SKY-MAP.ORG
 首页 开始 To Survive in the Universe News@Sky 天文图片 收集 论坛 Blog New! 常见问题 新闻 登录

# HD 8879

### 图像

DSS Images   Other Images

### 相关文章

 Mid-Infrared Spectroscopy of Carbon Stars in the Small Magellanic CloudWe have observed a sample of 36 objects in the Small Magellanic Cloud(SMC) with the Infrared Spectrometer on the Spitzer Space Telescope.Nineteen of these sources are carbon stars. An examination of the near-and mid-infrared photometry shows that the carbon-rich and oxygen-richdust sources follow two easily separated sequences. A comparison of thespectra of the 19 carbon stars in the SMC to spectra from the InfraredSpace Observatory (ISO) of carbon stars in the Galaxy revealssignificant differences. The absorption bands at 7.5 and 13.7 μm dueto C2H2 are stronger in the SMC sample, and theSiC dust emission feature at 11.3 μm is weaker. Our measurements ofthe MgS dust emission feature at 26-30 μm are less conclusive, butthis feature appears to be weaker in the SMC sample as well. All ofthese results are consistent with the lower metallicity in the SMC. Thelower abundance of SiC grains in the SMC may result in less efficientcarbon-rich dust production, which could explain the excessC2H2 gas seen in the spectra. The sources in theSMC with the strongest SiC dust emission tend to have redder infraredcolors than the other sources in the sample, which implies moreamorphous carbon, and they also tend to show stronger MgS dust emission.The weakest SiC emission features tend to be shifted to the blue; thesespectra may arise from low-density shells with large SiC grains. Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisitedAs part of a reanalysis of galactic Asymptotic Giant Branch (AGB) starsat infrared (IR) wavelengths, we discuss a sample (357) of carbon starsfor which mass loss rates, near-IR photometry and distance estimatesexist. For 252 sources we collected mid-IR fluxes from the MSX (6C) andthe ISO-SWS catalogues. Most stars have spectral energy distributions upto 21 μm, and some (1/3) up to 45 μm. This wide wavelengthcoverage allows us to obtain reliable bolometric magnitudes. Theproperties of our sample are discussed with emphasis on ~70 stars withastrometric distances. We show that mid-IR fluxes are crucial toestimate the magnitude of stars with dusty envelopes. We construct HRdiagrams and show that the luminosities agree fairly well with modelpredictions based on the Schwarzschild's criterion, contrary to what iswidely argued in the literature. A problem with the brightness of Cstars does not appear to exist. From the relative number of Mira andSemiregular C-variables, we argue that the switch between these classesis unlikely to be connected to thermal pulses. The relevance of the twopopulations varies with the evolution, with Miras dominating the finalstages. We also analyze mass loss rates, which increase for increasingluminosity, but with a spread that probably results from a dependence ona number of parameters (like e.g. different stellar masses and differentmechanisms powering stellar winds). Instead, mass loss rates are wellmonitored by IR colours, especially if extended to 20 μm and beyond,where AGB envelopes behave like black bodies. From these colours theevolutionary status of various classes of C stars is discussed. Carbon Stars in the Infrared Telescope in Space SurveyWe have identified 139 cool carbon stars in the near-infraredspectrophotometric survey of the Infrared Telescope in Space (IRTS) fromthe conspicuous presence of molecular absorption bands at 1.8, 3.1, and3.8 μm. Among them, 14 are new bright (K~4.0-7.0) carbon stars. Wefind a trend relating the 3.1 μm band strength to the K-L'color index, which is known to correlate with mass-loss rate. This couldbe an effect of a relation between the depth of the 3.1 μm featureand the degree of development of the extended stellar atmosphere wheredust starts to form. Properties of detached shells around carbon stars. Evidence of interacting windsThe nature of the mechanism responsible for producing the spectacular,geometrically thin, spherical shells found around some carbon stars hasbeen an enigma for some time. Based on extensive radiative transfermodelling of both CO line emission and dust continuum radiation for allobjects with known detached molecular shells, we present compellingevidence that these shells show clear signs of interaction with asurrounding medium. The derived masses of the shells increase withradial distance from the central star while their velocities decrease. Asimple model for interacting winds indicates that the mass-loss rateproducing the faster moving wind has to be almost two orders ofmagnitudes higher (~10-5 Mȯ yr-1)than the slower AGB wind (a few 10-7 Mȯyr-1) preceding this violent event. At the same time, thepresent-day mass-loss rates are very low indicating that the epoch ofhigh mass-loss rate was relatively short, on the order of a few hundredyears. This, together with the number of sources exhibiting thisphenomenon, suggests a connection with He-shell flashes (thermalpulses). We report the detection of a detached molecular shell aroundthe carbon star DR Ser, as revealed from newsingle-dish CO (sub-)millimetre line observations. The properties of theshell are similar to those characterising the young shell aroundU Cam. First results from the ESO VLTI calibrators programThe ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators. Three-micron spectra of AGB stars and supergiants in nearby galaxiesThe dependence of stellar molecular bands on the metallicity is studiedusing infrared L-band spectra of AGB stars (both carbon-rich andoxygen-rich) and M-type supergiants in the Large and Small MagellanicClouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. Thespectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CHand HCN bands for carbon-rich AGB stars. The equivalent width ofacetylene is found to be high even at low metallicity. The high C2H2abundance can be explained with a high carbon-to-oxygen (C/O) ratio forlower metallicity carbon stars. In contrast, the HCN equivalent width islow: fewer than half of the extra-galactic carbon stars show the 3.5μm HCN band, and only a few LMC stars show high HCN equivalent width.HCN abundances are limited by both nitrogen and carbon elementalabundances. The amount of synthesized nitrogen depends on the initialmass, and stars with high luminosity (i.e. high initial mass) could havea high HCN abundance. CH bands are found in both the extra-galactic andGalactic carbon stars. One SMC post-AGB star, SMC-S2, shows the 3.3μm PAH band. This first detection of a PAH band from an SMC post-AGBstar confirms PAHs can form in these low-metallicity stars. None of theoxygen-rich LMC stars show SiO bands, except one possible detection in alow quality spectrum. The limits on the equivalent widths of the SiObands are below the expectation of up to 30 Å for LMC metallicity.Several possible explanations are discussed, mostly based on the effectof pulsation and circumstellar dust. The observations imply that LMC andSMC carbon stars could reach mass-loss rates as high as their Galacticcounterparts, because there are more carbon atoms available and morecarbonaceous dust can be formed. On the other hand, the lack of SiOsuggests less dust and lower mass-loss rates in low-metallicityoxygen-rich stars. The effect on the ISM dust enrichment is discussed. The mass loss of C-rich giantsThe mass loss rates, expansion velocities and dust-to-gas density ratiosfrom millimetric observations of 119 carbon-rich giants are compared, asfunctions of stellar parameters, to the predictions of recenthydrodynamical models. Distances and luminosities previously estimatedfrom HIPPARCOS data, masses from pulsations and C/O abundance ratiosfrom spectroscopy, and effective temperatures from a new homogeneousscale, are used. Predicted and observed mass loss rates agree fairlywell, as functions of effective temperature. The signature of the massrange M≤4 Mȯ of most carbon-rich AGB stars is seenas a flat portion in the diagram of mass loss rate vs. effectivetemperature. It is flanked by two regions of mass loss rates increasingwith decreasing effective temperature at nearly constant stellar mass.Four stars with detached shells, i.e. episodic strong mass loss, andfive cool infrared carbon-rich stars with optically-thick dust shells,have mass loss rates much larger than predicted values. The latter(including CW Leo) could be stars of smaller masses (M≃ 1.5-2.5Mȯ) while M≃ 4 Mȯ is indicated formost of the coolest objects. Among the carbon stars with detachedshells, R Scl returned to a predicted level (16 times lower) accordingto recent measurements of the central source. The observed expansionvelocities are in agreement with the predicted velocities at infinity ina diagram of velocities vs. effective temperature, provided the carbonto oxygen abundance ratio is 1≤ɛ C/ɛO≤2, i.e. the range deduced from spectra and modelatmospheres of those cool variables. Five stars with detached shellsdisplay expansion velocities about twice that predicted at theireffective temperature. Miras and non-Miras do populate the same locus inboth diagrams at the present accuracy. The predicted dust-to-gas densityratios are however about 2.2 times smaller than the values estimatedfrom observations. Recent drift models can contribute to minimize thediscrepancy since they include more dust. Simple approximate formulaeare proposed.This research has made use of the Simbad database operated at CDS.Partially based on data from the ESA HIPPARCOS astrometry satellite.Table 3 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/429/235 Merrill-Sanford bands in Large Magellanic Cloud carbon starsFrom a sample of 304 carbon stars in the central parts of the LargeMagellanic Cloud (LMC), ~27 per cent have Merrill-Sanford (MS) bands ofthe SiC2 molecule. The data are based on a uniform set ofspectra taken with 2dF on the Anglo-Australian Telescope, and giveuseful statistics on the incidence of MS bands and on their correlation(or otherwise) with other properties. All of these are red stars, coolerthan 3100 K. The proportion of stars showing the bands is highestamongst the coolest stars, but not all very cool stars show the bands.There is no evidence that MS bands are more common in J-type stars(carbon stars with a high 13C/12C ratio) than inN-type carbon stars, at least within this sample of LMC stars. There isno apparent correlation with stellar variability, or between thephotospheric temperature [as measured by (J-K)] and the occurrence ofthe hot' MS bands from excited molecular states. Classification of Spectra from the Infrared Space Observatory PHT-S DatabaseWe 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). Molecular and dust features of 29 SiC carbon AGB starsWe have reduced and analyzed the Infrared Space Observatory (ISO)Short-Wavelength Spectrometer (SWS) spectra of 29 infrared carbon starswith a silicon carbide feature at 11.30 μm, 17 of which have not beenpreviously published. Absorption or emission features of C2,HCN, C2H2, C3 and silicon carbide (SiC)have been identified in all 17 unpublished carbon stars. In addition,two unidentified absorption features at 3.50 and 3.65 μm are listedfor the first time in this paper. We classify these 29 carbon stars intogroups A, B, C and D according to the shapes of their spectral energydistribution, and this classification seems to show an evolutionarysequence of carbon stars with an SiC feature. Moreover we have found thefollowing results for the different groups: on average, the relativeintegrated flux of the 3.05 μm C2H2+HCNabsorption feature increases gradually from group A to B and C; that ofthe 5.20 μm C3 absorption feature becomes gradually weakerfrom group A to B and C; that of the 11.30 μm SiC emission featureincreases gradually from group A to B and C but weakens in group D; andin contrast, that of the 13.70 μm C2H2absorption feature weakens gradually from group A to B and C but becomesstronger in group D. We suggest that the evolution of the IR spectra ofcarbon stars along the sequence A to D is a result of the followingphenomena: as the near-IR black-body temperature (Tnir)decreases, the circumstellar envelope becomes thicker; also theeffective temperature (Teff) of the photosphere of thecentral star decreases gradually and the C/O ratio increases from A toB.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) with the participation of ISAS and NASA. MgS in detached shells around carbon stars. Mining the mass-loss historyWe investigate the dust composition of detached shells around carbonstars, with a focus to understand the origin of the coolmagnesium-sulfide (MgS) material around warm carbon stars, which hasbeen detected around several of these objects\citep{2002A&A...390..533H}. We build a radiative transfer model ofa carbon star surrounded by an expanding detached shell of dust. Theshell contains amorphous carbon grains and MgS grains. We find that asmall fraction of MgS dust (2% of the dust mass) can give a significantcontribution to the IRAS 25 μm flux. However, the presence of MgS inthe detached shell cannot be inferred from the IRAS broadband photometryalone but requires infrared spectroscopy.We apply the model to the detached-shell sources R Scl and U Cam, bothexhibiting a cool MgS feature in their ISO/SWS spectra. We use the shellparameters derived for the molecular shell, using the CO submillimetremaps \citep{1999A&A...351L...1L,2001A&A...368..969S}. Themodels, with MgS grains located in the detached shell, explain the MgSgrain temperature, as derived from their ISO spectra, very well. Thisdemonstrates that the MgS grains are located at the distance of thedetached shell, which is a direct indication that these shells originatefrom a time when the stellar photosphere was already carbon-rich. In thecase of R Scl, the IRAS photometry is simultaneously explained by thesingle shell model. In the case of U Cam, the IRAS photometry is underpredicted, pointing to a contribution from cooler dust located evenfarther away from the star than the molecular shell.We present a simple diagnostic to constrain the distance of the shellusing the profile of the MgS emission feature. The emission featureshifts to longer wavelength with decreasing grain temperature. One cantherefore infer a temperature and a corresponding distance to the starfrom the observed profile. Such a diagnostic might prove useful forfuture studies of such systems with SIRTF or SOFIA.based on observations obtained with ISO, an ESA project with instrumentsfunded by ESA member states (especially the PI countries: France,Germany, the Netherlands and UK) with the participation of ISAS andNASA.} Australia Telescope Compact Array imaging of circumstellar HCN line emission from R SclWe present radio-interferometric observations of HCN J=1->0 lineemission from the carbon star R Scl, obtained with the interim 3-mmreceivers of the Australia Telescope Compact Array. The emission isresolved into a central source with a Gaussian FWHM of 1 arcsec,which we identify as the present mass loss envelope. Using a simplephotodissociation model and constraints from single-dish HCN spectra, weargue that the present mass-loss rate is low,  2 ×10-7 Mȯ yr-1, supporting the ideathat R Scl had to experience a brief episode of intense mass loss inorder to produce the detached CO shell at 10 arcsec radius inferredfrom single-dish observations. Detailed radiative transfer modellingyields an abundance of HCN relative to H2, fHCN,of 10-5 in the present-day wind. There appears to be adiscrepancy between model results obtained with higher transitionsingle-dish data included and those from the J=1->0 interferometerdata alone, in that the interferometer data suggest a smaller envelopesize and larger HCN abundance than the single-dish data. The lack of HCNin the detached shell, fHCN 2× 10-7, isconsistent with the rapid photodissociation of HCN into CN as it expandsaway from the star. New Laboratory Spectra of Isolated β-SiC Nanoparticles: Comparison with Spectra Taken by the Infrared Space ObservatoryWe present new laboratory infrared spectra of matrix-isolated β-SiCnanoparticles, which perfectly match the band profile of the 11+ μmfeature observed in carbon stars. The new laboratory spectra differ inthe shape of the band profile from former measurements of SiCnanoparticles thanks to the matrix-isolation technique which allows toobtain spectra of nonagglomerated particles. The final spectra arecorrected for the influence of the surrounding medium (argon-matrix) bya computational technique proposed by Papoular et al. (1998).Furthermore, we study the influence of nitrogen incorporation into theSiC lattice, which introduces a strong near-infrared absorption owing tosurface-plasmon excitation (Mutschke et al. 1999). Our laboratoryspectra are compared with Infrared Space Observatory observations ofseveral carbon stars showing an 11 μm feature either in emission orin absorption. We discuss the implications of the new laboratory resultsfor the interpretation of the spectra of carbon stars.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the UK) and with the participation of ISAS and NASA.The SWS is a joint project of SRON and MPE. Infrared Colors and Variability of Evolved Stars from COBE DIRBE DataFor a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima. Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable starsWe analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993 How many Hipparcos Variability-Induced Movers are genuine binaries?Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also 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/399/1167 Imaging polarimetry of stellar light scattered in detached shells around the carbon stars R Scl and U AntImaging polarimetry has been used to study the extended, detachedcircumstellar shells around the bright carbon stars RScl and U Ant. The observations were donein two narrow band filters centred on the resonance lines of neutral Kand Na, but much broader than the expected line widths. The polarimetricdata reveal brightness distributions, in both cases, which are inperfect agreement with previous observations of scattered light obtainedthrough direct imaging techniques. The total intensity images towardsR Scl outline, in both filters, circular disk-likedistributions out to a radius of ~21arcsec , where the intensity dropssharply. The polarised intensity images reveal, however, that thescattering occurs in a geometrically thin shell. The degree ofpolarisation reaches values of ~35% in both filters. The imagingpolarimetry observations of U Ant reveal a somewhatmore complex structure, where the existence of several shells can bediscerned. The polarised scattered light comes from a component, at aradius of ~50arcsec from the star, which lies outside the region wherethe bulk of the light is scattered. The latter comes from a dominatingshell at ~43arcsec , which coincides spatially with the detached gasshell inferred from CO radio line data, and there may be another twoshells inside this. The polarisation degree reaches ~50% in the outercomponent. We model, with a code based on the Monte Carlo method, thescattered emission under the assumption of dust scattering, using theobserved polarised brightness distributions as constraints. In the caseof R Scl we found that the polarised, as well as thetotal, light distributions can be explained by scattering in a 2arcsecwide shell of radius 20arcsec containing a dust mass of~2*E-6 Msun. This dust shell is also responsiblefor the thermal dust emission measured by IRAS. There is room, up to 30%of the total scattered flux, for other scattering agents. Comparisonwith CO radio line data shows that this dust shell probably lies outsidethe detached CO gas shell. In the case of U Ant themodelling explains the outer component in terms of a 5arcsec wide shellat a radius of about 52arcsec with a dust mass of ~4*E-6Msun. This is also the dust shell responsible for theemission measured by IRAS. However, the bulk of the scattered lightcannot in this case be due to scattering by dust. In accordance with adiscussion in a previous paper we attribute the remaining, unpolarised,scattering to the KI and Na D resonance lines. In both cases we foundevidence that a dust shell has separated from the rest of thecircumstellar medium. This may be due to gas-grain drift, or tohydrodynamical effects, which may also explain the complexmultiple-shell structure seen towards U Ant. Themodel results are very dependent on the grain size distribution, and theobservational data can only be reconciled with a very steep decline ingrain size.Based on observations using the 3.6 m telescope of the European SouthernObservatory, La Silla, Chile. Hipparcos red stars in the HpV_T2 and V I_C systemsFor Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/397/997 Very Large Telescope Spectra of Carbon Stars in the Large Magellanic Cloud and Their Metallicity DependenceVery Large Telescope (VLT) L-band spectra of six carbon stars in theLarge Magellanic Cloud are presented. The stars show absorption bands at3.1 μm (HCN and C2H2), and 3.8 μm, which isprobably due to C2H2. Two LMC stars show strong3.5 μm HCN absorption. The equivalent widths of the 3.1 μm and 3.8μm bands are systematically larger in LMC carbon stars than in carbonstars in the solar neighborhood. Moreover, the ratio of the equivalentwidths of the 3.8 and 3.1 μm bands is much larger in the LMC,suggesting a higher ratio of n(C2H2)/n(HCN). Thestronger absorption bands are in contrast to the assumption that if theelemental abundances are scaled from the carbon star's abundances in thesolar neighbor, the abundances of these molecules are less at lowermetallicity. We argue for a systematically larger C/O ratio in LMCcarbon stars. In the Galactic carbon stars n(C)/n(O)~1.05-1.1 onaverage; our chemical model shows that the stronger molecular bands inthe LMC carbon stars could be explained with n(C)/n(O)>1.2. Thehigher C/O ratio can also explain the higher ratio ofn(C2H2)/n(HCN) in LMC stars than in the solarneighborhood. Based on observations obtained at the European SouthernObservatory, Chile (proposal 68.D-0660). Polarimetry of evolved stars. II. The carbon star R SclWe present broadband optical polarimetry of the carbon star R Scl,primarily in the VRCIC bands. Polarimetricvariability was detected on time-scales from hours to years. Thepolarization of R Scl showed a ~ lambda -4wavelength-dependence, which we attribute to scattering by smallamorphous carbon dust grains. We deduce the properties of thecircumstellar condensations in which the scattering takes place, andfind consistency with infrared data if the condensations move ineccentric orbits. We also conclude that the condensations are tidallydispersed on a time-scale ~ 1 day. On longer time-scales, polarimetricvariations may possibly be linked with photometric variations. Carbon-rich giants in the HR diagram and their luminosity functionThe 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 The carrier of the 30'' mu m emission feature in evolved stars. A simple model using magnesium sulfideWe present 2-45 mu m spectra of a large sample of carbon-rich evolvedstars in order to study the 30'' mu m feature. We find the 30'' mu mfeature in a wide range of sources: low mass loss carbon stars, extremecarbon-stars, post-AGB objects and planetary nebulae. We extract theprofiles from the sources by using a simple systematic approach to modelthe continuum. We find large variations in the wavelength and width ofthe extracted profiles of the 30'' mu m feature. We modelled the wholerange of profiles in a simple way by using magnesium sulfide (MgS) dustgrains with a MgS grain temperature different from the continuumtemperature. The systematic change in peak positions can be explained bycooling of MgS grains as the star evolves off the AGB. In severalsources we find that a residual emission excess at ~ 26 mu m can also befitted using MgS grains but with a different grains shape distribution.The profiles of the `30'' mu m feature in planetary nebulae arenarrower than our simple MgS model predicts. We discuss the possiblereasons for this difference. We find a sample of warm carbon-stars withvery cold MgS grains. We discuss possible causes for this phenomenon. Wefind no evidence for rapid destruction of MgS during the planetarynebula phase and conclude that the MgS may survive to be incorporated inthe ISM. Based on observations obtained with ISO, an ESA project withinstruments funded by ESA Member states (especially the PI countries:France, Germany, The Netherlands and the United Kingdom) with theparticipation of ISAS and NASA. Appendix A (Figs. A.1 and A.2) is onlyavailable in electronic form at http://www.edpsciences.org CO 1st overtone spectra of cool evolved stars: Diagnostics for hydrodynamic atmosphere modelsWe present spectra covering the wavelength range 2.28 to 2.36 mu m at aresolution of Delta lambda = 0.0007 mu m (or R = 3500) for a sample of24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants,3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengthscovered include the main parts of the 12C16O v =2-0 and 3-1 overtone bands, as well as the v = 4-2 and 13CO v= 2-0 bandhead regions. CO lines dominate the spectrum for all the starsobserved, and at this resolution most of the observed features can beidentified with individual CO R- or P-branch lines or blends. Theobserved transitions arise from a wide range of energy levels extendingfrom the ground state to E/k > 20 000 K. We looked for correlationsbetween the intensities of various CO absorption line features and otherstellar properties, including IR colors and mass loss rates. Two usefulCO line features are the v = 2-0 R14 line, and the CO v = 2-0 bandhead.The intensity of the 2-0 bandhead shows a trend with K-[12] color suchthat the reddest stars (K-[12] > 3 mag) exhibit a wide range in 2-0bandhead depth, while the least reddened have the deepest 2-0 bandheads,with a small range of variation from star to star. Gas mass loss ratesfor both the AGB stars and the red supergiants in our sample correlatewith the K-[12] color, consistent with other studies. The data implythat stars with dot M_gas < 5x 10-7 Msuny-1 exhibit a much narrower range in the relative strengthsof CO 2-0 band features than stars with higher mass loss rates. Therange in observed spectral properties implies that there are significantdifferences in atmospheric structure among the stars in this sample.Figures 4-9, 11-14, 16, 17, 19-21, 23, 24 are only avalaible inelectronic form at http://www.edpsciences.org An improved mass-loss description for dust-driven superwinds and tip-AGB evolution modelsWe derive an improved description of dust-driven stellar mass-loss forthe cool winds of carbon-rich tip-AGB stars. We use pulsating windmodels in which the mass loss is driven by radiation pressure on dustgrains, for C-rich chemistry. From a larger set of these models,selected for representative dynamical (pulsational velocity amplitudeDelta v, period P) and chemical (theepsilonC/epsilonO abundance ratio) inputparameters, an improved approximative mass-loss formula has been derivedwhich depends only on the stellar parameters (effective temperatureTeff, luminosity L and mass M). Due to the detailedconsideration of the chemistry and the physics of the dust nucleationand growth processes, there is a particularly strong dependence of themass-loss rate dot {M} (in Msun/yr) on Teff: log{dot {M}} = 8.86 - 1.95 * log {M/{Msun}} - 6.81 * log {T/K} +2.47 * log {L/{Lsun}}. The dependence of the model mass-losson the pulsational period has explicitly been accounted for inconnection with the luminosity dependence, by applying an observedperiod-luminosity relation for C-rich Miras. We also apply the improvedmass-loss description to our evolution models, and we revisit theirtip-AGB mass-loss histories and the total masses lost, in comparison toour earlier work with a preliminary mass-loss description. While thereis virtually no difference for the models in the lower mass range ofconsideration (Mi = 1.0 to ~ 1.3 Msun), we nowfind more realistic, larger superwind mass-loss rates for larger stellarmasses: i.e., dot {M} between ~ 0.4 and 1.0 x 10-4Msun/yr for Mi between 1.85 and 2.65Msun, removing between 0.6 and 1.2 Msun,respectively, during the final 30 000 yrs on the tip-AGB. Nature of OH maser and SiO thermal emission towards carbon star: IRAS 05373-0810 (V1187 Ori)We present observational evidence that IRAS 05373-0810 is a genuinecarbon star with an ISO SWS spectrum closely resembling that of R Scl.Modelling of the spectral energy distribution of IRAS 05373-0810suggests that the star has luminosity of order of 8000 Lsunand loses mass at a rate of about 2-3*E-7 Msunyr-1. The detected OH maser emission at 1612, 1665 and 1667MHz and SiO thermal emission at 86.85 GHz towards IRAS 05373-0810 is notassociated with this source. The available observations imply that theselines, typical for O-rich sources, come from the molecular cloud L 1641in the Orion star forming region (OH) and, very likely, from the NGC2149 molecular complex (SiO). Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) andwith participation of ISAS and NASA. Stars with the Largest Hipparcos Photometric AmplitudesA list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars. General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third EditionThe catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars. Discovery of Two New HCN Maser Lines in Five Carbon StarsA survey with the Heinrich Hertz Submillimeter Telescope of HCN emissionfrom mass-losing carbon stars has revealed masers in the J=3-2 and 4-3transitions of the (011c0) vibrational bending mode. Theselines have not previously been known to show maser action. Five stars-RScl, V384 Per, R Lep, Y CVn, and V Cyg-out of 12 observed were detectedas masers. Allowing for evidence of variability, this detection ratesuggests that these HCN lines are masers at least some of the time inthe majority of mass-losing carbon stars. The line widths and velocitiesimply that the maser action occurs in gas close to the star, where thecircumstellar envelope is just being accelerated outward. Polarimetry of 167 Cool Variable Stars: DataMulticolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool stars. High-Resolution Images of CO J=2-1 Emission from the Carbon Star V CygniThis paper presents observations of the CO J=2-1 emission from thecircumstellar envelope of the mass-losing carbon star V Cyg. Theobservations were made with the Caltech Millimeter Array. A previouslypublished single-dish map was used to construct short-spacingvisibilities not sampled by the interferometer data, thereby recoveringmissing flux in extended low brightness emission. The images have anangular resolution of ~1.2" with a velocity resolution of 1 MHz (1.3 kms-1). The channel maps are consistent with an expandingenvelope that is roughly spherical, but they also show evidence forasymmetric structure, as well as small-scale clumping. We compare theseobservations, as well as other published spectra, with statisticalequilibrium models for CO in a circumstellar envelope. Models that fitthe spherically averaged data must invoke a mass-loss rate, M, that hasdecreased with time by a factor of ~2-3 over the past several hundredyears. The model kinetic temperature structure in radius,TK(r), decreases as r-0.8 out tor~6×1015 cm and levels off to a constant value atTK=23 K beyond. The secular change in M may be related tochanges in the stellar luminosity or temperature, as predicted by recentnumerical hydrodynamic models for mass loss. The inferred kinetictemperature structure suggests that heating by the photoelectric effecton dust grains is important in the outer envelope.

• - 没有找到链接 -

### 下列团体成员

#### 观测天体数据

 星座: 玉夫座 右阿森松: 01h26m58.10s 赤纬: -32Â°32'35.0" 视星: 5.79 距离: 473.934 天文距离 右阿森松适当运动: 0 赤纬适当运动: 0 B-T magnitude: 12.544 V-T magnitude: 7.365