目录
图像
上传图像
DSS Images Other Images
相关文章
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
| CNO Abundances of Hydrogen-Deficient Carbon and R Coronae Borealis Stars: A View of the Nucleosynthesis in a White Dwarf Merger
We present high-resolution (R ~ 50, 000) observations of near-IRtransitions of CO and CN of the five known hydrogen-deficient carbon(HdC) stars and four R Coronae Borealis (RCB) stars. We perform anabundance analysis of these stars by using spectrum synthesis andstate-of-the-art MARCS model atmospheres for cool hydrogen-deficientstars. Our analysis confirms reports by Clayton and colleagues thatthose HdC stars exhibiting CO lines in their spectrum and the cool RCBstar S Aps are strongly enriched in 18O (with16O/18O ratios ranging from 0.3 to 16). Nitrogenand carbon are in the form of 14N and 12C,respectively. Elemental abundances for CNO are obtained from C I,C2, CN, and CO lines. Difficulties in deriving the carbonabundance are discussed. Abundances of Na from Na I lines and Sfrom S I lines are obtained. Elemental and isotopic CNO abundancessuggest that HdC and RCB stars may be related objects, and that theyprobably formed from a merger of an He white dwarf with a C-O whitedwarf.
| Fluorine in R Coronae Borealis Stars
Neutral fluorine (F I) lines are identified in the optical spectra ofseveral R Coronae Borealis stars (RCBs) at maximum light. These linesprovide the first measurement of the fluorine abundance in these stars.Fluorine is enriched in some RCBs by factors of 800-8000 relative to itslikely initial abundance. The overabundances of fluorine are evidencefor the synthesis of fluorine. These results are discussed in the lightof the scenario that RCBs are formed by accretion of an He white dwarfby a CO white dwarf. Sakurai's object (V4334 Sgr), a final He-shellflash product, shows no detectable F I lines.
| The evolutionary status of the semiregular variable QY Sge
Repeated spectroscopic observations made with the 6-m telescope of SAORAS yielded new data on the radial-velocity variability of the anomalousyellow supergiant QY Sge. The strongest and most peculiar feature in itsspectrum is the complex profile of Na I D lines, which contains a narrowand a very wide emission components. The wide emission component can beseen to extend from ?170 to +120 km/s, and at its central part itis cut by an absorption feature, which, in turn, is split into twosubcomponents by a narrow (16 km/s at r=2.5) emission peak. An analysisof all the Vr values leads us to adopt for the star a systemic velocityof Vr=?21.1 km/s, which corresponds to the position ofthe narrow emission component of Na I. The locations of emission-linefeatures of Na I D lines are invariable, which point to their formationin regions that are external to the supergiant’s photosphere.Differential line shifts of about 10 km/s are revealed. Emission in theH ? line is weaker than in Na I D lines, it fills the photosphericabsorption almost completely. The absorption lines in the spectrum of QYSge have a substantial width of FWHM ? 45 km/s. The method of modelatmospheres is used to determine the following parameters: the effectivetemperature T eff =6250±150 K, surface gravity lg g=2.0±0.2, and microturbulence velocity ? t=4.5±0.5 km/s. The chemical composition of the atmospherediffers only slightly from the solar composition: the metallicity of thestar is found to be somewhat higher than the solar metallicity with anaverage overabundance of iron-peak elements of [Met/H]?=+0.20. The star is found to be slightlyoverabundant in carbon and nitrogen, [ C/Fe]=+0.25, [ N/Fe]=+0.27. The?-process elements Mg, Si, and Ca are slightly overabundant, onthe average by [ ?/H]?=+0.12, and sulfuroverabundance is higher, [ S/?]=+0.29. The strong overabundance ofsodium, [ Na/Fe]=+0.75, is likely to be due to the dredge-up of thematter processed in the NeNa cycle. Heavy elements of the s-process areunderabundant relative to the Sun. On the whole, the observed propertiesof QY Sge do not give grounds for including this star into the group ofRCrB or RVTau-type objects.
| 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.
| RCoronae Borealis at the 2003 light minimum
A set of five high-resolution optical spectra of R CrB obtained in 2003March is discussed. At the time of the first spectrum (March 8), thestar was at V = 12.6, a decline of more than six magnitudes. By March31, the date of the last observation, the star at V = 9.3 was on therecovery to maximum light (V = 6). The 2003 spectra are compared withthe extensive collection of spectra from the 1995-1996 minimum presentedpreviously. Spectroscopic features common to the two minima include thefamiliar ones also seen in spectra of other R Coronae Borealis stars(RCBs) in decline: sharp emission lines of neutral and singly ionizedatoms, broad emission lines including HeI, [NII] 6583 Å, Na D andCaII H & K lines, and blueshifted absorption lines of Na D, and KIresonance lines. Prominent differences between the 2003 and 1995-1996spectra are seen. The broad Na D and Ca H & K lines in 2003 and1995-1996 are centred approximately on the mean stellar velocity. The2003 profiles are fit by a single Gaussian, but in 1995-1996 twoGaussians separated by about 200 km s-1 were required.However, the HeI broad emission lines are fit by a single Gaussian atall times; the emitting He and Na-Ca atoms are probably not colocated.The C2 Phillips 2-0 lines were detected as sharp absorptionlines and the C2 Swan band lines as sharp emission lines in2003, but in 1995-1996 the Swan band emission lines were broad and thePhillips lines were undetected. The 2003 spectra show CI sharp emissionlines at minimum light with a velocity changing in 5 d by about 20 kms-1 when the velocity of `metal' sharp lines is unchanged;the CI emission may arise from shock-heated gas. Reexamination ofspectra obtained at maximum light in 1995 shows extended blue wings tostrong lines with the extension dependent on a line's lower excitationpotential; this is the signature of a stellar wind, also revealed bypublished observations of the HeI 10830 Å line at maximum light.Changes in the cores of the resonance lines of AlI and Na D (variableblueshifts) and the CaII infrared (IR) lines (variable blueshifts andredshifts) suggest complex flow patterns near the photosphere. Thespectroscopic differences at the two mimima show the importance ofcontinued scrutiny of the declines of R CrB (and other RCBs). Thoroughunderstanding of the outer atmosphere and circumstellar regions of R CrBwill require such continued scrutiny.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München andGeorg-August-Universität Göttingen.E-mail: dll@anchor.as.utexas.edu (DLL)
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| 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.
| RCoronae Borealis stars at minimum light - UW Cen
Two high-resolution optical spectra of the R Coronae Borealis (R CrB)star UW Cen in decline are discussed. A spectrum from mid-1992 when thestar had faded by 3mag shows just a few differences with the spectrum atmaximum light. The ubiquitous sharp emission lines seen in R CrB at asimilar drop below maximum light are absent. In contrast, a spectrumfrom mid-2002 when the star was 5mag below maximum light shows an arrayof sharp emission lines and a collection of broad emission lines.Comparisons are made with spectra of R CrB obtained during the deep1995-1996 minimum. The many common features are discussed in terms of atorus-jet geometry.
| 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
| 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
| Long-Slit Observations of Extended C II ?1335 Emission around V854 Centauri and RY Sagittarii
We have obtained long-slit far-ultraviolet (1150-1730 Å) spectraof the R Coronae Borealis (RCB) stars V854 Cen and RY Sgr near maximumlight and pulsational phase zero with the Space Telescope ImagingSpectrograph on board the Hubble Space Telescope. The far-UV spectrum ofeach star shows a photospheric continuum rising steeply toward longerwavelengths and a prominent emission feature at C II ?1335. RYSgr displays a second, but fainter, emission attributed to Cl I?1351 (which is radiatively fluoresced by C II ?1335), butCl I is weak or absent in V854 Cen. Most surprisingly, the C II emissionof V854 Cen is significantly extended along the slit by +/-2.5", about6×103 AU at the distance of the star. The C II featureof RY Sgr exhibits no such gross extension. Nevertheless, subtlebroadenings of the C II emissions beyond the point response profilesuggest inner clouds of radius ~0.1" (250 AU) around both stars. V854Cen is only the third RCB star after R CrB and UW Cen known to have aresolved shell. Based on observations obtained with the NASA/ESA HubbleSpace Telescope, which is operated by STScI for the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
| 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.
| The DIRTY Model. I. Monte Carlo Radiative Transfer through Dust
We present the DIRTY (DustI Radiative Transfer, Yeah!) radiativetransfer model in this paper and a companion paper. This model computesthe polarized radiative transfer of photons from arbitrary distributionsof stars through arbitrary distributions of dust using Monte Carlotechniques. The dust re-emission is done self-consistently with the dustabsorption and scattering and includes all three important emissionpaths: equilibrium thermal emission, nonequilibrium thermal emission,and the aromatic features emission. The algorithm used for the radiativetransfer allows for the efficient computation of the appearance of amodel system as seen from any viewing direction. We present a simplemethod for computing an upper limit on the output quantity uncertaintiesfor Monte Carlo radiative transfer models that use the weighted photonapproach.
| 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.
| The Ever Changing Circumstellar Nebula Around UW Centauri
We will present new images of the unique reflection nebula surroundingthe R Coronae Borealis Star, UW Cen. This nebula changes its appearancesignificantly on timescales of a year or less as different parts areilluminated by light from the central star modulated by shifting thickdust clouds near its surface. These dust clouds form and dissipate atirregular intervals causing the well-known declines in the R CoronaeBorealis star lightcurve. In this way, the central star acts like alighthouse shining through holes in the dust clouds and lighting updifferent portions of the nebula. The new images may enable us to usethe light echo to calculate an accurate distance to UW Cen, investigatethe morphology of the nebula in detail to study whether this object isrelated to planetary nebulae and the final helium shell flash stars anduse the illumination of the nebula to discern the pattern of newcircumstellar dust clouds. Understanding the RCB stars is a key test forany theory which aims to explain hydrogen deficiency in post-AsymptoticGiant Branch stars.
| Post-AGB Variables and Stellar Mass-Loss
Not Available
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| 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.
| Dust formation above cool magnetic spots in evolved stars
We examine the structure of cool magnetic spots in the photospheres ofevolved stars, specifically asymptotic giant branch (AGB) stars and RCoronae Borealis (RCB) stars. We find that the photosphere of a coolmagnetic spot will be above the surrounding photosphere of AGB stars,which is the opposite of the situation in the Sun. This results from thebehaviour of the opacity, which increases with decreasing temperature,which again is the opposite of the behaviour of the opacity near theeffective temperature of the Sun. We analyse the formation of dust abovethe cool magnetic spots, and suggest that the dust formation isfacilitated by strong shocks, driven by stellar pulsations, which runthrough and around the spots. The presence of both the magnetic fieldand cooler temperatures makes dust formation easier as the shock passesabove the spot. We review some observations supporting the proposedmechanism, and suggest further observations to check the model.
| The Ever-changing Circumstellar Nebula around UW Centauri
We present new images of the reflection nebula surrounding the R CoronaeBorealis Star, UW Centauri. This nebula, first detected in 1990, haschanged its appearance significantly. At the estimated distance of UWCen, this nebula is approximately 0.6 lt-yr in radius, so the nebulacannot have physically altered in only 8 years. Instead, the morphologyof the nebula appears to change as different parts are illuminated bylight from the central star that is modulated by shifting thick dustclouds near its surface. These dust clouds form and dissipate atirregular intervals, causing the well-known declines in the R CoronaeBorealis (RCB) stars. In this way, the central star acts like alighthouse shining through holes in the dust clouds and lighting updifferent portions of the nebula. The existence of this nebula providesclues to the evolutionary history of RCB stars, possibly linking them tothe planetary nebulae and the final helium shell flash stars.
| The rapid evolution of the born-again giant Sakurai's object
The extraordinarily rapid evolution of the born-again giant Sakurai'sobject following discovery in 1996 has been investigated. The evolutioncan be traced both in a continued cooling of the stellar surface anddramatic changes in chemical composition on a timescale of a mere fewmonths. The abundance alterations are the results of the mixing andnuclear reactions which have ensued due to the final He-shell flashwhich occurred during the descent along the white dwarf cooling track.The observed changes in the H and Li abundances can be explained byingestion and burning of the H-rich envelope and Li-production throughthe Cameron-Fowler mechanism. The rapidly increasing abundances of thelight s-elements (including Sc) are consistent with current s-processingby neutrons released from the concomitantly produced (13) C. However,the possibility that the s-elements have previously been synthesizedduring the AGB-phase and only mixed to the surface in connection withthe final He-shell flash in the pre-white dwarf cannot be convincinglyruled out either. Since Sakurai's object shows substantial abundancesimilarities with the R CrB stars and has recently undergone R CrB-likevisual fading events, the ``birth'' of an R CrB star may have beenwitnessed for the first time ever. Sakurai's object thus lends strongsupport for the suggestion that at least some of the R CrB stars havebeen formed through a final He-shell flash in a post-AGB star.
| Stellar evolution in fast forward
Not Available
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
