Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Dust around R Coronae Borealis Stars. I. Spitzer/Infrared Spectrograph Observations
Spitzer/infrared spectrograph (IRS) spectra from 5 to 37 ?m for acomplete sample of 31 R Coronae Borealis stars (RCBs) are presented.These spectra are combined with optical and near-infrared photometry ofeach RCB at maximum light to compile a spectral energy distribution(SED). The SEDs are fitted with blackbody flux distributions andestimates are made of the ratio of the infrared flux from circumstellardust to the flux emitted by the star. Comparisons for 29 of the 31 starsare made with the Infrared Astronomical Satellite (IRAS) fluxes fromthree decades earlier: Spitzer and IRAS fluxes at 12 ?m and 25 ?mare essentially equal for all but a minority of the sample. For thisminority, the IRAS to Spitzer flux ratio exceeds a factor of three. Theoutliers are suggested to be stars where formation of a dust cloud ordust puff is a rare event. A single puff ejected prior to the IRASobservations may have been reobserved by Spitzer as a cooler puff at agreater distance from the RCB. RCBs which experience more frequentoptical declines have, in general, a circumstellar environmentcontaining puffs subtending a larger solid angle at the star and aquasi-constant infrared flux. Yet, the estimated subtended solid anglesand the blackbody temperatures of the dust show a systematic evolutionto lower solid angles and cooler temperatures in the interval betweenIRAS and Spitzer. Dust emission by these RCBs and those in the LMC issimilar in terms of total 24 ?m luminosity and [8.0]-[24.0]color index.
| Are C60 Molecules Detectable in Circumstellar Shells of R Coronae Borealis Stars?
The hydrogen-poor, helium-rich, and carbon-rich character of the gasaround R Coronae Borealis (RCB) stars has been suggested to be a sitefor formation of C60 molecules. This suggestion is notsupported by observations reported here showing that infraredtransitions of C60 are not seen in a large sample of RCBstars observed with the Infrared Spectrograph on the Spitzer SpaceTelescope. The infrared C60 transitions are seen, however, inemission and blended with polycyclic aromatic hydrocarbon (PAH) featuresin spectra of DY Cen and possibly also of V854 Cen, the two leasthydrogen-deficient (hydrogen deficiency of only ~10-100) RCB stars. Thespeculation is offered that C60 (and the PAHs) in themoderately H-deficient circumstellar envelopes may be formed by thedecomposition of hydrogenated amorphous carbon but fullerene formationis inefficient in the highly H-deficient environments of most RCBs.
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A2
| DY Persei, the coolest metal-poor R CrB carbon star
We have derived the atmospheric parameters of the R CrB carbon star DYPer. The spectrum of DY Per near its maximum brightness was obtainedusing the 2.6-m ZTSh telescope of the Crimean Astrophysical Observatory,and has a resolution of about 1.74 Å per pixel. We compare theabsolute observed and theoretical spectral energy distributions (SEDs)of DY Per for ?? 430-730 nm for the first time. Themodel atmospheres were computed using a code written by Ya.V. Pavlenkoin the classical approximation, taking into account the main opacitysources in carbon-star atmospheres. The theoretical SEDs were computedusing the list of atomic lines from the VALD database and the molecularline lists from CD-ROM No. 18 of Kurucz’s database. The estimatedby spectral synthesis effective temperature of DY Per is in the range of2900-3100 K, if we assume log g = 0. We find a metal deficiency inthe atmosphere of DY Per. Quantitative estimates of the overallmetallicity, carbon and nitrogen abundances, and the H/He ratio aresomewhat uncertain: ?2 ? [Fe/H] ? ?0.5, 0.65 ?[C/Fe] ? 1.35, 0.0 ? [N/Fe] ? 0.8, 1/9 ? H/He ? 9/1.These high H/He values do not quite agree with qualitative observationalestimates made by other authors. Our results confirm that DY Per is aunique stellar object. This is the coolest and possibly the mostmetal-poor of all known R CrB stars. We conclude that the largedeficiencies of metals and hydrogen lead to [C/Fe] values in theatmosphere of DY Per characteristic of R CrB stars.
| 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
| Pulsation-decline relationships in R Coronae Borealis stars
Decline onset times were measured in long-term visual light curves forfive R Coronae Borealis (RCB) variable stars. These included RYSgr andV854Cen, the two RCB stars previously reported to have a relationshipbetween dust formation events and pulsational variations. Analysis ofthe decline epochs showed that all decline onsets for a given star obeya linear ephemeris tied to the object's dominant radial pulsationperiod. Thus, in addition to confirming the pulsation-declinecorrelation for RYSgr and V854Cen, this same behaviour was demonstratedin UWCen, RCrB and SAps for the first time. This general result firmlyestablishes the connection between radial pulsation and mass loss in theRCB stars. The dominant pulsation period of ~40d for all of theseobjects therefore represents the characteristic time-scale on whichthese stars produce dust.
| Post-AGB stars as testbeds of nucleosynthesis in AGB stars
We construct a data base of 125 post-AGB objects (including R CrB andextreme helium stars) with published photospheric parameters (effectivetemperature and gravity) and chemical composition. We estimate themasses of the post-AGB stars by comparing their position in the (logT{eff}, log g) plane with theoretical evolutionary tracks ofdifferent masses. We construct various diagrams, with the aim of findingclues to AGB nucleosynthesis. This is the first time that a large sampleof post-AGB stars has been used in a systematic way for such a purposeand we argue that, in several respects, post-AGB stars should be morepowerful than planetary nebulae to test AGB nucleosynthesis. Our mainfindings are that: the vast majority of objects which do not showevidence of N production from primary C have a low stellar mass(Mstar < 0.56 Mȯ); there is no evidencethat objects which did not experience 3rd dredge-up have a differentstellar mass distribution than objects that did; there is clear evidencethat 3rd dredge-up is more efficient at low metallicity. The sample ofknown post-AGB stars is likely to increase significantly in the nearfuture thanks to the ASTRO-F and follow-up observations, making theseobjects even more promising as testbeds for AGB nucleosynthesis.
| 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.
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also 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/399/1167
| 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.
| Recent declines of RS Telescopii, UW Centauri, and V Coronae Australis
Not Available
| 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
| Multicolor Variability of the Carbon Star DY Per
Photographic monitoring in the I(0.81), R(0.63), v, b and u passbandswith the Baldone Schmidt telescope in 1975--2002, the infrared J, H, K,L and M photometry carried out in 1995--2002 with the 0.7 m telescope ofthe Crimean Astrophysical Observatory and with the 1.25 m telescope ofthe Crimean Station of the Sternberg Astronomical Institute, as well asphotovisual magnitude estimates on the Sonneberg Observatory Patrolplate archive of 1963--1980 are used to study the photometric behaviorof DY Per, an unusual carbon star. The characteristics of lightvariations of DY Per are compared with those of long period variables,RCB stars and LMC DY Per-like stars.
| Abundance analyses of cool extreme helium stars
Extreme helium stars (EHes) with effective temperatures from 8000 to13000K are among the coolest EHes and overlap the hotter R CrB stars ineffective temperature. The cool EHes may represent an evolutionary linkbetween the hot EHes and the R CrB stars. Abundance analyses of fourcool EHes, BD+1°4381 (FQ Aqr), LS IV -14°109, BD -1°3438 (NOSer) and LS IV -1°002 (V2244 Oph), are presented. All these starsshow evidence of H- and He-burning at earlier stages of their evolution.To test for an evolutionary connection, the chemical compositions ofcool EHes are compared with those of hot EHes and R CrB stars. Relativeto Fe, the N abundance of these stars is intermediate between those ofhot EHes and R CrB stars. For the R CrB stars, the metallicity M derivedfrom the mean of Si and S appears to be more consistent with thekinematics than that derived from Fe. When metallicity M derived from Siand S replaces Fe, the observed N abundances of EHes and R CrB starsfall at or below the upper limit corresponding to thorough conversion ofinitial C and O to N. There is an apparent difference between thecomposition of R CrB stars and EHes, the former having systematicallyhigher [N/M] ratios. The material present in the atmospheres of many RCrB stars is heavily CN- and ON-cycled. Most of the EHes have onlyCN-cycled material in their atmospheres. There is an indication that theCN- and ON-cycled N in EHes was partially converted to Ne byα-captures. If EHes are to evolve to R CrB stars, fresh C in EHeshas to be converted to N; the atmospheres of EHes have just sufficienthydrogen to raise the N abundance to the level of R CrB stars. If Ne isfound to be normal in R CrB stars, the proposal that EHes evolve to RCrB stars fails. The idea that R CrB stars evolve to EHes is ruled out;the N abundance in R CrB stars has to be reduced to the level of EHes,as the C/He, which is observed to be uniform across EHes, has to bemaintained. Hence the inferred [N/M], C/He and [Ne/M] ratios, and theH-abundances of these two groups indicate that the EHes and the R CrBstars may not be on the same evolutionary path. The atmospheres ofH-deficient stars probably consist of three ingredients: a residue ofnormal H-rich material, substantial amounts of H-poor CN(O)-cycledmaterial, and C- (and O-) rich material from gas exposed to He-burning.This composition could be a result of final He-shell flash in a singlepost-AGB star (FF scenario), or a merger of two white dwarfs (DDscenario). Although the FF scenario accounts for Sakurai's object andother stars (e.g., the H-poor central stars of planetary nebulae),present theoretical calculations imply higher C/He and O/He ratios thanare observed in EHes and R CrB stars. Quantitative predictions arelacking for the DD scenario.
| Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars
The Hipparcos Intermediate Astrometric Data for carbon stars have beenreprocessed using an algorithm which provides an objective criterion forrejecting anomalous data points and constrains the parallax to bepositive. New parallax solutions have been derived for 317 cool carbonstars, mostly of types R and N. In this paper we discuss the results forthe R stars. The most important result is that the early R stars (i.e.,R0 - R3) have absolute magnitudes and V-K colors locating them among redclump giants in the Hertzsprung-Russell diagram. The average absolutemagnitude MK for early R-type stars (with V - K < 4) hasbeen derived from a Monte-Carlo simulation implicitly incorporating allpossible biases. It appears that the simulated magnitude distributionfor a population with a true Gaussian distribution of mean MK= -2.0 and intrinsic standard deviation 1.0 mag provides a satisfactorymatch to the observed distribution. These values are consistent with theaverage absolute magnitude MK = -1.6 for clump red giants inthe solar neighborhood (Alves 2000). Further, early R-type stars arenon-variable, and their infrared photometric properties show that theyare not undergoing mass loss, properties similar to those of the redclump giants. Stars with subtypes R4 - R9 tend to be cooler and havesimilar luminosity to the N-type carbon stars, as confirmed by theirposition in the (J-H, H-K) color-color diagram. The sample of earlyR-type stars selected from the Hipparcos Catalogue appears to beapproximately complete to magnitude K0 ~ 7, translating intoa completeness distance of 600 pc if all R stars had MK= -2(400 pc if MK= -1). With about 30 early R-type stars in thatvolume, they comprise about 0.04% (0.14% for MK= -1) of thered clump stars in the solar neighborhood. Identification with the redclump locates these stars at the helium core burning stage of stellarevolution, while the N stars are on the asymptotic giant branch, wherehelium shell burning occurs. The present analysis suggests that for asmall fraction of the helium core burning stars (far lower than thefraction of helium shell-burning stars), carbon produced in the interioris mixed to the atmosphere in sufficient quantities to form a carbonstar. Based on observations from the Hipparcos astrometric satelliteoperated by the European Space Agency (ESA 1997).
| 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.
| Post-AGB Variables and Stellar Mass-Loss
Not Available
| 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.
| 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
| Spectral analysis of the low-gravity extreme helium stars LSS 4357, LS II+33.5 deg and LSS 99
We have carried out quantitative analyses of three very low surfacegravity extreme helium stars with very similar spectra. Their effectivetemperatures of ~ 16 000K fill a gap in a nearly continuous sequence ofextreme helium stars all having similar luminosity-to-mass ratios, butextending from effective temperatures around 12 000 K to more than 20000 K. Because of the low surface gravities and extremely rich linespectra, the model atmosphere calculations have been reviewed, andlarge-scale spectral synthesis techniques have been introduced to theanalyses for the first time. In addition to the high carbon and nitrogenabundances usually seen in extreme helium stars, two of the programmestars have extremely high oxygen abundances, comparable with or greaterthan their carbon abundances. Based on observations obtained at theEuropean Southern Observatory, La Silla, Chile.
| 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.
| Spectral analysis of the high-gravity extreme helium star LS IV+6 deg 2
The optical spectrum of the early B hydrogen-deficient star LS IV+6 deg2 has been analyzed. It is hottest high-gravity extreme helium star(EHe) yet studied. The He I spectrum shows all predicted permitted andforbidden transitions in absorption. LS IV+6 deg 2 is a comparativelymetal-rich EHe star; abundances of C, N, O, Ne, Mg, Al, and P aretypical of other EHes, whilst Si and S are somewhat deficient. With thesurface parameters given, LS IV+6 deg 2 lies close to the boundary ofthe helium star pulsation instability finger near Teff of about 27,000K. Available data indicate that the radial velocity is variable, butgive no indication of amplitude or period.
| On anticorrelation in changes of visual and infrared brightness of stars with the R Coronae Borealis type variability.
Not Available
| 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)
| 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.
|
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: | Telescopium |
| Ascensió Recta: | 18h18m51.23s |
| Declinació: | -46°32'53.4" |
| Magnitud Aparent: | 10.703 |
| Moviment propi RA: | -2.7 |
| Moviment propi Dec: | -6.3 |
| B-T magnitude: | 11.68 |
| V-T magnitude: | 10.784 |
Catàlegs i designacions:
|
INHABITED SKY
