Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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.
| 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
| 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
| 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
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 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?/A+A/390/1033
| 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.
| 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).
| 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.}
| Identification of Variable Stars in Indus
Not Available
| The Chromospheres of Carbon 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.
| 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
| The carbon-rich dust sequence - Infrared spectral classification of carbon stars
We have developed a classification system for the infrared spectralemission from carbon stars using a sample of 96 bright carbon-richvariables associated with the asymptotic giant branch. In addition tothe stellar contribution, most spectra include the 11.2 micron emissionfeature from SiC and either a smooth, cool continuum from amorphouscarbon or a secondary emission feature at 9.0 microns. We haveidentified a carbon-rich dust sequence along which the amorphous carboncomponent grows while the 9.0 micron feature declines in strength. Alongthis spectral sequence, the proportion of Mira variables increases, asdoes the period of variability, the mass-loss rate, and the thickness ofthe circumstellar shell. Thus the carbon-rich dust sequence appears tobe an evolutionary sequence. One class of spectra shows a particularlystrong 9.0 micron feature, enhanced C/O ratio, and several other unusualproperties that suggest a different sequence, perhaps related to Jstars.
| The PL relation of galactic carbon LPVs. The distance modulus to LMC
We present a period-luminosity (PL) diagram of 115 galactic carbon-richlong period variables (LPVs) observed by the HIPPARCOS satellite, in theform of the (MK,log P) relation. Our plot is compared to thediagram of carbon variables observed in the Large Magellanic Cloud(LMC). Both diagrams are found very similar and three samples aredelineated: long period variables close to the PL relation of Feast etal. (1989), short period-overluminous variables and a few underluminousLPVs, respectively Samples 1, 2 and 3. The used data were deduced fromexpectations of true parallaxes (Knapik et al. 1997) which arestatistically free of the Lutz-Kelker effect. The remaining bias due tothe non-gaussian distribution of absolute magnitudes is avoided: anon-linear parametric method is applied in Sect. 4 to the analysis ofthe PL relation for Sample 1 (72 LPVs). We obtainMK=(-3.99+/-0.13)log P+(2.07+/-0.15), in good agreement withthe slope found for LMC variables by Reid et al. (1995). The LMCdistance modulus then derived is mu =18.50+/-0.17. A well-defined upperlimit (ul) for long period stars in Sample 1 is found, with similarslopes in both the Galaxy (-4.85) and LMC (-4.72). No correction formetallicity was applied to the results. This research has made use ofthe Simbad database operated at CDS, Strasbourg, France.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| The Chromospheric Radiation on the Circumstellar Shell of TX PISCIUM
Not Available
| Interstellar extinction and the intrinsic spectral distribution of variable carbon stars.
We present a new method of evaluation of the extinction by interstellardust on cool carbon variables. These late-type stars show no markedrelationship between spectral classification (the R, N- and C-types) andphotometric colour indices. The pair method is thus ruled out, at leastin the form currently in use for early-type or intermediate stars. Ourmethod makes use of the whole spectral energy distributions from UV toIR. A sample of 60 unreddened carbon variables is delineated and newcolour-colour diagrams are proposed where the reddening vector is nearlyperpendicular to their narrow intrinsic locus. Six photometric groups(or boxes : CV1 to 6) are derived among unreddened stars. They show acontinuous range of spectral energy distributions from "bluer" to"redder", and mean colour indices are obtained. A pair method isdescribed where each presumably reddened star is compared to these meanunreddened stars, a given extinction law being assumed. As anillustration, the results are shown for a sample of 133 well-documentedstars. The mean extinction law usually adopted for the diffuseinterstellar medium (R_V_=~3.1) is shown to provide good fits. Thethreshold for reddening detection turns to be E(B-V)=~0.02-0.03A goodcorrelation is observed when the derived colour excesses are compared tovalues from maps in the literature. The mean rate of visual extinctionamounts to =~1.25+/-1.1 , ranging from 0.37 nearl=~240° (intercloud) to 2.1 (cloud + intercloud) in two structurescorrelated with Gould's belt.
| Semiregular variables of types SRa and SRb. Energy distributions and stellar parameters.
Semiregular variables of type SRa and SRb are studied by fittingcombinations of blackbodies to new near infrared and IRAS data. Thefitted parameters T^*^, T^d^ and R^d^/R^*^ are related to physicallymeaningful quantities. For the O-rich cases all `blue' objects can bereasonably well approximated by only one blackbody whereas the `red' andthe `Mira' SRVs need two. The T^*^ values, reflecting mainly offsetted(-500K), effective temperatures for objects with small to moderate massloss, are significantly higher in the `blue' SRV cases. A smalldifference between the T^*^ distributions of `red' and `Mira' SRVs seemsto be present: with a few exceptions most of the `Mira' T^*^ values areconcentrated in the cold half of the `red' SRV distribution. Carbon-richobjects differ significantly from the O-rich ones in their fitparameters. Sometimes `unphysically' low T^*^ are found - a result ofcircumstellar reddening in the high mass loss cases. Also lower valuesof T^d^, accompanied by normal T^*^s and large shell radii are common.S-stars populate a similar region as the optically thin carbon stars.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Semiregular variables of types SRa and SRb. New JHKL'M-photometry for 200 stars.
This paper presents new JHKL'M observations of 200 Semiregular variables(SRVs) of types SRa and SRb. The sample was defined in Kerschbaum &Hron (1992a, Paper I) by means of a certain limit in bolometricalmagnitude. From the sample of 350 objects, 260 now have near infrared(NIR) photometry - for 60 of these stars data from the literature areused. In total 290 datasets are available because of some multipleobservations. We briefly compare the photometry obtained at differentobservatories. Small but significant differences are found. A firstanalysis of the photometry supports one of the main findings of Paper I.The, in many aspects inhomogeneous, O-rich semiregular variables oftypes SRa and SRb can be successfully split in two subgroups called the`blue' and `red'/`Mira' SRVs. A separation of the `red' SRVs fromintrinsic Miras additionally requires variability information.
| Evolution from visual to infrared carbon stars: Interrupted mass-loss model
a set of self-consistent model calulations of the evolution from visualcarbon stars to infrared carbon stars is presented. A two-shell systemmodel, the interrurpted mass-loss model (oxygen-rich detached shell andnewly forming SiC dust shell), has been developed. This model overcomesthe shortcomings of the single detached-shell model (Chan and Kwok 1988)and the SiC shell model (Chan and Kwok 1990). A new mass-loss formula,which describes the gradually increasing mass-loss rate at the earlystage of the infrared carbon stars, is introduced to simulate the effectof the newly forming SiC shell. The model tracks successfully explain a'C' -shaped distribution of carbon stars in the color-color diagram. Theenergy distributions of approximately 110 transition objects are fittedwith devloping SiC dust shells using the interrupted mass-loss model.The existence of these transition objects with infrared propertiesintermediate between visual and infrared carbon stars, lends futhersupport to the idea of an evolutionary link between visual and infraredcabon stars.
| On the abundance of technetium in stellar atmospheres.
Not Available
| A Study of Circumstellar Envelopes around Bright Carbon Stars. II. Molecular Abundances
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJS...87..305O&db_key=AST
| A study of circumstellar envelopes around bright carbon stars. I - Structure, kinematics, and mass-loss rate.
Results are presented of a survey of circumstellar CO emission on asample of bright carbon stars, which is relatively complete out to about900 pc from the sun. A total of 68 detections were made. All objectswithin 600 pc of the sun were detected. It is suggested that the largemajority of all carbon stars have circumstellar envelopes. TheCO-emitting parts of these envelopes have angular sizes less than about15 arcsec. The median gas expansion velocity is 12.5 km/s, and theexpansion velocities for the majority of the objects fall in the range9-15 km/s. The median mass-loss rate is 1.5 x 10 exp -7 solar mass/yr,and the mass loss rate for the majority of stars lies within the narrowrange (0.8-2.5) x 10 exp -7 solar mass/yr. Circumstellar andphotospheric HCN, CN, and CS abundances are estimated and compared for asample for bright carbon stars. The chemistry in the envelope around RScl is determined.
| Circumstellar shells resolved in the IRAS survey data. I - Data processing procedure, results, and confidence tests
We have examined the IRAS 60 and 100 micron survey data covering 512evolved stars and young planetary nebulae for evidence of spatiallyresolved structure. A simple model, consisting of a central unresolvedsource surrounded by a resolved isothermal shell, was fitted to the datafor each star. Seventy-six stars were found to be resolved in the 60micron data. Tests have been performed to verify that the extendedstructure seen is not an artifact of the data-processing algorithm.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Indiano |
| Ascensione retta: | 21h20m09.50s |
| Declinazione: | -45°01'20.0" |
| Magnitudine apparente: | 6 |
| Distanza: | 571.429 parsec |
| Moto proprio RA: | 1.3 |
| Moto proprio Dec: | 4.2 |
| B-T magnitude: | 9.444 |
| V-T magnitude: | 6.43 |
Cataloghi e designazioni:
|
INHABITED SKY
