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Публикации по объекту
Variability of Luminous Stars in the Large Magellanic Cloud Using 10 Years of ASAS Data
Motivated by the detection of a recent outburst of the massive luminousblue variable LMC-R71, which reached an absolute magnitude MV= –9.3 mag, we undertook a systematic study of the opticalvariability of 1268 massive stars in the Large Magellanic Cloud, using arecent catalog by Bonanos et al. as the input. The ASAS All Star Catalogprovided well-sampled light curves of these bright stars spanning 10years. Combining the two catalogs resulted in 599 matches, on which weperformed a variability search. We identified 117 variable stars, 38 ofwhich were not known before, despite their brightness and largeamplitude of variation. We found 13 periodic stars that we classify aseclipsing binary (EB) stars, 8 of which are newly discovered brightmassive EBs composed of OB-type stars. The remaining 104 variables areeither semi- or non-periodic, the majority (85) being red supergiants(RSGs). Most (26) of the newly discovered variables in this category arealso RSGs with only three B and four O stars.
| A Spitzer Space Telescope Far-Infrared Spectral Atlas of Compact Sources in the Magellanic Clouds. I. The Large Magellanic Cloud
We present far-infrared spectra, λ = 52-93 μm, obtained withthe Spitzer Space Telescope in the spectral energy distribution mode ofits MIPS instrument, of a representative sample of the most luminouscompact far-infrared sources in the Large Magellanic Cloud (LMC). Theseinclude carbon stars, OH/IR asymptotic giant branch (AGB) stars,post-AGB objects and planetary nebulae, the R CrB-type star HV 2671, theOH/IR red supergiants (RSGs) WOH G064 and IRAS 05280 – 6910, thethree B[e] stars IRAS 04530 – 6916, R 66 and R 126, the Wolf-Rayetstar Brey 3a, the luminous blue variable (LBV) R 71, the supernovaremnant N 49, a large number of young stellar objects (YSOs), compact HII regions and molecular cores, and a background galaxy at a redshift zsime 0.175. We use the spectra to constrain the presence and temperatureof cold dust and the excitation conditions and shocks within the neutraland ionized gas, in the circumstellar environments and interfaces withthe surrounding interstellar medium (ISM). First, we introduce aspectral classification scheme. Then, we measure line strengths, dusttemperatures, and IR luminosities. Objects associated with starformation are readily distinguished from evolved stars by their colddust and/or fine-structure lines. Evolved stars, including the LBV R 71,lack cold dust except in some cases where we argue that this is swept-upISM. This leads to an estimate of the duration of the prolificdust-producing phase ("superwind") of several thousand years for bothRSGs and massive AGB stars, with a similar fractional mass lossexperienced despite the different masses. We tentatively detect lineemission from neutral oxygen in the extreme RSG WOH G064, which suggestsa large dust-free cavity with implications for wind driving. In N 49,the shock between the supernova ejecta and ISM is revealed inspectacular fashion by its strong [O I] λ63 μm emission andpossibly water vapor; we estimate that 0.2 M sun of ISM dustwas swept up. On the other hand, some of the compact H II regionsdisplay pronounced [O III] λ88 μm emission. The efficiency ofphotoelectric heating in the interfaces of ionized gas and molecularclouds is estimated at 0.1%-0.3%. We confirm earlier indications of alow nitrogen content in the LMC. Evidence for solid state emissionfeatures is found in both young and evolved objects, but the carriers ofthese features remain elusive; some of the YSOs are found to containcrystalline water ice. The spectra constitute a valuable resource forthe planning and interpretation of observations with the Herschel SpaceObservatory and the Stratospheric Observatory For Infrared Astronomy.
| Spitzer IRS Spectra of Luminous 8 ?m Sources in the Large Magellanic Cloud: Testing Color-based Classifications
We present archival Spitzer Infrared Spectrograph (IRS) spectra of 19luminous 8 ?m selected sources in the Large Magellanic Cloud (LMC).The object classes derived from these spectra and from an additional 24spectra in the literature are compared with classifications based on TwoMicron All Sky Survey (2MASS)/MSX (J, H, K, and 8 ?m) colors in orderto test the "JHK8" (Kastner et al.) classification scheme. The IRSspectra confirm the classifications of 22 of the 31 sources that can beclassified under the JHK8 system. The spectroscopic classification of 12objects that were unclassifiable in the JHK8 scheme allow us tocharacterize regions of the color-color diagrams that previously lackedspectroscopic verification, enabling refinements to the JHK8classification system. The results of these new classifications areconsistent with previous results concerning the identification of themost infrared-luminous objects in the LMC. In particular, while the IRSspectra reveal several new examples of asymptotic giant branch (AGB)stars with O-rich envelopes, such objects are still far outnumbered bycarbon stars (C-rich AGB stars). We show that Spitzer IRAC/MIPScolor-color diagrams provide improved discrimination between redsupergiants and oxygen-rich and carbon-rich AGB stars relative to thosebased on 2MASS/MSX colors. These diagrams will enable the most luminousIR sources in Local Group galaxies to be classified with high confidencebased on their Spitzer colors. Such characterizations of stellarpopulations will continue to be possible during Spitzer's warm missionthrough the use of IRAC [3.6]-[4.5] and 2MASS colors.
| Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud
We present a catalog of 1750 massive stars in the Large Magellanic Cloud(LMC), with accurate spectral types compiled from the literature, and aphotometric catalog for a subset of 1268 of these stars, with the goalof exploring their infrared properties. The photometric catalog consistsof stars with infrared counterparts in the Spitzer SAGE survey database,for which we present uniform photometry from 0.3 to 24 μm in theUBVIJHKs +IRAC+MIPS24 bands. The resulting infraredcolor-magnitude diagrams illustrate that the supergiant B[e], redsupergiant, and luminous blue variable (LBV) stars are among thebrightest infrared point sources in the LMC, due to their intrinsicbrightness, and at longer wavelengths, due to dust. We detect infraredexcesses due to free-free emission among ~900 OB stars, which correlatewith luminosity class. We confirm the presence of dust around 10supergiant B[e] stars, finding the shape of their spectral energydistributions (SEDs) to be very similar, in contrast to the variety ofSED shapes among the spectrally variable LBVs. The similar luminositiesof B[e] supergiants (log L/L sun >= 4) and the rare, dustyprogenitors of the new class of optical transients (e.g., SN 2008S andNGC 300 OT), plus the fact that dust is present in both types ofobjects, suggests a common origin for them. We find the infrared colorsfor Wolf-Rayet stars to be independent of spectral type and their SEDsto be flatter than what models predict. The results of this studyprovide the first comprehensive roadmap for interpreting luminous,massive, resolved stellar populations in nearby galaxies at infraredwavelengths.
| RAVE spectroscopy of luminous blue variables in the Large Magellanic Cloud
Context: The RAVE spectroscopic survey for galactic structure andevolution obtains 8400-8800 Å spectra at 7500 resolving power atthe UK Schmidt Telescope using the 6dF multi-fiber positioner. More than300 000 9 ≤ IC ≤ 12 and |b| ≥ 25° southern starshave been observed to date. Aims: This paper presents the firstintrinsic examination of stellar spectra from the RAVE survey, aimed atevaluating their diagnostic potential for peculiar stars and atcontributing to the general understanding of luminous blue variables(LBVs). Methods: We used the multi-epoch spectra for all sevenLBVs observed, between 2005 and 2008, in the Large Magellanic Cloud(LMC) by the RAVE survey. Results: We demonstrate that RAVEspectra possess significant diagnostic potential when applied topeculiar stars and, in particular, LBVs. The behaviour of the radialvelocities for both emission and absorption lines, and the spectralchanges between outburst and quiescence states are described and foundto agree with evidence gathered at more conventional wavelengths. Thewind outflow signatures and their variability are investigated, withmulti-components detected in S Doradus. Photoionisation modelling of therich emission line spectrum of R 127 shows evidence of a massivedetached ionised shell that was ejected during the 1982-2000 outburst.Surface inhomogeneities in the nuclear-processed material, brought tothe surface by heavy mass loss, could have been observed in S Doradus,even if alternative explanations are possible. We also detect thetransition from quiescence to outburst state in R 71. Finally, ourspectrum of R 84 offers one of the clearest views of its cool companion.
| The Far Ultraviolet Spectroscopic Explorer Legacy in the Magellanic Clouds: An Online Stellar Sight Line Atlas
We present an atlas of stellar sight line data from the Far UltravioletSpectroscopic Explorer (FUSE) satellite for 287 stars in the MagellanicClouds, obtained from eight years of satellite operations. The intent ofour project is to make this rich ensemble data set accessible to a broadcommunity of researchers in a standardized format that will enable easyidentification of subsets of these data that are appropriate forpursuing specific science programs. We present the data in astandardized manner, showing key interstellar lines on a velocity scale,optical (MCELS) and infrared (Spitzer) sight line context images, andoverview plots of the spectral region containing the O VI??1032, 1038 doublet, and the entire 905-1187 Åspectral range observed with FUSE. Objects with multiple observationshave had their data summed to directly provide the highestsignal-to-noise ratio available. These data are accessible online as aHigh Level Science Product through the Multimission Archive at SpaceTelescope (MAST). In this article, we describe the data sets andprocessing, the atlas materials, and the MAST interface in detail, andalso provide examples of how to use these materials.
| Mass loss and the evolution of massive stars
We present the results of mass-loss predictions for hot stars. Mass lossis an important aspect for the evolution of massive stars, therotational properties of the progenitors of gamma-ray bursts, and isessential in assessing whether the most massive stars explode aspair-instability supernovae, or avoid them due to mass loss. As aresult, the rate of mass loss is critical for our understanding of thechemical enrichment of the interstellar medium. Of topical interest isthe question whether luminous blue variables are the progenitors ofsupernovae. Although there is a growing body of evidence to suggestthis, it remains as yet unexplained by modern stellar evolution models.
| The Large Magellanic Cloud's Top 250: Classification of the Most Luminous Compact 8 μm Sources in the Large Magellanic Cloud
To ascertain the nature of the brightest compact mid-infrared (mid-IR)sources in the Large Magellanic Cloud (LMC), we have applied an updatedversion of Buchanan et al.'s Two Micron All Sky Survey (2MASS)-MidcourseSpace Experiment (MSX) color classification system, which is based onthe results of Spitzer Space Telescope spectroscopy, to a mid-IR (8μm) flux-limited sample of 250 LMC objects for which 2MASS and MSXphotometry is available. The resulting 2MASS-MSX ("JHK8") color-basedclassifications of these sources, which constitute the mostmid-IR-luminous objects in the LMC, were augmented, cross-checked, andcorrected where necessary via a variety of independent means, such thatonly 46 sources retain tentative classifications and only 10 sourcescannot be classified at all. The sample is found to consist primarily ofcarbon-rich asymptotic giant branch (AGB) stars (35%), red supergiants(RSGs) (18%), and compact H II regions (32%), with additional, smallpopulations of oxygen-rich AGB stars (~5%), dusty, early-typeemission-line stars (~3%), and foreground, O-rich AGB stars in the MilkyWay (~3%). The very large ratio of C-rich to O-rich objects among theluminous and heavily dust-enshrouded AGB stars in our LMC IR sourcesample is consistent with the hypothesis that carbon stars form easilyin lower metallicity environments. We demonstrate that very luminousC-rich and O-rich AGB stars and RSGs, identified here primarily on thebasis of their JHK8 colors, also appear as distinct clusters in SpitzerInfrared Array Camera (IRAC)/Multiband Imaging Photometer for Spitzer(MIPS) color-color diagrams. Thus, in principle, the infraredspectrograph (IRS)-based IR photometric classification techniquesapplied here to the LMC can be applied to any external galaxy whose mostluminous IR point sources are detectable and resolvable by 2MASS andSpitzer.
| A Three-Decade Outburst of the LMC Luminous Blue Variable R127 Draws to a Close
The paradigmatic luminous blue variable R127 in the Large MagellanicCloud has been found in the intermediate, peculiar early-B state, andsubstantially fainter in visual light, signaling the final decline fromits major outburst that began between 1978 and 1980. This transformationwas detected in 2008 January, but archival data show that it beganbetween early 2005 and early 2007. In fact, significant changes from themaximum, peculiar A-type spectrum, which was maintained from 1986through 1998, had already begun the following year, coinciding with asteep drop in visual light. We show detailed correspondences between thespectrum and light, in which the decline mimics the rise. Moreover,these trends are not monotonic but are characterized by multiple spikesand dips, which may provide constraints on the unknown outburstmechanism. Intensive photometric and spectroscopic monitoring of R127should now resume, to follow the decline presumably back to thequiescent Ofpe/WN9 state, in order to fully document the remainder ofthis unique observational opportunity.
| On the Role of the WNH Phase in the Evolution of Very Massive Stars: Enabling the LBV Instability with Feedback
We propose the new designation ``WNH'' for luminous Wolf-Rayet (WR)stars of the nitrogen sequence with hydrogen in their spectra. Thesehave been commonly referred to as WNL stars (WN7h, for example), butthis new shorthand avoids confusion because there are late-type WN starswithout hydrogen and early-type WN stars with hydrogen. Clearlydifferentiating WNH stars from H-poor WN stars is critical whendiscussing them as potential progenitors of Type Ib/c supernovae andgamma-ray bursts-the massive WNH stars are not likely Type Ib/csupernova progenitors, and are distinct from core He burning WR stars.We show that masses of WNH stars are systematically higher than for bonafide H-poor WR stars (both WN and WC), with little overlap. Also,hydrogen mass fractions of the most luminous WNH stars are higher thanthose of luminous blue variables (LBVs). While on the main sequence, astar's mass is reduced due to winds and its luminosity slowly rises, sothe star increases its Eddington factor, which in turn stronglyincreases the mass-loss rate, pushing it even closer to the Eddingtonlimit. Accounting for this feedback, observed properties of WNH starsare a natural and expected outcome for very luminous stars approachingthe end of core H burning. Feedback from the strong WNH wind itselfplays a similar role, enabling the eruptive instability seensubsequently as an LBV. Altogether, for initial masses above 40-60Msolar, we find a strong and self-consistent case thatluminous WNH stars are pre-LBVs rather than post-LBVs (for lower initialmass, the case is less clear). The steady march toward increasedmass-loss rates from feedback also provides a natural explanation forthe continuity in observed spectral traits from O3 V to O3 If* to WNHnoted previously.
| Wra 751, a luminous blue variable developing an S Doradus cycle
Aims. The object Wra 751 is a luminous blue variable that latelyexhibits strong changes in light and color. We summarise the availablephotometry of Wra 751, present new photometric observations, and discussthese data with special attention on the systematic differences betweenthe various data sources. In addition, we establish an empiricalrelationship between b-y and B-V for this class of stars. Methods: Wra751 is a strong-active member of the S Dor class, and it exhibitsvery-long term S Doradus phases with an amplitude of about twomagnitudes in V and a cycle length of several decades. The associatedB-V colour-index amplitude is about 0.4 mag. At this moment this LBV,which is the reddest member of the class, is going through the bright(and red) stage of a long-term S Dor cycle. Results: The S Dorbehaviour of this system shows some resemblence to the temporalcharacteristics of the Galactic LBV AG Car, because the time scales andamplitudes of light and colour variability are very similar.Individual photometric measurements (Tables 2-4) are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/484/463
| On the evolution and fate of super-massive stars
Context: We study the evolution and fate of solar compositionsuper-massive stars in the mass range 60-1000 Mȯ. Ourstudy is relevant for very massive objects observed in young stellarcomplexes as well as for super-massive stars that could potentially formthrough runaway stellar collisions. Aims: We predict the outcomes ofstellar evolution by employing a mass-loss prescription that isconsistent with the observed Hertzsprung-Russell Diagram location of themost massive stars. Methods: We compute a series of stellar models withan appropriately modified version of the Eggleton evolutionary code. Results: We find that super-massive stars with initial masses up to 1000Mȯ end their lives as objects less massive than≃150 Mȯ. These objects are expected to collapseinto black holes (with M ⪉ 70 ~Mȯ) or explode aspair-instability supernovae. Conclusions: We argue that ifultraluminous X-ray sources (ULXs) contain intermediate-mass blackholes, these are unlikely to be the result of runaway stellar collisionsin the cores of young clusters.
| Estimating the Structure and Geometry of Winds from Luminous Blue Variables via Fitting the Continuum Energy Distributions
By combining the UV spectra from IUE with photometric data in theoptical band, we present a quantitative study on the continuum energydistributions of LBVs to determine the structure and geometry of LBVwinds. It is shown that the shape of continuum energy distributionsaround the Balmer jump is sensitive to the velocity law of LBV winds. Asimple, spherically symmetric wind model including free-bound andfree-free radiation is constructed to compute the continuum energydistributions of LBVs. By matching theoretical ones to the observedcontinuum energy distributions around the Balmer jump, we have obtainedvalue of the exponent of the velocity law β in both minimum andmaximum state for five LBVs, i.e., AG Car, HR Car, R40, S Dor, and R127.We have found that β is about 0.5-0.7 in the minimum state andlarger than 1.5 in the maximum state. Transitions in the ionizationstates of metals between the minimum and maximum state of LBVs, whichlead to changes in the radiative acceleration due to spectral lines, aremost likely responsible for such effect on the velocity law. We havealso determined the geometry of the wind and found that a sphericallysymmetric wind model can well reproduce the observed continuum energydistributions of the five LBVs. Based on these results we suggest thatthe wind of LBVs be basically quasi-spherical, maybe with some clumpystructure in the spherical wind to produce some observed asphericalfeatures.
| Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gm
Photometric and spectroscopic observations of the faint Supernovae (SNe)2002kg and 2003gm, and their precursors, in NGC 2403 and NGC 5334,respectively, are presented. The properties of these SNe are discussedin the context of previously proposed scenarios for faint SNe: low-massprogenitors producing underenergetic SNe; SNe with ejecta constrained bya circumstellar medium; and outbursts of massive Luminous Blue Variables(LBVs). The last scenario has been referred to as `Type V SNe', `SNimpostors' or `fake SNe'.The faint SN 2002kg reached a maximum brightness of MV =-9.6, much fainter than normal Type II SNe. The precursor of SN 2002kgis confirmed to be, as shown in previous work, the LBV NGC 2403-V37.Late-time photometry of SN 2002kg shows it to be only 0.6 mag fainter at500 d than at the epoch of discovery. Two spectra of SN 2002kg, with anapproximately 1-yr interval between observations, show only minordifferences. Strong FeII lines are observed in the spectra of SN 2002kg,similar to both the LBV NGC 2363-V1 and the Type IIn SN 1995G. Thespectrum of SN 2002kg does show strong resolved [NII] atλλ6549,6583 Å. The identified progenitor of SN2003gm is a bright yellow star, consistent with a F5-G2 supergiant,similar to the identified progenitor of SN 2004et. SN 2003gm, at theepoch of discovery, was of similar brightness to the possible fake SN1997bs and the Type IIP SNe 1999br and 2005cs. Photometrically SN 2003gmshows the same decrease in brightness, over the same time period as SN1997bs. The light curve and the spectral properties of SN 2003gm arealso consistent with some intrinsically faint and low-velocity Type IISNe. The early-time spectra of SN 2003gm are dominated by Balmeremission lines, which at the observed resolution, appear similar to SN2000ch. On the basis of the post-discovery photometric and spectroscopicobservations presented here, we suggest that SN 2003gm is a similarevent to SN 1997bs, although the SN/LBV nature of both of these objectsis debated. At 226 d post-discovery the spectrum of SN 2003gm isstrongle contaminated by HII region emission lines, and it cannot beconfirmed that the precursor star has disappeared. The presence ofstrong [NII] lines, near Hα, is suggested as a possible means ofidentifying objects such as SN 2002kg/NGC 2403-V37 as being LBVs -although not as a general classification criterion of all LBVsmasquerading as SNe.
| New Insights Into the S Doradus Phenomenon and the Micro-Variations of Eight Luminous Blue Variables
Here we investigate the photometric histories of the luminous bluevariables (LBVs) eta, AG and HR Car within our Galaxy, S Dor, R127,R110, and R71 in the LMC, and R40 in the SMC by collecting all availablephotometry.
| The Discovery of a P Cygni Analog in M31
We present spectroscopy and discuss the photometric history of apreviously obscure star in M31. The spectrum of the star is an extremelyclose match to that of P Cygni, one of the archetypes of luminous bluevariables (LBVs). The star has not shown much variability over the past40 years (<0.2 mag), although small-scale (0.05 mag) variations overa year appear to be real. Nevertheless, the presence of a subarcsecondextension around the star is indicative of a past outburst, and from thenebula's size (0.5 pc diameter) we estimate that the outburst took placeroughly 2000 years ago. P Cygni itself exhibits a similar photometricbehavior and has a similar nebula (0.2 pc diameter). We argue that thismay be more typical behavior for LBVs than commonly assumed. The star'slocation in the H-R diagram offers substantial support for stellarevolutionary models that include the effects of rotation, as the star isjust at a juncture in the evolutionary track of a 85 Msolarstar. The star is likely in a transition from an O star to a late-typeWN Wolf-Rayet star.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope (HST), obtained from the Data Archive at the Space TelescopeScience Institute, which is operated by the Association for Universitiesfor Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
| The Detection of Crystalline Silicates in Ultraluminous Infrared Galaxies
Silicates are an important component of interstellar dust, and thestructure of these grains (amorphous or crystalline) is sensitive to thelocal physical conditions. We have studied the infrared spectra of asample of ultraluminous infrared galaxies (ULIRGs). Here we report thediscovery of weak, narrow absorption features at 11, 16, 19, 23, and 28μm, characteristic of crystalline silicates, superimposed on thebroad absorption bands at 10 and 18 μm due to amorphous silicates ina subset of this sample. These features betray the presence offorsterite (Mg2SiO4), the magnesium-rich endmember of the olivines. Previously, crystalline silicates have only beenobserved in circumstellar environments. The derived fraction offorsterite to amorphous silicates is typically 0.1 in these ULIRGs. Thisis much larger than the upper limit for this ratio in the interstellarmedium of the Milky Way, 0.01. These results suggest that the timescalefor injection of crystalline silicates into the ISM is short in amerger-driven starburst environment (e.g., as compared to the total timeto dissipate the gas), pointing toward massive stars as a prominentsource of crystalline silicates. Furthermore, amorphization due tocosmic rays, which is thought to be of prime importance for the localISM, lags in vigorous starburst environments.
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| Infrared Observations of the Candidate LBV 1806-20 and Nearby Cluster Stars1,
We report near-infrared photometry, spectroscopy, and speckle imaging ofthe hot, luminous star we identify as candidate LBV 1806-20. We alsopresent photometry and spectroscopy of three nearby stars, which aremembers of the same star cluster containing LBV 1806-20 and SGR 1806-20.The spectroscopy and photometry show that LBV 1806-20 is similar in manyrespects to the luminous ``Pistol star,'' albeit with some importantdifferences. They also provide estimates of the effective temperatureand reddening of LBV 1806-20 and confirm distance estimates, leading toa best estimate for the luminosity of this star of greater than5×106Lsolar. The nearby cluster stars havespectral types and inferred absolute magnitudes that confirm thedistance (and thus luminosity) estimate for LBV 1806-20. If we dropkinematic measurements of the distance(15.1+1.8-1.3 kpc), we have a lower limit on thedistance of greater than 9.5 kpc and on the luminosity of greater than2×106Lsolar, based on the cluster stars. Ifwe drop both the kinematic and cluster star indicators for distance, anammonia absorption feature sets yet another lower limit to the distanceof greater than 5.7 kpc, with a corresponding luminosity estimate ofgreater than 7×105 Lsolar for the candidateLBV 1806-20. Furthermore, on the absis of very high angular resolutionspeckle images, we determine that LBV 1806-20 is not a cluster of starsbut is rather a single star or binary system. Simple arguments based onthe Eddington luminosity lead to an estimate of the total mass of LBV1806-20 (single or binary) exceeding 190Msolar. We discussthe possible uncertainties in these results and their implications forthe star formation history of this cluster.Based on data obtained at the Palomar Observatory 200 inch telescope,which is operated by the California Institute of Technology, the JetPropulsion Laboratory, and Cornell University.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| The Missing Luminous Blue Variables and the Bistability Jump
We discuss an interesting feature of the distribution of luminous bluevariables (LBVs) on the H-R diagram, and we propose a connection withthe bistability jump seen in the winds of early-type supergiants. Thereappears to be a deficiency of quiescent LBVs on the S Doradusinstability strip at luminosities between log(L/Lsolar)~=5.6and 5.8. The upper boundary, interestingly, is also where thetemperature-dependent S Doradus instability strip intersects thebistability jump at about Teff~=21,000 K. Because ofincreased opacity, winds of early-type supergiants are slower and denseron the cool side of the bistability jump, and we postulate that this maytrigger optically thick winds that inhibit quiescent LBVs from residingthere. We conduct numerical simulations of radiation-driven winds for arange of temperatures, masses, and velocity laws atlog(L/Lsolar)=5.7 to see what effect the bistability jumpshould have. We find that for relatively low stellar masses, theorder-of-magnitude increase in the wind density at the bistability jumpleads to the formation of a modest to strong pseudophotosphere thatmight alter a star's apparent position on the H-R diagram. The effect isstrongest for LBVs approaching 10 Msolar, where thepseudophotospheres are sufficiently extended to make an early B-typestar appear as a yellow hypergiant. Thus, the proposed mechanism will bemost relevant for LBVs that are post-red supergiants [curiously, theupper boundary at log(L/Lsolar)~=5.8 coincides with the upperluminosity limit for red supergiants]. Further work is obviously needed,especially with regard to a possible evolutionary connection between the``missing'' LBVs and the most luminous red supergiants and yellowhypergiants. Specifically, yellow hypergiants such as IRC +10420 andρ Cas occupy the same luminosity range as the missing LBVs and showapparent temperature variations at constant luminosity. If these yellowhypergiants do eventually become Wolf-Rayet stars, we speculate thatthey may skip the normal LBV phase, at least as far as their apparentpositions on the H-R diagram are concerned.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Radiative Instability in Stellar Envelopes
A general analytic criterion for radiative instability in the envelopeof a nonexplosive star is derived, based on a generalization of theEddington limit for the star's luminosity, an expression for thestationary mass-loss acceleration, and an assumed universality of thecritical mean density of the envelope. The derived criterion has beenformulated as the minimum rate of mass loss needed to achieve asuper-Eddington state. Expressed as a simple function of luminosity andeffective temperature, it is applied to luminous blue variables (LBVs),Wolf-Rayet stars, and ordinary B-type supergiants. Assuming that thebrightest LBVs as well as most of the Wolf-Rayet stars arepost-main-sequence objects, these objects must have radiatively unstableenvelopes.
| Dusty ring nebulae around new candidate Luminous Blue Variables
We report on the discovery of a further two ring nebulae in theMidcourse Space Experiment (MSX) Galactic Plane Survey;G24.73+0.69 and G26.47+0.02.Morphologically, both appear similar to the nebulae found around theLuminous Blue Variable (LBV) candidates G79.29+0.46and Wra 17-96. A central, unresolved point source wasidentified in both cases - positional coincidence with the starStRS 237 was found forG26.47+0.02, while no optical counterpart could beidentified for G24.73+0.69. However, subsequent nearIR broadband imaging of the G24.73+0.69 fieldidentified a very red - (J-K) ~ 2 mag - stellar counterpart to thecentral object. Near-IR spectroscopy of both objects reveal richemission line spectra dominated by H I, He I and low excitation metals,suggesting classification as luminous B supergiants and revealing astriking superficial similarity to the other MSX ring sources and knownLBVs. We utilised a NLTE model atomsphere code to model the K bandspectra and near-IR spectral energy distributions of the central starsin order to determine their physical parameters. Adopting a distance, d=5.2 kpc to G24.73+0.69 yields a temperature, T= 12kK, luminosity, log (L/Lsun) = 5.6 and mass loss rate, dot{M}=1x 10-5 Msun yr-1.G26.47+0.02 appears to be a more extreme object;adopting d= 6.5 kpc results in T= 17 kK, log (L/Lsun) = 6.0and dot {M}=9x 10-5 Msun yr-1, placingit at the Humphreys-Davidson limit for massive stellar objects. Analysisof the spatially resolved mid-IR fluxes of both objects reveal extendedperiods of enhanced mass loss, resulting in comparatively low massnebulae, with chemistries dominated by O-rich dust (with a population ofsmall Fe grains existing co-spatially with the silicate dust).Comparison to the other MSX ring nebulae sources reveals a homogeneousgroup of objects, with both stellar and nebular properties consistentwith known LBVs. With both spectroscopic and/or photometric variabilityobserved for those sources with multiepoch observations, we propose aclose affinity between both classes of object and suggest that long termmonitoring of the MSX sources will reveal them to be bona fide LBVs.
| On the structure and kinematics of nebulae around LBVs and LBV candidates in the LMC
We present a detailed analysis of the morphology and kinematics ofnebulae around LBVs and LBV candidates in the Large Magellanic Cloud.HST images and high-resolution Echelle Spectra were used to determinethe size, shape, brightness, and expansion velocities of the LBV nebulaearound R 127, R 143, and S 61. For S Dor, R 71, R 99, and R 84 wediscuss the possible presence of nebular emission, and derive upperlimits for the size and lower limits on the expansion velocities ofpossible nebulae. Including earlier results for the LBV candidates S 119and Sk-69o279 we find that in general the nebulae around LBVsin the LMC are comparable in size to those found in the Milky Way. Theexpansion velocities of the LMC nebulae, however, are significantlylower - by about a factor of 3 to 4 - than those of galactic nebulae ofcomparable size. Galactic and LMC nebulae show about the same diversityof morphologies, but only in the LMC do we find nebulae with outflow.Bipolarity - at least to some degree - is found in nebulae in the LMC aswell as in the Milky Way, and manifests a much more general featureamong LBV nebulae than previously known.
| Confirmation of the Luminous Blue Variable nature of AFGL 2298
We present new photometric and spectroscopic observations of the stellarsource AFGL 2298 (= IRAS18576+0341) that has recently been proposed as a candidateLuminous Blue Variable (LBV). Our data confirm that the star is a highlyluminous B supergiant which is both spectroscopically andphotometrically variable. Assuming a distance of 10 kpc, comparison ofthe 2001 June data to synthetic spectra suggest stellar parameters ofT=12.5 kK, log(L/Lsun)=6.2 and dot M=5*E-5Msun yr-1. Data obtained in 2002 August indicatean increase in both temperature (=15 kK) and mass loss rate(=1.2*E-4 Msun yr-1) at constantbolometric luminosity. These physical parameters place AFGL2298 at the Humphreys-Davidson limit for the most luminousstars known. We conclude that the position of AFG2298 in the HR diagram, the significant variability observedbetween 1999-2002 and presence of a massive ejection nebula areconsistent with a classification of AFGL 2298 as abona fide LBV.
| Discovery and Quantitative Spectral Analysis of an Ofpe/WN9 (WN11) Star in the Sculptor Spiral Galaxy NGC 300
We have discovered an Ofpe/WN9 (WN11 following Smith et al.) star in theSculptor spiral galaxy NGC 300, the first object of this class foundoutside the Local Group, during a recent spectroscopic survey of bluesupergiant stars obtained at the ESO Very Large Telescope. The lightcurve over a 5 month period in late 1999 displays a variability at the0.1 mag level. The intermediate-resolution spectra (3800-7200 Å)show a very close resemblance to the Galactic luminous blue variable(LBV) AG Carinae during minimum. We have performed a detailed non-LTEanalysis of the stellar spectrum and have derived a chemical abundancepattern that includes H, He, C, N, O, Al, Si, and Fe, in addition to thestellar and wind parameters. The derived stellar properties and the Heand N surface enrichments are consistent with those of other Local GroupWN11 stars in the literature, suggesting a similar quiescent or post-LBVevolutionary status. Based on observations obtained at the ESO VeryLarge Telescope [65.N-0389(B) and 67.D-0006(A)].
| Predictions of variable mass loss for Luminous Blue Variables
We present radiation-driven wind models for Luminous Blue Variables(LBVs) and predict their mass-loss rates. We study the effects of lowermasses and modified abundances in comparison to the normal OBsupergiants, and we find that the main difference in mass loss is due tothe lower masses of LBVs. In addition, we find that the increase inhelium abundance changes the mass-loss properties by small amounts (upto about 0.2 dex in log dot {M}), while CNO processing is relativelyunimportant for the mass-loss rate. A comparison between our mass losspredictions and the observations is performed for four relativelywell-studied LBVs. The comparison shows that (i) the winds of LBVs aredriven by radiation pressure on spectral lines, (ii) the variable massloss behaviour of LBVs during their S Doradus-type variation cycles isexplained by changes in the line driving efficiency, notably due to therecombination/ionisation of Fe IV/III and Fe III/II, and finally, (iii)the winds of LBVs can be used to derive their masses, as exemplified bythe case of AG Car, for which we derive a present-day mass of 35Msun.
| Outflow from and asymmetries in the nebula around the LBV candidate Sk-69o279
We present and discuss new long-slit Echelle spectra of the LMC LBVcandidate Sk-69o279 and put them in context with previousimages and spectra. While at first glance it resembles a simplespherically expanding symmetric shell, we find a considerably morecomplex morphology and kinematics. The spectra indicate thatmorphologically identified deviations from sphericity are outflows offaster material out of the main body of Sk-69o279. Themorphological as well as the kinematic similarity with other LBV nebulaemakes it likely that Sk-69o279 is an LBV candidate and posesthe question: in how far are outflows out of expanding LBV nebulae ageneral property of such nebulae - at least during some phases of theirevolutions.
| Possible Recovery of SN 1961V in Hubble Space Telescope Archival Images
SN 1961V in NGC 1058 was originally classified by Fritz Zwicky as a``Type V'' supernova. However, it has been argued that SN 1961V was nota genuine supernova, but instead the superoutburst of an ?Carinae-like luminous blue variable star. In particular, Filippenko etal. used pre-refurbishment Hubble Space Telescope (HST) Wide FieldPlanetary Camera (WFPC) images and the known radio position of SN 1961Vto conclude that the star survived the eruption and is likely coincidentwith a V~25.6, V-I~1.9 mag object. Recently, Stockdale et al. recoveredthe fading SN 1961V at radio wavelengths and argue that its behavior issimilar that of some Type II supernovae. We have analyzedpost-refurbishment archival HST WFPC2 data and find that the new radioposition is still consistent with the Filippenko et al. object, whichhas not changed in brightness or color, but is also consistent with anadjacent, fainter (I~24.3 mag), and very red (V-I>1.0 mag) object. Wesuggest that this fainter object could be the survivor of SN 1961V.Forthcoming HST observations may settle this issue. Based onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive of the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| An Infrared Ring Nebula around MSX5C G358.5391+00.1305: The True Nature of Suspected Planetary Nebula Wray 17-96 Determined via Direct Imaging and Spectroscopy
The Midcourse Space Experiment (MSX) Galactic plane survey discovered anearly perfectly circular ring nebula around the suspected planetarynebula Wray 17-96. Using near-IR spectral typing and modeling of themid-IR nebula, we find that Wray 17-96 is more likely a candidate to bea luminous blue variable (LBV) surrounded by a large spherical ejectashell. It is very similar to the G79.29+0.46 LBV candidate in Cygnus andthe Pistol Star. The K-band spectrum and the mid-IR data indicate astellar temperature of 13,000 K. The most likely distance to the sourceis 4.5 kpc, leading to a luminosity of 1.8×106Lsolar. We suggest that the nebula consists of multipleshells and that an evolution from oxygen-rich to carbon-rich chemistrymay be indicated.
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Наблюдательные данные и астрометрия
| Созвездие: | Столовая Гора |
| Прямое восхождение: | 05h02m07.39s |
| Склонение: | -71°20'13.1" |
| Видимая звёздная величина: | 10.841 |
| Собственное движение RA: | -0.3 |
| Собственное движение Dec: | 0.3 |
| B-T magnitude: | 10.812 |
| V-T magnitude: | 10.839 |
Каталоги и обозначения:
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