目录
图像
上传图像
DSS Images Other Images
相关文章
The Galactic R Coronae Borealis Stars: The C2 Swan Bands, the Carbon Problem, and the 12C/13C Ratio
Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficientcarbon (HdC) stars are analyzed by synthesizing the C2 Swanbands (1, 0), (0, 0), and (0, 1) using our detailed line list and theUppsala model atmospheres. The (0, 1) and (0, 0) C2 bands areused to derive the 12C abundance, and the (1, 0)12C13C band to determine the12C/13C ratios. The carbon abundance derived fromthe C2 Swan bands is about the same for the adopted modelsconstructed with different carbon abundances over the range 8.5 (C/He =0.1%) to 10.5 (C/He = 10%). Carbon abundances derived from C I lines areabout a factor of four lower than the carbon abundance of the adoptedmodel atmosphere over the same C/He interval, as reported by Asplund etal., who dubbed the mismatch between adopted and derived C abundance asthe "carbon problem." In principle, the carbon abundances obtained fromC2 Swan bands and that assumed for the model atmosphere canbe equated for a particular choice of C/He that varies from star tostar. Then, the carbon problem for C2 bands is eliminated.However, such C/He ratios are in general less than those of the extremehelium stars, the seemingly natural relatives to the RCB and HdC stars.A more likely solution to the C2 carbon problem may lie in amodification of the model atmosphere's temperature structure. Thederived carbon abundances and the 12C/13C ratiosare discussed in light of the double degenerate and the final flashscenarios.
| Tracking down R Coronae Borealis stars from their mid-infrared WISE colours
Context. R Coronae Borealis stars (RCBs) are hydrogen-deficient andcarbon-rich supergiant stars. They are very rare, with only ~50 actuallyknown in our Galaxy. Interestingly, RCBs are strongly suspected of beingthe evolved merger product of two white dwarfs and could therefore be animportant tool for understanding supernovae type Ia in the doubledegenerate scenario. Constraints on the spatial distribution and theformation rate of such stars are needed to picture their origin and testit in the context of actual population synthesis results. Aims:It is crucial to increase the number of known RCBs significantly. Withan absolute magnitude MV ~ -5 and a bright/hot circumstellarshell made of amorphous carbon grains, RCBs are so distinctive that weshould nowadays be able to find them everywhere in our Galaxy usingpublicly available catalogues. In the optical, the search is difficultbecause RCBs are known to undergo unpredictable photometric declines;however, mono-epoch mid-infrared data can help us to discriminate RCBsamong other dust-producing stars. The aim is to produce from themid-infrared WISE and near-infrared 2MASS catalogues a new catalogue ofreasonable size, enriched with RCB stars. Methods: Colour-colourcuts used on all stars detected are the main selection criteria. Theselection efficiency was monitored using the 52 known RCBs located inthe sky area covered by the WISE first preliminary data release. Results: It has been found that selection cuts in mid-infraredcolour-colour diagrams are a very efficient method of distinguishingRCBs from other stars. An RCB enriched catalogue made of only 1602 starswas produced, with a high detection efficiency of about 77%. Spectralenergy distributions of 49 known RCBs and 5 known HdCs are alsopresented with estimates of their photosphere and circumstellar shelltemperatures. Conclusions: The newly released WISE all skycatalogue has proven to be a valuable resource in finding RCB stars.Actual scenarios predict that between 100 and 500 RCBs exist in ourGalaxy. The newly created RCB enriched catalogue is an important steptowards significantly increasing the number of known RCB stars andtherefore better understanding their origin.Tables 3-5 ara available in electronic form at http://www.aanda.orgFull Table 5 is onlyavailable at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/539/A51
| 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.
| 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
| Do Hydrogen-Deficient Carbon Stars have Winds?
We present high resolution spectra of the five known hydrogen-deficientcarbon (HdC) stars in the vicinity of the 10830 Å line of neutralhelium. In R Coronae Borealis (RCB) stars the He I line is known to bestrong and broad, often with a P Cygni profile, and must be formedin the powerful winds of those stars. RCB stars have similar chemicalabundances as HdC stars and also share greatly enhanced 18Oabundances with them, indicating a common origin for these two classesof stars, which has been suggested to be white dwarf mergers. A narrowHe I absorption line may be present in the hotter HdC stars, but no lineis seen in the cooler stars, and no evidence for a wind is found in anyof them. The presence of wind lines in the RCB stars is stronglycorrelated with dust formation episodes so the absence of wind lines inthe HdC stars, which do not make dust, is as expected.
| 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.
| 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
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] 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/464/377
| 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.
| Tycho-2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infraredemission due to the presence of circumstellar material. To identify suchstars, we have positionally correlated the infrared Mid-Course SpaceExperiment (MSX) Point Source Catalogue and the Tycho-2 opticalcatalogue. Near-mid-infrared colour criteria have been developed toselect infrared excess stars. The search yielded 1938 excess stars; overhalf (979) have never previously been detected by IRAS. The excess starswere found to be young objects such as Herbig Ae/Be and Be stars, andevolved objects such as OH/IR (infrared) and carbon stars. A number ofB-type excess stars were also discovered whose infrared colours couldnot be readily explained by known catalogued objects.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| HATNET Variability Survey in the High Stellar Density ``Kepler Field'' with Millimagnitude Image Subtraction Photometry
The Hungarian-made Automated Telescope Network (HATnet) is an ongoingproject to detect transiting extrasolar planets using small-aperture (11cm diameter) robotic telescopes. In this paper, we present the resultsfrom using image subtraction photometry to reduce a crowded stellarfield observed with one of the HATnet telescopes (HAT-5). This field waschosen to overlap with the planned Kepler mission. We obtained I-bandlight curves for 98,000 objects in a 67 square degree field of viewcentered at J2000 (α,δ) =(19h44m00s0, +37°32'00.0"), near theGalactic plane in the constellations Cygnus and Lyra. These observationsinclude 788 exposures of 5 minutes' length over 30 days. For thebrightest stars (I~8.0) we achieved a precision of 3.5 mmag, falling to0.1 mag at the faint end (I~14). From these light curves we identify1617 variable stars, of which 1439 are newly discovered. The fact thatnearly 90% of the variables were previously undetected furtherdemonstrates the vast number of variables yet to be discovered evenamong fairly bright stars in our Galaxy. We also discuss some of themost interesting cases. These include V1171 Cyg, a triple system withthe inner two stars in a P=1.462 day period eclipsing orbit and theouter star a P=4.86 day Cepheid; HD 227269, an eccentric eclipsingsystem with a P=4.86 day period that also shows P=2.907 day pulsations;WW Cyg, a well-studied eclipsing binary; V482 Cyg, an R Coronae Borealisstar; and V546 Cyg, a PV Telescopii variable. We also detect a number ofsmall-amplitude variables, in some cases with full amplitude as low as10 mmag.
| The R Coronae Borealis stars: carbon abundances from forbidden carbon lines
Spectra of several R Coronae Borealis (RCB) stars at maximum light havebeen examined for the [CI] 9850- and 8727-Åabsorption lines. The9850-Åline is variously blended with an FeII and CN lines, butpositive identifications of the [CI] line are made for RCrB and SUTau.The 8727-Åline is detected in the spectrum of the five starsobserved in this wavelength region. Carbon abundances are derived fromthe [CI] lines using the model atmospheres and atmospheric parametersused by Asplund et al.Although the observed strength of a CI line is constant from cool to hotRCB stars, the strength is lower than predicted by an amount equivalentto a factor of 4 reduction of the gf-value of a line. Asplund et al.dubbed this `the carbon problem' and discussed possible solutions.The [CI] 9850-Åline seen clearly in RCrB and SUTau confirms themagnitude of the carbon problem revealed by the CI lines. The [CI]8727-Åline measured in five stars shows an enhanced carbonproblem. The gf-value required to fit the observed [CI] 8727-Ålineis a factor of 15 less than the well-determined theoretical gf-value. Wesuggest that the carbon problem for all lines may be alleviated to someextent by a chromospheric-like temperature rise in these stars. The risefar exceeds that predicted by our non-local thermodynamic equilibriumcalculations, and requires a substantial deposition of mechanicalenergy.
| 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
| 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.
| Behavior of the Line Profiles of the Sodium Doublet and the Calcium Triplet in the Spectrum of R CrB in the Shallow Minima of 1998-1999
The behavior of profiles of the Na I D line and of the infrared Ca IItriplet for the star R Coronae Borealis (R CrB) during shallow lightminima of 1998-1999 is traced using high-resolution spectra. During alight maximum, the sodium lines had an absorption profile with a shiftof —(2-4) km/sec. During a light minimum, a narrow emissionfeature, which has an almost constant absolute intensity and a shift of—(8-10) km/sec, and an intense circumstellar absorption feature,which has a variable profile and a variable relative shift correspondingto an increase to 220 km/sec in the velocity of mass ejection, appearedin the cores of absorption lines. For several days before the onset of alight minimum, all three calcium lines exhibited a narrow emissionfeature in the line core with a shift of —(1-5) km/sec. All thesubsequent changes in a line involved mainly the shape of the absorptionline profile. The narrow emission feature's absolute intensity andrelative position were maintained during all our observations. Thebehavior of the Na I D line profiles can be described qualitativelywithin the framework of the model of a spherical dust shell.
| Aufsuchkarten fur intrinsic Variable Stars in Brno.
Not Available
| 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.
| Some Characteristics of the Circumstellar Shells of Stars with Variability of the R Coronae Borealis Type
The transformation of the line and molecular spectra of R CrB fromabsorption to emission at the 1985 minimum is traced from spectroscopicobservations, and it is concluded for the first time, from a comparisonwith light curves and color indices, that the rapid variation of thecolor index U - B by -0m.6 during fading is due exclusively to thistransformation. The transformation of the spectrum can result in anincrease in the star's brightness in the UBV photometric bands by about1m.4, 0m.75, and up to 0m.75, respectively. This removes one of the mainobjections to using the concept of a homogeneous spherical shell tointerpret the R Coronae Borealis phenomenon. High-resolutionspectroscopic observations of SV Sge, a star with variability of the RCoronae Borealis type, showed the presence of an intense circumstellarcomponent in Na I D lines with a violet shift corresponding to 240km/sec, which is unrelated to visible brightness minima. It is suggestedthat a stream of matter with such a velocity through the star's vicinityis the cause of the excitation not only of the broad emission featuresobserved during brightness minima upon fading of the photosphericemission but also of the narrow ones. Assuming that a brightness minimumis associated with the temporary formation of another sphericalscreening shell, some characteristics of the permanent shell aredetermined. Its optical depth and geometrical thickness are 0.2-0.7 andat least 0.4 of its radius, respectively.
| 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.
| Identifications for Wachmann's Variablesin the southern Cygnus Starcloud
Not Available
| 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.
| Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars.
We have constructed line-blanketed model atmospheres for thehydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, aswell as for the similar hydrogen-deficient carbon (HdC) stars and thecool extreme helium (EHe) stars. Improved continuum opacities have beenused together with realistic line absorption data for atomic andmolecular transitions. The observed dereddened fluxes of R CrB arecompared with the calculated model fluxes and found to agree best with amodel effective temperature of 6900K, while the infrared flux methodgives between 6600 and 6900K, depending on the nature of the flux excessin the J and H bands compared to the model fluxes. The excess maycorrespond to a recently formed dust cloud close to the star, with atypical temperature around 2000K and a dust mass of~10^-11^Msun_. The agreement for the ultraviolet fluxdistribution is also very satisfactory as seen from IUE spectra of RCrB.Theoretical broad band photometry is presented and effectivetemperatures of RCrB and HdC stars estimated. The constructed modelsshow a significantly steeper temperature gradient compared to previouslyexisting models as a result of the line opacity. Due to the cool surfaceand high abundance of carbon, molecular bands of e.g. C_2_ and CO arevisible in the spectra even at as high effective temperatures as 7000K.Furthermore, the high temperatures encountered at depth explain theobserved Hei and CII lines for T_eff_ down to ~7000K. In the innerlayers (τ_Ross_ > 3) the models show density inversions relatedto the ionization zone of helium. For certain low gravity models theluminosity exceeds the local Eddington limit and hence gas pressureinversions occur as well, which could be related to the decline eventsof RCrB stars.
| V854 Centauri - the first 3000 days.
Not Available
| The MACHO Project Large Magellanic Cloud Variable Star Inventory. IV. New R Coronae Borealis Stars
We report the discovery of two new R Coronae Borealis (RCB) stars in theLMC using the MACHO project photometry database. The identification ofboth stars has been confirmed spectroscopically. One is a cool RCB star(T_eff_ ~ 5000 K), characterized by very strong Swan bands of C_2_ andviolet bands of CN, and weak or absent Balmer lines, G band, and^12^C^13^C bands. The second star is an example of a hot RCB star, ofwhich only three were previously known to exist in the Galaxy and nonein the LMC. Its spectrum is characterized by several C II lines inemission. Both stars have shown deep declines of {DELTA}V >= 4 mag inbrightness. The new stars are significantly fainter at maximum lightthan the three previously known LMC RCB stars. The amount of reddeningtoward these stars is somewhat uncertain, but both seem to have absolutemagnitudes, M_V_ ~ 0.5 mag fainter than the other three stars. Estimatesof M_bol_ find that the hot RCB star lies in the range of the otherthree stars, while the cool RCB star is fainter. The two cool LMC RCBstars are the faintest at M_bol_. The discovery of these two new starsbrings to five the number of known RCB stars in the LMC, and itdemonstrates the utility of the MACHO photometric database for thediscovery of new RCB stars.
| On the Frequency of the Decline Events of R CrB Type Variables
All the available light curves of R CrB stars have been collected inorder to find a connection between their inter-fade periods and otherphysical parameters. The frequency of the fadings seems to be stronglycorrelated with the [C/H] abundance ratio, in the sense that the smalleris the hydrogen deficiency of the variables the more enhanced activitycan be observed.
| The R Coronae Borealis Stars
This year marks the bicentennial of the discovery of the variability ofR Coronae Borealis. The R Coronae Borealis (RCB) stars are distinguishedfrom other hydrogen-deficient objects by their spectacular dustformation episodes. They may decline by up to 8 magnitudes in a fewweeks revealing a rich emission-line spectrum. Their atmospheres haveunusual abundances with very little hydrogen and an overabundance ofcarbon and nitrogen. The RCB stars are thought to be the product of afinal helium shell flash or the coalescence of a binary white-dwarfsystem. Dust may form in non-equilibrium conditions created behindshocks caused by pulsations in the atmospheres of these stars. The RCBstars are interesting and important, first because they represent arare, or short-lived stage of stellar evolution, and second becausethese stars regularly produce large amounts of dust so they arelaboratories for the study of dust formation and evolution. (SECTION:Invited Review Paper)
|
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:
|
