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Detached shells as tracers of asymptotic giant branch-interstellar medium bow shocks
New Spitzer imaging observations have revealed the structure around theMira variable star R Hya to be a one-sided parabolic arc 100 arcsec tothe west, stretching from north to south. We successfully model R Hyaand its surroundings in terms of an interaction of the stellar wind froman asymptotic giant branch (AGB) star with the interstellar medium (ISM)the star moves through. Our three-dimensional hydrodynamic simulationreproduces the structure as a bow shock into the oncoming ISM. Wepropose this as another explanation of detached shells around suchstars, which should be considered alongside current theories of internalorigin. The simulation predicts the existence of a tail ofram-pressure-stripped AGB material stretching downstream. Indicationsfor such a tail behind R Hya are seen in IRAS maps.
| Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud
We investigate the molecular bands in carbon-rich asymptotic giantbranch (AGB) stars in the Large Magellanic Cloud (LMC), using theInfrared Spectrograph (IRS) onboard the Spitzer Space Telescope (SST)over the 5-38 μm range. All 26 low-resolution spectra show acetylene(C2H2) bands at 7 and 14 μm. The hydrogencyanide (HCN) bands at these wavelengths are very weak or absent. Thisis consistent with low nitrogen abundances in the LMC. The observed 14μm C2H2 band is reasonably reproduced by anexcitation temperature of 500 K. There is no clear dilution of the 14μm C2H2 band by circumstellar dust emission.This 14-μm band originates from molecular gas in the circumstellarenvelope in these high mass-loss rate stars, in agreement with previousfindings for Galactic stars. The C2H2columndensity, derived from the 13.7 μm band, shows a gas mass-loss rate inthe range 3 × 10-6 to 5 ×10-5Msolaryr-1. This is comparable withthe total mass-loss rate of these stars estimated from the spectralenergy distribution. Additionally, we compare the line strengths of the13.7 μm C2H2 band of our LMC sample with thoseof a Galactic sample. Despite the low metallicity of the LMC, there isno clear difference in the C2H2 abundance amongLMC and Galactic stars. This reflects the effect of the third dredge-upbringing self-produced carbon to the surface, leading to highcarbon-to-oxygen ratio at low metallicity.
| Challenging the Carbon Star Dust Condensation Sequence: Anarchist C Stars
There have been several investigations of the evolution of themid-infrared (IR) dust features in carbon star spectra based on IRAS LRSdata, but these studies are somewhat contradictory. In order tounderstand these differences in interpretations and to develop anunderstanding of the carbon star dust sequence, we have reexamined 26IRAS LRS spectra of carbon stars that have also been observedspectroscopically by ISO SWS. The low resolution and narrow wavelengthcoverage of the IRAS LRS data hinder determination of the effect ofmolecular absorptions in these spectra. This has led to incorrectestimations of the continuum levels in these spectra, which has a hugeeffect on the continuum-divided and continuum-subtracted spectra used toanalyze trends in the shape, strength, and position of the mid-IRfeatures. The higher resolution and broader wavelength coverage of theISO data allow more accurate fitting of the underlying continuum. Wehave reassessed the trends in shape, strength, and position of the ~11μm silicon carbide (SiC) feature and the apparent emergence of the ~9μm feature. We find that there are no correlations between thespectral parameters. We also investigate whether any of these parameterscorrelate with the strength of the molecular bands; no correlation wasfound. Moreover, we show that the apparent 9 μm feature is probablyan artifact. We discuss the implications of this study in terms of botha carbon star condensation sequence and the application of this study tothe larger IRAS data set.
| UIR bands in the ISO SWS spectrum of the carbon star TU Tauri
Polycyclic Aromatic Hydrocarbon (PAH) molecules are thought to form inthe ejecta of carbon-rich Asymptotic Giant Branch (AGB) stars. Fifty ISOSWS spectra of warm carbon-rich AGB stars have been investigated for theUnidentified IR (UIR) emission due to fluorescence by PAH molecules. Inthis sample the binary star TU Tau, which has a blue companion, showsinteresting spectral structure in the appropriate wavelength regions.The profiles of the UIR bands in TU Tau have been derived by comparingto suitable carbon-star spectra. The presence of the UIR bands in TU Tauis attributed to UV photons originating from the A2 companion star whichare necessary to excite PAH molecules. The absence of the UIR bands inthe remainder of the sample is ascribed to the lack of UV (or visible)photons in their environment. Hence, the absence of UIR bands does notnecessarily imply the absence of circumstellar PAHs in these sources.The derived UIR band profiles have been compared to UIR band profilesfrom Reflection Nebulae, Planetary Nebulae (PNe), Hii regions, YoungStellar Objects, evolved stars and Galaxies. The profiles of TU Tau areshown to have the most resemblance to those from PNe. Integrated bandflux ratios have also been determined and compared to object type fluxratio correlations found in other studies. Here no definite match wasfound. TU Tau is the only binary with a blue companion in this sample ofAGB stars. In line with earlier studies, we suggest that the bluephotons provided by this companion are required for efficient excitationof PAH molecules in AGB ejecta. In addition, we argue that these bluephotons may promote complex chemistry in the ejecta of TU Tau. Thesimilarity in the peak profiles observed in the spectrum of TU Tau withthose in the spectra of PNe indicates that PAHs are formed in thecircumstellar envelope of carbon-rich AGB stars and make it largelyunmodified into the PNe phase. The variations in the band strengthratios between the different objects has been linked to the ionizationstate of PAHs and reflects the different physical environments betweenthis AGB star and PNe. In contrast, the variation in UIR band profilesbetween stellar ejecta and the Interstellar Medium (ISM) are largelyattributed to chemical modifications during the ISM phase.
| Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisited
As 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.
| Properties of detached shells around carbon stars. Evidence of interacting winds
The 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.
| Three-micron spectra of AGB stars and supergiants in nearby galaxies
The 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.
| 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
| The mass loss of C-rich giants
The 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
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Australia Telescope Compact Array imaging of circumstellar HCN line emission from R Scl
We 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.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We 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
| Imaging polarimetry of stellar light scattered in detached shells around the carbon stars R Scl and U Ant
Imaging 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 systems
For 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
| s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-elementcontent in Galactic carbon stars of N type. Abundances of Sr, Y, Zr(low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-masss-elements, or hs) are derived using the spectral synthesis techniquefrom high-resolution spectra. The N stars analyzed are of nearly solarmetallicity and show moderate s-element enhancements, similar to thosefound in S stars, but smaller than those found in the only previoussimilar study (Utsumi 1985), and also smaller than those found insupergiant post-asymptotic giant branch (post-AGB) stars. This is inagreement with the present understanding of the envelope s-elementenrichment in giant stars, which is increasing along the spectralsequence M-->MS-->S-->SC-->C during the AGB phase. Wecompare the observational data with recent s-process nucleosynthesismodels for different metallicities and stellar masses. Good agreement isobtained between low-mass AGB star models (M<~3 Msolar)and s-element observations. In low-mass AGB stars, the13C(α, n)16O reaction is the main source ofneutrons for the s-process a moderate spread, however, must exist in theabundance of 13C that is burnt in different stars. Bycombining information deriving from the detection of Tc, the infraredcolors, and the theoretical relations between stellar mass, metallicity,and the final C/O ratio, we conclude that most (or maybe all) of the Nstars studied in this work are intrinsic, thermally pulsing AGB stars;their abundances are the consequence of the operation of third dredge-upand are not to be ascribed to mass transfer in binary systems.
| 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
| The carrier of the ``30'' mu m emission feature in evolved stars. A simple model using magnesium sulfide
We 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
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The 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.
| The 85Kr s-Process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type redgiants. These objects belong to the class of asymptotic giant branchstars, experiencing thermal instabilities in the He-burning shell(thermal pulses). Mixing episodes called third dredge-up enrich thephotosphere with newly synthesized 12C in the He-rich zone,and this is the source of the high observed ratio between carbon andoxygen (C/O>=1 by number). Our spectroscopic abundance estimatesconfirm that, in agreement with the general understanding of the lateevolutionary stages of low- and intermediate-mass stars, carbonenrichment is accompanied by the appearance of s-process elements in thephotosphere. We discuss the details of the observations and of thederived abundances, focusing in particular on rubidium, a neutrondensity sensitive element, and on the s-elements Sr, Y, and Zr belongingto the first s-peak. The critical reaction branching at 85Kr,which determines the relative enrichment of the studied species, isdiscussed. Subsequently, we compare our data with recent models fors-processing in thermally pulsing asymptotic giant branch stars, atmetallicities relevant for our sample. A remarkable agreement betweenmodel predictions and observations is found. Thanks to the differentneutron density prevailing in low- and intermediate-mass stars,comparison with the models allows us to conclude that most C(N) starsare of low mass (M<~3 Msolar). We also analyze the12C/13C ratios measured, showing that most of themcannot be explained by canonical stellar models. We discuss how thisfact would require the operation of an ad hoc additional mixing,currently called cool bottom process, operating only in low-mass starsduring the first ascent of the red giant branch and, perhaps, alsoduring the asymptotic giant branch.
| High-Resolution Images of CO J=2-1 Emission from the Carbon Star V Cygni
This 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.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table 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/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| Imaging of detached shells around the carbon stars R Scl and U Ant through scattered stellar light
We present the first optical images of scattered light from large,detached gas/dust shells around two carbon stars, RScl and U Ant, obtained in narrow bandfilters centred on the resonance lines of neutral K and Na, and in aStröm}gren b filter (only U Ant). They confirmresults obtained in CO radio line observations, but also reveal new andinteresting structures. Towards R Scl the scatteringappears optically thick in both the K and Na filters, and both imagesoutline almost perfectly circular disks with essentially uniformintensity out to a sharp outer radius of ~21arcsec . These disks arelarger - by about a factor of two - than the radius of the detachedshell which has been marginally resolved in CO radio line data. InU Ant the scattering in the K filter appears to be,at least partially, optically thin, and the image is consistent withscattering in a geometrically thin (~3arcsec ) shell (radius ~43arcsec )with an overall spherical symmetry. The size of this shell agrees verywell with that of the detached shell seen in CO radio line emission. Thescattering in the Na filter appears more optically thick, and the imagesuggests the presence of at least one, possibly two, shells inside the43arcsec shell. There is no evidence for such a multiple-shell structurein the CO data, but this can be due to considerably lower masses forthese inner shells. Weak scattering appears also in a shell which islocated outside the 43arcsec shell. The present data do not allow us toconclusively identify the scattering agent, but we argue that most ofthe emission in the K and Na filter images is to due to resonance linescattering, and that there is also a weaker contribution from dustscattering in the U Ant data. Awaiting newobservational data, our interpretation must be regarded as tentative.Based on observations using the 3.6 m telescope of the European SouthernObservatory, La Silla, Chile.
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| Models of circumstellar molecular radio line emission. Mass loss rates for a sample of bright carbon stars
Using a detailed radiative transfer analysis, combined with an energybalance equation for the gas, we have performed extensive modelling ofcircumstellar CO radio line emission from a large sample of opticallybright carbon stars, originally observed by Olofsson et al. (ApJS, 87,267). Some new observational results are presented here. We determinesome of the basic parameters that characterize circumstellar envelopes(CSEs), e.g., the stellar mass loss rate, the gas expansion velocity,and the kinetic temperature structure of the gas. Assuming a sphericallysymmetric CSE with a smooth gas density distribution, created by acontinuous mass loss, which expands with a constant velocity we are ableto model reasonably well 61 of our 69 sample stars. The derived massloss rates depend crucially on the assumptions in the circumstellarmodel, of which some can be constrained if enough observational dataexist. Therefore, a reliable mass loss rate determination for anindividual star requires, in addition to a detailed radiative transferanalysis, good observational constraints in the form of multi-lineobservations and radial brightness distributions. In our analysis we usethe results of a model for the photodissociation of circumstellar CO byMamon et al. (1988). This leads to model fits to observed radialbrightness profiles that are, in general, very good, but there are alsoa few cases with clear deviations, which suggest departures from asimple r-2 density law. The derived mass loss rates spanalmost four orders of magnitude, from ~ 5 10-9Msun yr-1 up to ~ 2 10-5Msun yr-1, with the median mass loss rate being ~3 10-7 Msun yr-1. We estimate that themass loss rates are typically accurate to ~ 50% within the adoptedcircumstellar model. The physical conditions prevailing in the CSEs varyconsiderably over such a large range of mass loss rates. Among otherthings, it appears that the dust-to-gas mass ratio and/or the dustproperties change with the mass loss rate. We find that the mass lossrate and the gas expansion velocity are well correlated, and that bothof them clearly depend on the pulsational period and (with largerscatter) the stellar luminosity. Moreover, the mass loss rate correlatesweakly with the stellar effective temperature, in the sense that thecooler stars tend to have higher mass loss rates, but there seems to beno correlation with the stellar C/O-ratio. We conclude that the massloss rate increases with increased regular pulsation and/or luminosity,and that the expansion velocity increases as an effect of increasingmass loss rate (for low mass loss rates) and luminosity. Five, of theremaining eight, sample stars have detached CSEs in the form ofgeometrically thin CO shells. The present mass loss rates and shellmasses of these sources are estimated. Finally, in three cases weencounter problems using our model. For two of these sources there areindications of significant departures from overall spherical symmetry ofthe CSEs. Carbon stars on the AGB are probably important in returningprocessed gas to the ISM. We estimate that carbon stars of the typeconsidered here annually return ~ 0.05 Msun of gas to theGalaxy, but more extreme carbon stars may contribute an order ofmagnitude more. However, as for the total carbon budget of the Galaxy,carbon stars appear to be of only minor importance. Presented in thispaper is observational data collected using the Swedish-ESOsubmillimetre telescope, La Silla, Chile, the 20\,m telescope at OnsalaSpace Observatory, Chalmers Tekniska Högskola, Sweden, and the NRAO12\,m telescope located at Kitt Peak, USA.}
| The Neutral Envelopes around AGB and Post-AGB Objects Their Structure and Kinematics
Not Available
| Gasdynamics of Detached Shells Around Carbon Stars With Variable Mass Loss
Gasdynamic features of detached shells around carbon stars with variablemass loss rate are investigated in detail numerically. It is shown thata shell is unstable and also, 2D perturbations are less developed that3D ones. The structure of perturbed flows corresponding to differentevolution scenarios is compared. The results obtained seem to bepromising for interpretation of observations, in particular, therecently obtained detailed data of TT Cyg.
| The 12C/13C-ratio in cool carbon stars
We present observations of circumstellar millimetre-wave 13COline emission towards a sample of 20 cool carbon stars. Using a detailedradiative transfer model we estimate the circumstellar12CO/13CO-ratios, which we believe accuratelymeasure the important stellar 12C/13C-ratios. Forthose optically bright carbon stars where it is possible, our derived12C/13C-ratios are compared with the photosphericresults, obtained with different methods. Our estimates agree well withthose of Lambert et al. \citeyearpar{Lambert86}. It is shown that astraightforward determination of the12CO/13CO-ratio from observed line intensityratios is often hampered by optical depth effects, and that a detailedradiative transfer analysis is needed in order to determine reliableisotope ratios. Presented in this paper is observational data collectedusing the Swedish-ESO submillimetre telescope, La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Chalmers Tekniska Högskola,Sweden, and the NRAO 12 m telescope located at Kitt Peak, USA.
| On the origin of thin detached gas shells around AGB stars. Insights from time-dependent hydrodynamical simulations
We have applied two different computer codes to study the time-dependenthydrodynamics of circumstellar gas/dust shells of AGB stars in theirfinal stages of evolution. A two-component radiation hydrodynamics codeis designed to model a stellar wind driven by radiation pressure on dustgrains. Combined with detailed stellar evolution calculations, thisapproach allows us to simulate the dynamical response of the AGB windenvelope and the emergent spectral energy distribution to temporalchanges of the stellar luminosity and mass loss rate. A completelyindependent one-component, Godunov-type hydrodynamics code, which isparticularly well suited to resolve shock fronts, is used to check theresults obtained with the numerically more diffusive two-component code.First, we verify that a presumed short episode of high mass losstranslates into a correspondingly narrow, high-density shell movingthrough the circumstellar envelope, provided that the mass loss rate,and hence the outflow velocity, is essentially constant during the massejection. In principle, this scenario remains a viable explanation forthe existence of the very thin molecular shells recently detected aroundsome carbon-rich AGB stars. Second, we discovered that an alternativemechanism producing very thin shells of greatly enhanced gas density canoperate in the dusty outflows from AGB stars: the interaction of afaster inner wind running into a slower outer wind, sweeping up matterat the interface between both type of winds. Based on differentnumerical simulations and on a simple analytical model, we show thatthis mechanism easily leads to the formation of very thin shells withoutthe need to invoke large variations of the mass loss rate on very shorttime scales. Finally, we demonstrate that a typical helium-shell flashinduces both a mass loss `eruption' and a two-wind interaction due tothe increased outflow velocity during the high mass loss phase, leadingto the formation of a thin compressed gas shell. Very likely, thismechanism is responsible for the origin of the CO shells found aroundsome semiregular, optically visible carbon stars, the most prominentexample being TT Cygni.
| A high-resolution study of episodic mass loss from the carbon star TT Cygni
CO radio line observations with the IRAM Plateau de Bure interferometershow that the carbon star TT Cyg is surrounded by a large (radius ~35arcsec or 2.7x1017 cm), geometrically thin (average width ~2farcs 5 or 1.9x1016 cm) shell of gas, which has a remarkableoverall spherical symmetry (e.g., its radius varies by less than +/-3%).It expands with a velocity of ~ 12.6 km s-1. The emitting gasis very evenly distributed in the shell when averaged over a solid angleof about 0.2 steradians. We estimate a molecular hydrogen density of ~250 cm-3, a gas kinetic temperature of ~ 100 K, and a mass of~ 0.007 M_sun for the shell if the medium is homogeneous. There is noevidence for matter immediately inside or outside the shell, nor isthere any evidence for structure in the radial direction of the shellbrightness distribution (it is essentialy perfectly fitted withGaussians). The shell centre is displaced ~ 1farcs 7 (position angle ~-20degr ) with respect to the star. We favour an interpretation of thisdisplacement in terms of TT Cyg being a member of a binary system. Weput forward several arguments for a shell medium that consists almostentirely of a large number of small (la 1arcsec ) clumps (in which casethe density required to fit the observational data is much higher, ~104 cm-3, and the kinetic temperature isconsiderably lower, la 20 K). TT Cyg is presently losing mass at amodest rate, ~ 3x10-8 M_sunpyr, and with a low expansionvelocity, ~ 3.8 km s-1. This is inferred from CO lineemission from a region centred on the present position of the star. Thesystemic velocity is estimated, from both the centre and the shellemission, to be -27.3+/-0.1 km s-1 in the LSR system. Allquantitative results are obtained assuming the Hipparcos distance of 510pc. These data strongly support that TT Cyg has recently ( ~7x103 yr ago) gone through a period of drastically varyingmass loss properties. We discuss briefly two scenarios: a short period(a few hundred years) of very intense mass loss (a rate in excess of10-5 M_sunpyr), and a related scenario with a more modestmass ejection and where most of the shell gas is swept-up from aprevious, slower stellar wind. It is presently not possible to favourany of these two scenarios, but we suggest that in either case it is acoordinated mass ejection that caused the shell formation. The He-shellflash phenomenon in AGB-stars can provide this coordination, and it alsofits the time scales involved.
| Distance Determination of Mass-Losing Stars
Based on the Principal Component Analysis on IRAS colors and the radiodata, the distances to 183 mass-losing red giant stars were determinedusing the radial velocity and Oort's galactic rotation model for azero-point calibration in the distance modulus. Also, based on therequirement of higher accuracy of the distance determination, themass-losing red giant stars were divided into two groups by means of thefirst-principal component representing an intrinsic photometric propertyof the expanding shell; then, the distances were estimated to be log{d(kpc)}=0.458 p_2+0.09+/-0.13 for group 1 and log {d(kpc)}=0.325p_2+0.45+/-0.15 for group 2, where p_2 is the principal componentcorresponding to the distance, as obtained from the IRAS flux, which wasassumed to be inversely proportional to the square of the distance.Thus,these two groups differ from each other not only by theirphotometric properties, but also by their average distances, by a factorof about 2. Systematic differences exist between the two groups in theirpopulation characteristics and in their evolutionary stages.
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Observation and Astrometry data
| Constellation: | Ασπίς |
| Right ascension: | 18h50m20.00s |
| Declination: | -07°54'27.0" |
| Apparent magnitude: | 6.8 |
| Distance: | 395.257 parsecs |
| Proper motion RA: | 8 |
| Proper motion Dec: | -4.1 |
| B-T magnitude: | 11.061 |
| V-T magnitude: | 7.157 |
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