Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Very Large Excesses of 18O in Hydrogen-deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
We have found that at least seven hydrogen-deficient carbon (HdC) and RCoronae Borealis (RCB) stars, have 16O/18O ratiosclose to and in some cases less than unity, values that are orders ofmagnitude lower than measured in other stars (the solar value is 500).Greatly enhanced 18O is evident in every HdC and RCB we havemeasured that is cool enough to have detectable CO bands. The three HdCstars measured have 16O/18O<1, lower valuesthan any of the RCB stars. These discoveries are important clues indetermining the evolutionary pathways of HdC and RCB stars, for whichtwo models have been proposed: the double degenerate (white dwarf [WD]merger) and the final helium-shell flash (FF). No overproduction of18O is expected in the FF scenario. We have quantitativelyexplored the idea that HdC and RCB stars originate in the mergers of CO-and He-WDs. The merger process is estimated to take only a few days,with accretion rates of 150 Msolar yr-1 producingtemperatures at the base of the accreted envelope of(1.2-1.9)×108 K. Analysis of a simplified one-zonecalculation shows that nucleosynthesis in the dynamically accretingmaterial may provide a suitable environment for a significant productionof 18O, leading to very low values of16O/18O, similar to those observed. We also findqualitative agreement with observed values of 12C/13C and with the CNO elemental ratios. H-admixture during theaccretion process from the small H-rich C/O WD envelope may play animportant role in producing the observed abundances. Overall, ouranalysis shows that WD mergers may very well be the progenitors ofO18-rich RCB and HdC stars, and that more detailedsimulations and modeling are justified.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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
| Hydrogen-Deficient Cargon Star HM Lib
The high and medium resolution spectra of a nonvariablehydrogen-deficient carbon star (HdC) HM Lib were analyzed and thechemical composition derived. The atmospheric parameters of HM Lib areTeff = 6000+/-200 K, log g = 1.3+/-0.5 and the microturbulentvelocity xi_t = 6.8+/-1.0 km/s. The high hydrogen deficiency isconfirmed. The abundances of other elements are close to those found forthe majority of warmer and variable R CrB stars. The medium resolutionspectrum of another HdC star HD 182040 is compared to that of HM Lib. Inthe spectrum of HD 182040 the CN red system bands are weaker due to 0.3dex lower N abundance.
| 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
| 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.
| A Search for an Emission-Line Region in the Hydrogen-deficient Carbon Star HD 182040
A long-exposure short-wavelength IUE spectrum of the hydrogen-deficientcarbon (HdC) star HD 182040 does not show any detectable emission in theC II lambda1335 line. It is not certain whether this absence of emissionis intrinsic or due to the large uncertainties in the distance, absoluteluminosity, and reddening toward HD 182040. If intrinsic, this absenceof emission along with the absence of an IR excess from circumstellardust imply that little or no mass loss is occurring at present from thisstar even though it is very similar spectroscopically to the R CoronaeBorealis (RCB) stars. This result also suggests that the emission-linegas and the circumstellar dust may result from the same mass-lossmechanism in the RCB stars.
| HIPPARCOS observations of hydrogen-deficient carbon stars
Parallax measurements for 21 hydrogen-deficient carbon stars have beenmade by the Hipparcos satellite. These stars include most of thebrighter R Coronae Borealis (RCB) variables, other coolhydrogen-deficient carbon (HdC) stars, and several higher-temperatureextreme helium (eHe) stars. Most of these stars have either negative orstatistically insignificant parallaxes, indicating that they lie beyondthe detection capability of Hipparcos. Although the distances to thegalactic hydrogen-deficient carbon stars remain unknown, at least theHipparcos observations do confirm that these objects must have highluminosity like the LMC RCB stars, for which M_bol = -4 to -5. Basedupon Hipparcos proper motions, we derive UVW velocities for the RCB andHdC stars, assuming M_bol = -3 and -5. The UW-velocity dispersion of theRCB/HdC stars is similar to that already reported for the eHe stars,further supporting that these groups of stars have predominantly bulgedistributions. However, UW Cen may be a second example of a halo RCBstar currently seen transitting the galactic plane.
| The R Coronae Borealis stars - II. Further inferences from the infrared data
The extensive infrared JHKL photometry of R Coronae Borealis (RCB)variables reported in Paper I is further discussed, especially inrelation to the temperature and formation of circumstellar (carbon)particles. Evidence is given for a range of dust temperatures in theshell consistent with the random dust-puff model. The mean dusttemperature is higher when the flux from the dust is increasing thanwhen it is decreasing. This is also consistent with the puff model.There is no evidence from any of the stars that a significant proportionof the dust is at temperatures greater than ~1500K. It is suggested thatthe dust forms at ~1500K above the cool regions of large convectioncells in a quasi-steady Eddington-driven outflow. Dust formation canthen take place relatively close to the stellar surface, as appearsnecessary in order to explain the details of the RCB-type obscurationevents. Enhanced C_2-band absorption seen occasionally at maximum lightin RCB stars also finds a natural explanation in this model. Relateddata on the HdC stars are also discussed.
| The R Coronae Borealis stars - I. Infrared photometry and long-term variations
Extensive JHKL photometry is given for 12 R Coronae Borealis (RCB)stars, covering periods of up to 23 years. Limited infrared observationsof a few other RCB stars and five HdC stars are also reported. Thesedata are used to study the long-term variations of the stars (at J) andof the circumstellar dust (at L). All of the RCB stars show variationsin the flux from the dust on time-scales of from a few hundred days to afew thousand days. Dust flux amplitudes at L of up to 3mag are found,the larger amplitudes being associated with the longer time-scales.Secular variations over about 10000d are also sometimes seen. R CrBitself is atypical in showing evidence of semiregular variations (period~1260d). There is no direct relation between the occurrence ofobscuration minima and changes in the flux from the dust. However, thereis possibly a statistical relation between the dust flux and thefrequency of the obscuration minima. Models involving fixed geometry forthe ejection of dust from the star appear to be ruled out and the datasupport the random dust-puff model. High infrared excesses and highlevels of obscuration activity seem to be associated with a higher thanaverage hydrogen abundance.
| The radial velocity variations of cool hydrogen-deficient carbon stars
We have obtained a series of radial velocities for many of the brightgalactic R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC)stars, in order to characterize the pulsations of these types of coolhydrogen-deficient carbon stars. By comparing these velocities with theresults of long-term photometric monitoring, we investigate thepulsation properties of these objects, the link between pulsations andmass loss, and the relationship between these stars and the higher-T_effhydrogen-deficient stars, the extreme helium (eHe) stars. We find thatmost of the RCB stars have radial velocity and V light amplitudes of10-20kms^-1 and 0.2-0.3 mag, respectively. Pulsationally more-activestars such as RY Sgr are rare. Only one other RCB star, RT Nor, has asimilar photometric amplitude to RY Sgr, but this star has a much lowerradial velocity amplitude. With only one exception, HD 175893, all ofthe HdC stars have a lower pulsation amplitude than the RCB stars. Sincethe RCB and HdC stars appear to be distinguished by the large-amplitudedeclines and infrared excesses of the RCB stars, we suggest that thepulsation amplitude dictates whether significant mass loss occurs inthese objects. The radial velocity-to-light amplitude (RV/V) ratioappears to be temperature dependent, but we suspect that this is not anintrinsic effect. The 7000-K group of RCB stars haveRV/V~50kms^-1mag^-1, which is similar to radially pulsating Cepheids.The similar pulsation properties of the RCB and HdC stars and the coolereHe stars provide further evidence that these types of stars are closelyrelated.
| Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars.
We have constructed line-blanketed model atmospheres for thehydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, aswell as for the similar hydrogen-deficient carbon (HdC) stars and thecool extreme helium (EHe) stars. Improved continuum opacities have beenused together with realistic line absorption data for atomic andmolecular transitions. The observed dereddened fluxes of R CrB arecompared with the calculated model fluxes and found to agree best with amodel effective temperature of 6900K, while the infrared flux methodgives between 6600 and 6900K, depending on the nature of the flux excessin the J and H bands compared to the model fluxes. The excess maycorrespond to a recently formed dust cloud close to the star, with atypical temperature around 2000K and a dust mass of~10^-11^Msun_. The agreement for the ultraviolet fluxdistribution is also very satisfactory as seen from IUE spectra of RCrB.Theoretical broad band photometry is presented and effectivetemperatures of RCrB and HdC stars estimated. The constructed modelsshow a significantly steeper temperature gradient compared to previouslyexisting models as a result of the line opacity. Due to the cool surfaceand high abundance of carbon, molecular bands of e.g. C_2_ and CO arevisible in the spectra even at as high effective temperatures as 7000K.Furthermore, the high temperatures encountered at depth explain theobserved Hei and CII lines for T_eff_ down to ~7000K. In the innerlayers (τ_Ross_ > 3) the models show density inversions relatedto the ionization zone of helium. For certain low gravity models theluminosity exceeds the local Eddington limit and hence gas pressureinversions occur as well, which could be related to the decline eventsof RCrB stars.
| V854 Centauri - the first 3000 days.
Not Available
| 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.
| Hydrogen-Deficient Stars: Recent Work at the SAAO
Not Available
| The Photometric Characteristics of Cool Hydrogen Deficient Carbon Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1990MNRAS.247...91L
| The 70th Name-List of Variable Stars
Not Available
| Photometry of R Coronae Borealis and hydrogen-deficient carbon stars, II 1986.
Not Available
| Photometry of hydrogen deficient stars.
Not Available
| A radial velocity survey of extremely hydrogen-deficient stars
A radial velocity survey of hot extremely hydrogen-deficient starsconfirms HDE 320156 to be a single-lined spectroscopichydrogen-deficient binary. The three hydrogen-deficient stars detectedall show weak C-lines. Little evidence is found for the small-amplituderadial velocity variations known to be present among the extreme-helium(EHe) stars. The heliocentric radial velocity distributions andkinematics of the known EHe stars are found to be consistent with an oldstellar population which is strongly concentrated towards the Galacticcenter, and evidence suggests that the EHe stars are all single.
| Photometry and Radial Velocities of Southern Carbon Stars
Not Available
| A general catalogue of cool carbon stars
Not Available
| The barium and R type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972MNRAS.159..403E&db_key=AST
| The hydrogen-deficient carbon stars
Abstract image available at:http://adsabs.harvard.edu/abs/1967MNRAS.137..119W
| The magnitudes, colors and motions of stars of spectral class R.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958AJ.....63..477V&db_key=AST
| The Spectra of Certain Stars whose Atmospheres may BE Deficient in Hydrogen.
Abstract image available at:http://adsabs.harvard.edu/abs/1953ApJ...117...25B
| Report.
Not Available
| Radial Velocities of 283 Stars of Spectral Classes R and N.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1944ApJ....99..145S&db_key=AST
| Radial Velocities of Stars of Spectral Class R
Not Available
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Sagittarius |
| Ascensió Recta: | 18h58m47.29s |
| Declinació: | -29°30'18.1" |
| Magnitud Aparent: | 9.434 |
| Moviment propi RA: | 1 |
| Moviment propi Dec: | -4 |
| B-T magnitude: | 10.921 |
| V-T magnitude: | 9.557 |
Catàlegs i designacions:
|
