目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available 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/qcat?J/A+A/465/993
| Discovery of a Nearby Twin of SN 1987A's Nebula around the Luminous Blue Variable HD 168625: Was Sk -69 202 an LBV?
Spitzer Space Telescope images of the luminous blue variable (LBV)candidate HD 168625 reveal the existence of a bipolar nebula severaltimes larger than its previously known equatorial dust torus. The outernebula of HD 168625 has a full extent of ~80" or 0.85 pc, and one of thelobes has a well-defined polar ring. The nebula is a near twin of thetriple-ring system around SN 1987A. Because of these polar rings, andaccounting for stellar/progenitor luminosity, HD 168625 is an evencloser twin of SN 1987A than the B supergiant Sher 25 in NGC 3603. HD168625's nebula was probably ejected during a giant LBV eruption and notduring a red supergiant phase, so its similarity to the nebula around SN1987A may open new possibilities for the creation of SN 1987A's rings.Namely, the hypothesis that Sk -69 202 suffered an LBV-like eruptionwould avert the complete surrender of single-star models for its bipolarnebula by offering an alternative to an unlikely binary merger scenario.It also hints that LBVs are the likely progenitors of some Type IIsupernovae, and that HD 168625's nebula is a good example of apre-explosion environment.Based on observations made with the Spitzer Space Telescope, which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| Cleaning Up η Carinae: Detection of Ammonia in the Homunculus Nebula
We report the first detection of ammonia in the Homunculus Nebula aroundη Carinae, which is also the first detection of emission from apolyatomic molecule in this or any other luminous blue variable (LBV)nebula. Observations of the NH3 (J,K)=(3,3) inversiontransition made with the Australia Telescope Compact Array revealemission at locations where infrared H2 emission had beendetected previously, near the strongest dust emission in the core of theHomunculus. We also detect ammonia emission from the so-called strontiumfilament in the equatorial disk. The presence of NH3 aroundη Car hints that molecular shells around some Wolf-Rayet stars couldhave originated in prior LBV eruptions, rather than in cool redsupergiant winds or the ambient interstellar medium. Combined with thelack of any CO detection, NH3 seems to suggest that theHomunculus is nitrogen-rich like the ionized ejecta around η Car. Italso indicates that the Homunculus is a unique laboratory in which tostudy unusual molecule and dust chemistry, as well as their rapidformation in a nitrogen-rich environment around a hot star. We encouragefuture observations of other transitions like NH 3 (1, 1) and(2, 2), related molecules like N2H+, and renewedattempts to detect CO.
| The Asymmetrical Wind of the Candidate Luminous Blue Variable MWC 314
We present the results of long-term spectropolarimetric andspectroscopic monitoring of MWC 314, a candidate luminous blue variablestar. We detect the first evidence of Hα variability in MWC 314and find no apparent periodicity in this emission. The total R-bandpolarization is observed to vary between 2.21% and 3.00% at a positionangle consistently around ~0°, indicating the presence of atime-variable intrinsic polarization component, and hence anasymmetrical circumstellar envelope. We find suggestive evidence thatMWC 314's intrinsic polarization exhibits a wavelength-independentmagnitude varying between 0.09% and 0.58% at a wavelength-independentposition angle covering all four quadrants of the Stokes Q-U plane.Electron scattering off of density clumps in MWC 314's wind isconsidered as the probable mechanism responsible for these variations.
| MWC 930 - a new luminous blue variable candidate
We present the results of optical high-resolution and near-infraredlow-resolution spectroscopy and multicolour optical and near-infraredphotometry of the emission-line star MWC 930. The spectrum is rich withFeII emissions, most of which have P Cyg-type profiles. The emissionlines are strong and narrow, indicating a powerful stellar wind with alow terminal velocity (v&infy;~ 140kms-1). Thephotospheric absorption lines are broad and show splitting, which mightbe due to the object's binarity. MWC 930 is most probably located in theNorma spiral arm at a distance of D= 3-4kpc. This strong and slow windas well as the star's luminosity (logL/Lsolar~ 5.5) and theinfrared excess shape suggest that MWC 930 is an unusual B-typesupergiant, most likely undergoing the luminous blue variableevolutionary phase.
| Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations
A multifrequency, multiconfiguration study has been made of the compactradio sources in the Galactic center Quintuplet and Arches stellarclusters using the Very Large Array. Ten radio sources have beendetected in the Quintuplet cluster. The majority of these radio sourceshave rising spectral indices and are positionally coincident with youngmassive stars that are known to have powerful stellar winds. We concludethat the three most compact of these sources are produced by stellarwind emission; thus, mass-loss rates can be derived and have an averagevalue of 3×10-5 Msolar yr-1. Theremainder of the sources are likely to be a combination of stellar windemission and free-free emission from surrounding ionized gas. In threecases, the radio sources have no stellar counterpart, and the radioemission is thought to arise from compact or ultracompact H II regions.If so, these sources would be the first detections of embedded massivestars to be discovered in the Galactic center clusters. The radio nebulaassociated with the Pistol star resembles the nebula surrounding theluminous blue variable star η Car and may be related to the stellarwind of the Pistol star. Ten compact radio sources are also detected inthe Arches cluster and are interpreted to be stellar wind sources,consistent with previous findings. Several of the sources show moderatevariability (10%-30%) in their flux density, possibly related to anonthermal component in the wind emission. A number of radio sources inboth clusters have X-ray counterparts, which have been interpreted to bethe shocked, colliding winds of massive binary systems.
| 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.
| Current day mass loss rate for Luminous Blue Variable IRAS 18576+0341
In this letter we report on the first set of multi-frequency and highangular resolution radio observations of IRAS 18576+0341. The radioobservations have revealed an extended, asymmetric and quite structuredradio nebula and allowed us to locate the central core of this highlyobscured new galactic Luminous Blue Variable. From the analysis of radioproperties of IRAS 18576+0341 estimates of important physical parametersof the central star have been determined. In particular, an effectivetemperature of Teff (2.6 ± 0.2) ×104 K, corresponding to a B0-B0.5 supergiant, has beenderived. The most notable result is the determination of the current daymass-loss (dot {M}= 3.7 × 10-5 Mȯyr-1), which is of particular importance since mass-loss ratedetermination from radio observations appears to be a more reliablemethod compared to the others based on different diagnostics.
| On the population of galactic Luminous Blue Variables
We report the first results of a long term infrared monitoring campaignof known and candidate galactic Luminous Blue Variables (LBVs). Inparticular, we are able to confirm the LBV nature ofG24.73+0.69, a luminous mid-B supergiant associatedwith a dusty ejection nebula. We find that prior to 2003 SeptemberG24.73+0.69 exhibited low amplitude (Δ JHK 0.4 mag) variability, but in the ~200 day period between 2003September-2004 April it abruptly brightened by ~0.7 mag in the broadbandJ filter. Subsequently, a further ~0.4 mag increase was observed between2004 April-October, resulting in an overall difference of ~1.1 magbetween (current) photometric mimimum and maximum; similar variabilityalso being observed in the H and K bands. In light of the numerousrecent IR studies of the galactic hot star population we also compile anupdated census of confirmed and candidate galactic LBVs, reporting 12and 23 members respectively for each class. Finally, we utilise this newcensus to construct an H-R diagram for the galactic LBV population,resulting in a striking confirmation of the LBV-minimum light strip.
| Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565
| 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.
| HD 183143: A Hypergiant
We present spectroscopic evidence that the luminosity of HD 183143 ishigher by one magnitude than thought previously. The star is yet anotherB6-8 Ia-0 white hypergiant of the Galaxy. Its absolute visual magnitudeis close to -8 mag, and its distance is close to 2 kpc. We describespectroscopic manifestations of the nonstationary behavior of itsatmosphere and wind.
| The Nature of SN 1961V
The nature of SN 1961V has been uncertain. Its peculiar optical lightcurve and slow expansion velocity are similar to those of superoutburstsof luminous blue variables (LBVs), but its nonthermal radio spectralindex and declining radio luminosity are consistent with decades-oldsupernovae (SNe). We have obtained Hubble Space Telescope STIS imagesand spectra of the stars in the vicinity of SN 1961V and find object 7identified by Filippenko and coworkers to be closest to the optical andradio positions of SN 1961V. Object 7 is the only point source detectedin our STIS spectra, and only its Hα emission is detected; itcannot be the SN or its remnant because of the absence of forbiddenlines. While the Hα line profile of object 7 is remarkably similarto that of η Carinae, the blue color (similar to an A2 Ibsupergiant) and lack of appreciable variability are unlike knownpostoutburst LBVs. We have also obtained Very Long Baseline Arrayobservations of SN 1961V at 18 cm. The nondetection of SN 1961V places alower limit on the size of the radio-emitting region, 7.6 mas or 0.34pc, which implies an average expansion velocity in excess of 4400 kms-1, much higher than the optical expansion velocity measuredin 1961. We conclude the following: (1) An SN occurred in the vicinityof SN 1961V a few decades ago. (2) If the SN 1961V light maximumoriginates from a giant eruption of a massive star, object 7 is the mostprobable candidate for the survivor, but its blue color and lack ofsignificant variability are different from a postoutburst η Car. (3)The radio SN and object 7 could be physically associated with each otherthrough a binary system. (4) Object 7 needs to be monitored to determineits nature and relationship to SN 1961V.
| The Dust Ring of Luminous Blue Variable Candidate HD 168625: Infrared Observations and Model Calculations
We present a 2.218 μm image from the Hubble Space Telescope/NearInfrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 μmimage from ISOPHOT of the dust ring surrounding the luminous bluevariable (LBV) candidate HD 168625, together with new temperature andoptical depth maps derived from mid-IR images. The shell is detachedfrom the star in the near-IR, and substructure in the overall toroidalshell is visible. The far-IR image constrains the extent of the dustshell to ~25" in diameter, providing an upper radius limit for modeling.The temperature maps and the NICMOS image show evidence for very smalltransiently heated dust grains in the shell. The opacity maps showhigher optical depth in the limbs, consistent with interpretation of thedust shell as an equatorially enhanced torus inclined ~60° withrespect to the observer. An overall trend in the dust emission locationwith wavelength is observed and interpreted as a variation with respectto location in the nebula of either the dust grain size distribution orgas-to-dust mass ratio. Radiative transfer calculations using 2-DUSTindicate that a mass-loss event occurred ~5700 yr ago with a rate of(1.9+/-0.1)×10-4Msolaryr-1,creating a dust torus that currently has a τV~0.22 in theequatorial plane and a dust mass of(2.5+/-0.1)×10-3Msolar. Using publishedvalues for the gas mass, we find a gas-to-dust mass ratio of 840, whichis ~4 times higher than current estimates for the interstellar medium.In addition to a high equator-to-pole density ratio (~31) torus, anelliptical midshell is needed to reproduce the appearance and spectralenergy distribution of the dust. Therefore, HD 168625 is an excellentexample of proposed models of LBV nebulae in which a stellar windinteracts with a preexisting density contrast and creates a blowout inthe polar direction perpendicular to the equatorial ring. Thecircumstellar shell is much lower in mass than that of LBV ηCarinae, suggesting that HD 168625 had a lower mass progenitor.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 these observations are associatedwith proposal 7898. Based also on observations with ISO, an ESA projectwith instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom) andwith the participation of ISAS and NASA.
| 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.
| The Carina Spiral Feature: Strömgren-Hβ photometry approach. I. The photometric data-base
A data-base collating all uvbybeta photometry available at present forO-B9 stars brighter than 10th visual magnitude in the field of theCarina Spiral Feature is presented. The completeness and homogeneity ofthe data-base are discussed.Based on CDS data.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/523
| Using spectroscopic analysis of unclassified supergiants to investigate massive stars in transition. II. The superluminous HD 327083
New optical high-resolution (R~50 000) spectra of the star HD 327083 arereported in this paper. The Balmer and Fe II lines show P Cygni profileswhile He I lines are all in pure absorption, resembling the spectra ofthe Luminous Blue Variable HR Carinae. However, the blue absorptioncomponents of the Balmer lines are more pronounced and the Fe II linesare less numerous and less intense in HD 327083. A spectroscopicanalysis of the Balmer lines (Hα , Hβ , Hγ and Hdelta) with a non-LTE wind model has been performed. The derived stellarparameters for HD 327083 are: log (Lstar/Lsun) =6.0, Tstar = 11 500 K, dot M = 8.0 x 10-5Msun/yr and He/H = 0.2 (by number). From these results weconcluded that HD 327083 is a superluminous object in an evolved phaseof an evolutive track of MZAMS ~ 60 Msun. It islikely to be about to enter in a Luminous Blue Variable phase.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under the agreement with theObservatório Nacional, Brazil.
| 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.
| Discovery of Highly Dynamic Matter Enhancements along the Polar Axis and Equatorial Plane in the Luminous Blue Variable Binary HD 5980
HD 5980 is a W-R+O eclipsing binary in the Small Magellanic Cloud thathas attracted considerable attention since it underwent a luminous bluevariable (LBV)-type outburst in 1994. Since then, intense spectroscopicand photometric monitoring have revealed the presence of periodicvariability on a timescale of 6-7 hr that cannot be explained by ourcurrent understanding of the system. In this paper, we present the firstresults from our polarimetric observations around secondary eclipse whenthe assumed Wolf-Rayet (W-R) star passes in front of the assumed LBVcomponent (phase φ=0.36). These data confirm the presence ofstochastic polarimetric variability on a typical timescale of ~30minutes, reaching a very high amplitude of almost 1% on our last of fivenights. At that time, we also found that the mass loss, which isresponsible for the polarimetric variability, presented fluctuations inaxial symmetry ranging from very rapid density enhancements along theorbital plane to polar ejections. We propose that either a fast rotatormodel for the W-R star or the presence of a close orbiting neutron starcould qualitatively explain the observed polarimetric, spectroscopic,and photometric behavior. The evidence presented in this paper iscrucial for an understanding of the erratic behavior of this importantbinary and should be taken into account in any consistent model of thesystem.
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| An outflow from the nebula around the LBV candidate S 119
We present an analysis of the kinematic and morphological structure ofthe nebula around the LMC LBV candidate S 119. On HST images, we find apredominantly spherical nebula which, however, seems to be much betterconfined in its eastern hemisphere than in the western one. Thefilamentary western part of the nebula is indicative of matter flowingout of the nebula's main body. This outflow is evidenced by ourlong-slit echelle spectra. They show that, while most of the nebula hasan expansion velocity of 25.5 km s-1, the outflowing materialreaches velocities of almost 140 km s-1, relative to thesystemic one. A ROSAT HRI image shows no trace of S 119 and thus noindications of hot or shocked material.
| Massive Stars in Transition: Spectroscopic Analysis of HR Carinae
We analysed the high-resolution data of the LBV (or S Dor variable) HRCarinae, taken with the FEROS spectrograph in the wavelength range 3900to 8200 Å . We performed a spectral analysis of the Balmer lineswith a non-LTE expanding atmosphere code in order to derive its stellarparameters and to infer its evolutive status.
| High-resolution spectroscopy of two LBV cycles of HR Car
Luminous Blue Variables (LBVs or S Dor Variables) are showingcharacteristic variability of very long timescales of a decade and more.During their expansion and contraction phases, they move across a widepart of the H-R diagram with dramatic changes of their stellar windcharacteristics, radius and temperature. We present results of ourlong-term spectroscopic monitoring campaigns for the galactic star HRCar with optical spectra taken over 13 years and two minimum and maximumphases observed during this time.
| MASSIVE STARS IN THE LOCAL GROUP: Implications for Stellar Evolution and Star Formation
The galaxies of the Local Group serve as important laboratories forunderstanding the physics of massive stars. Here I discuss what isinvolved in identifying various kinds of massive stars in nearbygalaxies: the hydrogen-burning O-type stars and their evolved He-burningevolutionary descendants, the luminous blue variables, red supergiants,and Wolf-Rayet stars. Primarily I review what our knowledge of themassive star population in nearby galaxies has taught us about stellarevolution and star formation. I show that the current generation ofstellar evolutionary models do well at matching some of the observedfeatures and provide a look at the sort of new observational data thatwill provide a benchmark against which new models can be evaluated.
| Detection of 12CO J=1-->0 and J=2-->1 Emission from the Luminous Blue Variable AG Carinae: Circumstellar Envelope or Disk?
We present the first detection of 12CO J=2-->1 andJ=1-->0 emission from the luminous blue variable AG Carinae. We showthat AG Car resides in a region that is very rich in molecular gas withcomplex motions. We find evidence of a slow outflow of molecular gas,expanding at ~=7 km s-1. This emission appears spatiallyunresolved. We argue that it is spatially localized, rather thanextended, and possibly associated with the immediate circumstellarregion of AG Car. Both detected CO lines are characterized by apseudo-Gaussian profile with FWHM~=15 km s-1, indicating aslowly expanding region of molecular gas in close proximity to the hotcentral star. We have explored two possible scenarios to explain theobserved profile: a circumstellar envelope, similar to carbon stars, ora circumstellar disk. The option of the circumstellar disk is preferablebecause (1) it is consistent with additional independent indications ofthe existence of wind asymmetries in close proximity to the centralstar, found from spectropolarimetry and analysis of the UV and opticalline profiles, and (2) it provides the conditions of density andshielding necessary for the survival of the CO molecules in proximity tosuch a hot star (Teff~=14,000-20,000 K). On the assumptionthat the CO emission originated when AG Car was in an evolved state, wederive a lower limit to the CO mass of 6.5×10-3Msolar. We also estimate that the CO fraction is~=2.3×10-3 of the total mass of molecular gas, whichthen would amount to 2.8 Msolar. This is smaller, but stillcomparable to, the mass of ionized gas present in the circumstellarenvironment (4.2 Msolar), with the implication that themolecular gas fraction can contribute significantly to the overall masslost from the central star in its post-main-sequence evolution.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Infrared [FeII] emission in the circumstellar nebulae of luminous blue variables
After a serendipitous discovery of bright [FeII]λ16435 emissionin nebulae around η Carinae and P Cygni, infrared spectra of otherluminous blue variables (LBV) and LBV candidates were obtained. Brightinfrared [FeII] emission appears to be a common property among LBVs withprominent nebulae; this is an interesting discovery because strong[FeII]λ16435 is typically seen in shock-excited objects likesupernova remnants and outflows from newly formed massive stars, as wellas in active galactic nuclei (AGN), where the excitation mechanism isuncertain. This paper presents spectra in the H-band (1.5 to 1.75 μm)for the central stars and nebulae of η Car, AG Car, P Cyg, Wra 751,HR Car, HD 168625, HD 160529, R 127 and S Doradus. Seven of nine targetsshow bright [FeII]λ16435 in their nebulae, while it is absent inall central stars except the LBV candidate Wra 751. The two objects (SDor and HD 160529) without prominent [FeII]λ16435 are not yetknown to have nebulae detected in optical images, and both lack brightthermal infrared emission from dust. The possible excitation mechanismsfor this line and the implications of its discovery in LBV nebulae arediscussed; there are good reasons to expect shock excitation in someobjects, but other mechanisms cannot be ruled out.
| 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.
| Multiwavelength Study of the Nebula Associated with the Galactic LBV Candidate HD 168625
We present high-resolution HST imaging of the nebula associated with theGalactic luminous blue variable candidate HD 168625, together with ISOimaging and Anglo-Australian Telescope echelle spectroscopy. The overallnebular morphology is elliptical, with the major axis at P.A.~=120°.The dimensions of the nebula are 12"×16.7" at Hα and15.5"×23.5" at 4 μm. In the HST Hα image the nebula isresolved into a complex structure of filaments and arcs of differentbrightness. The asymmetry is lost in the HST continuum image, where thenebula appears more diffuse and richer in filaments and clumps with theshape of cometary tails. At 11.3 μm the nebular emission peaks in twodiametrically opposite lobes located on the nebula boundaries and alongits major axis. A very faint loop is also visible at optical wavelengthsnorth and south of the shell. We suggest that the nebula is an ellipsoidwith projected sizes of 14" and 9" (0.19 pc×0.12 pc) along theright ascension and declination directions, respectively. This ellipsoidis expanding at 19 km s-1 and is dynamically as old as ~=4800yr; it probably interacts with the stellar wind and the loop so thatpolycyclic aromatic hydrocarbon emission is detected from its caps,i.e., the lobes seen in the ISO images. The chemistry of the loopsuggests that it is composed of unprocessed material, probably from thelocal interstellar medium swept by the stellar wind. Based in part onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute (STScI), which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Also based on observations with the Infrared SpaceObservatory, an ESA project with instruments funded by ESA member states(especially the PI countries: France, Germany, Netherlands, and the UK),and with the participation of ISAS and NASA. Also based on observationsobtained at the Anglo-Australian Observatory, Siding Spring.
|
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:
|
INHABITED SKY
