Contents
Images
Upload your image
DSS Images Other Images
Related articles
Correlations between Lithium and Technetium Absorption Lines in the Spectra of Galactic S Stars
Correlations between the presence of the 6707 Å line of lithiumand the resonance lines of technetium (4238 and 4262 Å) in a largesample of Galactic S stars are analyzed. Half of the sample stars areintrinsic S stars (those exhibiting technetium in their spectra), and1/3 of these stars also have strong lithium lines in their spectra.Stars having both lithium and technetium in their spectra areinterpreted as intermediate-mass thermally pulsating asymptotic giantbranch (TP-AGB) stars in which lithium is produced by the Cameron-Fowlermechanism. The production of lithium is predicted to occur inhigh-luminosity (Mbol<=-6) TP-AGB stars by the hot-bottomburning (HBB) mechanism. Data on the carbon isotope ratios of stars inour sample agree with the predictions of HBB; however, oxygen isotoperatios in these stars do not agree with the predictions of HBB.Furthermore, the available luminosities for our sample stars are belowthe minimum value necessary for HBB to occur in available models.Cool-bottom processing (CBP) is one possible explanation for thepresence of lithium in the spectra of these stars. Intrinsic S starshaving technetium but no lithium in their spectra are interpreted aslower mass (1.5-3 Msolar) thermally pulsating AGB stars thathave not undergone CBP. Extrinsic S stars constitute the remaining halfof the sample. Carbon and oxygen isotope ratios, as well as the lack oftechnetium and lithium in the spectra of these stars, are consistentwith these being low-mass red giant branch stars (1-2Msolar), with mass transfer from a now extinct thermallypulsating AGB star being responsible for the enhanced abundance ofs-process elements.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| 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
| Spectral Classification of Faint Carbon Stars
R--N classification of 187 faint carbon stars is based on the classicalcriteria adjusted to the yellow-red spectral region, with two newcriteria added -- the ratios of the red CN bands 6206/6332 (Å) and6478/6631 (Å).
| 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
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| High Angular Resolution Observations of Late-Type Stars
This paper presents speckle observations of Mira (o Cet) and late-typestars with the PISCO speckle camera of Pic du Midi during the period1995-1998. A survey for binarity among a sample of late-type stars wasperformed, which led to seven positive detections out of 36 objects.Photometric and color variations of the companion of Mira were searchedfor, but no significant brightness variations could be found over atimescale of ~5-10 minutes. The position and photometry measurements,the restored images with high angular resolution of the binary systemMira A-B (ADS 1778) are in full agreement with Hubble Space Telescopedata obtained at the same epoch. A new orbit has been derived for MiraA-B.
| CO 1st overtone spectra of cool evolved stars: Diagnostics for hydrodynamic atmosphere models
We present spectra covering the wavelength range 2.28 to 2.36 mu m at aresolution of Delta lambda = 0.0007 mu m (or R = 3500) for a sample of24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants,3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengthscovered include the main parts of the 12C16O v =2-0 and 3-1 overtone bands, as well as the v = 4-2 and 13CO v= 2-0 bandhead regions. CO lines dominate the spectrum for all the starsobserved, and at this resolution most of the observed features can beidentified with individual CO R- or P-branch lines or blends. Theobserved transitions arise from a wide range of energy levels extendingfrom the ground state to E/k > 20 000 K. We looked for correlationsbetween the intensities of various CO absorption line features and otherstellar properties, including IR colors and mass loss rates. Two usefulCO line features are the v = 2-0 R14 line, and the CO v = 2-0 bandhead.The intensity of the 2-0 bandhead shows a trend with K-[12] color suchthat the reddest stars (K-[12] > 3 mag) exhibit a wide range in 2-0bandhead depth, while the least reddened have the deepest 2-0 bandheads,with a small range of variation from star to star. Gas mass loss ratesfor both the AGB stars and the red supergiants in our sample correlatewith the K-[12] color, consistent with other studies. The data implythat stars with dot M_gas < 5x 10-7 Msuny-1 exhibit a much narrower range in the relative strengthsof CO 2-0 band features than stars with higher mass loss rates. Therange in observed spectral properties implies that there are significantdifferences in atmospheric structure among the stars in this sample.Figures 4-9, 11-14, 16, 17, 19-21, 23, 24 are only avalaible inelectronic form at http://www.edpsciences.org
| 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.}
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| 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.
| Hipparcos parallaxes for Mira-like long-period variables
This paper concerns the calibration of the K period-luminosity relationfor Mira variables using Hipparcos parallaxes. K magnitudes areavailable for 255 Mira-like variables which were observed by Hipparcos.Period-luminosity zero-points are evaluated for various subgroups ofdata. The best solution for oxygen-rich Miras, which uses 180 stars,omitting the short-period red group (which had different kinematics fromthe short-period blue stars) and the low-amplitude variables, provides azero-point of σ2σ2π +(0.4605)2π2PL(K)σ2K + σ2PL(K),0.84+/-0.14mag, which implies a distance modulus for the LargeMagellanic Cloud of σK = 0.3ΔK√N,18.64+/-0.14mag, or perhaps slightly greater if a metallicity correctionis required, in good agreement with the value derived from Cepheids. Thezero-point of the period-luminosity relation for carbon stars is brieflydiscussed. Linear diameters are derived for red variables with measuredangular diameters and parallaxes, and are used to examine thelong-standing question of the pulsation mode(s) of these stars. Evidenceis presented to suggest that most of them are pulsating in the same modeand, if published model atmospheres are correct, this is probably thefirst overtone. Some discussion is given of sequences in theperiod-luminosity and period-colour diagrams and their bearing on thepulsation mode problem.
| Multiperiodicity in semiregular variables. I. General properties
We present a detailed period analysis for 93 red semiregular variablesby means of Fourier and wavelet analyses of long-term visualobservations carried out by amateur astronomers. The results of thisanalysis yield insights into the mode structure of semiregular variablesand help to clarify the relationship between them and Mira variables.After collecting all available data from various international databases(AFOEV, VSOLJ, HAA/VSS and AAVSO) we test the accuracy and reliabilityof data. We compare the averaged and noise-filtered visual light curveswith simultaneous photoelectric V-measurements, the effect of the lengthversus the relatively low signal-to-noise ratio is illustrated by periodanalysis of artificial data, while binning effects are tested bycomparing results of frequency analyses of the unbinned and averagedlight curves. The overwhelming majority of the stars studied showmultiperiodic behaviour. We found two significant periods in 44variables, while there are definite signs of three periods in 12 stars.29 stars turned out to be monoperiodic with small instabilities in theperiod. Since this study deals with the general trends, we wanted tofind only the most dominant periods. The distribution of periods andperiod ratios is examined through the use of the (log P_0, log P_1) and(log P_1, log P_0/P_1) plots. Three significant and two less obvioussequences are present which could be explained as the direct consequenceof different pulsational modes. This hypothesis is supported by theresults for multiperiodic variables with three periods. Finally, thesespace methods are illustrated by several interesting case studies thatshow the best examples of different special phenomena such as long-termamplitude modulation, amplitude decrease and mode switching.
| 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.
| Spectrophotometry of carbon stars.
Not Available
| Quantitative analysis of carbon isotopic ratios in carbon stars. II. The effect of model atmosphere on the iso-intensity method
We discuss the analysis of (12C/({13)) C} ratios in cool carbon starspresented by de Laverny & Gustafsson (\cite{dLG98}), who questionedthe reliability of the iso-intensity method used by Ohnaka & Tsuji(\cite{OT96}). We show that the systematic discrepancy of (12C/({13)) C}ratios between Lambert et al. (\cite{lambert86}) and Ohnaka & Tsuji(\cite{OT96}) cannot be attributed to the uncertainty of theiso-intensity method. The analysis of the iso-intensity method done byde Laverny & Gustafsson (\cite{dLG98}) differs from that of Ohnaka& Tsuji (\cite{OT96}), defining the abscissa of curves of depthgrowth in a completely different manner. Namely, we derived the abscissadirectly from model atmospheres, while they simply assumed a singleexcitation temperature whose value is never accurately derived. The highsensitivity of the iso-intensity method to model atmospheres, reportedin their work, can be attributed to an incorrect definition of theabscissa of curves of depth growth. In fact, we show that thedetermination of (12C/({13)) C} ratios by the iso-intensity method isnot so sensitive to model atmospheres (atmospheric structure itself andstellar parameters) as they claim, when the abscissa is properlycalculated. In addition, we demonstrate that our model atmospheres canreproduce photometric and spectrophotometric observations fairly well.Therefore, their conclusion that the iso-intensity method is risky andunreliable for determining (12C/({13)) C} ratios in cool carbon starscannot be justified.
| Observations and modelling of spectral energy distributions of carbon stars with optically thin envelopes
We present broad-band photometry in the optical, near-infrared andsubmillimetre, and mid-infrared spectrophotometry of a selection ofcarbon stars with optically thin envelopes. Most of the observationswere carried out simultaneously. Beside the emission feature at 11.3mumdue to silicon carbide grains in the circumstellar environment, many ofour mid-infrared spectra show an emission feature at 8.6mum. All theobserved spectral energy distributions exhibit a very large far-infraredflux excess. Both these features are indeed common to many carbon starssurrounded by optically thin envelopes. We have modelled the observedspectral energy distributions by means of a full radiative transfertreatment, paying particular attention to the features quoted above. Thepeak at 8.6mum is usually ascribed to the presence of hydrogenatedamorphous carbon grains. We find also that the feature at 8.6mum mightbe reproduced by assuming that the stars have a circumstellarenvironment formed of both carbon- and oxygen-rich dust grains, althoughthis is in contrast with what one should expect in a carbon-richenvironment. The far-infrared flux excess is usually explained by thepresence of a cool detached dust shell. Following this hypothesis, ourmodels suggest a time-scale for the modulation of the mass-loss rate ofthe order of some 10^3yr.
| 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.
| Circumstellar Shells of the Mass-Losing Asymptotic Giant Branch Stars: Limits for the Dust-Driven Winds
We investigated whether the radiation pressure on dust grains alone mayaccount for driving winds in semiregular, irregular and non-variable AGBstars. A simple theoretical model is employed to calculate the limitsfor the radiatively driven winds and to compare theoretical predictionswith observations. Present analysis indicates, that for most of theobjects in the studied sample of 67 oxygen-rich and 40 carbon-richmass-losing AGB stars radiation pressure on dust grains alone is noteffective enough to drive the observed circumstellar outflows.
| 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.
| Circumstellar molecular radio line intensity ratios
We have observed a sample of 61 AGB--stars (39 M--stars and 22 C--stars)in circumstellar CO, CS, HCN, SiO, SiS, and SO radio line emission. Themain results presented are based on the use of line intensity ratios, awell defined observational quantity that can be used to infer importantconclusions as well as to provide constraints on models. Taken togetherthe data are fully consistent with the facts that for this sample thecircumstellar envelopes have the same basic chemistry (i.e., C/O<1 or>1) as the central stars, and that the mass loss rates have notchanged drastically over periods between 10(2) --10(3) years. TheHCN({\jtra10})/SiO({\jtra21}) intensity ratio discriminatesunambiguously between {``}normal{''} circumstellar envelopes withC/O<1 (O--CSEs) and >1 (C--CSEs), while the CS({\jtra21}),HCN({\jtra10}), SiO({\jtra21}), and SiS({\jtra54}) intensity ratios withrespect to CO({\jtra10}) are not perfect for this purpose, and neitheris the SiS({\jtra54})/SiO({\jtra21}) intensity ratio. The data furthershows that SO and the C-bearing molecule HCN are ubiquitously present inO--CSEs, and that their line intensities in O--CSEs are qualitativelyconsistent with the fact that the molecules are formed in aphoto--induced circumstellar chemistry in a quantity that depends on themass loss rate. Hence, both species can in principle be used to estimatethe mass loss rate, and the tight relation between the SO(J_K=3_2->2_1) and CO({\jtra10}) intensities in O--CSEs shows that SO lineemission may even be a good mass loss rate estimator. On the contrary,the SiO({\jtra21}) luminosity appears to be essentially independent ofthe mass loss rate in O--CSEs, possibly due to a larger influence frommolecular adhesion onto grains. These results explain why theHCN({\jtra10})/SiO({\jtra21}) intensity ratio increases with the massloss rate in O--CSEs, and there is no need to invoke e.g. a spread inC/O--ratios for the M--stars to explain the large range of this ratio.Maser emission is very likely present in the HCN({\jtra10}) line inC--CSEs, and it seems to be sensitively dependent on the mass loss rate,i.e., it appears only for dot M la 5x 10(-7) M_sunpyr. Based on timemonitoring of this emission towards the C--stars W Ori and X TrA, wesuggest that the strongest maser features are due to radialamplification in the {\ftra21} transition. The predominance ofredshifted maser emission could be caused by an additional amplificationin the {\ftra11} transition. We find no evidence for a similar maser inO--CSEs.
| Baldone Schmidt Telescope Plate Archive and Catalogue
The article presents information on the archive and catalogue of theastrophotos taken with the Schmidt telescope of the Institute ofAstronomy of the University of Latvia (until July 1, 1997 --Radioastrophysical Observatory of the Latvian Academy of Sciences) inthe period 1967--1998. The archive and catalogue contain more than 22000direct and 2300 spectral photos of various sky regions. Information onthe types of photo materials and color filters used as well as on mostfrequently photographed sky fields or objects is given. The catalogue isavailable in a computer readable form at the Institute of Astronomy ofthe University of Latvia and at the Astrophysical Observatory in Baldone(Riekstukalns, Baldone, LV-2125, Latvia), e-mail: astra@latnet.lv.
| Carbon Stars
Absolute magnitudes are estimated for carbon stars of various subtypesin the Hipparcos catalogue and as found in the Magellanic Clouds.Stellar radii fall within the limits of 2.4-4.7 AU. The chemicalcomposition of carbon stars indicates that the C-N stars show nearlysolar C/H, N/H, and ^12C/^13C ratios. This indicates that much of the Cand N in our Galaxy came from mass-losing carbon stars. Special carbonstars such as the C-R, C-H, and dC stars are described. Mass loss fromasymptotic giant branch (AGB) carbon stars, at rates up to several x10^-5 M{solar} year^-1, contributes about half of the total mass returnto the interstellar medium. R stars do not lose mass and may becarbon-rich red giants. The mass loss rates for Miras are about 10 timeshigher than for SRb and Lb stars, whose properties are similar enough toshow that they are likely to belong to the same population. Thedistribution of carbon star mass loss rates peaks at about 10^-7M{solar} year^-1, close to the rate of growth of the core mass anddemonstrative of the close relationship between mass loss and evolution.Infrared spectroscopy shows that dust mixtures can occur. Detachedshells are seen around some stars; they appear to form on the timescales of the helium shell flashes and to be a normal occurrence incarbon star evolution.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Κύκνος |
| Right ascension: | 21h43m16.33s |
| Declination: | +38°01'03.0" |
| Apparent magnitude: | 7.954 |
| Distance: | 2439.024 parsecs |
| Proper motion RA: | -4.8 |
| Proper motion Dec: | -3.6 |
| B-T magnitude: | 13.753 |
| V-T magnitude: | 8.433 |
Catalogs and designations:
|
