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Публикации по объекту
Chemical composition of A-F type post-AGB candidates
An abundance analysis has been conducted for a sample of ninepost-asymptotic giant branch (AGB) candidate stars; eight of them havenot been explored before. We find four very promising objects likeHD105262, HD53300 and CpD-62° 5428 among them. We find strongevidence of dust-gas separation through selective depletion ofrefractive elements in HD105262. The same effect is also observed inHD53300, CpD-62° 5428 and HD114855 although abundance peculiaritiesare relatively smaller for the last two stars. We find strong enrichmentof nitrogen for HD725, HD842, HD1457, HD9233 and HD61227 but no furtherevidence to support their post-AGB nature. We have compared the observed[N/C] ratios of these stars with the predictions of evolutionary modelswhich include the rotation-induced mixing.
| 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
| SPITZER survey of dust grain processing in stable discs around binary post-AGB stars
Aims: We investigate the mineralogy and dust processing in thecircumbinary discs of binary post-AGB stars using high-resolution TIMMI2and SPITZER infrared spectra. Methods: We perform a full spectralfitting to the infrared spectra using the most recent opacities ofamorphous and crystalline dust species. This allows for theidentification of the carriers of the different emission bands. Our fitsalso constrain the physical properties of different dust species andgrain sizes responsible for the observed emission features. Results: In all stars the dust is oxygen-rich: amorphous andcrystalline silicate dust species prevail and no features of acarbon-rich component can be found, the exception being EP Lyr, where amixed chemistry of both oxygen- and carbon-rich species is found. Ourfull spectral fitting indicates a high degree of dust grain processing.The mineralogy of our sample stars shows that the dust is constituted ofirregularly shaped and relatively large grains, with typical grain sizeslarger than 2 ?m. The spectra of nearly all stars show a high degreeof crystallinity, where magnesium-rich end members of olivine andpyroxene silicates dominate. Other dust features of e.g. silica oralumina are not present at detectable levels. Temperature estimates fromour fitting routine show that a significant fraction of grains must becool, significantly cooler than the glass temperature. This shows thatradial mixing is very efficient is these discs and/or indicatesdifferent thermal conditions at grain formation. Our results show thatstrong grain processing is not limited to young stellar objects and thatthe physical processes occurring in the discs are very similar to thosein protoplanetary discs.Based on observations obtained at the European Southern Observatory(ESO), La Silla, observing program 072.D-0263, on observations made withthe 1.2 m Flemish Mercator telescope at Roque de los Muchachos, Spain,the 1.2 m Swiss Euler telescope at La Silla, Chile and on observationsmade with the SPITZER Space Telescope (program id 3274), which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology under a contract with NASA.Appendices A and B are only available in electronic form athttp://www.aanda.org
| On the Connection Between CWA and RVA Stars
Phase plots of ASAS-3 data of a large number of CWA and RVA variablestars reveal that there is no clear distinction in period or light curveshape between these two types of stars.
| The Chemical Compositions of the Type II Cepheids-The BL Herculis and W Virginis Variables
Abundance analyses from high-resolution optical spectra are presentedfor 19 type II Cepheids in the Galactic field. The sample includes bothshort-period (BL Her) and long-period (W Vir) stars. This is the firstextensive abundance analysis of these variables. The C, N, and Oabundances with similar spreads for the BL Her and W Vir show evidencefor an atmosphere contaminated with 3α process and CN-cyclingproducts. A notable anomaly of the BL Her stars is an overabundance ofNa by a factor of about 5 relative to their presumed initial abundances.This overabundance is not seen in the W Vir stars. The abundanceanomalies running from mild to extreme in W Vir stars but not seen inthe BL Her stars are attributed to dust-gas separation that provides anatmosphere deficient in elements of high condensation temperature,notably, Al, Ca, Sc, Ti, and s-process elements. Such anomalies havepreviously been seen among RV Tau stars which represent a long-periodextension of the variability enjoyed by the type II Cepheids. Commentsare offered on how the contrasting abundance anomalies of BL Her and WVir stars may be explained in terms of the stars' evolution from theblue horizontal branch.
| 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
| Defining the instability strip of pulsating post-AGB binary stars from ASAS and NSVS photometry
We analyse public domain time-series photometric observations of 30known and candidate binary post-AGB stars for measuring pulsation andorbital periods. We derive periodicities for 17 stars for the first timein the literature. Besides identifying five new RV Tauri type pulsatingvariables (three with the RVb phenomenon, i.e. long-term changes of themean brightness), we find multiply periodic (or possibly irregular)post-AGB stars on the two edges of the instability strip. Thetemperature dependence of the peak-to-peak light-curve amplitudesclearly indicates the changes in excitation as post-AGB stars evolvethrough the strip. One object, the peculiar Type II Cepheid ST Pup,showed a period increase from 18.5 to 19.2 d, which is consistent withthe known period fluctuations in the past. In HD 44179, the central starof the Red Rectangle nebula, we see very similar asymmetric light curvethan was measured 10-15 yr ago, suggesting a very stable circumstellarenvironment. In contrast to this, HD 213985 shows coherent but highlynon-repetitive brightness modulation, indicating changes in thecircumstellar cloud on a similar time-scale to the orbital period.
| Condensation temperature trends among stars with planets
Results from detailed spectroscopic analyses of stars hosting massiveplanets are employed to search for trends between abundances andcondensation temperatures. The elements C, S, Na, Mg, Al, Ca, Sc, Ti, V,Cr, Mn, Fe, Ni and Zn are included in the analysis of 64 stars withplanets and 33 comparison stars. No significant trends are evident inthe data. This null result suggests that accretion of rocky material onto the photospheres of stars with planets is not the primary explanationfor their high metallicities. However, the differences between the solarphotospheric and meteoritic abundances do display a weak but significanttrend with condensation temperature. This suggests that the metallicityof the Sun's envelope has been enriched relative to its interior byabout 0.07 dex.
| Keplerian discs around post-AGB stars: a common phenomenon?
Aims.We aim at showing that the broad-band SED characteristics of oursample of post-AGB stars are best interpreted, assuming thecircumstellar dust is stored in Keplerian rotating passivediscs.Methods.We present a homogeneous and systematic study of theSpectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects.The selection criteria to define the whole sample were tuned to coverthe broad-band characteristics of known binary post-AGB stars. The wholesample includes 20 dusty RV Tauri stars from the General Catalogue ofVariable Stars (GCVS). We supplemented our own Geneva optical photometrywith literature data to cover a broad range of fluxes from the UV to thefar-IR.Results.All the SEDs display very similar characteristics: alarge IR excess with a dust excess starting near the sublimationtemperature, irrespective of the effective temperature of the centralstar. Moreover, when available, the long wavelength fluxes show ablack-body slope indicative of the presence of a component of large mmsized grains.Conclusions.We argue that in all systems, gravitationallybound dusty discs are present. The discs must be puffed-up to cover alarge opening angle for the central star and we argue that the discshave some similarity with the passive discs detected around youngstellar objects. We interpret the presence of a disc to be a signaturefor binarity of the central object, but this will need confirmation bylong-term monitoring of the radial velocities. We argue that dusty RVTauri stars are those binaries which happen to be in the Population IIinstability strip.
| 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.
| Detection of Zinc in the Very Metal-Poor Post-AGB Star HR 4049
We report on the detection of two Zn I lines at4722.15Å and 4810.53Å in the high-quality spectrum of thevery metal-poor post-AGB star HR4049, which was obtained with the HighDispersion Spectrograph attached to the Subaru Telescope. The strengthsof these lines indicate an appreciable underabundance of Zn by ~ -1.3dex relative to the Sun. The fact that this volatile element, similarlyto others belonging to the same group (e.g., C, N, O, S), does notconform to the extreme depletion ( > 4 dex) of refractory metals(e.g., Fe), strongly suggests that grain formation has something to dowith the origin of the chemical peculiarity. This (not extremely butsignificantly) subsolar value of [Zn/H] is quantitatively discussed inconnection with those of other volatile species, especially with respectto S. We also detected a new Fe II line at5159.03Å along with the already known Fe II4923.93Å line; based on these two lines the Fe abundance of HR4049is determined to be ~ 2.8 ([Fe/H] ~ -4.7).
| The Cepheids of Population II and Related Stars
The Type II Cepheids include most intrinsic variables with periodsbetween 1 and about 50 days, except for the classical Cepheids and theshortest semiregular variables of type M. The Type II Cepheids may bedivided in groups by period, such that the stars with periods beween 1and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20days (RV Tau class) have differing evolutionary histories. The chemicalcomposition of Type II Cepheids reflects the material they were madefrom as modified by their internal nuclear evolution and mixing.Finally, RV Tau stars are affected by mass loss by dust and speciesattached to the dust. The populations to which the various classes ofType II Cepheids are assigned constitute important clues to the originand evolution of the halo of our Galaxy and the dwarf spheroidal systemsfrom which at least part of the halo seems to have been accreted.
| Photometry and radial velocities of cepheids and other variable stars in the Galaxy and the LMC
UBVRIc and radial velocity measurements are presented for Galactic andLMC Cepheids, and for several variables of other type. The photometrycomprises 168 objects with 1790 phases, and the speedometry 15 objectswith 97 phases.
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| Photoelectric Observations of Southern Cepheids in 2001
A total of 2097 photometric observations in the BVIc systemare presented for 117 Cepheids located in the southern hemisphere. Themain purpose of the photometry is to provide new epochs of maximumbrightness for studying Cepheid period changes, as well as to establishcurrent light elements for the Cepheids.
| Version 2000 of the Catalogue of Galactic Planetary Nebulae
The ``Catalogue of Galactic Planetary Nebulae (Version 2000)'' appearsin Abhandlungen aus der Hamburger Sternwarte, Band XII in the year 2001.It is a continuation of CGPN(1967) and contains 1510 objects classifiedas galactic PNe up to the end of 1999. The lists of possible pre-PNe andpossible post-PNe are also given. The catalogue is restricted only tothe data belonging to the location and identification of the objects. Itgives identification charts of PNe discovered since 1965 (published inthe supplements to CGPN) and those charts of objects discovered earlier,which have wrong or uncertain identification. The question ``what is aplanetary nebula'' is discussed and the typical values of PNe and oftheir central stars are summarized. Short statistics about thediscoveries of PNe are given. The catalogue is also available in theCentre de Données, Strasbourg and at Hamburg Observatory viainternet. The Catalogue is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/843
| 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.
| Chemical Composition and Evolution of Post-AGB Stars
Not Available
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| The carbon Cepheid RT Trianguli Australis: additional evidence of triple-α and CNO cycling
We have used echelle spectra of resolving power 35000 to derive chemicalabundances and the 12C/13C ratio in the 1.9-dcarbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparisonstar. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. Inaddition, C and N are substantially in excess, and a small deficiency inO is present. We interpret these anomalies as resulting from theappearance on the stellar surface of material enriched in 12Cby the 3-α process, followed by CNO cycling to convert12C to 13C and 14N. In addition, some16O has been processed to 14N. The partialprocessing of 16O to 14N indicates thatsubstantial 17O may be present. Proton capture seems to haveenhanced 23Na from the Ne isotopes.
| Using BV Photometry to Distinguish between Type I and Type II Cepheids
We use Johnson BV photometry of Galactic type I and type II Cepheids ina variety of diagrams to see whether the two populations can beadequately separated. The diagrams include the Fourier parametersphi_21, phi_31, phi_41, R_21, R_31, R_41, the loop width in the V-(B-V)plane, the relative phasing of V and B-V extrema, and the amplituderatios of B-V to V, all versus period. We found the R_i1 diagramsuseless in this regard, while a plot of the phasing parameter versusamplitude ratio was best, closely followed by the phi_i1 diagrams. Amajor difficulty was arriving at a sample of Cepheids that could bereliably taken to be definitely type II. We found it necessary to useonly stars that would have been more than 2 kpc from the Galactic planehad they been type I for this sample, and even then it is uncertainwhether such overlap between types as we find in our diagrams is due toinadequacy of the tests or is due to type I stars' having contaminatedour type II sample. We point out evidence strongly suggesting that typeI stars can be found at least up to 2 kpc from the plane. Selectioneffects in our sample of type II Cepheids resulted in only one BLHerculis star (short-period type II) being included, so our conclusionsbasically apply only to W Virginis stars.
| The RV\ Tauri phenomenon and binarity
We present accurate radial velocity measurements on the pulsatingextremely iron-deficient post-AGB object HD 52961 and the RV Tauri starEN TrA (HD 131356) proving them to be binaries. Our long-termphotometric monitoring campaign shows that the RV Tauri photometricclass ``b'' phenomenon in HD 52961 is due to variable circumstellarextinction during orbital motion. By comparing carefully theobservational characteristics of RV Tauri stars and the class ofextremely iron-deficient post-AGB objects we conclude that binarity is awidespread phenomenon in the RV Tauri class of objects. The observedchemical depletion patterns, weak circumstellar CO emission, peculiarspectral energy distribution and the difference in photospheric class ofthe RV Tauri objects can all be naturally explained by assuming that thecircumstellar material is not freely expanding, but trapped in thebinary system. Based on observations collected at the European SouthernObservatory (proposals codes 51.7-0052; 51.7-0053; 52.7-0048; 58.E-0462;59.E-0432; 61.E-0426); with the Swiss telescopes at ESO and OHP and theAPT telescope at Mt. Hopkins
| IRAS Detections of Metal-poor Red Giants
A number of relatively bright metal-poor red giants from the HD and BDcatalogs are found to have been detected by the IRAS satellite. Data forthese stars have been retrieved from the IRAS Point Source Catalog (PSC)and/or the Faint Source Catalog (FSC). The majority of metal-poor giantsin these samples fall along relatively well-defined sequences in plotsof V-[12] versus B-V and V-I; for these stars, the 12 μm fluxdetected is presumed to arise from the photosphere. Only a subset ofstars detected at 12 μm were detected at 25 μm these are displayedin a plot of [12]-[25] versus V-[12]. There are a small number of giantsthat exhibit notable 12 and/or 25 μm excesses relative to the meansequences defined by the bulk of the sample. Those stars with the mostunambiguous evidence for infrared excesses are variable stars, eitherlong-period or semiregular variables or RV Tauri stars. As such, thosestars exhibiting infrared excesses in the metal-poor giant sample arelikely in the asymptotic giant branch (AGB) or post-AGB phase ofevolution. There is no clear evidence for nonvariable first-ascent redgiants having been detected among the infrared-excess stars. In fact,some metal-poor red giants known to exhibit outflows in theirchromospheres do not show infrared excesses. A Population II starascending the red giant branch for the first time appears to have toolow a mass-loss rate to be recognizable as an infrared-excess star inthe IRAS PSC or FSC.
| Circumstellar C_2_, CN, and CH^+^ in the optical spectra of post-AGB stars.
We present optical high-resolution spectra of a sample of sixteenpost-AGB stars and IRC +10216. Of the post-AGB stars, ten show C_2_Phillips (A^1PI_u_-X^1SIGMA^+^_g_) and Swan(d^3PI_g_-a^3PI_u_) and CN Red System(A^2PI-X^2SIGMA^+^) absorption, one CH^+^(A^1PI-X^1SIGMA^+^) emission, one CH^+^absorption, and four without any molecules. We find typically T_rot_~43-399, 155-202, and 18-50K, logN~14.90-15.57, 14.35, and15.03-16.47cm^-2^ for C_2_, CH^+^, and CN respectively, and0.6<=N(CN)/N(C_2_)<=11.2. We did not detect isotopic lines, whichplaces a lower limit on the isotope ratio of ^12^C/^13^C>20. Thepresence of C_2_ and CN absorption is correlated with cold dust(T_dust_<=300K) and the presence of CH^+^ with hot dust(T_dust_>=300K). All objects with the unidentified 21?m emissionfeature exhibit C_2_ and CN absorption, but not all objects with C_2_and CN detections exhibit a 21?m feature. The derived expansionvelocity, ranging from 5 to 44km/s, is the same as that derived from COmillimeter line emission. This unambiguously proves that these lines areof circumstellar origin and are formed in the AGB ejecta (circumstellarshell expelled during the preceding AGB phase). Furthermore there seemsto be a relation between the C_2_ molecular column density and theexpansion velocity, which is attributed to the fact that a higher carbonabundance of the dust leads to a more efficient acceleration of the AGBwind. Using simple assumptions for the location of the molecular linesand molecular abundances, mass-loss rates have been derived from themolecular absorption lines and are comparable to those obtained from COemission lines and the infrared excess.
| Abundance Analyses of Field RV Tauri Variables. III. DY Aquilae, SS Geminorum, CT Orionis, and CE Virginis
Analyses of the photospheric compositions of the four field RV Tauristars, DY Aql, SS Gem, CT Ori, and CE Vir, indicate that to varyingdegrees they have experienced fractionation processes that havepreferentially depleted their atmospheres of elements with highcondensation temperatures. This corroborates our previous studies, whichshowed similar patterns in five other field RV Tauri stars. Two stars inour sample, DY Aql and CE Vir, were found to have strong Li I resonancelines, with corresponding lithium abundances near log epsilon (Li) =0.0; this is not significantly different from that found in less evolvedM giants. These are also the coolest stars displaying a correlation ofphotospheric abundances with condensation temperatures.
| Abundance Analyses of the Field RV Tauri Variables: EP Lyrae, DY Orionis, AR Puppis, and R Sagittae
Analyses of the photospheric compositions of the four field RV Tauristars, EP Lyr, DY Ori, AR Pup, and R Sge, indicate that to varyingdegrees they have experienced fractionation processes that havepreferentially depleted their atmospheres of elements with highcondensation temperatures. The depletion, as indicated by, for instance,[S/Fe], is greatest for DY Ori, [S/Fe] = 2.5, and least for R Sge,[S/Fe] = 0.9. The initial composition, presumably indicated by thesulfur abundance, was nearly solar for AR Pup, R Sge, and DY Ori, whileit was about 0.6 dex less than solar for EP Lyr. This implies that theRV Tauri stars as a group may not be as metal-poor as previouslythought---they are instead "metal-depleted." The field RV Tauri's arenot halo stars, but probably belong to the thick disk. This brings toseven the number of type II Cepheids that show such a trend; the otherthree are IW Car and V1 in omega Cen, RV Tauri stars, and ST Pup, a WVirginis star. The 12C/13C ratios for EP Lyr and DY Ori are 9 +/- 1 and6 +/- 3, respectively, indicating that CN-cycled material has been mixedwith their surface layers. This is consistent with the general consensusthat RV Tau stars are in a post-AGB evolutionary stage. There is alsoevidence that EP Lyr has a stellar mass companion, but additionalobservations are required to calculate an orbit; hence, EP Lyr could bea link to the group of metal-depleted, high-latitude A--F supergiants,all of which are binaries.
| Chemical composition of optically bright post-AGB stars.
We present a detailed LTE chemical analysis of 10 optically brightF-type post-AGB objects on the basis of the analysis of high-resolutionoptical spectra and compare the results with similar objects discussedin the literature. The iron content is low on average, and so confirmsthe old and hence low-mass nature of the supergiants, with a noticableexception of HD 95767. We emphasize the fact that the chemical patternsobserved are very diverse : several different classes can bedistinguished. Only a minor fraction of the objects are conform tostandard post third dredge-up theory. Only in HD 187885 (Van Winckel etal., 1996A&A...306L..37V), HD 56126 (Klochkova, 1995MNRAS.272..710K)and HD 158616 (this paper) is there conclusive chemical evidence thatthey occur in a post-AGB evolutionary phase : a high total CNOabundance, for HD 187885 a supersolar He content and-above all-a largeoverabundance of s-process elements. The other objects, together withother well studied high galactic latitude F-supergiants, display nos-process enhancement but even depletion in some cases. The high Nabondance and the mildly enhanced total CNO abundance indicate that theatmospheres of these objects contain a mixture of CNO-cycled materialand He-burning products. For some sources, however, this enhancement ofthe total CNO abundance is barely significant. HD 107369, the onlyobject in our sample with neither Hα emission nor observed IRexcess, displays also unique chemical patterns among our sample stars (aC deficiency coupled with a moderate Fe depletion of [Fe/H]=-1.1). Thisstar is the only object in our sample showing similar chemical patternsto the metal poor B stars at high galactic latitude (Conlon et al.,1993, in ASP Conf. Ser., Vol. 45, p. 146). Our chemical analysis doestherefore not point to an evolutionary connection between the dustyhigh-latitude supergiants and the metal-poor B stars, but rathersuggests that the latter evolve from stars such as HD 107369.
| New and misclassified Planetary Nebulae
Altogether 86 objects discovered in the period 1991-1994 have beenaccepted by the author as new PN (Table 1). In Appendices A and B tothis table the lists of possible pre-PN as well as of possible post-PN,respectively, are given. Table 2 presents 6 misclassified objects whichwe suggest removing from CGPN.
| The carbon Cepheid V553 Cen: evidence of triple-alpha and CNO cycling
We have obtained high-resolution spectroscopic observations of V553 Cen,a short-period type II Cepheid. We have confirmed the findings ofprevious investigators that V553 Cen is C- and N-rich. For the firsttime, we show that V553 Cen is also O-poor and Na-rich, and that^12C/^13C=4+/-1. We do not see s-process element enhancements in thisstar. When compared to historical observations, our radial velocitymeasurements indicate that V553 Cen does not have a binary companion.Hence the abundance peculiarities we observe on the surface are due toV553 Cen altering its own composition by nuclear reaction sequences anddredge-up episodes. These results imply that V553 Cen has experiencedCN, ON and triple-alpha processing, and has mixed the products of thesereactions to its surface. The chemical composition of V553 Cen and thekinematics of the carbon Cepheids indicate that they are of the samepopulation as early R stars. The evolutionary processes that haveproduced such objects are not known, but are suspected to be due tohelium core flashing in low-mass stars of normal metallicity, assuggested by both Lloyd Evans and Dominy.
| ST Pup: a binary Type II Cepheid with a peculiar chemical composition
Analysis of recent photometric observations of the W Vir star ST Pupreveals that it has experienced one sudden period change in the last 35years. This is in sharp contrast to its erratic behaviour in the firsthalf of this century when the period changed much more frequently.Application of the surface brightness version of the Baade-Wesselinkmethod to the observations yields M_v=-3.86+/-0.2. Using spectroscopicobservations spanning almost 7yr, we have discovered that ST Pup is asingle-lined spectroscopic binary. The orbital period, 410.4+/-2.9d, isconsiderably longer than that of the other three known binary Type IICepheids, which average about 100d. It has a very low orbitaleccentricity, implying that orbital circularization and mass transferhave already taken place in this system. We estimate that[Fe/H]=-1.47+/-0.13 from a local thermodynamic equilibrium (LTE) fineanalysis of high-resolution spectra obtained near minimum light. Thevalue of C/O is 0.3, demonstrating that ST Pup is not a carbon star eventhough it displays strong CH and CN bands near minimum light. Most ofthe elements show an abundance pattern that correlates with condensationtemperature similar to that of diffuse interstellar clouds and peculiarhigh-latitude A-F supergiants, which have been interpreted as low-masspost-AGB stars that have undergone selective mass-loss via grainformation. However, the Ca/Ti abundance ratio in the atmosphere of STPup is too high to be consistent with this interpretation. The Caanomaly may have been caused by overionization of Ca^+ by Lyalphaphotons when the star was losing mass. An infrared excess is seen at theJHKL and IRAS bands, indicating the presence of dust spanning a largerange in distance from ST Pup.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Корма |
| Прямое восхождение: | 06h48m56.41s |
| Склонение: | -37°16'33.3" |
| Видимая звёздная величина: | 9.901 |
| Собственное движение RA: | -2.4 |
| Собственное движение Dec: | 3.9 |
| B-T magnitude: | 10.6 |
| V-T magnitude: | 9.959 |
Каталоги и обозначения:
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