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.
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Chemical composition of evolved stars of high galactic latitude
We have carried out abundance analysis for a sample of high galacticlatitude supergiants in search of evolved stars.We find that HD 27381 has atmospheric parameters and an abundancepatternvery similar to that of the post-AGB star HD 107369.HD 10285 and HD 25291 are moderately metal-poor andshow the influence of mixing that has brought the productsof NeNa cycle to the surface.The high galactic latitude B supergiant HD 137569 shows selectivedepletion of refractory elements normally seen in post-AGB stars.We find that the high velocity B typestar HD 172324 shows moderate deficiency of Fe group elements butthe CNO abundances are verysimilar to that of disk B supergiants. The observed variations inthe radial velocities, transient appearance of emission componentsin hydrogen line profiles and doublingof O I lines at 7774 Å support the possibility of this star beinga pulsating variable or a binary star.
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The Post-AGB Star HD 101584
High resolution spectra of a post-AGB star HD 101584 were analyzed.Profiles and radial velocities of various emission and absorption lineswere measured. The primary star of the system hasTeff=8500±1000 K and logg=2.0+0.5-1.0. The abundances of C, N ,O, Na andMg are close to the solar values but Si is underabundant. The dustshell surrounding the system is modeled.
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A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars
A new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram.
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An analysis of the optical spectra of the post-asymptotic giant branch stars LSIV -12 111 and HD 341617
High spectral resolution and signal-to-noise observations of theabsorption and emission line spectra in two post-asymptotic-giant-branch(PAGB) stellar candidates, LSIV -12 111 and HD 314617 are discussed. Theabsorption line spectra have been analysed using non-LTE modelatmosphere techniques to determine stellar atmospheric parameters andchemical compositions, both in absolute terms and relative to a standardstar, HD 13841. The atmospheric parameters differ from previousestimates based on LTE model atmospheres, probably due to non-LTEeffects. In turn these imply stellar masses that are generally largerthan have been previously estimated. Both PAGB candidates have relativeuniform underabundances of metals with mean values of -0.35 dex for LSIV-12 111 and -0.50 dex for HD 314617.Furthermore, their abundance patterns are remarkably similar to thatobserved for optically bright, F-type PAGBs. From the emission spectra,the plasma parameters and angular size of the circumstellar material areconstrained, and these are consistent with previous estimates and with aPAGB evolutionary stage.
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High-resolution Keck I spectroscopy of Galactic halo post-asymptotic giant branch stars
Absolute and differential abundance analyses have been performed fromhigh-resolution, high signal-to-noise ratio optical (Keck I) spectra forthree evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 andLS IV -04 01. Their derived atmospheric parameters indicate that allthree objects are undergoing a post-asymptotic giant branch (post-AGB)phase of evolution. A differential abundance analysis reveals HD 341617as having a mild carbon deficiency of 0.74 dex, possibly due to the starhaving evolved off the AGB before the onset of the third dredge-up.Although such carbon underabundances are typical of hot post-AGBobjects, the same trend is not observed in PG 1704 + 222, where thecarbon abundance is found to be consistent with those derived fornitrogen and oxygen. Hence, a dredge-up scenario need not be invoked toexplain the chemical composition of PG 1704 + 222. For LS IV -04 01 noiron deficiency is apparent relative to magnesium and silicon, and hencea gas-dust separation event in the AGB progenitor need not be invokedfor this star.
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Atmospheric abundances in post-AGB candidates of intermediate temperature
Detailed atmospheric abundances have been calculated for a sample of -Gsupergiant stars with IR fluxes and/or high galactic latitudes. HD172481 and HD 158616 show clear indications of being post-AGB stars thathave experienced third dredge-up. HD 158616 is carbon-rich while theabundance pattern of HD 172481 and its large Li enhancement givessupport to the hot bottom burning scenario that explains paucity ofcarbon-rich stars among AGB stars. HD 172324 is very likely a hotpost-AGB star that shows a strong carbon deficiency. HD 725, HD 218753and HD 331319 also appear to be evolved objects between the red giantand the AGB. HD 9167, HD 173638 with a few exceptions, reflect solarabundances and no signs of post red giant evolution. They are mostlikely young massive disk supergiants. Further analysis ofproto-Planetary Nebula HDE 341617 reveals that He lines show signs ofvelocity stratification. The emission lines have weakened considerablysince 1993. The envelope expands at 19 km s-1 relative to thestar. Atmospheric abundances, evolutionary tracks and isochrones areused to estimate masses and ages of all stars in the sample. Based onobservations obtained at the Haute-Provence Observatory, France. Tables11 and 12 are only available in electronic form athttp://www.edpsciences.org
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The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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Winds in the atmospheres of central stars of Planetary Nebulae (Invited Review)
Not Available
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Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
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A study of dust shells around high latitude supergiants
A sample of 22 stars with infrared excess emission and many of whichhave supergiant-like spectra was surveyed in the submillimeter continuumat 438, 761 and 1100μm using the James Clerk Maxwell Telescope (JCMT,Mauna Kea, Hawaii). Most of the sources are at high galactic latitudesbut stars with similar properties at lower galactic latitudes are alsoincluded. For 16 sources it is believed that they are post AsymptoticGiant Branch stars (post-AGB stars) and 6 of them are detected. The restof the sample consists of 2 binaries, 1 Herbig Ae star, 1 Luminous BlueVariable and 2 sources for which the post-AGB nature is uncertain. Theobservations are combined with optical, near infrared and IRAS data andare fitted with a spherically symmetric and optically thin dust shellmodel. It is found that a large fraction of the excess emissionoriginates from relatively hot dust near the star. Only 2 sources lacksuch a hot dust component. For all stars with a hot dust component itwas found that the emission longward of 60μm cannot be explained withonly one dust shell. Several possible explanations are discussed and itis concluded that the extra excess at far IR and submm wavelengths iscaused by a second colder and thus more distant dust component. For thepost-AGB stars this component is believed to be the remnant AGB shell.The relatively low stellar temperatures (<10,000 K) in combinationwith the relatively large ages of the remnant AGB shell (>10^3^ yr)confirm earlier suggestions that many of these stars are very slowlyevolving towards higher temperatures, as expected for low mass post-AGBstars. Evidence was found that stars may stop losing mass and evolve offthe AGB at temperatures below 5,000 K (as assumed for the Schoenbernertracks) causing a slower evolution towards higher temperatures.
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Catalogue of Population II A-F supergiants - UU Herculis and related stars
The catalogue of Population II A-F supergiants collects the availableinformation on 18 UU Herculis type and related stars. For each entry,the following information is presented: star name, equatorial andgalactic coordinates, spectral type, mean magnitude V, color index B-V,color excess E(B-V), metallicity Fe/H, absolute magnitude MV, effectivetemperature Te, and gravity log g. A list of six suspected UUHerculis-type stars is also presented.
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POST AGB candidates - Selection and IR properties
The properties of a group of stars is examined, that are selected on thebasis of the properties of two known post-AGB stars, HR 4049 and HD213985. It is shown that the stars from this sample that probably arepost-AGB stars all show IR excess. This excess can be due to hot (T =1000 K), cool (T = 300 K) or hot and cool dust. The stars showing thehot dust all have mass loss rates that are substantially larger than themass loss rates of the stars showing only cool dust. This indicates thatmass loss continues after the AGB at a substantial rate (about 10 to the-7th solar mass/yr). Selection criteria are given on the basis of whichpost-AGB stars can be selected from the literature.
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New UBVRI photometry for 900 supergiants
A description is presented of the results obtained in connection with asystematic program of supergiant photometry on the Johnson UBVRI system.During the eight years after the start of the program, almost 1000 starshave been observed, about 400 three or more times each. The originalselection of stars used the spectral type catalog of Jaschek et al.(1964) to choose supergiants. Since observations were possible from bothChile and Canada, no declination limits were imposed, and no particularselection criteria were imposed other than to eliminate carbon stars.These are so red as to require enormous extrapolations of thetransformation equations.
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Prediction of spectral classification from photometric observations - Application of the UVBY beta photometry and the MK spectra classification. II - General case
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&A....85...93M&db_key=AST
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Prediction of spectral classification from photometric observations-application to the UVBY beta photometry and the MK spectral classification. I - Prediction assuming a luminosity class
An algorithm based on multiple stepwise and isotonic regressions isdeveloped for the prediction of spectral classification from photometricdata. The prediction assumes a luminosity class with reference touvbybeta photometry and the MK spectral classification. The precisionattained is about 90 percent and 80 percent probability of being withinone spectral subtype respectively for luminosity groups I and V and forluminosity groups III and IV. A list of stars for which discrepanciesappear between photometry and spectral classification is given.
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Detection of errors in spectral classification by cluster analysis
Cluster analysis methods are applied to the photometric catalogue ofuvby-beta measurements by Hauck and Lindemann (1973) and point out 249stars the spectral type of which should be reconsidered or thephotometric indices of which should be redetermined.
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Spectrum Variations in A-Type Supergiants
Abstract image available at:http://adsabs.harvard.edu/abs/1970ApJ...162..547R
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A Catalog of Magnetic Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958ApJS....3..141B&db_key=AST
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Some high-velocity Population I stars.
Not Available
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A High-Velocity Supergiant, HD 172324
Not Available
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Revised Standards for Supergiants on the System of the Yerkes Spectral Atlas.
Not Available
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INHABITED SKY
