目录
图像
上传图像
DSS Images Other Images
相关文章
Ultraviolet spectroscopy of the extended solar corona
The first observations of ultraviolet spectral line profiles andintensities from the extended solar corona (i.e., more than 1.5 solarradii from Sun-center) were obtained on 13 April 1979 when arocket-borne ultraviolet coronagraph spectrometer of theHarvard-Smithsonian Center for Astrophysics made direct measurements ofproton kinetic temperatures, and obtained upper limits on outflowvelocities in a quiet coronal region and a polar coronal hole. Followingthose observations, ultraviolet coronagraphic spectroscopy has expandedto include observations of over 60 spectral lines in coronal holes,streamers, coronal jets, and solar flare/coronal mass ejection (CME)events. Spectroscopic diagnostic techniques have been developed todetermine proton, electron and ion kinetic temperatures and velocitydistributions, proton and ion bulk flow speeds and chemical abundances.The observations have been made during three sounding rocket flights,four Shuttle deployed and retrieved Spartan 201 flights, and the Solarand Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy ofthe extended solar corona has led to fundamentally new views of theacceleration regions of the solar wind and CMEs. Observations with theUltraviolet Coronagraph Spectrometer (UVCS) on SOHO revealedsurprisingly large temperatures, outflow speeds, and velocitydistribution anisotropies in coronal holes, especially for minor ions.Those measurements have guided theorists to discard some candidatephysical processes of solar wind acceleration and to increase and expandinvestigations of ion cyclotron resonance and related processes.Analyses of UVCS observations of CME plasma properties and the evolutionof CMEs have provided the following: temperatures, inflow velocities andderived values of resistivity and reconnection rates in CME currentsheets, compression ratios and extremely high ion temperatures behindCME shocks, and three dimensional flow velocities and magnetic fieldchirality in CMEs. Ultraviolet spectroscopy has been used to determinethe thermal energy content of CMEs allowing the total energy budget tobe known for the first time. Such spectroscopic observations are capableof providing detailed empirical descriptions of solar energetic particle(SEP) source regions that allow theoretical models of SEP accelerationto be tailored to specific events, thereby enabling in situ measurementsof freshly emitted SEPs to be used for testing and guiding the evolutionof SEP acceleration theory. Here we review the history of ultravioletcoronagraph spectroscopy, summarize the physics of spectral lineformation in the extended corona, describe the spectroscopic diagnostictechniques, review the advances in our understanding of solar windsource regions and flare/CME events provided by ultraviolet spectroscopyand discuss the scientific potential of next generation ultravioletcoronagraph spectrometers.
| Physical parameters and wind properties of galactic early B supergiants
We present optical studies of the physical and wind properties, plus CNOchemical abundances, of 25 O9.5-B3 Galactic supergiants. We employnon-LTE, line blanketed, extended model atmospheres, which provide amodest downward revision in the effective temperature scale of early Bsupergiants of up to 1-2 kK relative to previous non-blanketed results.The so-called "bistability jump" at B1 (Teff 21 kK)from Lamers et al. is rather a more gradual trend (with large scatter)from v&infy;/vesc3.4 for B0-0.5 supergiantsabove 24 kK to v&infy;/vesc 2.5 for B0.7-1supergiants with 20 kK ≤ Teff ≤ 24 kK, andv&infy;/vesc 1.9 for B1.5-3 supergiants below20 kK. This, in part, explains the break in observed UV spectralcharacteristics between B0.5 and B0.7 subtypes as discussed by Walbornet al. We compare derived (homogeneous) wind densities with recentresults for Magellanic Cloud B supergiants and generally confirmtheoretical expectations for stronger winds amongst Galacticsupergiants. However, winds are substantially weaker than predictionsfrom current radiatively driven wind theory, especially at mid-Bsubtypes, a problem which is exacerbated if winds are already clumped inthe Hα line forming region. In general, CNO elemental abundancesreveal strongly processed material at the surface of Galactic Bsupergiants, with mean N/C and N/O abundances 10 and 5 times higher thanthe Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating thelowest degree of processing in our sample, and HD 152236 (B1.5Ia+) the highest.
| Young Stars far from the Galactic Plane: Runaways from Clusters
Quite recently, a significant number of OB stars far from the galacticplane have been found, situated at z-distances ranging from severalhundreds of pc to several kpc. The short lifetimes of these stars poseproblems for their interpretation in terms of the standard picture ofstar formation. Different mechanisms have been put forward to explainthe existence of these stars, either within the conventional view, orpostulating star formation in the galactic halo itself. These mechanismsrange from arguing that they are misidentified evolved or abnormalstars, to postulating powerful ejection mechanisms for young disk stars;in situ formation also admits several variants. We have collected fromthe literature a list of young stars far from the plane, for which theevidence of youth seems convincing. We discuss two possible formationmechanisms for these stars: ejection from the plane as the result ofdynamical evolution of small clusters (Poveda et al. 1967) and in situformation, via induced shocks created by spiral density waves (Martos etal. 1999). We compute galactic orbits for these stars, and identify thestars that could be explained by one or the other mechanism. We findthat about 90 percent of the stars can be accounted for by the clusterejection mechanism, that is, they can be regarded as runaway stars inthe galactic halo.
| Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.
| Fast Line-Profile Variability in the Spectra of O Stars
A program of the search for and analysis of profile variability in thespectra of bright O supergiants with a time resolution of 5-30 min isdescribed. Preliminary results of the spectroscopic observations of thestars lambda Ori, alpha Cam, and 19 Cep with the 1-m SpecialAstrophysical Observatory telescope in 2001 are presented. Line-profilevariability was detected in the spectra of all the stars studied;variability in the H_alpha and C III lambda5696 lines in the spectrum oflambda Ori has been found for the first time. The variability amplitudeis 4-5% for 19 Cep and 1-2% for alpha Cam and lambda Ori on time scalesfrom several hours to 3 or 4 days, and the width of the variablefeatures reaches 2 Angstroms (100 km/s). We detected cyclical variationsin the He II lambda4686 and C III lambda5696 line profiles in thespectrum of lambda Ori on time scales of 1.3-1.6 days. Rapid profilevariations on time scales of 3.5-7 h were found in the violet parts ofthe H_alpha and He I lambda4715 line profiles in the spectrum of lambdaOri A.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| On the Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329
Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB030329 obtained in 2003 May using the Subaru 8.2 m Telescope arepresented. The properties of the supernova (SN) are investigated througha comparison with spectra of the Type Ic hypernovae SN 1997ef and SN1998bw (hypernovae being a tentatively defined class of SNe with verybroad absorption features: these features suggest a large velocity ofthe ejected material and possibly a large explosion kinetic energy).Comparison with spectra of other hypernovae shows that the spectrum ofSN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame daysafter the gamma-ray burst (GRB; for z=0.1685), are similar to those ofSN 1997ef obtained ~34-42 days after the fiducial time of explosion ofthat SN. The match with SN 1998bw spectra is not as good (at rest7300-8000 Å), but again spectra obtained ~33-43 days after GRB980425 are preferred. This indicates that the SN may have intermediateproperties between SNe 1997ef and 1998bw. On the basis of the analogywith the other hypernovae, the time of explosion of SN 2003dh is thenconstrained to be between -8 and +2 days of the GRB. The Si and O PCygni lines of SN 2003dh seem comparable to those of SN 1997ef, whichsuggests that the ejected mass in SN 2003dh may match that in SN 1997ef.Polarization was marginally detected at optical wavelengths. This isconsistent with measurements of the late afterglow, implying that itoriginated mostly in the interstellar medium of the host galaxy.Based on data obtained at the Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan (NAOJ).
| Variable Na I Absorption toward ρ Leonis: Biased Neutral Formation in the Diffuse Interstellar Medium?
We present multiepoch KPNO coudé feed telescope observations ofinterstellar Na I and Ca II absorption toward the bright star ρ Leo.Comparisons of the Na I profiles observed over a period of 8 yr revealsignificant temporal variations in the Na I column in at least onecomponent, implying that there is ``structure'' at scales on the orderof the proper motion (~12 AU). Archival Hubble Space Telescope (HST)Goddard High Resolution Spectrograph observations of the C Ifine-structure excitation in the variable component suggest that thedensity is <~20 cm-3, significantly lower than thedensities inferred in past H I 21 cm and Na I studies. We suggest thatthe bulk of the trace neutral species are in the density peaks within aninterstellar cloud. The patchy distribution of these species naturallygives rise to the large fluctuations seen on scales of 10-1000 AU inpast temporal and binary studies. This picture predicts that the scalesover which fluctuations will be observed vary as a function of theionization rate of a species.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Science Institute. STScI isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555.
| Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe post-asymptotic giant branch star von Zeipel 1128 (l=42.5d, b=+78.7dd=10.2 kpc; z=10.0 kpc vLSR=-140+/-8 km s-1),located in the globular cluster Messier 3. The FUSE observations coverthe wavelength range 905-1187 Å at ~20 km s-1 (FWHM)resolution. These data exhibit many photospheric and interstellarabsorption lines, including absorption from ions associated with thewarm neutral, warm ionized, and highly ionized phases of theinterstellar medium along this sight line. We derive interstellar columndensities of H I, P II, Ar I, Fe II, Fe III, S III, and O VI, with lowerlimits for C II, C III, N I, O I, and Si II. Though the individualvelocity components within the absorption profiles are unresolved byFUSE, a comparison of the velocity distribution of depleted or ionizedspecies with the neutral species suggests that the thick-disk materialalong this sight line is infalling onto the Galactic plane, whilematerial near the plane is seen closer to rest velocities. Ionizedhydrogen represents >~12%, most likely ~45%, of the total hydrogencolumn along this sight line, most of it associated with the warmionized phase. The warm ionized and neutral media toward von Zeipel 1128have very similar gas-phase abundances and kinematics: the neutral andionized gases in this region of the thick disk are closely related.Strong O VI absorption is seen with the same central velocity as thewarm ionized gas, although the O VI velocity dispersion is much higher(σ≡sqrt(2)b=32 km s-1). Virtually all of the O VIis found at velocities where lower ionization gas is seen, suggestingthat the O VI and WNM/WIM probes are tracing different portions of thesame structures (e.g., the O VI may reside in interfaces surrounding theWNM/WIM clouds). We see no evidence for interstellar absorptionassociated with the globular cluster Messier 3 itself or with thecircumstellar environment of von Zeipel 1128. Neither high-velocitycloud absorption (with |vLSR|>~125 km s-1) norhigh velocity dispersion gas (with σ~60 km s-1) is seentoward von Zeipel 1128.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| A search for O VI in the winds of B-type stars
We have conducted a survey of FUSE spectra of 235 Galactic B-type starsin order to determine the boundaries in the H-R diagram for theproduction of the superion {O Vi} in their winds. By comparing thelocations and morphology of otherwise unidentified absorption featuresin the vicinity of the {O Vi} resonance doublet with the bona fide windprofiles seen in archival IUE spectra of the resonance lines of {N V},{Si Iv} and {C Iv}, we were able to detect blueshifted {O Vi} lines inthe spectra of giant and supergiant stars with temperature classes aslate as B1. No features attributable to {O Vi} were detected in dwarfslater than B0, or in stars of any luminosity class later than B1,although our ability to recognize weak absorption features in thesestars is severely restricted by blending with photospheric andinterstellar features. We discuss evidence that the ratio of the ionfractions of {O Vi} and {N V} is substantially different in the winds ofearly B-type stars than O-type stars.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| Identification of the naphthalene cation in space?
A painstaking comparison of laboratory gas-phase spectra of thenaphthalene cation with an extensive set of astrophysical spectra wascarried out. In the vicinity of both naphthalene features, found in thelaboratory experiment, we detected relevant interstellar features. Thestrongest of them is situated at 6710.5Å, which is reasonablyclose to the laboratory feature at 6706.5Å and which is also ofcomparable width, i.e. about 20Å. The second feature was found at6493Å - also close to the 6488.4-Å laboratory band of asimilar width. The structures can be observed only in spectra of O starsbecause of the growing stellar line contamination in colder objects. Thepossible third feature is intertwined with a strong stellar helium line,but the unusual strength of the latter suggests that this naphthalenecation band is present as well as the two former ones. We estimated thecolumn density of the carrier, for the reddening value E(B-V)=1.0, asroughly 4×1015cm-2. The lack of precisewavelength coincidence between laboratory and observed features,however, makes the identification uncertain and further laboratory andobservational works are both highly desirable.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
| A High-Resolution Survey of Interstellar K I Absorption
We present high-resolution (FWHM ~0.4-1.8 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar K Iabsorption toward 54 Galactic stars. These new K I spectra revealcomplex structure and narrow, closely blended components in many linesof sight. Multicomponent fits to the line profiles yield estimates forthe column densities, line widths, and velocities for 319 individualinterstellar cloud components. The median component width (FWHM) and thetrue median separation between adjacent components are both <~1.2 kms-1. The median and maximum individual component K I columndensities, about 4×1010 and 1012cm-2, correspond to individual component hydrogen columndensities of about 2×1020 and 1021cm-2 and E(B-V)~0.03 and 0.17, respectively. If T istypically ~100 K, then at least half the individual components havesubsonic internal turbulent velocities. We also reexamine therelationships between the column densities of K I, Na I, C I, Li I,Htot, H2, and CH. The four trace neutral speciesexhibit essentially linear relationships with each other over wideranges in overall column density. If C is uniformly depleted by 0.4 dex,then Li, Na, and K are each typically depleted by 0.6-0.7 dex. The totalline of sight values for N(K I) and N(Na I) show roughly quadraticdependences on N(Htot), but the relationships for theensemble of individual clouds could be significantly steeper. Thesequadratic (or steeper) dependences appear to rule out significantcontributions to the ionization from cosmic rays, X-rays, and/or chargeexchange with C II in most cases. Charge exchange with negativelycharged large molecules may often be more important than radiativerecombination in neutralizing most singly ionized atomic species in coolH I clouds, however-suggesting that the true ne,nH, and thermal pressures may be significantly smaller thanthe values estimated by considering only radiative recombination. BothN(CH) and N(H2) are nearly linearly proportional to N(K I)and N(Na I) [except for 1015cm-2<~N(H2)<~1019cm-2, over which H2 makes the transition to theself-shielded regime]. Those relationships appear also to hold for manyindividual components and component groups, suggesting thathigh-resolution spectra of K I and Na I can be very useful forinterpreting lower resolution molecular data. The scatter about allthese mean relationships is generally small (<~0.1-0.2 dex), ifcertain consistently ``discrepant'' sight lines are excluded-suggestingthat both the relative depletions and the relative ionization of Li, C,Na, and K are generally within factors of 2 of their mean values.Differences noted for sight lines in Sco-Oph, in the Pleiades, near theOrion Trapezium, and in the LMC and SMC may be due to differences in thestrength and/or shape of the ambient radiation fields, perhaps amplifiedby the effects of charge transfer with large molecules.
| Rejection of the C-7 Diffuse Interstellar Band Hypothesis
Using the new high-resolution (~8 km s-1) echellespectrograph on the 3.5 m telescope at the Apache Point Observatory, wehave begun a high-sensitivity survey of the diffuse interstellar bands(DIBs) in a large sample of reddened stars. Now that we are 2 years intothis long-term survey, our sample includes over 20 reddened stars thatshow at least one of the DIBs that had been suggested to be caused byC-7, based on the gas-phase measurement of theC-7 spectrum by J. P. Maier's group. Thehigh-quality astronomical data from this larger sample of stars, alongwith the spectroscopic constants from the new laboratory work recentlyreported by Maier's group, have enabled us to examine more carefully theagreement between C-7 and the DIBs. We find thatnone of the C-7 bands match the DIBs in wavelengthor expected profile. One of the DIBs (λ5748) attributed toC-7 is actually a stellar line. The two strongestDIBs attributed to C-7 (λ6270 andλ4964) are not correlated in strength, so they cannot share thesame carrier. On the whole, we find no evidence supporting thehypothesis that C-7 is a carrier of the DIBs.
| Results from UVCS and LASCO Observation of the Sungrazing Comet C/2000 C6
During 2000 February 9-10, the Large Angle Spectrometric Coronagraph andUltraviolet Coronal Spectrometer (UVCS) instruments aboard the Solar andHeliospheric Observatory observed comet C/2000 C6, a member of theKreutz family of sungrazing comets. A tail nearly 0.5 Rsolarin length was detected in Lyα emission. UVCS was able to observethe comet at four heights as it approached the Sun. A jump in theLyα brightness between 5.71 and 4.56 Rsolar suggeststhat the nucleus fragmented, exposing more area to solar illuminationand increasing the outgassing rate. We interpret the Lyαluminosity in terms of the outgassing rate and use this to estimate thediameter of the nucleus. The Lyα emission fades as H I is ionized,providing estimates of the density in the coronal streamer encounteredby the comet.
| Far-Ultraviolet Spectra of B Stars near the Ecliptic
Spectra of B stars in the wavelength range of 911-1100 Å have beenobtained with the Espectrógrafo Ultravioleta de RadiaciónDifusa (EURD) spectrograph on board the Spanish satellite MINISAT-01with ~5 Å spectral resolution. International Ultraviolet Explorer(IUE) spectra of the same stars have been used to normalize Kuruczmodels to the distance, reddening, and spectral type of thecorresponding star. The comparison of eight main-sequence stars studiedin detail (α Vir, ɛ Tau, λ Tau, τ Tau, αLeo, ζ Lib, θ Oph, and σ Sgr) shows agreement withKurucz models, but observed fluxes are 10%-40% higher than the models inmost cases. The difference in flux between observations and models ishigher in the wavelength range between Lyα and Lyβ. Wesuggest that Kurucz models underestimate the far-ultraviolet (FUV) fluxof main-sequence B stars between these two Lyman lines. Computation offlux distributions of line-blanketed model atmospheres including non-LTEeffects suggests that this flux underestimate could be due to departuresfrom LTE, although other causes cannot be ruled out. We found that thecommon assumption of solar metallicity for young disk stars should bemade with care, since small deviations can have a significant impact onFUV model fluxes. Two peculiar stars (ρ Leo and ɛ Aqr) and twoemission-line stars (ɛ Cap and π Aqr) were also studied. Ofthese, only ɛ Aqr has a flux in agreement with the models. The resthave strong variability in the IUE range and/or uncertain reddening,which makes the comparison with models difficult. Based on thedevelopment and utilization of the Espectrógrafo Ultravioleta deRadiación Difusa, a collaboration of the Spanish InstitutoNacional de Técnica Aeroespacial and the Center for EUVAstrophysics, University of California, Berkeley.
| EURD: The Mission and the Stellar Absolute Fluxes of B-Type Stars
We present here stellar spectra of B stars obtained with the EURDspectrograph, one of the three instruments on board MINISAT-01. EURD isa spectrograph specially designed to detect diffuse radiation in thewavelength range between 350 and 1100 Å with 5 Å spectralresolution. EURD main scientific targets are: the spectrum ofinterstellar medium, atmospheric airglow, decaying neutrinos, Moon andearly type stars.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| The Normal Energy Distributions in Stellar Spectra: Giants and Supergiants
We have derived the normal spectral energy distributions for thoseearly-type subgiants, giants, and supergiants that were not investigatedin our earlier studies, which were in most cases also not included inthe studies of Sviderskiene. Color indices computed using our normalenergy distributions are in good agreement with normal colors derivedfrom observations in the Vilnius photometric system. The reliability ofour distribution curves is also demonstrated by comparisons of observedand computed (W-B)-(B-V) two-color diagrams in the WBVR system. Normalcolor indices for the photometric WBVR system are derived.
| An Empirical Test of the Mg II λ1240 Doublet Branching Ratio and Oscillator Strength
We empirically confirm the theoretical branching ratio and oscillatorstrength for the weak Mg II 3s-4p doublet at 1240 Å as determinedby Theodosiou & Federman. We use the independent methods of apparentoptical depth analysis for the sight lines toward μ Col, γ Ara,and ρ Leo and profile component fitting for the sight line towardμ Col in order to determine the branching ratio. We findf1239/f1240=1.74+/-0.06, in agreement with thetheoretical value of 1.78+/-0.03. Profile fitting for the line of sighttoward μ Col gives an f-value for the doublet of9.71+/-0.32x10-4, which agrees with both the theoreticalvalue of Theodosiou & Federman and the empirical value ofFitzpatrick. Based on observations obtained with the NASA/ESA HubbleSpace Telescope through the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NASA-26555.
| Site characterisation for the IUCAA telescope
Not Available
| Ionized Gas in Damped LYalpha Systems and Its Effects on Elemental Abundance Studies
Recent high-resolution observations of metal absorption lines inhigh-redshift damped Lyalpha systems have shown that Al III, a tracer ofmoderately ionized gas, very often has a velocity structureindistinguishable from that of low-ionization gas. Regions of ionizedand neutral hydrogen in these systems are likely cospatial. The higherionization Si IV and C IV absorption shows a much weaker or nonexistentcorrelation with the low-ionization material, implying that the regionstraced by Al III are photoionized by a soft (stellar) spectrum, by ahard (power-law) spectrum with a very low ionization parameter, or acombination of both. We discuss the ionization of the damped Lyalphasystems and use photoionization equilibrium models to make quantitativeestimates of its effects on abundance studies in these systems. We showthat ionization effects may be large enough to account for the observeddispersion in absolute metal abundances in damped Lyalpha systems,causing systematically higher abundances in lower column densitysystems. The observed Si^+/Fe^+ and Zn^+/Cr^+ ratios may systematicallyoverestimate the intrinsic Si/Fe and Zn/Cr ratios, respectively, ifionized gas is present in these systems, thereby mimicking the effectsof alpha-element enrichment or dust depletion.
| Dust in the Ionized Medium of the Galaxy: GHRS Measurements of AL III and S III
We present interstellar absorption-line measurements of the ions S IIIand Al III toward the stars beta^1 Sco, mu Col, xi Per, zeta Oph, rhoLeo, and HD 18100 using archival data from the Goddard High-ResolutionSpectrograph on board the Hubble Space Telescope. The ions Al III and SIII trace heavily depleted and nondepleted elements, respectively, inionized gas along the sight lines to these late-O/early-B stars. We usethe photoionization equilibrium code CLOUDY to derive the ionizationcorrection relating the ratio N(Al III) / N(S III) to the gas-phaseabundance [Al/S]_i [=log (N(Al)/N(S))_i-log(Al/S)_solar] in the ionizedgas. For spectral types considered here, the corrections range from 0.1to 0.3 dex and are independent of the assumed ionization parameter,i.e., the ratio of ionizing photon density to mean electron density.Using the results of these photoionization models, we find [Al/S]_i~-1.0in the ionized gas toward beta^1 Sco, xi Per, and zeta Oph; along thelow-density path toward mu Col we find [Al/S]_i~-0.8. Since S is notdepleted onto grains, these values of [Al/S]_i (~[Al/H]_i) imply thatAl-bearing grains are present in the ionized nebulae around these stars.If the warm ionized medium (WIM) of the Galaxy is photoionized by OBstars, the observations of rho Leo and HD 18100 imply [Al/S]_i=-0.4 to-0.5 in the WIM and thus the presence of dust grains containing Al inthis important phase of the interstellar medium. While photoionizationappears to be the most likely origin of the ionization for Al III and SIII, we cannot rule out confusion from the presence of hot,collisionally ionized gas along the sight lines to beta^1 Sco and HD18100. We find that [Al/S]_i in the ionized gas along the six sightlines is anticorrelated with the electron density and average sight-lineneutral density. The degree of grain destruction in the ionized mediumof the Galaxy is not much higher than in the warm neutral medium. Theexistence of grains in the ionized regions studied here has importantimplications for the thermal balance of these regions.
| Diffuse interstellar bands in low E(B-V) objects
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1998MNRAS.295..437G&db_key=AST
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
