目录
图像
上传图像
DSS Images Other Images
相关文章
Veraenderliche fuer den kleinen Feldstecher.
Not Available
| M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption
Variable A in M33 is a member of a rare class of highly luminous,evolved stars near the upper luminosity boundary that show sudden anddramatic shifts in apparent temperature due to the formation ofoptically thick winds in high mass loss episodes. Recent optical andinfrared spectroscopy and imaging reveal that its ``eruption,'' begun in~1950, has ended, having lasted ~45 yr. Our current observations showmajor changes in its wind from a cool, dense envelope to a much warmerstate surrounded by low-density gas with rare emission lines of Ca II,[Ca II], and K I. Its spectral energy distribution has unexpectedlychanged, especially at the long wavelengths, with a significant decreasein its apparent flux, while the star remains optically obscured. Weconclude that much of its radiation is now escaping out of our line ofsight. We attribute this to the changing structure and distribution ofits circumstellar ejecta, corresponding to the altered state of its windas the star recovers from a high mass loss event.
| The Circumstellar Environments of NML Cygni and the Cool Hypergiants
We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H2Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H2O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant μ Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.
| Spectroscopic binary orbits from photoelectric radial velocities. Paper 180: zeta Aurigae
Not Available
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
Not Available
| Analysis of the Na, Mg, Al, and Si Abundances in the Atmospheres of Red Giants of Different Spectral Subgroups
We analyze the Na, Mg, Al, and Si abundances in the atmospheres of morethan 40 stars, includingred giants of different spectral subgroups(normal red giants, mild and classical barium stars) and severalsupergiants. All these elements exhibit abundance excesses, with theoverabundance increasing with the star’s luminosity. Thedependence of the overabundances for each of these elements on theluminosity (or log g) is the same for all the spectral subgroups,testifying to a common origin: they are all products of hydrogen burningin the NeNa and MgAl cycles that have been dredged up from the stellarinteriors to the outer atmospheric layers by convection that graduallydevelops during the star’s evolution from the main sequence to thered-giant stage. The sodium abundances derived for several stars arelower than for other stars with similar atmospheric parameters. The agesand kinematic characteristics of these two groups of stars suggest thatthey probably belong to different stellar generations.
| 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.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Variable Star Network: World Center for Transient Object Astronomy and Variable Stars
Variable Star Network (VSNET) is a global professional-amateur networkof researchers in variable stars and related objects, particularly intransient objects, such as cataclysmic variables, black-hole binaries,supernovae, and gamma-ray bursts. The VSNET has been playing apioneering role in establishing the field of transient object astronomy,by effectively incorporating modern advances in observational astronomyand global electronic networks, as well as collaborative progress intheoretical astronomy and astronomical computing. The VSNET is now oneof the best-featured global networks in this field of astronomy. Wereview the historical progress, design concept, associated technology,and a wealth of scientific achievements powered by VSNET.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| Kinematics of the envelope of the post-AGB star V510 PupNucleus of a future planetary nebula
We have carried out a detailed identification of lines in the opticalspectrum of the post-AGB star V510 Pup associated with the infraredsource IRAS 08005-2356 based on observations with high spectralresolution. Absorption lines of the ions FeII, TiII, CrII, and YII arepresent at wavelengths from 4549 to 8546 Å. The absorption by YIIand other s-process elements is anomalously strong, and the absorptionis also strong in the circumstellar C2 Swan bands. The profiles of mostof the lines (of hydrogen and metals) display P Cygniabsorption-emission profiles. All the absorption lines are shiftedtoward the blue, suggesting an outflow of stellar material. Theexpansion velocity of the envelope derived from the Swan bands arisingthere is V exp =42 km/s. The highest wind velocity determined from theabsorption wings of the FeII(42) P Cygni profiles reaches 240 km/s.Based on the star’s kinematic characteristics and the amount ofinterstellar absorption, it is at a distance of d≈3-4 kpc, whichcorresponds to an absolute magnitude of M v≈-6m.
| 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.
| The Origin and Variability of the CO Near-IR Band in the Yellow Hypergiant Rho Casseopeiae.
We report the discovery of variability in the CO 1st overtonero-vibrational band at 2.3 micron in the yellow hypergiant RhoCassiopeiae.Rho Cas belongs to a rare class of peculiar post-red hypergiantsbelieved to be SN II progenitors. We present the near-IR low- to highresolution spectra of this star obtained over 5 pulsation epochs. Wediscover that the CO band transforms from pure absorption during theoutburst minimum in Oct 2000 to prominent emission during the phases offast atmospheric expansion in Oct 1998 and Sept 2004.Our preliminary modeling of the recent high-resolution emission spectrumrules out the photospheric nature of the CO band and indicates astructure in the the atmosphere where the gas temperature drops below4000K and then increases outwards. The relation to the optical emissionspectrum and the P Cyg profiles are also discussed - in search of cluesfor the mechanisms behind the severe mass loss and chromospheric heatingin extreme supergiants.We would like to thank the UKIRT and the VATT observer support teams forassistance with observations, reductions and retrieving archival data.Financial support has been partially provided by the NASA Spitzer andFUSE GI-E068 grants.
| A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars
A new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram.
| A gallery of cool hypergiants --- imaging their circumstellar environments
High resolution imaging with HST-WFPC2 of seven of the most luminousevolved cool stars shows that the three OH/IR stars NML Cyg, VX Sgr andS Per have circumstellar nebulae while no ejecta was detected for theM-type supergiant μ Cep and the yellow hypergiants ρ Cas, HR 8752and HR 5171a.
| Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram
Current observations of the S Dor/LBVs and candidates and theimplications for their important role in massive star evolution arereviewed. Recent observations of the cool hypergiants are altering ourideas about their evolutionary state, their atmospheres and winds, andthe possible mechanisms for their asymmetric high mass loss episodeswhich may involve surface activity and magnetic fields. Recent resultsfor IRC+10 420, ρ Cas and VY CMa are highlighted. S Dor/LBVs ineruption, and the cool hypergiants in their high mass loss phases withtheir optically thick winds are not what their apparent spectra andtemperatures imply; they are then `impostors' on the H-R diagram. Theimportance of the very most massive stars, like η Carinae and the`supernovae impostors' are also discussed.
| Aus der Sektion "Halb- und unregelmassige".
Not Available
| Aus der Sektion Kataklysmische und eruptive: Aktivitaten zwischen April und August 2003.
Not Available
| Aus der Sektion "Halb-und Unregelmassige".
Not Available
| High-Resolution Spectroscopy of FU Orionis Stars
High-resolution spectroscopy was obtained of the FU orionis stars FU Oriand V1057 Cyg between 1995 and 2002 with the SOFIN spectrograph at theNordic Optical Telescope and with HIRES at Keck I. During these years FUOri remained about 1 mag (in B) below its 1938-39 maximum brightness,but V1057 Cyg (B~10.5 at peak in 1970-1971) faded from about 13.5 to14.9 and then recovered slightly. Their photospheric spectra resemblethat of a rotationally broadened, slightly veiled supergiant of abouttype G0 Ib, with veqsini=70 km s-1 for FU Ori, and55 km s-1 for V1057 Cyg. As V1057 Cyg faded, P Cyg structurein Hα and the IR Ca II lines strengthened and a complexshortward-displaced shell spectrum of low-excitation lines of theneutral metals (including Li I and Rb I) increased in strength,disappeared in 1999, and reappeared in 2001. Several SOFIN runs extendedover a number of successive nights so that a search for rapid and cyclicchanges in the spectra was possible. These spectra show rapidnight-to-night changes in the wind structure of FU Ori at Hα,including clear evidence of sporadic infall. The equivalent width of theP Cyg absorption varied cyclically with a period of 14.8 days, withphase stability maintained over three seasons. This is believed to bethe rotation period of FU Ori. The internal structure of itsphotospheric lines also varies cyclically, but with a period of 3.54days. A similar variation may be present in V1057 Cyg, but the data aremuch noisier and that result uncertain. As V1057 Cyg has faded and thecontinuum level fallen, the emission lines of a preexistinglow-excitation chromosphere have emerged. Therefore we believe that the``line doubling'' in V1057 Cyg is produced by these central emissioncores in the absorption lines, not by orbital motion in an inclinedKeplerian disk. No convincing dependence of veqsini onwavelength or excitation potential was detected in either FU Ori orV1057 Cyg, again contrary to expectation for a self-luminous accretiondisk. It was found also that certain critical lines in the near infraredare not accounted for by synthetic disk spectra. It is concluded that arapidly rotating star near the edge of stability, as proposed by Larson,can better account for these observations. The possibility is alsoconsidered that FUor eruptions are not a property of ordinary T Tauristars but may be confined to a special subspecies of rapidly rotatingpre-main-sequence stars having powerful quasi-permanent winds.
| High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001
We present an overview of the spectral variability of the peculiarF-type hypergiant ρ Cas, obtained from our long-term monitoringcampaigns over the past 8.5 yr with four spectrographs in the northernhemisphere. Between 2000 June and September an exceptional variabilityphase occurred when the V brightness dimmed by about a full magnitude.The star recovered from this deep minimum by 2001 April. It is the thirdoutburst of ρ Cas on record in the last century. We observe TiOabsorption bands in high-resolution near-IR spectra obtained with theUtrecht Echelle Spectrograph during the summer of 2000. TiO formation inthe outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by atleast 3000 K, and the TiO shell is driven supersonically withM~=5.4×10-2 Msolar yr-1. Strongepisodic mass loss and TiO have also been observed during the outburstsof 1945-1947 and 1985-1986. A detailed analysis of the exceptionaloutburst spectra is provided, by comparing with high-resolution opticalspectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse(Iab). During the outburst, central emission appears above the localcontinuum level in the split Na D lines. A prominent optical emissionline spectrum appears in variability phases of fast wind expansion. Theradial velocity curves of Hα and of photospheric metal absorptionlines signal a very extended and velocity-stratified dynamic atmosphere.The outburst spectra indicate the formation of a low-temperature,optically thick circumstellar gas shell of 3×10-2Msolar during 200 days, caused by dynamic instability of theupper atmosphere of this pulsating massive supergiant near the Eddingtonluminosity limit. We observe that the mass-loss rate during the outburstis of the same order of magnitude as has been proposed for the outburstsof η Carinae. We present calculations that correctly predict theoutburst timescale, whereby the shell ejection is driven by the releaseof hydrogen ionization recombination energy.
| Line profile variations in classical Cepheids. Evidence for non-radial pulsations?
We have investigated line profiles in a large sample of Cepheid spectra,and found four stars that show unusual (for Cepheids) line profilestructure (bumps or/and asymmetries). The profiles can be phasedependent but the behavior persists over many cycles. The asymmetriesare unlikely to be due to the spectroscopic binarity of these stars orthe specific velocity field in their atmospheres caused by shock waves.As a preliminary hypothesis, we suggest that the observed features onthe line profiles in the spectra of X Sgr, V1334 Cyg, EV Sct and BG Crucan be caused by the non-radial oscillations. It is possible that thesenon-radial oscillations are connected to resonances between the radialmodes (3fd2 , 7fd5 or 10fd0 ).Based on spectra collected at McDonald 2.1 m and Kitt Peak 4 m (USA),CTIO 4 m (Chile) and MSO 1.8 m (Australia).
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| Crossing the Yellow Void: Spatially Resolved Spectroscopy of the Post-Red Supergiant IRC +10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define theempirical upper luminosity boundary in the H-R diagram. During theirpost-red supergiant evolution, these massive stars enter a temperaturerange (6000-9000 K) of increased dynamical instability, high mass loss,and increasing opacity, a semiforbidden region that de Jager and hiscollaborators have called the ``yellow void.'' We report HST/STISspatially resolved spectroscopy of IRC +10420 and its reflection nebulawith some surprising results. Long-slit spectroscopy of the reflectedspectrum allows us to effectively view the star from differentdirections. Measurements of the double-peaked Hα emission profileshow a uniform outflow of gas in a nearly spherical distribution,contrary to previous models with an equatorial disk or bipolar outflow.Based on the temperature and mass-loss rate estimates that are usuallyquoted for this object, the wind is optically thick to the continuum atsome and possibly all wavelengths. Consequently, the observed variationsin apparent spectral type and inferred temperature are changes in thewind and do not necessarily mean that the underlying stellar radius andinterior structure are evolving on such a short timescale. To explainthe evidence for simultaneous outflow and infall of material near thestar, we propose a ``rain'' model, in which blobs of gas condense inregions of lowered opacity outside the dense wind. With the apparentwarming of its wind, the recent appearance of strong emission, and adecline in the mass-loss rate, IRC +10420 may be about to shed itsopaque wind, cross the yellow void, and emerge as a hotter star. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| The Millenium Outburst of the Yellow Hypergiant ρ Cassiopeiae
We report the largest mass-loss rate of ~5 % of the solar mass per year,directly observed in any stellar object so far during the recentoutburst of the Yellow Hypergiant ρ Cas. In the fall of 2000, theenigmatic cool luminous supergiant dimmed by more than a full visualmagnitude, thereby changing its spectral type form early F- to earlyM-type. Our spectral monitoring reveals that the effective temperaturedecreased by more than 3000 K during the event, from above 7000 K tobelow 4000 K in less than ~200 days. It is the third outburst of ρCas on record in the last century. The optical spectrum becomescomparable to that of the red supergiant Betelgeuse, and revealsstrongly blue-shifted molecular absorption bands of titanium-oxide(TiO). We determine from the newly formed TiO bands a gas mass-loss rateof the same order of magnitude as has been proposed for the giantoutbursts of the Luminous Blue Variable η Carinae. The outburst ofρ Cas produces an outward propagating circumstellar shock wave,driven by recombination of hydrogen gas in its wake, resulting in atremendous cooling of the entire outer atmosphere. Over the past 18months since the outburst we observe a very prominent inverse P Cygniprofile in Balmer Hα . Strong emission in this line has not beforebeen observed in ρ Cas over this long period of time. The opticalspectroscopic monitoring signals an unusal strong collapse of the upperHα atmosphere, which we also observed in the months before theoutburst event. The recent observations may therefore signal that a newand stronger outburst of ρ Cas is imminent. This reseach has beensupported in part by a Space Telescope Science Institute grant to theSmithsonian Astrophysical Observatory.
| A hidden population of Wolf-Rayet stars in the massive galactic cluster Westerlund 1
We report the discovery of a hitherto undetected population ofWolf-Rayet stars in the young galactic open cluster Westerlund1. Optical spectroscopy of the cluster identified 11 suchobjects; provisional classification suggests that 6 are nitrogen rich(WN) and 5 carbon rich (WC). Including the previously identified Blue,Yellow & Red Super- & Hypergiants, Westerlund1 clearly has a very rich population of massive post-MainSequence objects. To date, the post-MS population of Westerlund1 is significantly larger than that of any other galacticyoung open cluster - with the possible exception of theArches - implying that it is potentially amongst themost massive young clusters yet identified in the Local Group. Based onobservations collected at the European Southern Observatory, La Silla,Chile (ESO 67.D-0211).
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is 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/390/967
|
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:
|
