Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Infrared Study of J-Type Carbon Stars Based on Infrared Astronomical Satellite, Two Micron All Sky Survey, and Infrared Space Observatory Data
We collected 113 J-type carbon stars from the published literature.Observations from 2MASS, IRAS, and ISO show that, except for silicatecarbon stars in the J-type carbon star domain, the infrared propertiesof the other J-type carbon stars are quite similar to those of ordinarycarbon stars. The above results imply that the chemical peculiarity ofenhanced 13C for J-type carbon stars is not reflected in theinfrared region. In addition, the possible evolutionary scenario andbinarity for J-type carbon stars are also discussed.
| Challenging the Carbon Star Dust Condensation Sequence: Anarchist C Stars
There have been several investigations of the evolution of themid-infrared (IR) dust features in carbon star spectra based on IRAS LRSdata, but these studies are somewhat contradictory. In order tounderstand these differences in interpretations and to develop anunderstanding of the carbon star dust sequence, we have reexamined 26IRAS LRS spectra of carbon stars that have also been observedspectroscopically by ISO SWS. The low resolution and narrow wavelengthcoverage of the IRAS LRS data hinder determination of the effect ofmolecular absorptions in these spectra. This has led to incorrectestimations of the continuum levels in these spectra, which has a hugeeffect on the continuum-divided and continuum-subtracted spectra used toanalyze trends in the shape, strength, and position of the mid-IRfeatures. The higher resolution and broader wavelength coverage of theISO data allow more accurate fitting of the underlying continuum. Wehave reassessed the trends in shape, strength, and position of the ~11μm silicon carbide (SiC) feature and the apparent emergence of the ~9μm feature. We find that there are no correlations between thespectral parameters. We also investigate whether any of these parameterscorrelate with the strength of the molecular bands; no correlation wasfound. Moreover, we show that the apparent 9 μm feature is probablyan artifact. We discuss the implications of this study in terms of botha carbon star condensation sequence and the application of this study tothe larger IRAS data set.
| 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
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are 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/399/1083
| 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
| 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 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
| Modeling of C stars with core/mantle grains: Amorphous carbon + SiC
A set of 45 dust envelopes of carbon stars has been modeled. Among them,34 were selected according to their dust envelope class (as suggested bySloan et al. \cite{Sloan98}) and 11 are extreme carbon stars. The modelswere performed using a code that describes the radiative transfer indust envelopes considering core/mantle grains composed by an alpha -SiCcore and an amorphous carbon (A.C.) mantle. In addition, we have alsocomputed models with a code that considers two kinds of grains - alpha-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dustenvelopes of evolved carbon stars, while two homogeneous grains are moreable to reproduce thinner dust envelopes. Our results suggest that thereexists an evolution of dust grains in the carbon star sequence. In thebeginning of the sequence, grains are mainly composed of SiC andamorphous carbon; with dust envelope evolution, carbon grains are coatedin SiC. This phenomena could perhaps explain the small quantity of SiCgrains observed in the interstellar medium. However, in this work weconsider only alpha -SiC grains, and the inclusion of beta -SiC grainscan perhaps change some of these results.
| 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 atmospheres of cool, helium-rich white dwarfs
We have modified our version of the marcs stellar atmosphere code tomake it possible to compute white dwarf atmospheres, and we haveconstructed and present here the necessary input data for suchcalculations. With this new tool we have computed a number ofhelium-rich white dwarf atmospheres, synthetic spectra, and broad bandcolours, and we compare them here with observational data from theliterature. The major differences between our code and existing whitedwarf codes in the literature, are (1) a detailed computation of thechemical equilibrium between all relevant ions, neutral atoms, andmolecules, (2) a detailed line-by-line calculation of the molecularopacity, and (3) a detailed quantum mechanical description of thecollision-induced absorption (CIA) processes. The molecular line datainclude a few hundred million spectral lines from CO, CH, CN, CS,C2, C3, C2H, HCN, andC2H2. The sampling of the lines is done by theopacity sampling (OS) method. The CIA data are calculated from ab initiodipole and potential surfaces suitable for high temperatures. Resultsfor the H2-H2 absorption coefficient have beenpresented elsewhere, while the H2-He CIA data are obtainedspecifically for this study, and are presented at frequencies from 800to 20 000 cm-1 and at temperatures from 1 000 to 7 000 K. Thenew data are available at www.astro.ku.dk/~aborysow/programs/. Our codeis particularly useful for description of the cooler white dwarfs. Thesehave received increased attention during recent years because of betterobservational facilities. They are important for understanding theconditions in the early phase of the evolution of our Galaxy and as atool to determine the age of the Galactic disk and halo. We demonstratethe importance of including all elements (i.e. not only hydrogen andhelium as is done in several existing models in the literature), and ofincluding collision-induced opacities and molecular line opacities inthe atmospheric models. Our results are in good agreement with existingphotometry of cool white dwarfs. In particular, we can explain thescatter in observational colour-colour diagrams as a consequence ofsmall variations in the C/O ratio. Finally we discuss, based on our newmodels, the possible role of molecular C2H in so-calledC2H white dwarfs.
| 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.
| 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.
| 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.
| The R Stars: Carbon Stars of a Different Kind
After $\sim$16 years of radial-velocity observations of a sample of 22R-type carbon stars, no evidence for binary motion has been detected inany of them. This is surprising considering that approximately 20\% ofnormal late-type giants are spectroscopic binaries, and the fraction isclose to 100\% in barium, CH, and subgiant/dwarf CH and barium stars. Itis suggested, therefore, that a process that has caused the mixing ofcarbon to the surface of these stars cannot act in a wide binary system.Possibly, the R stars were once all binaries, but with separations thatwould not allow them to evolve completely up the giant and asymptoticgiant branchs without coalescing. This coalescence may be the agentwhich causes carbon produced in the helium-core flash to be mixedoutwards to a region where convection zones can bring it to the surfaceof the star. (SECTION: Stars)
| Circumstellar envelopes of J-type carbon stars.
Normal J-type carbon stars without silicate features have been modelled,considering two kinds of grains simultaneously: amorphous carbon (A.C.)and silicon carbide (SiC). The temperature of central stars and somecharacteristics of circumstellar shell as extinction opacities and itsextension were determined by fitting the flux curves. The SiC/A.C. ratioas well as the energy distributions and temperature law have beenstudied. The results show that these J-type carbon stars have thinenvelopes (with extinction opacities about 0.02) and intermediateSiC/A.C. ratios. Based on these results two alternative scenarios arediscussed: (1) Normal J-type carbon stars would be in an intermediategroup between Groups I and II introduced by Lorenz-Martins & Lefevre(1994). (2) J-type stars (normal and peculiar) could form an alternativeevolutive sequence, which differs from that proposed for ordinary carbonstars.
| A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and Barium Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..105..419B&db_key=AST
| 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.
| Fourier transform spectroscopy of the Swan (d3pig - a3piu) system of the jet-cooled C2 molecule
The Swan (d3pig - a3piu)system of the C2 molecule was produced in a jet-cooled corona excitedsupersonic expansion of helium using diazoacetonitrile as a percursormolecule. This spectrum was recorded using the McMath Fourier transformspectrometer of the National Solar Observatory at Kitt Peak. A total ofnine bands with v prime = 0 to 3 and v prime prime = 0 to 4 in the range16,570-22,760/cm were observed and rotationally analyzed. The C2molecules in this source had a rotational temperature of only 90 K sothat only the low-J lines were present in the spectrum. In some sensethe low temperatures in the jet source simulate conditions in theinterstellar medium. The Swan system of C2 was also produced in acomposite wall hollow cathode made Al4C3/Cu, and the rotationalstructure of the 1-0, 2-1, 3-2, 0-0, and 1-1 bands were analyzed. Thedata obtained from both these spectra were fitted together along withsome recently published line positions. The rotational constants, lambdadoubling parameters and the vibrational constants were estimated fromthis global fit. Our work on jet-cooled C2 follows similar work on theviolet and red systems of CN. A summary of this CN work is alsopresented. also presented.
| A Catalog of li Abundances and Equivalent Widths in a Sample of Galactic C-Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102..361B&db_key=AST
| Spectroscopic study of carbon stars with silicate features. 1: Observations
The results of an optical spectroscopic study of carbon stars withsilicate feature done at the Dominion Astrophysical Observatory (DAO) inVictoria in 1991 is presented. Four stars are confirmed to be C-13-richcarbon stars (J stars). Two stars are provisionally identified as Jstars. A preliminary spectral analysis is carried out. Two groups of Jstars were found in the IRAS color-color diagram. The 'red group'contains carbon stars with silicate emission feature which are good Jstar candidates; and the 'vertical strip group' contains standard Jstars which show weak or no SiC feature. It is suggested that thesources in the 'red group' represent the higher mass members of theasymptotic giant branch population. The enhancement of C-13 may beexplained by the envelope burning mechanism.
| 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.
| Revised MK spectral classification of the red carbon stars
The spectral classification of the red carbon stars has been broughtinto the revised MK system by combining some of the features of the oldR, N, and C classifications, as modified by Yamashita, and addingnumerical abundance indices. The new types are intended to (1) definethe population to which the star belongs, (2) allow quick interpolationbetween the detailed atmospheric analyses of individual stars, and (3)indicate the differences between carbon stars in different parts of ourgalaxy, and in other galaxies. It is the flexibility of the notation,allowing the incorporation of improved criteria, that should make thenew system useful.
| The radio properties of high-redshift quasars. I - Dual-frequency observations of 79 steep-spectrum quasars at Z over 1.5
We have made sensitive VLA observations of virtually all knownhigh-redshift radio-loud QSOs with extended radio morphologies. Theresulting images have angular resolutions of typically 0.4 and 0.15arcsec and are compiled here. This compilation of maps will permit theextraction of numerous radio source parameters, constituting a data basefor comprehensive statistical analyses to be explored in subsequentpapers. The data base is particularly useful for statistical studies.The key characteristics of the sample are the narrow redshift range, thenarrow distribution in linear resolutions of the maps, the narrowdistribution of map sensitivity to emitting regions of standard physicalcharacteristics, and the relatively narrow range of source luminosities.As a result of these properties, the sample enjoys relative freedom fromthe effects of parameter correlations with redshift and luminosity. Abrief discussion of the sample characteristics and some elaboration ofthe particular advantages of this sample are presented. Some cleartrends in the data are discussed briefly.
| Lithium Abundances in a Flux-Limited Sample of Galactic Carbon Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...272..455A
| Photoelectric photometry of carbon stars in the Vilnius system
A total of 368 observations of magnitudes and color indices of 178carbon stars in the Vilnius seven-color photometric system are given.The observations are presented in a catalog.
| Spectral indexes of cool carbon stars in the near-infrared region. II
Following the previous paper (Tanaka et al., 1990, hereafter as PaperI), near-infrared spectroscopic observations of cool carbon stars havebeen continued with the Fourier transform spectrometer and the 188 cmreflector at Okayama Astrophysical Observatory. Sixty-six spectra of 63carbon stars were analyzed. Eighteen stars were newly added to theprogram stars, and the total number of the analyzed carbon stars reached101. Three spectral indexes defined in Paper I; the color index, theCO-index, and the C2-index were measured. The same analysis was madeindependently as before, because the spectra were taken at differentphases of their variabilities and their signal-to-noise ratios were muchimproved by careful observations. General trends of the results obtainedin Paper I are confirmed.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cassiopea |
| Ascensione retta: | 03h11m25.32s |
| Declinazione: | +57°54'11.2" |
| Magnitudine apparente: | 7.571 |
| Distanza: | 313.48 parsec |
| Moto proprio RA: | -0.7 |
| Moto proprio Dec: | -3.2 |
| B-T magnitude: | 10.524 |
| V-T magnitude: | 7.815 |
Cataloghi e designazioni:
|
