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M43 (De Mairan's Nebula)


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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.

Irradiated and Bent Jets in the Orion Nebula
We present new images and proper-motion measurements of irradiatedoutflows from young stars in the outskirts of the Orion Nebula obtainedwith the Advanced Camera for Surveys onboard the Hubble Space Telescope.The images reveal many new outflows and new details in previouslydetected flows. The large-scale bipolar flows such as HH 502, 505, 874,and 876 contain multiple shocks whose velocities systematically decreasewith increasing distance from their sources as the flows bend away fromthe core of the Orion Nebula. We consider several mechanisms for bendingjets, including radiation pressure, the rocket effect on a neutral jet,and the impact of a side wind. While mild bends may be explained byeither radiation pressure or the rocket effect, the extreme bends ofsome Orion jets and LL Ori-type bow shocks are best explained by theinteraction of these outflows with a large-scale flow from the nebularcore. The jet Hα emission measures and geometry are used toestimate their mass-loss rates, which range from aboutM˙~10-9 to 10-6 Msolaryr-1. Many of the quasi-parabolic bow shocks that wrap aroundyoung stars in Orion (the LL Ori-type objects) exhibit large gapsbetween the observed jets and the parabolic fronts facing the core ofthe nebula. These may indicate the additional action of a wide-angleoutflow component such as a T Tauri stellar wind, a proplydphotoablation flow, or possibly a photoablation flow from the jetitself. The Hα surface brightness and radii of the LL Ori frontsare used to estimate that the M˙Vw products of thewide-angle flow components have values around 10-6Msolar yr-1 km s-1. The side-winddensity and velocity are also constrained. Outside the nebular core, thegreatest concentration of bent jets and LL Ori-type parabolic fronts islocated south and west of the Trapezium. The nonuniform, clumpy spatialdistribution of jets, outflow sources, and proplyds in the outskirts ofthe Orion Nebula indicate that star formation occurred in smallhierarchical clusters.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 NAS5-26555.

Star formation studies at TIFR
Studies of Galactic star forming regions being carried out at TIFR, issummarized. The techniques and typical results from these studies havebeen described brie y, which include: the balloon borne far infraredmapping in broad photometric bands (150 & 210mu m) as well asspectroscopic imaging in the fine structure [C II] line at 158 m; nearinfrared imaging in broad and narrow bands using the Mt. Abu InfraredTelescope & Himalayan Chandra Telescope; high resolution radiointerferometric imaging in the 330-1400 MHz bands using GMRT; and use ofdata products from international space missions like Midcourse SpaceExperiment (MSX).

New Silhouette Disks with Reflection Nebulae and Outflows in the Orion Nebula and M43
We report the detection of several new circumstellar disks seen insilhouette against background nebular light in the outskirts of theOrion Nebula and the neighboring H II region M43. These were detected aspart of our Hα survey of Orion with the Advanced Camera forSurveys on board the Hubble Space Telescope. Several of the disks showbipolar reflection nebulae, microjets, or pronounced temporalvariability of their central stars. The relatively large fraction ofbipolar reflection nebulae and microjets in our sample may be aselection effect caused by the faint nebular background far from theTrapezium. Two disks in our sample are large and particularlynoteworthy: A nearly edge-on disk, d216-0939, is located severalarcminutes northwest of M43 and resembles the famous HH 30 disk/jetsystem in Taurus. It drives the 0.15 pc long bipolar outflow HH 667 andexhibits a remarkable asymmetric reflection nebula caused by the tilt ofthe flared disk. With a diameter of ~2.6" (1200 AU), d216-0939 is aslarge as the giant edge-on silhouette disk d114-426 in the core of theOrion Nebula. The large disk d253-1536 is located in a binary systemembedded within an externally ionized giant proplyd in M43. The diskexhibits distortions that we attribute to tidal interactions with thecompanion star. The bipolar jet HH 668 emerges from the proplydionization front in a direction orthogonal to the disk and can be tracedto the young star embedded within it. A bow shock lies 54" south of thisbinary system along the outflow axis. Proper motions over a 1.4 yrbaseline confirm that these emission knots are indeed moving away fromd253-1536 with speeds as high as ~330 km s-1 in the HH 668microjet and slower motion farther from the star.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555.

Size distribution of circumstellar disks in the Trapezium cluster
In this paper we present results on the size distribution ofcircumstellar disks in the Trapezium cluster asmeasured from HST/WFPC2 data. Direct diameter measurements of a sampleof 135 bright proplyds and 14 silhouettes disks suggest that there is asingle population of disks well characterized by a power-lawdistribution with an exponent of -1.9 ± 0.3 between diskdiameters 100-400 AU. For the stellar mass sampled (from late G to lateM stars) we find no obvious correlation between disk diameter andstellar mass. We also find that there is no obvious correlation betweendisk diameter and the projected distance to the ionizing Trapezium OBstars. We estimate that about 40% of the disks in the Trapezium haveradius larger than 50 AU. We suggest that the origin of the Solarsystem's (Kuiper belt) outer edge is likely to be due to the starformation environment and disk destruction processes (photoevaporation,collisions) present in the stellar cluster on which the Sun was probablyformed. Finally, we identified a previously unknown proplyd and named it266-557, following convention.

Infrared Imaging of the Large Magellanic Cloud Star-forming Region Henize 206
Henize 206 is a region of star formation in the Large Magellanic Cloudof the approximate scale of the Orion belt and sword. Our Spitzer SpaceTelescope infrared images and Cerro Tololo Inter-American Observatory(CTIO) optical images show that the region is experiencing veryenergetic star formation. The radiation from young stars has excitedstrong polycyclic aromatic hydrocarbon (PAH) emission throughout Henize206, except on the side of the nebula with the prominent young supernovaremnant. As is also seen in early Spitzer observations of M81, starformation rates calculated from Hα for Henize 206 may miss thedeeply embedded young stars, compared with star formation ratescalculated from far infrared emission. For one of the highest surfacebrightness regions of Henize 206, we obtained snapshot exposures withthe Thermal-Region Camera Spectrograph on Gemini South to explore thecomplex structure. A few percent of the total flux from this brightestregion in Henize 206 emanates from infrared peaks of subparsec scale.

A Map of OMC-1 in CO J = 9-->8
The distribution of 12C16O J=9-->8 (1.037 THz)emission has been mapped in OMC-1 at 35 points with 84" resolution. Thisis the first map of this source in this transition and only the secondvelocity-resolved ground-based observation of a line in the THzfrequency band. There is emission present at all points in the map, aregion roughly 4'×6' in size, with peakantenna temperature dropping only near the edges. Away from the Orion KLoutflow, the velocity structure suggests that most of the emission comesfrom the OMC-1 photon-dominated region, with a typical line width of 3-6km s-1. Large velocity gradient modeling of the emission inJ=9-->8 and six lower transitions suggests that the lines originatein regions with temperatures around 120 K and densities of at least103.5 cm-3 near θ1C Ori and atthe Orion bar, and from 70 K gas at around 104cm-3 southeast and west of the bar. These observations areamong the first made with the 0.8 m Smithsonian AstrophysicalObservatory Receiver Lab Telescope, a new instrument designed to observeat frequencies above 1 THz from an extremely high and dry site innorthern Chile.

Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lyingbetween us and the main ionizing stars of the Orion Nebula. It isvisible in 21 cm H I absorption and in optical and UV absorption linesof H I and other species. Toward θ1 Ori C, the veilhas two remarkable properties, a high magnetic field (~100 μG) and asurprising lack of H2, given its total column density. Herewe compute photoionization models of the veil to establish its gasdensity and its distance from θ1 Ori C. We use agreatly improved model of the H2 molecule that determineslevel populations in 105 rotational/vibrational levels andprovides improved estimates of H2 destruction via theLyman-Werner bands. Our best-fit photoionization models place the veil1-3 pc in front of the star at a density of103-104 cm-3. Magnetic energy dominatesthe energy of nonthermal motions in at least one of the 21 cm H Ivelocity components. Therefore, the veil is the first interstellarenvironment in which magnetic dominance appears to exist. We find thatthe low ratio of H2/H0 (<10-4) is aconsequence of high UV flux incident on the veil due to its proximity tothe Trapezium stars and the absence of small grains in the region.

Radio Continuum Jets from Protostellar Objects
We have carried out a deep, 3.6 cm radio continuum survey of youngoutflow sources using the Very Large Array in its A configurationproviding subarcsecond resolution. The eight regions observed are Haro6-10 and L1527 IRS in Taurus, Haro 5a/6a in OMC 2/3, NGC 2023 MMS, NGC2264 IRS1, HH 108 IRAS/MMS in Serpens, L1228, and L1251A. In combinationwith our similar and previously published maps of eight otherstar-forming regions, we find only one region with a single source,while the other 15 regions have on average 3.9 nearby sources. Thissupports the view that isolated star formation is rare. We have selected21 objects, which are all young mostly Class I sources, and find abinary frequency of 33% in the separation range from 0.5" to 12". Thisis within the uncertainties comparable to the observed binary frequencyamong T Tauri stars in a similar separation range. Seven of the 21sources drive giant Herbig-Haro flows. Four of these seven are known tohave companions (three are triple systems), corresponding to 57%. Wediscuss these results in relation to the hypothesis that giantHerbig-Haro flows are driven by disintegrating multiple systems.

Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.

Mapping of large scale 158 mu m [CII] line emission: Orion A
We present the first results of an observational programme undertaken tomap the fine structure line emission of singly ionized carbon ([CII]157.7409 mu m) over extended regions using a Fabry Perot spectrometernewly installed at the focal plane of a 100 cm balloon-bornefar-infrared telescope. This new combination of instruments has avelocity resolution of ~ 200 km s-1 and an angularresolution of 1.5 arcmin. During the first flight, an area of 30 arcminx 15 arcmin in Orion A was mapped. These observations extend over alarger area than previous observations, the map is fully sampled and thespectral scanning method used enables reliable estimation of thecontinuum emission at frequencies adjacent to the [CII] line. The total[CII] line luminosity, calculated by considering up to 20% of themaximum line intensity is 0.04% of the luminosity of the far-infraredcontinuum. We have compared the [CII] intensity distribution with thevelocity-integrated intensity distributions of 13CO(1-0),CI(1-0) and CO(3-2) from the literature. Comparison of the [CII], [CI]and the radio continuum intensity distributions indicates that thelargescale [CII] emission originates mainly from the neutral gas, exceptat the position of M 43, where no [CI] emission corresponding to the[CII] emission is seen. Substantial part of the [CII] emission from hereoriginates from the ionized gas.The observed line intensities and ratios have been analyzed using thePDR models by \citet{kaufman99} to derive the incident UV flux andvolume density at a few selected positions. The models reproduce theobservations reasonably well at most positions excepting the [CII] peak(which coincides with the position of theta 1 Ori C).Possible reason for the failure could be the simplifying assumption of ahomogeneous plane parallel slab in place of a more complicated geometry.

Urban Astronomy: Observing the Messier Objects from the City
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Iron abundance in Galactic H II regions
Some recent results point to a possible discrepancy between the Feabundance in M42 as derived from the emission of the different Fe ions,but it is argued that the uncertainties are currently too high to decidewhether the discrepancy is real. The results of a recent analysis of theFe abundance in 7 Galactic H II regions are discussed as well as theimplications they could have for dust in H II regions.

Iron abundance in H II regions
Optical CCD spectra are used to determine the \element{Fe} abundances atseveral positions inside seven bright Galactic H Ii regions. Theobserved [Fe Iii] line ratios are compared with the predictions ofdifferent sets of collision strengths and transition probabilities forthis ion to select the atomic data providing the best fit to theobservations. The values found for the \element[++]{Fe} and\element[+]{Fe} abundances, along with ionization correction factors forthe contribution of \element[3+]{Fe}, obtained from available grids ofphotoionized models, imply that the Fe/O ratio in the ionized gas isbetween 2% and 30% of solar. The \element{Fe} abundances derived foreach area are correlated both with the degree of ionization and thecolour excess. A possible explanation is suggested, namely the presenceof a population of small grains, probably originating from thefragmentation of larger grains. These small grains would release\element{Fe} atoms into the gas after the absorption of energeticphotons; the small grains surviving this destruction process would beswept out of the ionized region by the action of radiation pressure orstellar winds. An indication of a further and more efficient destructionagent is given by the high \element{Fe} abundance derived for a positionsampling the optical jet H 399 in M20, where dust destruction due to shock waves has presumablytaken place. Based on observations made with the Isaac Newton Telescope,operated on the island of La Palma by the Isaac Newton Group in theSpanish Observatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.

Mapping of the Orion molecular cloud in the [CII] line and continuum at 158 μm
A fabry Perot Spectrometer (FPS) tuned to the astrophysicallyinteresting (CII) line at 157.7409 ?m, developed at ISAS (Japan) hasbeen successfully interfaced to the TIFR 100 cm far-infrared balloonborne telescope. A region (30? x 15?) of the Orion molecular cloud wasmapped with a spectral resolution of 1700 and angular resolution of ~1?. The spatial distribution of (CII) line emission at 157.7409 ?m andthe neighboring dust continuum are presented here.

A High-Density Thin Layer Confining the H II Region M42: Heinrich Hertz Telescope Measurements
We present Heinrich Hertz Telescope observations in the N=3-->2rotational transition of the CN radical toward selected positions of theTrapezium region and of the molecular ridge in the Orion molecularcloud. Two of the positions in the ridge were also observed in theN=2-->1 line of CN and 13CN. The N=3-->2 CN lines havebeen combined with observations of the N=2-->1 and N=1-->0transitions of CN and observations of the N=2-->1 of 13CNin order to estimate the physical conditions and CN abundances in themolecular gas. We analyze in detail the excitation of the CN lines andfind that the hyperfine ratios of the N=3-->2 line are always closeto the LTE optically thin values even in the case of optically thickemission. This is due to different excitation temperatures for thedifferent hyperfine lines. From the line-intensity ratios between thedifferent CN transitions we derive H2 densities of~105 cm-3 for the molecular ridge and densities of~3×106 cm-3 for the Trapezium region. The CNcolumn densities are 1 order of magnitude larger in the ridge than inthe Trapezium region, but the CN to H2 ratio is similar inboth the Trapezium and the ridge. The combination of the low CN columndensities, high H2 densities, and relatively high CNabundances toward the Trapezium region requires that the CN emissionarise from a thin layer with a depth along the line of sight of only~5×1015 cm. This high-density thin layer of moleculargas seems to be related to material that confines the rear side of the HII region Orion A. However, the molecular layer is not moving asexpected from the expansion of the H II region but is ``static'' withrespect to the gas in the molecular cloud. We discuss the implication ofa high-density ``static'' layer in the evolution of an H II region. Thiswork is based on measurements made with the Heinrich Hertz Telescope,which is operated by the Submillimeter Telescope Observatory on behalfof Steward Observatory and the Max-Planck-Institut fürRadioastronomie.

The Rich Ultraviolet Spectrum of Vibrationally Excited Interstellar H2 toward HD 37903
Based on recent high-resolution Hubble Space Telescope Space TelescopeImaging Spectrograph observations over the 1160-1360 Å wavelengthregion, we have discovered that HD 37903 (the illuminating star of thebright reflection nebula NGC 2023 in Orion) exhibits a rich spectrum ofvibrationally excited interstellar H2 absorption. We haveidentified a total of over 500 H2 lines arising from variousrotational levels (as high as J=13) in all 14 excited vibrational statesof the ground electronic state. Prior to this study, the only publishedUV absorption measurements of vibrationally excited interstellarH2 consisted of two weak lines toward the star ζOphiuchi. In terms of abundance, the v=3 J=1 H2 columndensity toward HD 37903 is over 200 times greater than that towardζ Oph. Overall, the populations of the excited H2rovibrational states toward HD 37903 exhibit a nonthermal distributionthat is consistent with the UV fluorescent excitation of dense(nH~104 cm-3) molecular gas locatedabout 0.5 pc from the B1.5 V star. We contrast these results with thoseprovided by near-IR H2 emission-line studies of NGC 2023 anddiscuss opportunities for future UV observations of vibrationallyexcited H2 toward HD 37903 and other sight lines. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationfor Research in Astronomy, Inc., under NASA contract NAS5-26555.

Irradiated Herbig-Haro Jets in the Orion Nebula and near NGC 1333
We report the discovery of a dozen Herbig-Haro jets illuminated by theLyman continuum (λ<912 Å) and/or softer far-ultraviolet(912<λ<2000 Å) radiation fields of nearby high-massstars. Five irradiated outflows lie in the outer parts of the OrionNebula (HH 502-506), and seven lie near the reflection nebula NGC 1333in the Perseus molecular cloud (HH 333-336 and HH 497-499). Thesestellar outflows are powered by optically visible low-mass young starsthat suffer relatively low extinction and seem not to be embedded withinopaque cloud cores. We propose that the UV radiation field has erodedresidual material left over from their formation on a timescale shortcompared to the ages of these star-forming regions. Many of theirradiated jets exhibit unusual C-shaped symmetry. In the outskirts ofthe Orion Nebula, most irradiated jets appear to bend away from the coreof the nebula. On the other hand, in NGC 1333, the C-shaped jets tend tobend back toward the cluster center. Jet bending in the Orion Nebula maybe dominated by either the outflow of material from the nebular core orby the rocket effect pushing on the irradiated portion of a mostlyneutral jet beam. But in NGC 1333, jet bending may indicate that thesource stars have been ejected from the cluster core. Many irradiatedjets are asymmetric with one beam much brighter than the other. Whenfully photoionized, irradiated jets may provide unique insights into thephysical conditions within outflows powered by young stars, permittingthe determination of the density and location of stellar ejecta even inthe absence of shocks. We present a model for the photoionization ofthese outflows by external radiation fields and discuss possiblemechanisms for producing the observed asymmetries. In particular, wedemonstrate that the UV radiation field may alter the amount of cloudmaterial entrained by the jet. Radiation-induced variations in massloading and beam heating can produce differences in the beam velocitiesand spreading rates, which in turn determine the surface brightness ofthe radiating plasma. In a bipolar irradiated jet in which both beamshave the same mass-loss rate and opening angle, the slower beam willappear brighter at a given distance from the source. On the other hand,if both beams spread orthogonal to the jet propagation direction withthe same speed (e.g., both beams have the same internal sound speed orshocks with similar physical conditions), the faster beam will appearbrighter at the same distance from the source. Thus, depending on theparameters, either the faster or slower beam of a jet can be brighter.Finally, we report the discovery of some large-scale bow shocks thatface the core of the Orion Nebula and surround visible young stars.These wind-wind collision fronts provide further evidence for alarge-scale mass flow originating near the nebular core.

High-Mass, OB Star Formation in M51: Hubble Space Telescope Hα and Paα Imaging
We have obtained Hα and Paα emission-line images coveringthe central 3'-4' of M51 using the WFPC2 and NICMOS instruments on theHubble Space Telescope to study the high-mass stellar population. The0.1"-0.2" pixels provide 4.6-9 pc resolution in M51, and theHα/Paα line ratios are used to obtain extinction estimates.A sample of 1373 Hα emission regions is cataloged using anautomated and uniform measurement algorithm. Their sizes are typically10-100 pc. The luminosity function for the Hα emission regions isobtained over the range LHα=1036 to2×1039 ergs s-1. The luminosity function isfitted well by a power law with dN/dlnL~L-1.01. The power lawis significantly truncated, and no regions were found with observedLHα above 2×1039 ergs s-1(uncorrected for extinction). (The maximum seen in ground-based studiesis approximately a factor of 5 higher, very likely because of theblending of multiple regions.) The extinctions derived here increase themaximum intrinsic luminosity to above 1040 ergss-1. The logarithmically binned luminosity function is alsosomewhat steeper (α=-1.01) than that found from ground-basedimaging (α=-0.5 to -0.8)-probably also a result of our resolvingregions that were blended in the ground-based images. The two-pointcorrelation function for the H II regions exhibits strong clustering onscales <=2", or 96 pc. To analyze the variations of H II regionproperties vis-à-vis the galactic structure, the spiral arm areaswere defined independently from millimeter-CO and optical continuumimaging. Although the arms constitute only 25% of the disk surface area,the arms contain 45% of the cataloged H II regions. The luminosityfunction is somewhat flatter in spiral arm regions than in the interarmareas (-0.72 to -0.95) however, this is very likely the result ofincreased blending of individual H II regions in the arms that havehigher surface density. No significant difference is seen in the sizesand electron densities of the H II regions in spiral arm and interarmregions. For 209 regions that had >=5 σ detections in bothPaα and Hα, the observed line ratios indicate visualextinctions in the range AV=0-6 mag. The mean extinction wasAV=3.1 mag (weighting each region equally), 2.4 mag(weighting each by the observed Hα luminosity), and 3.0 mag(weighting by the extinction-corrected luminosity). On average, theobserved Hα luminosities should be increased by a factor of ~10,implying comparable increases in global OB star cluster luminosities andstar formation rates. The full range of extinction-corrected Hαluminosities is between 1037 and 2×1040 ergss-1. The most luminous regions have sizes >=100 pc, so itis very likely that they are blends of multiple regions. This is clearbased on their sizes, which are much larger than the maximum diameter(<=50 pc) to which an H II region might conceivably expand within the~3×106 yr lifetime of the OB stars. It is alsoconsistent with the observed correlation (L~D2) between themeasured luminosities and sizes of the H II regions. We thereforegenerated a subsample of 1101 regions with sizes <=50 pc, which ismade up of those regions that might conceivably be ionized by a singlecluster. Their extinction-corrected luminosities range between2×1037 and 1039 ergs s-1, orbetween two-thirds of M42 (the Orion Nebula) and W49 (the most luminousGalactic radio H II region). The upper limit for individual clusters istherefore conservatively <=1039 ergs s-1,implying QLyc,up~=7×1050 s-1(with no corrections for dust absorption of the Lyman continuum or UVthat escapes to the diffuse medium). This corresponds to cluster masses<=5000 Msolar (between 1 and 120 Msolar). Thetotal star formation rate in M51 is estimated from theextinction-corrected Hα luminosities to be ~4.2 Msolaryr-1 (assuming a Salpeter initial mass function between 1 and120 Msolar), and the cycling time from the neutralinterstellar medium into these stars is 1.2×109 yr. Wedevelop a simple model for the UV output from OB star clusters as afunction of the cluster mass and age in order to interpret constraintsprovided by the observed luminosity functions. The power-law index atthe high-luminosity end of the luminosity function (α=-1.01)impliesN(Mcl)/dMcl~M-2.01cl. Thisimplies that high-mass star formation, cloud disruption due to OB stars,and UV production are contributed to by a large range of cluster masseswith equal effects per logarithmic interval of cluster mass. Thehigh-mass clusters (~1000 Msolar) have a mass such that theinitial mass function is well sampled up to ~120 Msolar, butthis cluster mass is <=1% of that available in a typical giantmolecular cloud. We suggest that OB star formation in a cloud coreregion is terminated at the point that radiation pressure on thesurrounding dust exceeds the self-gravity of the core star cluster andthat this is what limits the maximum mass of standard OB star clusters.This occurs at a stellar luminosity-to-mass ratio of ~500-1000(L/M)solar, which happens for clusters >=750Msolar. We have modeled the core collapse hydrodynamicallyand have found that a second wave of star formation may propagateoutward in a radiatively compressed shell surrounding the core starcluster-this triggered, secondary star formation may be the mechanismfor formation of the super-star clusters seen in starburst galaxies.

New Proplyds, Outflows, Shocks, and a Reflection Nebula in M43 and the Outer Parts of the Orion Nebula
Hubble Space Telescope WFPC2 images made as planned parallelobservations have produced emission-line images of fields in the outerportions of the Orion Nebula and near the center of the companion H IIregion, M43. Examination of these images have uncovered three new brightproplyds and one silhouette proplyd. Two of the bright proplyds liewithin M43 and are photoionized by its central star, NU Ori. The newbright proplyd in the Orion Nebula shows a monopolar microjet, andanalysis of its size and surface brightness indicates that it lies wellin the foreground. Symmetric shocks indicate bipolar flow around the K8estar V1348 Ori (304-539) near θ2 Ori C. Evidence formultiple outflows from a source southeast of 036-927, well south of thebright bar feature of the nebula, is indicated. A bright reflectionnebula was found around the B1.5 Vp star LP Ori (098-753). The form ofthis object indicates that this star is moving within the veil ofneutral material that lies in front of M42. Based in part onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS 5-26555.

Radio Continuum Structure of the Orion Nebula
We have imaged the large-scale radio continuum structure in the Orionregion with the Very Large Array at 330 MHz. Arcminute-resolutionmorphology of the extended emission in the H II regions M42 and M43 (NGC1976 and NGC 1982) and in the NGC 1973-75-77 nebulosity to the north arepresented. A low surface brightness thermal radio halo is detected inthe H II region M42: comparison with an optical photograph indicatesthat the radio emission distinguishes optical emission structures fromreflection nebulosities. In NGC 1977 we have discovered a compact,steep-spectrum radio source coincident with a bright optical rim.

Flare star activity in the open cluster Alpha Persei
The results from a flare star investigation in the open cluster AlphaPersei are presented. Photographic flare star monitoring and CCDphotometry of the discovered 4 new flare stars are made. The flare staractivity phenomenon is restricted to the classical flare stars (UV Cetitype) as classified in the GCVS. The V/V-I diagramme of the members ofthe cluster with the locations of the considered flare stars is given.Most of the flare stars are probable cluster members. The flarefrequency determined from the Rozhen flare star monitoring is very low -one flare event occurs for 38.5 hours effective observing time.Comparison with the flare activity of the Pleiades is made because ofthe small difference in the age and distance of the clusters.

A Multiwavelength Study of Outflows in OMC-2/3
We present new v=1-0 S(1) H2, 12CO J=2-->1, and12CO J=3-->2 observations of the star-forming clouds OMC-2and OMC-3, one of the densest known groupings of outflows from low-massyoung stellar objects (YSOs) in the sky. High-velocity 12COJ=2-->1 gas in this region suggests that previously discoveredH2 flows are driving and entraining molecular outflows.However, the large number of sources and flows within the narrowmolecular filament means it is difficult to make a firm association ofmolecular outflow gas with H2 flows, except for in the caseof the bipolar east-west H flow. A number of Herbig-Haro (HH) objects,including ones far to the west and east of the main ridge, areidentified with H2 knots. High-resolution spectroscopy in thev=1-0 S(1) line of 10 H2 knots shows line profiles consistentwith dual forward and reverse shocks. C-shock modeling suggest thatasymmetries seen in suspected bow shocks could be evidence of varyingmagnetic field orientations throughout the cloud. One of the bow shocksin the H flow, YBD-5, can be successfully modeled by a 100 kms-1 C-shock propagating into a magnetized, 106cm-3 medium, although the observations and limitations withinthe computer code itself do not entirely rule out J-shocks. Mass spectraof the H flow are broken power laws, which might be evidence for a jetthat has two entrainment mechanisms for accelerating ambient moleculargas into the outflow. The H2 luminosity in this flow is manytimes smaller than the CO mechanical luminosity, but this fact cannotrule out the possibility that a narrow highly collimated jet drives themolecular outflow, owing to uncertainties in extinction, outflow dynamictimes, cooling contributions from other lines, and the wind model used.Outflows from OMC-2/3 are likely to contribute to the turbulent pumpingof gas within the molecular ridge north of the Orion Nebula.High-velocity gas clumps north of the sources investigated here mayrepresent evidence of additional undiscovered outflows from young stars.

Distribution of Cold Dust in Orion A and B
Large-scale far-infrared (FIR) observations of the Orion complex at 205and 138 μm are presented with the aim of studying the distribution ofcold (<25 K) dust. The maps in these FIR bands extend over ~3600arcmin2 and cover regions around OMC-1, 2, and 3 in Orion Aand NGC 2023 and NGC 2024 in Orion B. Some limited regions have alsobeen mapped at 57 μm. A total of 15 sources in Orion A and 14 inOrion B (south) have been identified from our FIR maps. Dust temperaturedistribution in both Orion A and Orion B (south) have been determinedreliably using the maps at 205 and 138 μm obtained from simultaneousobservations using almost identical beams (1.6′ diameter). Thesetemperatures have been used to generate the map of τ150,the optical depth at 150 μm, for the Orion B region. The coldestsource detected is in OMC-3 and has temperature ~15 K. The diffuse FIRemission in the different subregions is found to vary between 25% and50% of the total FIR emission from that subregion.

JCMT/SCUBA Submillimeter Wavelength Imaging of the Integral-shaped Filament in Orion
We present the first high dynamic range and sensitivity images of thesubmillimeter wavelength continuum emission at 450 and 850 μm of the``integral-shaped filament'' in the northern portion of the Orion Acloud, which contains the nearest site of ongoing high-mass starformation. The images trace the morphology and spectral index ofoptically thin emission from interstellar dust, and they constrain thegrain temperature and emissivity. The images reveal a remarkable chainof compact sources embedded in a narrow (<1' = 0.14 pc), high columndensity filament that extends over the 50' (7 pc) length of the map,with faint extended structure surrounding it. While many compact sourcescontain extremely young protostars, others may be pre-collapse phasecloud cores. The brightest region, associated with OMC-1, contains aremarkable group of dust filaments that radiate radially away from thishigh-luminosity core and that coincide with the filaments of NH_3emission. The spectral index is uniform between 450 and 850 mum, exceptfor the ridge sources, the photoheated H II region edges including theOrion bar, and the location of molecular hydrogen shocks.

VLA Detection of Protostars in OMC-2/3
The OMC-2/3 molecular clouds contain one of the highest concentrationsof protostars known in nearby molecular clouds. We have observed an areaof about 6^'x15^' (0.8 pcx2 pc) covering the OMC-2/3 region with theVery Large Array in the D configuration at 3.6 cm, matching well thearea of a recent 1300 μm survey. We detected 14 sources, of which itis highly probable that 11 sources are either protostars or very youngstars. This testifies to the star-forming activity and extreme youth ofthe OMC-2/3 region. The 3.6 cm flux is free-free emission probably dueto shocks in outflowing material. Three of the sources are extended evenwith the relatively low resolution of the present observations, and twoof these may be collimated radio jets. The large fraction ofsubmillimeter continuum sources that have a radio continuum counterpartis evidence that outflow is already common at the very earliestevolutionary stages. No relation is found between the radio continuumflux and the 1300 μm flux of the associated submillimeter dustclumps.

The abundances of O, S, Cl, N, Ar, He and C in seven Galactic H bt II regions regions
An analysis of CCD spectra in the range lambda lambda4200 -8775,obtained at several positions of seven bright Galactic H Ii regions,with galactocentric distances in the range 6-10 kpc, is presented.Temperatures, densities and ionic abundances are calculated for all thepositions, and the relative trends followed by these parameters areestablished. It is shown that all these H Ii regions share the sameabundances of \element{O}, \element{S}, \element{Cl}, \element{N},\element{Ar} and \element{He}, within their expected uncertainties, aresult which is consistent with a flat or small abundance gradient inthe Galactic disc within +/-2 kpc from the Sun, as suggested by somestudies of the abundances in H Ii regions and B-type stars. The same setof homogeneously derived ionic abundances is also used to test theaccuracy of the fractional ionization predicted by available grids ofmodels for photoionized nebulae. Based on observations made with theIsaac Newton Telescope, operated on the island of La Palma by the IsaacNewton Group in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.

Fluorescence of [Fe II] in H II regions
A study of [Fe Ii] lines at various positions within the H Ii regionsM42 and M43 is presented. Therelative intensities of selected optical [Fe Ii] lines are shown to becorrelated with the intensity of the apparent nebular continuousspectrum. Since the continuum of H Ii regions is known to be mostlystellar radiation scattered by dust intermixed with the emitting gas,these correlations provide direct evidence for the existence offluorescent excitation in the formation process of the [Fe Ii] lines,irrespective of the prevailing physical state. Based on observationsmade with the Isaac Newton Telescope, operated on the island of La Palmaby the Isaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias.

Ionization Structure and Spectra of Iron in Gaseous Nebulae
The emission spectra and the ionization structure of the low ionizationstages of iron, Fe I--IV, in gaseous nebulae are studied. This workincludes: (i) new atomic data: photoionization cross sections, totale-ion recombination rates, excitation collision strengths, andtransition probabilities; (ii) detailed study of excitation mechanismsfor the [Fe II], [Fe III], and [Fe IV] emission, and spectroscopicanalysis of the observed IR, optical, and UV spectra; (iii) study of thephysical structure and kinematics of the nebulae and their ionizationfronts. Spectral analysis of the well observed Orion nebula is carriedout as a test case, using extensive collisional-radiative andphotoionization models. It is shown that the [Fe II] emission from theOrion nebula is predominantly excited via electron collisions in highdensity partially ionized zones; radiative fluorescence is relativelyless effective. Further evidence for high density zones is derived fromthe [O I] and [Ni II] spectral lines, as well as from the kinematicmeasurements of ionic species in the nebula. The ionization structure ofiron in Orion is modeled using the newly calculated atomic data, showingsome significant differences from previous models. The new modelsuggests a fully ionized H II region at densities on the order of $10^3$cm$^{-3}$, and a dynamic partially ionized H II/H I region at densitiesof $10^5-10^7$ \cm3. Photoionization models also indicate that theoptical [O I] and [Fe II] emission originates in high density partiallyionized regions within ionization fronts. The gas phase iron abundancein Orion is estimated from observed spectra.

CN emission in Orion. The high density interface between the H II region and the molecular cloud
We present high angular-resolution (12''-26'') large-scale mapping(19'x22') of the Orion A molecular complex in the N=1->0 andN=2->1 rotational transitions of the CN radical. The CN emission isnot only confined to the molecular ridge and the optical bar, but itreveals filamentary emission toward the north and southwest of theTrapezium cluster and surrounding M43. The morphology and the kinematicsof the CN emission supports the idea that the CN filaments represent theinterfaces between the molecular cloud and the major ionization frontsof M42 and M43. The CN lines have been used to estimate the physicalconditions of the molecular gas which confines the ionized material inM42 and M43. Surprisingly, the largest line intensity ratios between theN=1->0 and the N=2->1 lines are not observed toward the molecularridge or the ionization fronts, but towards the Trapezium cluster and inthe direction of the ionized gas in M43. Model calculations for the CNexcitation implies H_2 densities of ~ 10(5) cm(-3) toward the ridge andionization fronts and >6*E(6) cm(-3) toward the region surroundingthe Trapezium stars. This suggests that for the first time we havedetected the confining material behind M42. We estimate that the CNemission arises from a thin ( ~ 1.2*E(15) cm), dense (>6*E(6) cm(-3)) layer which might have been compressed by the expansion of the HIIregion. The morphology of the CN emission, dominated by the ionizationfronts of the HII regions, indicates that this molecule is an excellenttracer of regions affected by UV radiation. The comparison between theHC_3N and the CN abundance shows dramatic changes (more than 4 orders ofmagnitude) between the different features observed in OMC1. The[HC_3N]/[CN] abundance ratio varies from values of ~ 10(-3) for theionization fronts surrounding the HII regions, to 100 for the hot corein Orion. Our data shows that the [HC_3N]/[CN] abundance ratio is anexcellent tracer of photon dominated regions (PDRs) and hot cores withinregions of massive star formation.

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Observation and Astrometry data

Constellation:Orion
Right ascension:05h35m36.00s
Declination:-05°16'00.0"
Apparent magnitude:6.9

Catalogs and designations:
Proper NamesDe Mairan's Nebula
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MessierM 43
NGC 2000.0NGC 1982

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