Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
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
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Preliminary analysis of light curves of seven carbon stars
We present a preliminary analysis of the light curves of the followingcarbon stars: WZ Cas, VY UMa, Y CVn, RY Dra, T Lyr, HK Lyr and TT Cyg,constructed on the basis of our own BV photoelectric observationsobtained at the Brno Observatory in 1979-94 and Hipparcos observations.The analysis suggests that "semiregular" light curves of all studiedstars can faithfully be expressed by a superposition of long-termchanges and a set of medium-term harmonic variations (possiblypulsations) with periods from 50 to 500 days.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-elementcontent in Galactic carbon stars of N type. Abundances of Sr, Y, Zr(low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-masss-elements, or hs) are derived using the spectral synthesis techniquefrom high-resolution spectra. The N stars analyzed are of nearly solarmetallicity and show moderate s-element enhancements, similar to thosefound in S stars, but smaller than those found in the only previoussimilar study (Utsumi 1985), and also smaller than those found insupergiant post-asymptotic giant branch (post-AGB) stars. This is inagreement with the present understanding of the envelope s-elementenrichment in giant stars, which is increasing along the spectralsequence M-->MS-->S-->SC-->C during the AGB phase. Wecompare the observational data with recent s-process nucleosynthesismodels for different metallicities and stellar masses. Good agreement isobtained between low-mass AGB star models (M<~3 Msolar)and s-element observations. In low-mass AGB stars, the13C(α, n)16O reaction is the main source ofneutrons for the s-process a moderate spread, however, must exist in theabundance of 13C that is burnt in different stars. Bycombining information deriving from the detection of Tc, the infraredcolors, and the theoretical relations between stellar mass, metallicity,and the final C/O ratio, we conclude that most (or maybe all) of the Nstars studied in this work are intrinsic, thermally pulsing AGB stars;their abundances are the consequence of the operation of third dredge-upand are not to be ascribed to mass transfer in binary systems.
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/967
| Long-Term VRI Photometry of Small-Amplitude Red Variables. I. Light Curves and Periods
We report up to 5000 days of VRI photometry, from a robotic photometrictelescope, of 34 pulsating red giants, namely, TV Psc, EG And, Z Psc, RZAnd, 4 Ori, RX Lep, UW Lyn, η Gem, μ Gem, ψ1 Aur,V523 Mon, V614 Mon, HD 52690, Y Lyn, BC CMi, X Cnc, UX Lyn, RS Cnc, VYUMa, ST UMa, TU CVn, FS Com, SW Vir, 30 Her, α1 Her,V642 Her, R Lyr, V450 Aql, V1293 Aql, δ Sge, EU Del, V1070 Cyg, WCyg, and μ Cep, as well as a few variable comparison stars. V, R, andI variations are generally in phase. The length and density of the dataenable us to look for variations on timescales ranging from days toyears. We use both power-spectrum (Fourier) analysis and autocorrelationanalysis, as well as light-curve analysis; these three approaches arecomplementary. The variations range from regular to irregular, but inmost of the stars, we find a period in the range of 20-200 days, whichis probably due to low-order radial pulsation. In many of the stars, wealso find a period which is an order of magnitude longer. It may be dueto rotation, or it may be due to a new kind of convectively inducedoscillatory thermal mode, recently proposed by P. Wood.
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| Models of circumstellar molecular radio line emission. Mass loss rates for a sample of bright carbon stars
Using a detailed radiative transfer analysis, combined with an energybalance equation for the gas, we have performed extensive modelling ofcircumstellar CO radio line emission from a large sample of opticallybright carbon stars, originally observed by Olofsson et al. (ApJS, 87,267). Some new observational results are presented here. We determinesome of the basic parameters that characterize circumstellar envelopes(CSEs), e.g., the stellar mass loss rate, the gas expansion velocity,and the kinetic temperature structure of the gas. Assuming a sphericallysymmetric CSE with a smooth gas density distribution, created by acontinuous mass loss, which expands with a constant velocity we are ableto model reasonably well 61 of our 69 sample stars. The derived massloss rates depend crucially on the assumptions in the circumstellarmodel, of which some can be constrained if enough observational dataexist. Therefore, a reliable mass loss rate determination for anindividual star requires, in addition to a detailed radiative transferanalysis, good observational constraints in the form of multi-lineobservations and radial brightness distributions. In our analysis we usethe results of a model for the photodissociation of circumstellar CO byMamon et al. (1988). This leads to model fits to observed radialbrightness profiles that are, in general, very good, but there are alsoa few cases with clear deviations, which suggest departures from asimple r-2 density law. The derived mass loss rates spanalmost four orders of magnitude, from ~ 5 10-9Msun yr-1 up to ~ 2 10-5Msun yr-1, with the median mass loss rate being ~3 10-7 Msun yr-1. We estimate that themass loss rates are typically accurate to ~ 50% within the adoptedcircumstellar model. The physical conditions prevailing in the CSEs varyconsiderably over such a large range of mass loss rates. Among otherthings, it appears that the dust-to-gas mass ratio and/or the dustproperties change with the mass loss rate. We find that the mass lossrate and the gas expansion velocity are well correlated, and that bothof them clearly depend on the pulsational period and (with largerscatter) the stellar luminosity. Moreover, the mass loss rate correlatesweakly with the stellar effective temperature, in the sense that thecooler stars tend to have higher mass loss rates, but there seems to beno correlation with the stellar C/O-ratio. We conclude that the massloss rate increases with increased regular pulsation and/or luminosity,and that the expansion velocity increases as an effect of increasingmass loss rate (for low mass loss rates) and luminosity. Five, of theremaining eight, sample stars have detached CSEs in the form ofgeometrically thin CO shells. The present mass loss rates and shellmasses of these sources are estimated. Finally, in three cases weencounter problems using our model. For two of these sources there areindications of significant departures from overall spherical symmetry ofthe CSEs. Carbon stars on the AGB are probably important in returningprocessed gas to the ISM. We estimate that carbon stars of the typeconsidered here annually return ~ 0.05 Msun of gas to theGalaxy, but more extreme carbon stars may contribute an order ofmagnitude more. However, as for the total carbon budget of the Galaxy,carbon stars appear to be of only minor importance. Presented in thispaper is observational data collected using the Swedish-ESOsubmillimetre telescope, La Silla, Chile, the 20\,m telescope at OnsalaSpace Observatory, Chalmers Tekniska Högskola, Sweden, and the NRAO12\,m telescope located at Kitt Peak, USA.}
| The use of modern catalogue data for variable star comparisons: a practical investigation
Visual and photometric V band light curves are compared. The visual dataare recalibrated using the Johnson V magnitudes contained in the TychoCatalogue and the effect of this substitution assessed. The exerciseemphasises the fact that modern catalogue data is too contextual in itsnature to be plucked from its source and forced into service on anarbitrary basis.
| Long-Term VRI Photometry of Pulsating Red Giants
We report up to 5000 days of VRI photometry, from a robotic photometrictelescope, of 37 pulsating red giants, namely: TV Psc, EG And, Z Psc, RZAnd, 4 Ori, RX Lep, η Gem, μ Gem, UW Lyn, ψ 1 Aur,V523 Mon, V614 Mon, HD 52690, Y Lyn, BC CMi, X Cnc, UX Lyn, RS Cnc, VYUMa, ST UMa, TU CVn, FS Com, 35 Com, SW Vir, 30 Her, α1 Her, V642 Her, R Lyr, HD 174621, V450 Aql, V1293 Aql,δ Sge, EU Del, V1070 Cyg, W Cyg, μ Cep, and ν Cep. V, R, andI variations are generally in phase. The length and density of the dataenable us to look for variations on time scales ranging from days toyears. We use both power-spectrum (Fourier) analysis, andautocorrelation analysis, as well as light-curve analysis; these threeapproaches are complementary. The variations range from regular toirregular but, in most of the stars, we find a period in the range of 20to 200 days which is probably due to low-order radial pulsation. In manyof the stars, we also find a period which is an order of magnitudelonger. It may be due to rotation, or it may be due to a new kind ofconvectively-induced oscillatory thermal mode, recently proposed byPeter Wood. Supported by NASA, NSF, and NSERC Canada.
| On the distance and mass-loss rate of carbon stars showing the silicon carbide emission feature
The distances and the mass-loss rates of carbon stars are in generalvery poorly known. The various estimates of the distances, taken fromthe general literature, show considerable discrepancies, while theevaluations of the mass-loss rates can be in error by more than an orderof magnitude. In this work we have evaluated these two important stellarparameters for a previously selected sample of 55 carbon stars showingthe 11.3 mu m band, commonly attributed to silicon carbide (SiC) grains.To perform the calculation we have used the values of geometrical andphysical parameters of these sources obtained from the best fits oftheir observed spectra. Using the distance values derived in this wayand the 11.3 mu m band intensity, we have evaluated the absolute bandstrength and we have found that, in agreement with other authors, thereis a correlation between this quantity and the mass-loss rate. Thiscorrelation can be very useful to determine the mass-loss rate of othercarbon stars not included in our sample, by means of the intensity ofthe SiC band, without using the usual technique based on COobservations. The same procedure can be conveniently applied to the sameas well as to other carbon stars, whose spectra will be available to thecommunity in the next future (i.e. the infrared spectra of sourcesobserved by the Infrared Satellite Observatory, ISO).
| 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.
| Simple photometric observations of BR Canum Venaticorum (a troublesome comparison star)
During the last four years an Optec SSP3 photometer has been used toobtain lightcurves for various suspected and confirmed variable stars.Upon analysis the semi-regular variable BR CVn (formerly a comparisonstar for V CVn) showed clear periods of 712 and 71 days, both periodshaving average peak-to-peak amplitudes of less than 0.3 magnitude. Thetotal observed range using a V-band filter has been 6.47-7.17.
| Spectrophotometry of three S stars and thirteen carbon N stars
The absolute energy distributions in the visual spectra (lambdalambda3200-7600 Angstroms) of 13 N carbon stars and three S stars have beeninvestigated. The spectral resolution of the data is 50 Angstroms andthe relative rms error is 2-5%. Our data supplement and extend theAlmaty and other spectrophotometric catalogs that are lacking in thesetypes of objects. The results can be compared with model atmospheres andalso used in other studies.
| Circumstellar shells of the mass-losing asymptotic giant branch stars: limits for the dust-driven winds.
Not Available
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| 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
| 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.
| Spectra of Cool Stars in the J Band (1.0-1.3 μm) at Medium Resolution
A spectroscopic survey of 103 cool S-, C-, and M-type stars wasundertaken with the Kitt Peak cryogenic spectrograph on the 1.3 mtelescope to make a detailed search for new molecular bands in the1.0-1.3 μm J-band region. While the spectra have high signal-to-noiseratios and good repeatability, no strong new features were found. Higherresolution spectra of a small sample of representative stars observedwith the 4 m Fourier transform spectrometer were invoked in order toidentify the features in these spectra. With few exceptions, the detailsof these spectra are well understood. Laboratory spectra were obtainedto aid in the identification of some weak features. Spectral featuresfrom dominant lines of Ti, Fe, Al, Si, Mn, Na, and K, and molecularbands due to TiO, TiS, ZrO, ZrS, VO, H_2O, and CN, have been identifiedin the spectra. Several weak unidentified bands are found.
| 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.
| Absolute magnitudes of carbon stars from HIPPARCOS parallaxes
Hipparcos trigonometric parallaxes and photometric data for about 40bright carbon stars have been analysed. Individual absolute visual andbolometric magnitudes, normal color indices (B-V)_0, absorption valuesand distance moduli were determined. By comparison with stellarevolutionary tracks for initial mass 1<= M/M_ȯ<=4 it is foundthat the majority of CH- and R-stars are on the giant and subgiantbranches, but N-stars occupy a region -4
| Circumstellar emission from dust envelopes around carbon stars showing the silicon carbide feature
Spectroscopic and photometric data relative to a sample of 55 carbonstars showing the 11.3 mu m feature have been fitted in the wavelengthrange between 0.4 and 100 mu m by means of a radiative transfer modelusing the laboratory extinction spectra of amorphous carbon and siliconcarbide (SiC) grains. The transfer code allows to determine in aself-consistent way the grain equilibrium temperature of the variousspecies at different distances from the central star and gives all therelevant circumstellar parameters which can be very important for theevolutionary study of carbon stars. In order to get meaningfulinformation on the nature and physical properties of the dust grainsresponsible for the 11.3 mu m feature and the underlying continuum, thefitting procedure of the spectra has been applied individually to everysingle source. For this reason it has been possible to take into accountany variation in position and shape of the band from source to source.Our analysis show that all the sources, in addition to the amorphouscarbon grains accounting for the continuum emission, need always thepresence of alpha -SiC particles while some of them require also beta-SiC. Moreover, the presence of one or both types of SiC particles seemsnot correlated neither with the total optical thickness nor with anyother physical and geometrical parameters of the circumstellar envelope.The best-fit parameters found in this work have been used to calculatethe mass-loss rate from the central stars. The clear correlation, thatwe find between the strength of the SiC feature and the total massloss-rate, confirms the results already found by other authors for thesame kind of sources and derived from the observed CO emission lines.
| 12C/13C ratios and Li abundances in C stars: evidence for deep mixing?
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997MNRAS.289L..11A&db_key=AST
| 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.
| Medium-Resolution Spectra of Normal Stars in the K Band
An Atlas of 115 medium-resolution K-band (2.0--2.4 mu m) stellarspectra, spanning spectral types O--M and luminosity types I--V, ispresented. K-band spectra are also presented for one N- and one J-typecarbon star. A time series of spectra is presented for an S-type Miravariable. All the spectra are at a resolution of ~3000 (1.4 cm-1) andhave had the terrestrial absorption removed by dividing by a featurelessspectrum. The spectra are plotted with the major spectral featuresidentified and are available digitally.
| Complementary Trace Element Abundances in Meteoritic SiC Grains and Carbon Star Atmospheres
Equilibrium condensation calculations successfully explain thecomplementary trace element abundance patterns observed in carbon staratmospheres and circumstellar SiC grains found in meteorites. Fractionaltrace element condensation into SiC depletes the gas in refractory traceelements, while more volatile elements remain in the gas. The observedcomplementary patterns imply that dust forms relatively close to thestar, possibly during the minimum light phase in stellar variabilitycycles. Once the gas falls back onto the star during stellarcontraction, photospheric abundances become relatively enriched in morevolatile elements. The complementary trace element abundances linkcircumstellar SiC grains from meteorites to carbon star atmospheres.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Osa Mayor |
| Ascensión Recta: | 10h45m04.00s |
| Declinación: | +67°24'41.0" |
| Magnitud Aparente: | 6 |
| Distancia: | 347.222 parsecs |
| Movimiento Propio en Ascensión Recta: | 4.8 |
| Movimiento Propio en Declinación: | 0.5 |
| B-T magnitude: | 9.291 |
| V-T magnitude: | 6.241 |
Catálogos y designaciones:
|
INHABITED SKY
