Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Strong [O III] and [N II] emission lines in globular clusters from photoionized R Corona Borealis star winds
The globular cluster X-ray source CXO J033831.8-352604 in NGC 1399 hasrecently been found to show strong emission lines of [O III] and [N II]in its optical spectrum in addition to ultraluminous X-ray emission witha soft X-ray spectrum. It was further suggested that this systemcontained an intermediate-mass black hole which had tidally disrupted awhite dwarf, producing the strong emission lines without detectablehydrogen emission. We show that an alternative exists which can explainthe data more naturally in which the oxygen- and nitrogen-rich materialis ejected from an R Corona Borealis (RCB) star. The scenario we proposehere does not require an intermediate-mass black hole as the accretor,but also does not exclude the possibility.
| Metal abundances in the hottest known DO white dwarf (KPD 0005+5106)
We present the analysis of ultraviolet and optical spectra of KPD0005+5106. Redetermination of the atmospheric parameters is necessarybecause the recent discovery of highly ionised metals (Ne VIII, Ca X)indicates that the effective temperature is significantly higher thanpreviously thought. Here we announce the discovery of lines from highlyionised silicon, sulphur, and iron (Si VII, S VII, Fe X) which werenever found before in any stellar photosphere. Our analysis of theselines and those from helium and several other metals givesTeff = 200 000 ± 20 000 K, log g = 6.7 ± 0.3.Mass and luminosity follow from a comparison with evolutionary tracks: M= 0.64 M_? and log L/L_? = 3.7, so that, strictly speaking,the star is a helium-burning pre-white dwarf. The mass fractions of themetals in the helium-dominated atmosphere are in the range 0.7-4.3solar. Hydrogen is not detectable and we derive an upper abundance limitof 0.034 solar. This abundance pattern is probably unaffected bygravitational settling and radiative levitation. Its origin lies inprevious evolutionary stages. We discuss the link of KPD 0005+5106 toRCrB stars and the possibility that it results from a double-degeneratemerger event.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE was operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| Very Large Excesses of 18O in Hydrogen-deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
We have found that at least seven hydrogen-deficient carbon (HdC) and RCoronae Borealis (RCB) stars, have 16O/18O ratiosclose to and in some cases less than unity, values that are orders ofmagnitude lower than measured in other stars (the solar value is 500).Greatly enhanced 18O is evident in every HdC and RCB we havemeasured that is cool enough to have detectable CO bands. The three HdCstars measured have 16O/18O<1, lower valuesthan any of the RCB stars. These discoveries are important clues indetermining the evolutionary pathways of HdC and RCB stars, for whichtwo models have been proposed: the double degenerate (white dwarf [WD]merger) and the final helium-shell flash (FF). No overproduction of18O is expected in the FF scenario. We have quantitativelyexplored the idea that HdC and RCB stars originate in the mergers of CO-and He-WDs. The merger process is estimated to take only a few days,with accretion rates of 150 Msolar yr-1 producingtemperatures at the base of the accreted envelope of(1.2-1.9)×108 K. Analysis of a simplified one-zonecalculation shows that nucleosynthesis in the dynamically accretingmaterial may provide a suitable environment for a significant productionof 18O, leading to very low values of16O/18O, similar to those observed. We also findqualitative agreement with observed values of 12C/13C and with the CNO elemental ratios. H-admixture during theaccretion process from the small H-rich C/O WD envelope may play animportant role in producing the observed abundances. Overall, ouranalysis shows that WD mergers may very well be the progenitors ofO18-rich RCB and HdC stars, and that more detailedsimulations and modeling are justified.
| X Muscae-A Large Amplitude Red Variable
X Muscae is amongst the stars listed in the General Catalogue ofVariable Stars as "not studied." Visual observations by the authorduring the 5-year interval 1999 through 2003 indicate this is a largeamplitude variable star, probably of the Mira type. A visual magnituderange of 12.2 to fainter than 15.0 is found with a mean cycle of 266± 7 days. A preliminary ephemeris for determining maximumbrightness is JD 2452085 + 266 ± 7 E days.
| 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.
| 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.
| 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.
| A High-Resolution Spectrum of the R Coronae Borealis Star V2552 Ophiuchi
Photometry and low-resolution spectroscopy have added V2552 Oph to therare class of R Coronae Borealis variables. We confirm thisclassification of V2552 Oph through a comparison of our high-resolutionoptical spectrum of this star and that of R CrB and other F-type membersof the class. We show that V2552 Oph most closely resembles Y Mus and FHSct, stars in which Sr, Y, and Zr are enhanced.
| The Newly Active R Coronae Borealis Star, V2552 Ophiuchi
In 2001, V2552 Oph (CD -22°12017, Had V98) quickly faded by severalmagnitudes in a manner typical of the R Coronae Borealis (RCB) stars.Photometry of V2552 Oph obtained over 70 years previous to 2001 shows noindication of variability. Optical spectra of this star subsequentlyconfirmed that V2552 Oph is a member of the hydrogen-deficient,carbon-rich RCB class of variables. It resembles the warm(Teff~7000 K) RCB stars such as R Coronae Borealis itself.Other RCB stars, such as XX Cam and Y Mus, have experienced similarperiods of inactivity, going decades without significant dust formation.Further observations of V2552 Oph will be of great interest since thereis an opportunity to monitor an RCB star that may be moving fromprolonged inactivity into an active phase of dust production.
| 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.
| 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.
| 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.
| 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 R Coronae Borealis stars - II. Further inferences from the infrared data
The extensive infrared JHKL photometry of R Coronae Borealis (RCB)variables reported in Paper I is further discussed, especially inrelation to the temperature and formation of circumstellar (carbon)particles. Evidence is given for a range of dust temperatures in theshell consistent with the random dust-puff model. The mean dusttemperature is higher when the flux from the dust is increasing thanwhen it is decreasing. This is also consistent with the puff model.There is no evidence from any of the stars that a significant proportionof the dust is at temperatures greater than ~1500K. It is suggested thatthe dust forms at ~1500K above the cool regions of large convectioncells in a quasi-steady Eddington-driven outflow. Dust formation canthen take place relatively close to the stellar surface, as appearsnecessary in order to explain the details of the RCB-type obscurationevents. Enhanced C_2-band absorption seen occasionally at maximum lightin RCB stars also finds a natural explanation in this model. Relateddata on the HdC stars are also discussed.
| The R Coronae Borealis stars - I. Infrared photometry and long-term variations
Extensive JHKL photometry is given for 12 R Coronae Borealis (RCB)stars, covering periods of up to 23 years. Limited infrared observationsof a few other RCB stars and five HdC stars are also reported. Thesedata are used to study the long-term variations of the stars (at J) andof the circumstellar dust (at L). All of the RCB stars show variationsin the flux from the dust on time-scales of from a few hundred days to afew thousand days. Dust flux amplitudes at L of up to 3mag are found,the larger amplitudes being associated with the longer time-scales.Secular variations over about 10000d are also sometimes seen. R CrBitself is atypical in showing evidence of semiregular variations (period~1260d). There is no direct relation between the occurrence ofobscuration minima and changes in the flux from the dust. However, thereis possibly a statistical relation between the dust flux and thefrequency of the obscuration minima. Models involving fixed geometry forthe ejection of dust from the star appear to be ruled out and the datasupport the random dust-puff model. High infrared excesses and highlevels of obscuration activity seem to be associated with a higher thanaverage hydrogen abundance.
| The radial velocity variations of cool hydrogen-deficient carbon stars
We have obtained a series of radial velocities for many of the brightgalactic R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC)stars, in order to characterize the pulsations of these types of coolhydrogen-deficient carbon stars. By comparing these velocities with theresults of long-term photometric monitoring, we investigate thepulsation properties of these objects, the link between pulsations andmass loss, and the relationship between these stars and the higher-T_effhydrogen-deficient stars, the extreme helium (eHe) stars. We find thatmost of the RCB stars have radial velocity and V light amplitudes of10-20kms^-1 and 0.2-0.3 mag, respectively. Pulsationally more-activestars such as RY Sgr are rare. Only one other RCB star, RT Nor, has asimilar photometric amplitude to RY Sgr, but this star has a much lowerradial velocity amplitude. With only one exception, HD 175893, all ofthe HdC stars have a lower pulsation amplitude than the RCB stars. Sincethe RCB and HdC stars appear to be distinguished by the large-amplitudedeclines and infrared excesses of the RCB stars, we suggest that thepulsation amplitude dictates whether significant mass loss occurs inthese objects. The radial velocity-to-light amplitude (RV/V) ratioappears to be temperature dependent, but we suspect that this is not anintrinsic effect. The 7000-K group of RCB stars haveRV/V~50kms^-1mag^-1, which is similar to radially pulsating Cepheids.The similar pulsation properties of the RCB and HdC stars and the coolereHe stars provide further evidence that these types of stars are closelyrelated.
| Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars.
We have constructed line-blanketed model atmospheres for thehydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, aswell as for the similar hydrogen-deficient carbon (HdC) stars and thecool extreme helium (EHe) stars. Improved continuum opacities have beenused together with realistic line absorption data for atomic andmolecular transitions. The observed dereddened fluxes of R CrB arecompared with the calculated model fluxes and found to agree best with amodel effective temperature of 6900K, while the infrared flux methodgives between 6600 and 6900K, depending on the nature of the flux excessin the J and H bands compared to the model fluxes. The excess maycorrespond to a recently formed dust cloud close to the star, with atypical temperature around 2000K and a dust mass of~10^-11^Msun_. The agreement for the ultraviolet fluxdistribution is also very satisfactory as seen from IUE spectra of RCrB.Theoretical broad band photometry is presented and effectivetemperatures of RCrB and HdC stars estimated. The constructed modelsshow a significantly steeper temperature gradient compared to previouslyexisting models as a result of the line opacity. Due to the cool surfaceand high abundance of carbon, molecular bands of e.g. C_2_ and CO arevisible in the spectra even at as high effective temperatures as 7000K.Furthermore, the high temperatures encountered at depth explain theobserved Hei and CII lines for T_eff_ down to ~7000K. In the innerlayers (τ_Ross_ > 3) the models show density inversions relatedto the ionization zone of helium. For certain low gravity models theluminosity exceeds the local Eddington limit and hence gas pressureinversions occur as well, which could be related to the decline eventsof RCrB stars.
| V854 Centauri - the first 3000 days.
Not Available
| 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)
| R coronae borealis variables: A review of observational data
The principal photometric, spectroscopic, and polarimetric observationaldata for R Coronae Borealis variables are reviewed. Some properties ofthese stars are compared with those of other types of variables withsigns of intense mass loss.
| Observations of Variable Stars with the R-Coronae Phenomenon and Other Unique Objects
Not Available
| Faint Companions to UX Antliae
Not Available
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Légy |
| Rektaszcenzió: | 13h05m48.19s |
| Deklináció: | -65°30'46.6" |
| Vizuális fényesség: | 10.424 |
| RA sajátmozgás: | -10.7 |
| Dec sajátmozgás: | 1.1 |
| B-T magnitude: | 11.594 |
| V-T magnitude: | 10.521 |
Katalógusok és elnevezések:
|
