, Triton, comets, andpossibly in other icy bodies of the outer solar system like Kuiper Beltobjects. With the intense radiation fields that permeate virtually allunprotected regions of space, carbon monoxide ices can be processedthrough energetic particle bombardment (planetary magnetosphericparticles, solar wind, Galactic cosmic ray particles, and UV photons).In the present study we have investigated the effects by condensing a 1μm layer of carbon monoxide ice on a substrate at 10 K and irradiatedthe sample with energetic (keV) electrons. These simulate the energeticelectrons trapped in magnetospheres of planets and reproduce theirradiation effects of typical Galactic cosmic ray particles. A seriesof new carbon-chain (C3, C6) and carbon oxidespecies were observed including the linear isomers of C2O,C3O, C4O, C5O, C6/7O,CO2, C3O2, C4O2,and C5O2. A reaction model was proposed thatoutlines different reaction pathways to each of these products. Usingthis model, the kinetics of each route of reaction was quantified, andfrom this, the mechanisms and dynamics of the reactions can beunderstood. This work should aid in the astronomical detection of newmolecular species in solar system ices as well as building up acomprehensive reaction model to describe the chemical inventory of iceson interstellar dust grains.
|Coronal Abundances in Orion Nebula Cluster Stars|
Following the Chandra Orion Ultradeep Project (COUP) observation, wehave studied the chemical composition of the hot plasma in a sample of146 X-ray-bright pre-main-sequence stars in the Orion Nebula Cluster(ONC). We report measurements of individual element abundances for asubsample of 86 slightly absorbed and bright X-ray sources, usinglow-resolution X-ray spectra obtained from the Chandra ACIS instrument.The X-ray emission originates from a plasma with temperatures andelemental abundances very similar to those of active coronae in olderstars. A clear pattern of abundances versus first ionization potential(FIP) is evident if solar photospheric abundances are assumed asreference. The results are validated by extensive simulations. Theobserved abundance distributions are compatible with a single pattern ofabundances for all stars, although a weak dependence on flare loop sizemay be present. The abundance of calcium is the only one which appearsto vary substantially between stars, but this quantity is affected byrelatively large uncertainties. The ensemble properties of theX-ray-bright COUP sources confirm that the iron in the emitting plasmais underabundant with respect to both the solar composition and theaverage stellar photospheric values. Comparison of the present plasmaabundances with those of the stellar photospheres and those of thegaseous component of the nebula indicates a good agreement for all theother elements with available measurements, and in particular for thehigh-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. Weconclude that there is evidence of a significant chemical fractionationeffect only for iron, which appears to be depleted by a factor 1.5-3with respect to the stellar composition.
|Abundance Gradients in the Galaxy|
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
|A Molecular Line Survey of W3(OH) and W3 IRS 5 from 84.7 to 115.6 GHz: Observational Data and Analyses|
We have carried out observations toward the W3 complex and G34.3+0.15using the TRAO 14 m radio telescope to examine in detail the chemicalvariations occurring while molecular clouds evolve from the prestellarto the H II region phase. Observations include spectral surveys of theseobjects between 84.7 and 115.6 GHz; mapping observations toward W3(OH)with the emissions of CS (2-1), HCN (1-0), HNC (1-0), andHCO+ (1-0) and mapping of CS (2-1) emission toward W3 IRS 5.Chemical model calculations are used to estimate the age of W3(OH) bycomparing with the fractional abundances of detected molecules. We foundthat G34.3+0.15 and W3(OH) are at a similar evolutionary stage, althoughlarge differences in the fractional abundances are found inCH3CN and HC3N. Overall, the properties of thedetected species and abundances in three regions support the view thatchemistry varies as molecular clouds evolve from a cold, collapsingphase to a high-temperature phase, such as the hot core and H II phase.Chemical model calculations for W3(OH) indicate that the evolutionaryage of the cloud is 104-105 yr with temperature inthe range 10-60 K.
|From Ultracompact to Extended H II Regions. II. Cloud Gravity and Stellar Motion|
The dynamical evolution of H II regions with and without stellar motionin dense, structured molecular clouds is studied. Clouds are modeled inhydrostatic equilibrium, with Gaussian central cores and external halosthat obey ρ~r-2 and ρ~r-3 power laws.Cloud gravity is included as a time-independent, external force. Stellarvelocities of 0, 2, 8, and 12 km s -1 are considered,permitting stars to move from the central core toward the edge of thecloud. Ultracompact H II regions are seen to evolve into extended H IIregions as the stars move toward lower density regions. Our mainconclusion is that ultracompact H II regions are pressure-confinedentities while they remain embedded within dense cores. The confinementcomes from either ram or ambient pressures, or a combination of both.The survival of the ultracompact regions depends on the position of thestar with respect to the core center, the stellar lifetime, and thecrossing time of the cloud core. Stars with velocities less than thecloud dispersion velocity can produce cometary ultracompact H II regionsfor 2×104 yr or more, in statistical agreement withobservations. The sequence ultracompact H II --> compact H II -->extended H II shows a variety of structures induced by variousinstabilities. Some ultracompact H II regions with a core-halomorphology could be explained by self-blocking effects, when starsovertake and ionize leading, piled-up clumps of neutral gas.
|Near-Infrared Polarization Images of the Orion Nebula|
Wide-field (~8'×8') and deep near-infrared(JHKs bands) polarization images of the Orion Nebula arepresented. These data revealed various circumstellar structures asinfrared reflection nebulae (IRNe) around young stellar objects (YSOs),both massive and low-mass. We found the IRN around both IRc2 and BN tobe very extensive, suggesting that there might be two extended (>0.7pc) bipolar/monopolar IRNe in these sources. We discovered at least 13smaller scale (~0.01-0.1 pc) IRNe around less massive YSOs, includingthe famous source θ2 Ori C. We also suggest thepresence of many unresolved (<690 AU) systems around low-mass YSOsand young brown dwarfs showing possible intrinsic polarizations.Wide-field infrared polarimetry is thus demonstrated to be a powerfultechnique in revealing IRNe and hence potential disk/outflow systemsamong high-mass to substellar YSOs.
|A Multiwavelength Study of M17: The Spectral Energy Distribution and PAH Emission Morphology of a Massive Star Formation Region|
We combine diffuse emission photometry from GLIMPSE and several otherGalactic plane surveys covering near-IR through radio wavelengths tosynthesize a global spectral energy distribution (SED) for the M17complex. By balancing the integrated flux in the SED with the totalbolometric luminosity of all known O and early B stars in the ionizingcluster, we estimate a distance to M17 of1.6+0.3-0.1 kpc. At this distance, the observedtotal flux in the SED corresponds to a luminosity of2.4+/-0.3×106 Lsolar. We find that the SEDfrom the H II region peaks at shorter wavelengths and has aqualitatively different shape than the SED from the photodissociationregion (PDR). We find that polycyclic aromatic hydrocarbons (PAHs) aredestroyed over a short distance or edge at the boundary of the H IIregion. We demonstrate that this PAH destruction edge can be locatedeasily using GLIMPSE band-ratio images and confirm this using SpitzerIRS spectra. We investigate the relative roles of extreme ultraviolet(EUV) and X-ray photons in the destruction of PAHs, concluding thatX-rays are not an important PAH destruction mechanism in M17 or, byextension, in any other Galactic H II region. Our results support thehypothesis that PAHs are destroyed by EUV photons within H II regions.PAHs dominate the mid-IR emission in the neutral PDR beyond the ionizedgas.
|A Very High Spectral Resolution Study of Ground-State OH Masers in W3(OH)|
We present VLBA observations of the ground-state hydroxyl masers inW3(OH) at 0.02 km s-1 spectral resolution. Over 250 masersare detected, including 56 Zeeman pairs. Line shapes are predominantlyGaussian or combinations of several Gaussians, with normalizeddeviations typically of the same magnitude as in masers in otherspecies. Typical FWHM maser line widths are 0.15-0.38 km s-1and are larger in the 1665 MHz transition than in the other threeground-state transitions. The satellite-line 1612 and 1720 MHz masersshow no evidence of σ+/-2,3 components. The spatialpositions of most masers are seen to vary across the line profile, withmany spots showing clear, organized positional gradients. Equivalentline-of-sight velocity gradients in the plane of the sky typically rangefrom 0.01 to 1 km s-1 AU-1 (i.e., positionalgradients of 1-100 AU [km s-1]-1). Small velocitygradients in the 1667 MHz transition support theoretical predictionsthat 1667 MHz masers appear in regions with small velocity shifts alongthe amplification length. Deconvolved maser spot sizes appear to belarger in the line wings but do not support a spherical maser geometry.
|Large-Scale Flows from Orion South1,|
Multiple optical outflows are known to exist in the vicinity of theactive star formation region called Orion South (Orion-S). We havemapped the velocity of low-ionization features in the brightest part ofthe Orion Nebula, including Orion-S, and imaged the entire nebula withthe Hubble Space Telescope. These new data, combined with recenthigh-resolution radio maps of outflows from the Orion-S region, allow usto trace the origin of the optical outflows. It is confirmed that HH 625arises from the blueshifted lobe of the CO outflow from 136-359 inOrion-S, while it is likely that HH 507 arises from the blueshifted lobeof the SiO outflow from the nearby source 135-356. It is likely thatredshifted lobes are deflected within the photon-dominated region behindthe optical nebula. This leads to a possible identification of a newlarge shock to the southwest of Orion-S as being driven by theredshifted CO outflow arising from 137-408. The distant object HH 400 isseen to have two even further components, and these all are probablylinked to either HH 203, HH 204, or HH 528. Distant shocks on the westside of the nebula may be related to HH 269. The sources of multiplebright, blueshifted Herbig-Haro objects (HH 202, HH 203, HH 204, HH 269,and HH 528) remain unidentified, in spite of earlier claimedidentifications. Some of this lack of identification may arise from thefact that deflection in radial velocity can also produce a change indirection in the plane of the sky. The best way to resolve this openquestion is through improved tangential velocities of low-ionizationfeatures arising where the outflows first break out into the ionizednebula.Based on observations obtained at the Kitt Peak National Observatory,which is operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the National ScienceFoundation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Diffuse Ionized Gas inside the Dwarf Irregular Galaxy NGC 6822|
We have studied the differences between the diffuse ionized gas (DIG)and the H II regions along a slit position in the local dwarf irregulargalaxy NGC 6822. The slit position passes through the two most prominentH II regions: Hubble V and Hubble X. Important differences have beenfound in the excitation, ionization, and [N II] λ6584/Hαand [S II] λ6717/Hα line ratios between the DIG and the HII locations. Moreover, the values of all the line ratios are notsimilar to those in the DIG locations of spiral galaxies but are verysimilar to the values in other irregular galaxies, such as IC 10. Wealso determined the rate of recombination using the He I λ5875line. Finally, we obtained a picture of the ionization sources of theDIG. We consider that the leakage of photons from the H II regions mightexplain most of the line ratios, except [N II]/Hα, which might beexplained by turbulence.Based on observations collected at the European Southern Observatory,Chile, proposal 69.C-0203(A).
|Dynamical H II Region Evolution in Turbulent Molecular Clouds|
We present numerical radiation-hydrodynamic simulations of the evolutionof H II regions formed in an inhomogeneous medium resulting fromturbulence simulations. We find that the filamentary structure of theunderlying density distribution produces a highly irregular shape forthe ionized region, in which the ionization front escapes to largedistances in some directions within 80,000 years. In other directions,on the other hand, neutral gas in the form of dense globules persistswithin 1 pc of the central star for the full duration of our simulation(400,000 years). Divergent photoablation flows from these globulesmaintain an rms velocity in the ionized gas that is close to the ionizedsound speed. Simulated images in optical emission lines showmorphologies that are in strikingly detailed agreement with thoseobserved in real H II regions.
|The UKIRT wide-field camera|
Context: The infrared wide-field camera (WFCAM) is now in operation onthe 3.8 m UK Infrared Telescope on Mauna Kea. WFCAM currently has thefastest survey speed of any infrared camera in the world, and combinedwith generous allocations of telescope time, will produce deep maps ofthe sky from Z to K band. The data from a set of public surveys, knownas UKIDSS, will be initially available to astronomers in ESO memberstates, and later to the world. Aims: In order to maximise surveyspeed, the WFCAM field of view was required to be as large as possiblewhile incorporating conventional infrared-instrument design featuressuch as a cold re-imaged pupil stop and cryogenic optics andmechanisms. Methods: The solution adopted was to build a cryogenicSchmidt-type camera, mounted forward of the primary mirror, whichilluminates a very large 0.9° diameter focal plane, containing four2k × 2k HgCdTe Rockwell detectors. Results: Followingseveral commissioning periods during which the camera, focal plane andtelescope optical axes were successfully co-aligned, WFCAM now operatesclose to specifications, regularly achieving 0.7´´ FWHMimages over the full field. Projects which already report excellentresults include the detection of variability in young stellar clusters,as well as preliminary deep IR imaging of the Subaru and XMM-Newton deepfield.
|Determining the H+ Region / PDR Equation of State in Star-forming Regions|
The emission-line regions of starburst galaxies and active nuclei reveala wealth of spectroscopic information. A unified picture of therelationship among ionized, atomic, and molecular gas makes it possibleto better understand these observations. We performed a series ofcalculations designed to determine the equation of state-therelationship among density, temperature, and pressure-throughemission-line diagnostic ratios that form in the H+ regionand the photodissociation region (PDR). We consider a wide range ofphysical conditions in the H+ region. We connect theH+ region to the PDR by considering two constant pressurecases: one with no magnetic field and one in which the magnetic fieldoverwhelms the thermal pressure. We show that diagnostic ratios canyield the equation of state for single H+ regions adjacent tosingle PDRs, with the results being more ambiguous when consideringobservations of entire galaxies. As a test, we apply our calculations tothe Orion H+/PDR region behind the Trapezium. We find theratio of thermal to magnetic pressure in the PDR to be ~1.2. If magneticand turbulent energy are in equipartition, our results mean that themagnetic field is not the cause of the unexplained broadening in M42,but may significantly affect line broadening in the PDR. Since Orion isoften used to understand physical processes in extragalacticenvironments, our calculations suggest that magnetic pressure should beconsidered in modeling such regions.
|Visual/infrared interferometry of Orion Trapezium stars: preliminary dynamical orbit and aperture synthesis imaging of the θ1 Orionis C system|
Context: Located in the Orion Trapezium cluster, θ^1Ori C is oneof the youngest and nearest high-mass stars (O5-O7) known. Besides itsunique properties as a magnetic rotator, the system is also known to bea close binary. Aims: By tracing its orbital motion, we aim todetermine the orbit and dynamical mass of the system, yielding acharacterization of the individual components and, ultimately, also newconstraints for stellar evolution models in the high-mass regime.Furthermore, a dynamical parallax can be derived from the orbit,providing an independent estimate for the distance of the Trapeziumcluster. Methods: Using new multi-epoch visual and near-infraredbispectrum speckle interferometric observations obtained at the BTA 6 mtelescope, and IOTA near-infrared long-baseline interferometry, wetraced the orbital motion of the θ^1Ori C components over theinterval 1997.8 to 2005.9, covering a significant arc of the orbit.Besides fitting the relative position and the flux ratio, we appliedaperture synthesis techniques to our IOTA data to reconstruct amodel-independent image of the θ^1Ori C binary system. Results:The orbital solutions suggest a highly eccentricity (e≈0.91) andshort-period (P≈10.9 yrs) orbit. As the current astrometric data onlyallows rather weak constraints on the total dynamical mass, we presentthe two best-fit orbits. Of these two, the one implying a system mass of48 Mȯ and a distance of 434 pc to the Trapezium clustercan be favored. When also taking the measured flux ratio and the derivedlocation in the HR-diagram into account, we find good agreement for allobservables, assuming a spectral type of O5.5 for θ^1Ori C1 (M =34.0 Mȯ, T_eff = 39 900 K) and O9.5 for C2 (M = 15.5Mȯ, T_eff = 31 900 K). Using IOTA, we also obtainedfirst interferometric observations on θ^1Ori D, finding someevidence for a resolved structure, maybe by a faint, close companion. Conclusions: We find indications that the companion C2 is massiveitself, which makes it likely that its contribution to the intense UVradiation field of the Trapezium cluster is non-negligible. Furthermore,the high eccentricity of the preliminary orbit solution predicts a verysmall physical separation during periastron passage (˜1.5 AU, nextpassage around 2007.5), suggesting strong wind-wind interaction betweenthe two O stars.
|A New Water Vapor Megamaser|
We report on the detection of a new megamaser, the31,3-22,0 H2O line(ν0=183.310 GHz) in Arp 220, using the Institut deRadioastronomie Millimétrique (IRAM) 30 m telescope. The line isabout 350 km s-1 wide with a total luminosity of ~2.5 ×108 K km s-1 pc2. Although OHmegamasers were first discovered in this source, no emission is seen inthe 61,6-52,3 H2O transition(ν0=22.235 GHz), a line otherwise detected as a megamaserin about 50 sources to date. This fact puts interesting constraints onthe physical conditions of the central region of Arp 220 that arefurther strengthened by the HCN and HNC J=3-2 and J=1-0 luminosities [inthe range (1.5-10) × 108 K km s-1pc2]. A scenario with ~106 star-forming coressimilar to those found in Sgr B2 in the central kiloparsec of Arp 220would be compatible with these data and would explain the lack of 22 GHzH2O emission. This result opens up the possibility of usingthe 183 GHz H2O line as an additional tool to explore thephysical conditions in luminous and ultraluminous infrared galaxies(LIRGs and ULIRGs, respectively) and their starburst or active galacticnucleus (AGN) nature, with a potential interest for high angularresolution observations with the Atacama Large Millimeter Array (ALMA).
|A Correlation between Pre-Main-Sequence Stellar Rotation Rates and IRAC Excesses in Orion|
Early observations of T Tauri stars suggested that stars with evidenceof circumstellar accretion disks rotated slower than stars without suchevidence, but more recent results are not as clear. Near-IRcircumstellar disk indicators, although the most widely available, aresubject to uncertainties that can result from inner disk holes and/orthe system inclination. Mid-infrared observations are less sensitive tosuch effects, but until now, these observations have been difficult toobtain. The Spitzer Space Telescope now easily enables mid-infraredmeasurements of large samples of PMS stars covering a broad mass rangein nearby star-forming regions. Megeath and collaborators surveyed theOrion Molecular Clouds (~1 Myr) with the IRAC instrument (3.6, 4.5, 5.8,8 μm) as part of a joint IRAC and MIPS GTO program. We examine therelationship between rotation and Spitzer mid-IR fluxes for ~900 starsin Orion for stars between 3 and 0.1 Msolar. We find in theseSpitzer data the clearest indication to date that stars with longerperiods are more likely than those with short periods to have IRexcesses suggestive of disks.
|Herbig-Haro flows in L1641N|
Aims.To study the Herbig-Haro (HH) flows in L1641N,an active star formation region in the southern part of the Orion GMCand one of the most densely populated regions of HH objects in theentire sky. By mapping the velocities of these HH objects, combined withmid-IR observations of the young stars, the major flows in the regionand the corresponding outflow sources can be revealed. Methods: Wehave used the 2.56 m Nordic Optical Telescope (NOT) to observe two deepfields in L1641N, selected on the basis of previousshock studies, using the 2.12 μm transition of H2 (and aKS filter to sample the continuum) for a total exposure timeof 4.6 h (72 min K_S) in the overlapping region. The resultinghigh-resolution mosaic (0.23 arcsec pixel size, 0.75 arcsec seeing)shows numerous new shocks and resolves many known shocks into multiplecomponents. Using previous observations taken 9 yr earlier we calculatea proper motion map and combine this with Spitzer 24 μm observationsof the embedded young stars. Results: The combined H2 mosaic showsmany new shocks and faint structures in the HH flows. From the propermotion map we find that most HH objects belong to two major bi-polar HHflows, the large-scale roughly North-South oriented flow from centralL1641N and a previously unseen HH flow in easternL1641N. Combining the tangential velocity map withthe mid-IR Spitzer images, two very likely outflow sources are found.The outflow source of the eastern flow, L1641N-172, is found to be thecurrently brightest mid-IR source in L1641N and seemto have brightened considerably during the past 20 yr. We make the firstdetection of this source in the near-IR (K_S) and also find a near-IRreflection nebula pointing at the source, probably the illuminated wallsof a cone-shaped cavity cleared out by the eastern lobe of the outflow.Extending a line from the eastern outflow source along the proper motionvector we find that HH 301 and HH302 (almost 1 pc away) belong to this new HH flow.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.This work is based in part on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract with NASA.
|The interstellar medium of the Antennae galaxies|
To study the properties of the interstellar medium in the prototypicalmerging system of the Antennae galaxies (NGC 4038 and NGC 4039), we haveobtained 12CO (1 0), (2 1) and (3 2) line maps, as well as amap of the 870 μm continuum emission. Our results are analysed inconjunction with data from X-ray to radio wavelengths. In order todistinguish between exact coincidence and merely close correspondence ofemission features, we compare the morphological structure of thedifferent emission components at the highest available angularresolution. To constrain the physical state of the molecular gas, weapply models of photon dominated regions (PDRs) that allow us to fit COand [Cii] data, as well as other indicators of widespread PDRs in theAntennae system, particularly within the super giant molecular cloud(SGMC) complexes of the interaction region (IAR) between the twogalaxies. The modeled clouds have cores with moderately high gasdensities up to 4× 104 cm-3 and rather low kinetictemperatures (≤25 K). At present, all these clouds, including thosenear the galactic nuclei, show no signs of intense starburst activity.Thermal radio or mid-infrared emission are all observed to peak slightlyoffset from the molecular peaks. The total molecular gas mass of theAntennae system adds up to ~1010 Mȯ. In thevicinity of each galactic nucleus, the molecular gas mass, 1-2×10^9~Mȯ, exceeds that of the Galactic centre region by afactor of almost 100. Furthermore, the gas does not seem to deviate muchfrom the N_H_2/I_CO ratio typical of the disk of our Galaxy rather thanour Galactic centre. Alternative heating mechanisms with respect to PDRheating are discussed.
|Discovery of an Extreme MeV Blazar with the Swift Burst Alert Telescope|
The Burst Alert Telescope (BAT) on board Swift detected bright emissionfrom 15-195 keV from the source SWIFT J0746.3+2548 (J0746 in thefollowing), identified with the optically faint (R~19), z=2.979 quasarSDSS J074625.87+244901.2. Here we present Swift and multiwavelengthobservations of this source. The X-ray emission from J0746 is variableon timescales of hours to weeks in 0.5-8 keV and of a few months in15-195 keV, but there is no accompanying spectral variability in the0.5-8 keV band. There is a suggestion that the BAT spectrum, initiallyvery hard (photon index Γ~0.7), steepened to Γ~1.3 in a fewmonths, together with a decrease of the 15-195 keV flux by a factor ~2.The 0.5-8 keV continuum is well described by a power law withΓ~1.3 and spectral flattening below 1 keV. The latter can bedescribed with a column density in excess of the Galactic value withintrinsic column density NzH~1022cm-2, or with a flatter power law, implying a sharp(ΔΓ>~1) break across 16 keV in the quasar's rest frame.The spectral energy distribution of J0746 is double-humped, with thefirst component peaking at IR wavelengths and the second component atMeV energies. These properties suggest that J0746 is a blazar with highgamma-ray luminosity and low peak energy (MeV), stretching the blazarsequence to an extreme.
Submit a new link
Member of following groups:
Observation and Astrometry data
Catalogs and designations: