Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Oxygen Isotopic Ratios in Cool R Coronae Borealis Stars
We investigate the relationship between R Coronae Borealis (RCB) starsand hydrogen-deficient carbon (HdC) stars by measuring precise16O/18O ratios for five cool RCB stars. The16O/18O ratios are derived by spectrum synthesisfrom high-resolution (R ~ 50, 000) K-band spectra. Lower limits to the16O/17O and 14N/15N ratiosas well as Na and S abundances (when possible) are also given. RCB starsin our sample generally display less 18O than HdCstars—the derived 16O/18O ratios range from3 to 20. The only exception is the RCB star WX CrA, which seems to be anHdC-like star with 16O/18O = 0.3. Our result of ahigher 16O/18O ratio for the RCB stars must beaccounted for by a theory of the formation and evolution of HdC and RCBstars. We speculate that a late dredge-up of products of He burning,principally 12C and 16O, may convert an18O-rich HdC star into an 18O-poor RCB star as theH-deficient star begins its final evolution from a cool supergiant tothe top of the white dwarf cooling track.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available 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/469/799
| 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.
| Low-resolution spectroscopy of high Galactic latitude objects: A search for CH stars
Properties of CH stars like iron deficiency and enrichment of carbon andheavy elements can provide valuable inputs to our understanding ofnucleosynthesis. In particular, these parameters provide strongobservational constraints for theoretical studies of nucleosynthesis ofheavy elements at low-metallicity. Accurate identification andspectroscopic characterization of CH stars are therefore very essential.We have undertaken a programme with a prime objective to search forthese objects in a mixed sample of carbon stars taken from HamburggESO survey. The spectra of the objects were obtained using OMR at VBO,Kavalur and HFOSC at HCT, IAO, Hanle, during 2005 and 2006. Here, wereport a detection of twenty-one CH stars from a sample of sixtyobjects based on low-resolution spectral analysis. Estimated effectivetemperatures, ^{12}Cg^{13}C isotopic ratios, and their location in thetwo colour J-H vs H-K plot support their identification with the classof CH stars. Detection of these potential CH star candidates and theirspectral description is the main theme of this paper.
| A Sr-rich Star on the Main Sequence of ? Centauri
Abundance ratios relative to iron for carbon, nitrogen, strontium, andbarium are presented for a metal-rich main-sequence star ([Fe/H]=-0.74)in the globular cluster ? Centauri. This star, designated 2015448,shows depleted carbon and solar nitrogen but, more interestingly, showsan enhanced abundance ratio of strontium [Sr/Fe]~1.6 dex, while thebarium abundance ratio is [Ba/Fe]<=0.6 dex. At this metallicity oneusually sees strontium and barium abundance ratios that are roughlyequal. Possible formation scenarios of this peculiar object areconsidered.
| Detection of Near-Infrared CO Absorption Bands in R Coronae Borealis Stars
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich,pulsating, post-asymptotic giant branch stars that experience massiveirregular declines in brightness caused by circumstellar dust formation.The mechanism of dust formation around RCB stars is not well understood.It has been proposed that CO molecules play an important role in coolingthe circumstellar gas so that dust may form. We report on a survey forCO in a sample of RCB stars. We obtained H- and K-band spectra includingthe first- and second-overtone CO bands for eight RCB stars, theRCB-like star DY Per, and the final-helium-flash star FG Sge. The first-and second-overtone CO bands were detected in the cooler(Teff<6000 K) RCB stars, Z Umi, ES Aql, SV Sge, and DYPer. The bands are not present in the warmer (Teff>6000 K)RCB stars, R CrB, RY Sgr, SU Tau, and XX Cam. In addition,first-overtone bands are seen in FG Sge, a final-helium-flash star thatis in an RCB-like phase at present. Effective temperatures of the eightRCB stars range from 4000 to 7250 K. The observed photospheric COabsorption bands were compared to line-blanketed model spectra of RCBstars. As predicted by the models, the CO bands are strongest in thecoolest RCB stars and not present in the warmest. No correlation wasfound between the presence or strength of the CO bands and dustformation activity in the stars.
| Self-Correlation Analysis of R Coronae Borealis Stars: A Pilot Project
R. Coronae Borealis (RCB) stars are peculiar yellow supergiant starswhich suddenly and unpredictably decrease in brightness by up to severalmagnitudes, then slowly return to normal. Most (perhaps all) RCB starsalso pulsate, and the pulsations may be related to the ejection of thedust clouds which produce the fadings. As a pilot project, we haveapplied self-correlation analysis to two datasets: long-term photometryof R CrB itself by J. D. Fernie, and long-term photometry of severalsouthern RCB stars by P. L. Cottrell, L. Skuljan, and their colleagues.Self-correlation is a simple form of time series analysis which displaysthe cycle-to-cycle behavior of a variable star, averaged over a dataset.It is especially useful for semi-regular variables. Generally, theseasonal pulsation time scales and amplitudes which we derive are inagreement with Fourier analysis of the same datasets. In the case of RCrB, we confirm that there is apparent mode-switching from season toseason.
| Possible RCB-star DY Per: the Current Decline Will be Deep and Needs Observations
The light curve of a decline phase of deep decline event of DY Per isfirstly obtained. Blueing of the starlight during the minimum isconfirmed, at least two months time interval for the present low lightlevel is predicted, and importance of observations of the star at thisphase is expressed.
| Abundances of Neutron-Capture Elements in the Hot Extreme Helium Stars V1920 Cygni and HD 124448
Analysis of Hubble Space Telescope Space Telescope Imaging Spectrographultraviolet spectra of two hot extreme helium (EHe) stars, V1920 Cyg andHD 124448, provides the first measurements of abundances ofneutron-capture elements for EHe stars. Although the two stars havesimilar abundances for elements up through the iron group, they differstrikingly in their abundances of heavier elements: V1920 Cyg isenriched by a factor of 30 in light neutron-capture elements (Y/Fe andZr/Fe) relative to HD 124448. These differences in abundances ofneutron-capture elements among EHe stars are exhibited by the R CrBstars and are evidence supporting the view that there is an evolutionaryconnection between these two groups of hydrogen-deficient stars. Also,the abundances of Y and Zr in V1920 Cyg provide evidence that at leastone EHe star went through an s-process synthesis episode in its earlierevolution.Based on observations obtained with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| Polarimetry of evolved stars. III. RV Tau and R CrB stars
We present broadband optical polarimetry, and broadband optical andinfrared photometry, of eight RV Tau-type and five R CrB-type stars;much of the photometry and polarimetry was obtained simultaneously. Fornine of the objects polarimetric data is reported for the first time. Wehave estimated and subtracted the interstellar component ofpolarization, allowing us to determine the level of intrinsicpolarization. In some cases this is =~ 1%-2% even when the star is in abright photometric state. We consider this to be evidence for thepresence of permanent clumpy non-spherical dust shells around the RV Tauand R CrB-type stars we observed. Our polarimetric and photometric datalead us to conclude that, for most of our programme stars, neutralextinction must be significant in their circumstellar envelopes. Apartfrom the brightness variations due to pulsations and changes in theeffective temperature of stars, there is clear evidence ofwavelength-independent flux variations - with amplitude from 0fm 5 to1fm 0 - implying the presence of large (a>~ 0.15 mu m) dustparticles. Rapid ( ~ 2 hours) evolution of the infrared fluxdistribution at the level of ~ 0fm 6 in the JHKL bands was detected inthe RV Tau star R Sct.Table 2 is only available in electronic form at the CDS via anomymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/405Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
| Winds in R Coronae Borealis Stars
We present new spectroscopic observations of the He I λ10830 linein R Coronae Borealis (RCB) stars that provide the first strong evidencethat most, if not all, RCB stars have winds. It has long been suggestedthat when dust forms around an RCB star, radiation pressure acceleratesthe dust away from the star, dragging the gas along with it. The newspectra show that nine of the 10 stars observed have P Cygni orasymmetric blueshifted profiles in the He I λ10830 line. In allcases, the He I line indicates a mass outflow with a range of intensityand velocity. Around the RCB stars, it is likely that this state ispopulated by collisional excitation rather thanphotoionization/recombination. The line profiles have been modeled withan SEI code to derive the optical depth and the velocity field of thehelium gas. The results show that the typical RCB wind has a steepacceleration with a terminal velocity of V&infy;=200-350 kms-1 and a column density of N~1012 cm-2in the He I λ10830 line. There is a possible relationship betweenthe light curve of an RCB star and its He I λ10830 profile. Starsthat have gone hundreds of days with no dust formation episodes tend tohave weaker He I features. The unusual RCB star V854 Cen does not followthis trend, showing little or no He I absorption despite high mass-lossactivity. The He I λ10830 line in R CrB itself, which has beenobserved at four epochs between 1972 and 2001, seems to show a P Cygnior asymmetric blueshifted profile at all times, whether it is in declineor at maximum light.
| 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
| Photometry of R Coronae Borealis Stars during the Recovery Phase of their Declines
The photometric observations (UBVRI) of nine cool R Coronae Borealis(RCB) stars have been collected at Mt John University Observatory, NewZealand, over a period of twelve years. The analysis of themagnitude-colour and colour-colour diagrams for the recovery phasedemonstrates that all declines exhibit a similar asymptotic approach totheir normal brightness. Declines return to maximum brightness along aline with essentially the same slope that does not depend on the star orthe depth of the decline. Assuming a uniform obscuration of thephotosphere by the dust cloud during the recovery phase, the extinctionproperties of the material were determined. The ratio of total toselective extinction (RV) for the RCB stars in our sample isin the range 2.5 to 4.6, indicating that the obscuring dust hasextinction properties similar to that of the interstellar dust.Observations have been compared with the theoretical extinction curvesfor different sorts of grains.
| 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
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The stellar content of the Hamburg/ESO survey II. A large, homogeneously-selected sample of high latitude carbon stars
We present a sample of 403 faint high latitude carbon (FHLC) starsselected from the digitized objective prism plates of the Hamburg/ESOSurvey (HES). Because of the ~ 15 Å spectral resolution and highsignal-to-noise ratio of the HES prism spectra, our automated procedurebased on the detection of C2 and CN molecular bands permitshigh-confidence identification of carbon stars without the need forfollow-up spectroscopy. From a set of 329 plates (87 % of the survey),covering 6 400 deg2 to a magnitude limit of V ~ 16.5, weanalyze the selection efficiency and effective surface area of the HESFHLC survey to date. The surface density of FHLC stars that we detect(0.072+/- 0.005 deg-2) is 2-4 times higher than that ofprevious objective prism and CCD surveys at high galactic latitude, eventhough those surveys claimed a limiting magnitude up to 1.5 magnitudesfainter. This attests to the highest selection sensitivity yet achievedfor these types of stars. Based on observations collected at theEuropean Southern Observatory, Chile (Proposal IDs 145.B-0009 and63.L-0148). Table A.1 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/366
| The MACHO Project LMC Variable Star Inventory. X. The R Coronae Borealis Stars
We report the discovery of eight new R Coronae Borealis (RCB) stars inthe Large Magellanic Cloud (LMC) using the MACHO project photometrydatabase. The discovery of these new stars increases the number of knownRCB stars in the LMC to thirteen. We have also discovered four starssimilar to the Galactic variable DY Per. These stars decline much moreslowly and are cooler than the RCB stars. The absolute luminosities ofthe Galactic RCB stars are unknown since there is no direct measurementof the distance to any Galactic RCB star. Hence, the importance of theLMC RCB stars. We find a much larger range of absolute magnitudes(MV=-2.5 to -5 mag) than inferred from the small pre-MACHOsample of LMC RCB stars. It is likely that there is atemperature-MV relationship with the cooler stars beingintrinsically fainter. Cool (~5000 K) RCB stars are much more commonthan previously thought based on the Galactic RCB star sample. Using thefairly complete sample of RCB stars discovered in the MACHO fields, wehave estimated the likely number of RCB stars in the Galaxy to be ~3200.The SMC MACHO fields were also searched for RCB stars, but none werefound.
| 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
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| The R Coronae Borealis stars - atmospheres and abundances
An abundance analysis of the H-deficient and He- and C-rich R CoronaeBorealis (R CrB) stars has been undertaken to examine the ancestry ofthe stars. The investigation is based on high-resolution spectra andline-blanketed H-deficient model atmospheres. The models successfullyreproduce the flux distributions and all spectral features, bothmolecular bands and high-excitation transitions, with one importantexception, the C i lines. Since photoionization of C i dominates thecontinuous opacity, the line strengths of C i are essentiallyindependent of the adopted carbon abundance and stellar parameters. Allpredicted C i lines are, however, much too strong compared withobservations, with a discrepancy in abundance corresponding to 0.6 dexwith little star-to-star scatter. Various solutions of this ``carbonproblem'' have been investigated. A possible solution is that classicalmodel atmospheres are far from adequate descriptions of supergiants suchas the R CrB stars. We can also not exclude completely, however, thepossibility that the gf-values for the C i lines are in error. This issupported by the fact that the C ii, [C i] and C_2 lines are reproducedby the models with no apparent complications. In spite of the carbonproblem, various tests suggest that abundance ratios are little affectedby the uncertainties. Judging by chemical composition, the R CrB starscan be divided into a homogeneous majority group and a diverse minority,which is characterized by extreme abundance ratios, in particular asregards Si/Fe and S/Fe. All stars show evidence of H- and He-burning indifferent episodes as well as mild s-process enhancements. Four of themajority members are Li-rich, while overabundances of Na, Al, Si and Sare attributes of all stars. An anti-correlation found between the H andFe abundances of H-deficient stars remains unexplained. These enigmaticstars are believed to be born-again giants, formed either through afinal He-shell flash in a post-AGB star or through a merger of two whitedwarfs. Owing to a lack of theoretical predictions of the resultingchemical compositions, identification of the majority and minoritygroups with the two scenarios is unfortunately only preliminary.Furthermore, Sakurai's object and V854Cen exhibit aspects of both majority and minority groups,which may suggest that the division into two groups is too simplistic.
| The 1995-96 decline of R Coronae Borealis: high-resolution optical spectroscopy
A set of high-resolution optical spectra of R CrB acquired before,during and after its 1995-96 decline is discussed. All of the componentsreported from earlier declines are seen. This novel data set providesnew information on these components including several aspects notpreviously seen in declines of R CrB and other R Coronae Borealis stars.In the latter category is the discovery that the onset of the decline ismarked by distortions of absorption lines of high-excitation lines, andquickly followed by emission in these and in low-excitation lines. This`photospheric trigger' implies that dust causing the decline is formedclose to the star. These emission lines fade quickly. After 1995November 2, low-excitation narrow (FWHM ~12kms-1) emissionlines remain. These appear to be a permanent feature, slightlyblueshifted from the systemic velocity, and unaffected by the declineexcept for a late and slight decrease of flux at minimum light. Thelocation of the warm dense gas providing these lines is uncertain.Absorption lines unaffected by overlying sharp emission are greatlybroadened, weakened and redshifted at the faintest magnitudes whenscattered light from the star is a greater contributor than direct lighttransmitted through the fresh soot cloud. A few broad lines (FWHM~=300kms-1) are seen at and near minimum light withapproximately constant flux: prominent among these are the Hei tripletseries, Nai D and [Nii] lines. These lines are blueshifted by about30kms-1 relative to the systemic velocity, with no change invelocity over the several months for which the lines were seen. It issuggested that these lines, especially the Hei lines, arise from anaccretion disc around an unseen compact companion which may be alow-mass white dwarf. If so, R CrB is similar to the unusualpost-asymptotic giant branch star 89 Her.
| Is U Aquarii a Thorne-Żytkow Object?
The R Coronae Borealis (RCB) star U Aqr was known to show an unusualabundance pattern with respect to the post-iron peak elements. Theabundances of elements in the light s-process peak-Rb, Sr, Y, and Zr-aregreatly enhanced, whereas elements representative of the heavy s-processpeak, such as Ba, do not exhibit large abundance enhancements. Recentcalculations of the abundances of elements formed by rapid protoncapture in Thorne-Żytkow objects indicate that qualitatively theabundance pattern of post-iron peak elements in Thorne-Żytkowobjects appeared to match that observed in U Aqr. New high-resolutionspectra of U Aqr and two other stars are used to explore the possibilitythat U Aqr is a Thorne-Żytkow object. Our spectra show that on thebasis of the light s-process elements, in particular the absence of anenhancement of Mo, Cu, and Zn, it does not appear that U Aqr is a goodcandidate for a Thorne-Żytkow object. However, the light s-processabundance pattern resembles that of Sakurai's star that has recentlybegun to show RCB characteristics. We suggest that these stars representa possible source for the ``weak'' component of the solar systems-process distribution of heavy elements.
| Stellar evolution in fast forward
Not Available
| Dust extinction and intrinsic SEDs of carbon-rich stars. II. The hot carbon stars
The present work is an extension of a recent study by Knapik &Bergeat (\cite{knapik}, henceforth called Paper I) of the spectralenergy distributions (SEDs) of about 300 cool carbon-rich variables andof the interstellar extinction observed on their line of sights. Themethods were originally developed for Semi-Regular (SR) and Irregular(L)-variables. 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 carbon-rich giants with bluer SEDs, namely the hotcarbon (HC) stars, including many ``constant'' stars and a minority ofvariables: AC Her a RV Tau star, the R Coronae Borealis (RCB) stars andothers. Some HdC (i.e. carbon-rich hydrogen deficient stars) and Ba IIstars are also considered. The total number of studied HC stars amountsto about 140. With few exceptions, the colour excesses for interstellarextinction are found in good agreement with the field values from mapspublished in the literature, taking into account the approximatedistances to our stars from HIPPARCOS data (\cite{esa}, henceforthcalled ESA) or binarity. We propose a classification scheme with sixphotometric groups (or boxes: HC0 to HC5) from the bluest to the reddestSEDs. Oxygen-rich SEDs earlier than HC0, are attributed to the hotteststars (AC Her, most RCB-variables and a few others). Previous findingsare confirmed of a junction between oxygen-rich and carbon-rich SEDs atspectral type G. The latest (HC5) group is immediately close to theearliest one in Paper I, namely CV1. The sequence of groups then goesregularly from HC0 to CV6. Substantial infrared excesses with respect toour solutions are found in HD 100764 a HC1 carbon star, AC Her a G0g RVTau star, and the RCB stars classified in either HC or oxygen-groups.The colour excesses at maximum light can usually be attributed tointerstellar reddening, with neutral circumstellar (CS) reddening (largegrains) or no CS extinction at all on the line of sight (non sphericalgeometry) as possible explanations. The latter model (disc or patchydistribution through successive puffs) is favoured. Two RCB variablesfor which we exploit SEDs on a rising branch (V CrA) or minimum light(RS Tel), show CS laws, respectively a selective extinction compatiblewith small grains and an extinction partly neutral indicative of largegrains on the line of sight. 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{Tables~3 and 4 are only available in electronicform at the CDS via anonymous ftp 130.79.128.5
| 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.
| Parallaxes and Proper Motions of Prototypes of Astrophysically Interesting Classes of Stars. I. R Coronae Borealis Variables
Hipparcos data were obtained under the 1982 announcement of opportunityfor the seven brightest R CrB stars then known. None of the parallaxmeasurements is different from zero in a statistically significant way,though most of the proper motions are. The body of the data neverthelesssuggest that the stars belong to two rather different populations ineither luminosity, kinematics, or both. The best-known stars (R. CrBitself, RY Sgr, and perhaps others) probably are the very luminous, Msubscript v = -4 to -5, objects they are generally accused of being, butothers may belong to a fainter class for which there is some independentevidence among stars in the Large Magellanic Cloud. Alternatively, or inaddition, the apparently fainter stars may belong to a higher-velocitypopulation that we have had rather bad luck in sampling. Of two rashassumptions, the one that all seven stars are at the same distance of1200 pc leads to a velocity ellipsoid U superscript 2 superscript 1/2, Vsuperscript 2 superscript 1/2, W superscript 2> superscript 1/2 = 41,30, 35 km sec superscript -1, which is "not inconsistent" with valuestypical of (other) old disk populations, like carbon stars, long periodvariables, and planetary nebulae. The seemingly less rash assumptionthat all stars have M subscript v = -4.5 leads to U superscript 2superscript 1/2, V superscript 2 superscript 1/2, W superscript 2superscript 1/2, = 52, 60, and 104 km sec supercript -1, resembles noknown stellar population, at least not in our galaxy. (SECTION: Stars)
| The Spectrum Near Maximum Light of the Unusual R Coronae Borealis Variable DY Persei
DY Per was confirmed photometrically as an R CrB variable by Alksnis in1994. Spectra near maximum light show the high-speed ejection of matter(sodium at -174 km/sec in DY Per) characteristic of many of thesevariable stars. Moderate hydrogen deficiency may also be present. DY Peris remarkable in having a temperature many hundreds of degrees lowerthan most of the R CrB variables. It is unusual also in not showingclear spectroscopic evidence of high luminosit. (SECTION: Stars)
| On the Hydrogen Deficient Nature of Z UMi
Z Ursa Minoris was classified by Benson et al. (1994) a a R CoronaeBorealis (RCB) variable star from its light variations. Hydrogendeficiency, which is a defining feature of RCB stars, was notestablished. To investigate this aspect we have obtained high resolutionspectra in both blue (4200-4630\AA) and red (5050-7950\AA) regions.Lines of the CH molecule (G band) at about 4300\AA, which are present inspectra of N-type carbon stars are weak or absent in the spectrum of ZUMi indicating its hydrogen deficient nature and membership of the rareclass of RCB variables. (SECTION: Stars)
| A stellar endgame - the born-again Sakurai's object.
The surface chemical composition of this remarkable star shows that itis hydrogen-deficient, carbon-rich and enriched in the light s-processelements. Spectra taken in May and October 1996 indicate a decrease inthe surface hydrogen abundance by 0.7dex in five months along with anincrease in the abundances of Li, Sr, Y and Zr. The abundance changesare in agreement with the hypothesis of the star being a rapidlyevolving "born-again" AGB star experiencing a final He-shell flash,similar to FGSge. The ^12^C/^13^C ratio in October is very low, alsosuggesting hydrogen ingestion. By chemical composition, Sakurai's objectresembles the R Coronae Borealis (RCrB) stars.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Водолей |
| Прямое восхождение: | 22h03m19.69s |
| Склонение: | -16°37'35.3" |
| Видимая звёздная величина: | 10.845 |
| Собственное движение RA: | 0 |
| Собственное движение Dec: | 0 |
| B-T magnitude: | 11.852 |
| V-T magnitude: | 10.929 |
Каталоги и обозначения:
|
