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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.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| 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
| Carbon Stars in the Uves Paranal Observatory Project
The high resolution spectra of five cool carbon stars (X TrA, TW Hor, WOri, U Ant and U Hya) obtained in the UVES Paranal Observatory Projectwere used to examine the quality of R. A. Bell's line-list forsynthesizing the spectra of cool carbon stars. We propose theimprovements of this line-list and estimate the abundances of severalchemical elements.
| 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.
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For 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.
| 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
| A Wall of Dust around a Proto-Mira?
We present the discovery of a huge (19'×16')dust ring surrounding a bright (V=10.60) red star. The dust ring has, atD=700 pc, a diameter of 4 pc and a central hole of ~1.5 pc across. Partof the shell is also seen as an absorption nebulosity. The star isclassified as an M3 III asymptotic giant branch (AGB) star. Among AGBstars, its detached shell is of unrivaled size. Detached shells aroundAGB stars are normally interpreted in terms of thermal pulses. However,in this case, a significant fraction of the shell may consist ofswept-up ISM; the detached appearance can be explained with wind-ISMinteraction. We present a model in which the AGB wind has been stoppedby the surrounding ISM and the swept-up shell is now expanding at thesound speed. The model predicts that the ring will disperse over a fewtimes 105 yr and eventually will leave a large hole in theISM surrounding the AGB star or its future planetary nebula.
| 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
| Stars with the Largest Hipparcos Photometric Amplitudes
A 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 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.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor 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 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.}
| Modeling of C stars with core/mantle grains: Amorphous carbon + SiC
A set of 45 dust envelopes of carbon stars has been modeled. Among them,34 were selected according to their dust envelope class (as suggested bySloan et al. \cite{Sloan98}) and 11 are extreme carbon stars. The modelswere performed using a code that describes the radiative transfer indust envelopes considering core/mantle grains composed by an alpha -SiCcore and an amorphous carbon (A.C.) mantle. In addition, we have alsocomputed models with a code that considers two kinds of grains - alpha-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dustenvelopes of evolved carbon stars, while two homogeneous grains are moreable to reproduce thinner dust envelopes. Our results suggest that thereexists an evolution of dust grains in the carbon star sequence. In thebeginning of the sequence, grains are mainly composed of SiC andamorphous carbon; with dust envelope evolution, carbon grains are coatedin SiC. This phenomena could perhaps explain the small quantity of SiCgrains observed in the interstellar medium. However, in this work weconsider only alpha -SiC grains, and the inclusion of beta -SiC grainscan perhaps change some of these results.
| 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.
| 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.
| Hipparcos: The Stars
Not Available
| 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.
| Infrared studies of circumstellar matter of evolved stars
Not Available
| Circumstellar shells of the mass-losing asymptotic giant branch stars: limits for the dust-driven winds.
Not Available
| The young detached CO shell around U Camelopardalis
We report IRAM Plateau de Bure interferometer observations of the carbonstar U Cam in the COi and COii lines. The remarkableimages show that U Cam is surrounded by ageometrically thin, ~ 1016 cm, shell of gas at a distance of~ 6x1016 cm from the star, that expands with a velocity of ~23 km s-1. The estimated mass of the shell is low, ~10-3 M_sun. In addition, we detect emission that peaks at thestellar position. From this we estimate a present mass loss rate and gasexpansion velocity of ~ 2.5x10-7 M_sun yr-1 and 12km s-1, respectively. One possible explanation to thestructure of the circumstellar medium is that the shell was producedduring a very short period, ~ 150 yr, of high mass loss rate, ~10-5 M_sun yr-1, about 800 yr ago. UCam may fit into the scenario where a helium-shell flashmodulates the mass loss rate on short times scales.
| Irregular variables of type Lb. Energy distributions and stellar parameters
AGB variables of types Lb, SRa, SRb, and Mira are studied by fittingcombinations of blackbodies to visual, near infrared and IRAS data. Thispaper supplements an earlier work dealing with a smaller sample of SRaand SRb variables. The fitted parameters T*, T dand Rd/R* are related to physically meaningfulquantities. Also, quantities derived from the fits like the ratio of theluminosities of the two fitted blackbodies are confronted withindependent mass-loss estimators. For the O-rich Lb variables all of the`blue' objects can be reasonably well approximated by only one blackbodywhereas the `red' ones need two. Among the `blue' objects a significantfraction seem to be not on the AGB at all but a kind of `RGB pollution'.The T* values, reflecting mainly offseted (-500 K) effectivetemperatures for objects with small to moderate mass-loss, aresignificantly higher in the `blue' cases. Carbon-rich objects differsignificantly from the O-rich ones in their fit parameters. Sometimes`unphysically' low T* are found - a result of circumstellarreddening in the high mass-loss cases. Furthermore lower values ofT d, accompanied by normal T*s and large shellradii are common and can be related to the phenomenon of detachedshells. S-stars populate a similar region to the optically thin carbonstars in their fit properties.
| Dust extinction and intrinsic SEDs of carbon-rich stars. III. The Miras, CS, and SC stars
The present work is an extension of a recent study by Knapik &Bergeat (\cite{knapik97}), and Bergeat et al. (\cite{berge98b})henceforth called Papers I and II, respectively. The spectral energydistributions (SEDs) of about 440 carbon-rich stars and the interstellarextinction observed on their line of sights were analysed. The methodsoriginally developed for Semi-Regular (SR) and Irregular (L) variables(Paper I: our groups CV1 to CV6) were then extended (Paper II) to thehot carbon (HC) stars (our groups HC0 to HC5) and related objects (RCB,BaII and HdC stars). Shortly, this is a kind of a pair method making usesimultaneously of the whole SED from UV to IR. Our approach is appliedhere to the galactic cool carbon-rich variables which were notconsidered in Paper I, namely the carbon Miras and very cool non-Miras,and the CS and SC variables. The carbon Miras with infrared silicateemission are also studied. The photometric CV1 to CV6 classificationscheme of paper I is implemented, and we add here a later CV7-group anda specific SCV-group which corresponds to spectroscopic SC stars. Acontinuous S-SC-CS-C sequence is clearly supported by our results. Thecarbon stars with IR silicate emission included in our study do havecarbon-rich SEDs of the three consecutive groups HC5, CV1 and CV2. Theystand among the relatively hot carbon variables, in the 3600-3000 Krange in effective temperature. The carbon Miras are satisfactorilydescribed in this enlarged scheme. No specific extension is requiredsince non-Miras are also found in the CV7 and SCV-groups. The derivedgroup is however frequently phase-dependent in these large amplitudevariables. Additional selective extinction of circumstellar (CS) originis observed in variable amounts. The mean extinction law for theinterstellar diffuse medium as tabulated by Mathis (\cite{mathis}) isshown to be relevant. It applies to both interstellar and circumstellarextinction with a possible CS neutral extinction in addition which wouldremain undetected here. The corresponding colour excess E(B-V) is largerat minimum light or intermediate phases than what it is at maximum light(where it can amount to zero). It is associated to large IR excessesattributed to the emission from CS dust. Long-term variations onthousands of days may be interpreted in terms of varying CS dust opacityon the line of sight. The dust influence is discussed. It is shown thatscattering, if substantial on the line of sight in the observing lobe,has to be essentially wavelength-independent, i.e. due to large neutralscatterers, especially in high opacity objects like IRC +10216. Finally,with the HC0 to HC5 classification of HC stars (Paper II), we obtain afourteen groups sequence (HC0 to HC5 and then CV1 to CV7 from theearlier one to the latest one, and SCV for SC stars apart). The numberof studied stars amounts now to about 600 that is about 40 stars pergroup on the average when the oxygen-type SEDs are subtracted. Theeffective temperature calibration of this classification scheme iscurrently in preparation. This research has made use of the Simbaddatabase operated at CDS, Strasbourg, France.}\fnmsep\thanks{Partiallybased on data from the ESA HIPPARCOS astrometrysatellite}\fnmsep\thanks{Table~5 is only available in electronic form atthe CDS via anonymous ftp 130.79.128.5
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