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TASS Mark IV Photometric Survey of the Northern Sky
The Amateur Sky Survey (TASS) is a loose confederation of amateur andprofessional astronomers. We describe the design and construction of ourMark IV systems, a set of wide-field telescopes with CCD cameras thattake simultaneous images in the V and IC passbands. Weexplain our observational procedures and the pipeline that processes andreduces the images into lists of stellar positions and magnitudes. Wehave compiled a large database of measurements for stars in the northerncelestial hemisphere with V-band magnitudes in the range 7
| 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.
| 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.
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| Polarimetry of evolved stars. III. RV Tau and R CrB stars
We present broadband optical polarimetry, and broadband optical andinfrared photometry, of eight RV Tau-type and five R CrB-type stars;much of the photometry and polarimetry was obtained simultaneously. Fornine of the objects polarimetric data is reported for the first time. Wehave estimated and subtracted the interstellar component ofpolarization, allowing us to determine the level of intrinsicpolarization. In some cases this is =~ 1%-2% even when the star is in abright photometric state. We consider this to be evidence for thepresence of permanent clumpy non-spherical dust shells around the RV Tauand R CrB-type stars we observed. Our polarimetric and photometric datalead us to conclude that, for most of our programme stars, neutralextinction must be significant in their circumstellar envelopes. Apartfrom the brightness variations due to pulsations and changes in theeffective temperature of stars, there is clear evidence ofwavelength-independent flux variations - with amplitude from 0fm 5 to1fm 0 - implying the presence of large (a>~ 0.15 mu m) dustparticles. Rapid ( ~ 2 hours) evolution of the infrared fluxdistribution at the level of ~ 0fm 6 in the JHKL bands was detected inthe RV Tau star R Sct.Table 2 is only available in electronic form at the CDS via anomymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/405Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
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| 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).
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| Aus der Sektion "Halb-und Unregelmassige".
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| The Milton Bureau Revisited
Under the direction of Cecilia Payne-Gaposchkin and Sergei Gaposchkin, aprogram was subsidized by the Milton Fund of Harvard Observatory in 1937for the study of all variable stars then known to be brighter than tenthphotographic magnitude at maximum. This included some 1512 stars forwhich a grand total of 1,263,562 estimates of magnitude were made,ranging from a low of 16 (except for a few novae) to 4084 observationsper star. The sky had been divided into 54 fields, and the results ofthe measurements presented field by field in two volumes of the Annalsof Harvard Observatory. Then, in another volume, the results werediscussed in four sections, each dealing with a particular class ofvariable: 1, those of RV Tauri type; 2, the eclipsing variables; 3,Cepheids and RR Lyrae variables, and 4, the red variables, especiallyMira-type and semiregular variables.For the present paper, many of these results have been compared withmodern determinations in the 1985-87 version of the "General Catalogueof Variable Stars (GCVS)". In particular, there are numerous instancesof disagreement as to whether a star should be classified RV or SR.Although there are many instances where the Milton Bureau determinationsof types of variability differ from the types given in moderncatalogues, the reasons for the differences are generallyunderstandable.For 17 RV Tauri type stars in this survey multiple periods have now beendetermined. Many of these still deserve continued observations in orderto ascertain the constance of the periods and improve the accuracy oftheir longest reported periods.
| Chemical Composition and Evolution of Post-AGB Stars
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| Spectral Characteristics of RV Tauri Stars
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| Abundance Analyses of Field RV Tauri Stars. V. DS Aquarii, UY Arae, TW Camelopardalis, BT Librae, U Monocerotis, TT Ophiuchi, R Scuti, and RV Tauri
Abundance analyses are presented and discussed for eight RV Taurivariables. The RVB star UY Ara shows the abundance anomalies seen inother RVB stars, namely, elements that condense into grains at hightemperature are underabundant, but elements of low condensationtemperature are much less underabundant. This pattern is ascribed to aseparation of dust from gas with accretion of gas but not dust by theatmosphere. Abundances for two RVC stars with earlier results for otherRVC stars show that these intrinsically metal-poor stars do not showeffects of dust-gas separation. Analyses of five RVA stars show thatthese cooler stars are very largely unaffected by dust-gas separation.It is proposed that the deeper convective envelope of cooler starsdilutes anomalies resulting from dust-gas separation. Possible sites fordust formation and dust-gas separation-the dusty wind off the RV Taurivariable or a dusty circumbinary disk-are reviewed and observationaltests suggested.
| Abundance Analyses of Field RV Tauri Stars. IV. AD Aquilae, DS Aquarii, V360 Cygni, AC Herculis, and V453 Ophiuchi
Abundance analyses are presented and discussed for five RV Taurivariables. Three stars-DS Aqr, V360 Cyg, and V453 Oph-are RV C stars byspectroscopic classification, i.e., metal lines are weak. They are shownto be metal poor with [Fe/H] from -1.0 to -2.2 with normal relativeabundances of other elements. By contrast, AD Aql and AC Her are RV Bstars with an odd abundance pattern: elements that condense into grainsat a high temperatures are underabundant (i.e., [Fe/H] = -2.1 for ADAql) but elements with a low condensation temperatures are much lessunderabundant (i.e., [S/H] = 0.0 and [Zn/H] = -0.1 for AD Aql). Thisabundance pattern is ascribed to a separation of dust and gas in theupper atmosphere of the star. The present analyses with previouslypublished results are used to investigate the systematics of thedust-gas separation in RV Tauri variables. The process is apparentlyinoperative in stars with an initial metallicity of about [Fe/H] <~-1.0 RV C stars and similar variables in globular clusters are immune tothe dust-gas separation. The process achieves more severe effects in RVB than in RV A stars. The strength of the abundance anomalies attributedto dust-gas separation is not correlated with reported infraredexcesses. After correction for the effects of the dust-gas separation,there is no strong evidence from the abundances that evolution along theAGB and experience of the third dredge-up preceded the formation of themajority of the RV Tauri variables.
| No RV Tauri Stars in Globular Clusters?
The light curves of six globular cluster stars, the ones usuallyclassified as RV Tauri variables, were investigated in order to checkthe validity of the classification. Period analysis has shown -- as faras the poor quality of data allowed -- that these stars do not generallydisplay the formal period in their power spectra, and the ratio of theamplitudes of the existing frequencies is very different from thosefound in field RV Tauri stars. Their light curves do not support theclassification either, ie. the alternating minima are missing.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Stellar evolution of low and intermediate-mass stars. III. an application of evolutionary post-AGB models: the variable central star FG Sagittae.
Based on a set of evolutionary calculations for thermally pulsatingpost-AGB models we introduce a robust method to measure FG Sge's masswhich was found to be 0.61Msun_. The correspondingevolutionary timescale is consistent with the expansion age of theplanetary nebula. Assuming that FG Sge's surface was already enrichedwith heavy elements on the AGB we propose that during the central-starevolution these elements were removed from the superficial radiativelayers due to, e.g., dust/gas separation, leaving the deeper convectivelayers unchanged. During the flash the outward moving envelopeconvection mixed the stored heavy elements back to the surface.
| 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.
| The Nature of the Period Changes in RV Tauri Stars
We have studied the period changes in 15 RV Tauri stars, using theEddington- Plakidis hypothesis, which assumes that the (O-C) diagramscan be interpreted as a superposition of random errors in the measuredtimes of minimum, and random cycle-to-cycle fluctuations in the period.Except for three stars for which the data are sparse, the hypothesisfits the (O-C) data very well, suggesting that the assumptions arecorrect. The magnitude of the random period fluctuations does not appearto correlate with any obvious physical property of stars. (SECTION:Stars)
| 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.
| RV Tauri and the RVB phenomenon. I. Photometry of RV Tauri.
I present new photoelectric observations of RV Tauri. Period analysisrevealed that the amplitude of the long period variation has decreaseddrastically since the start of the century while the amplitude of the RVTauri-like variation remained practically the same. The change in theshort period shows the same kind of cyclic O-C diagram as the othervariables of this class. While a pulsational origin seems unlikely inthe case of the long term variation, its decreasing amplitude indicatesthat simple binarity could not be used in modelling the RVB phenomena.
| The nature of the period changes in RV Tauri stars.
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| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Near-infrared spectroscopy of post-AGB stars.
The results of a medium resolution near-infrared spectral survey of 18post-AGB candidate stars are presented. Most of the stars havenear-infrared hydrogen lines in absorption, which is normal for theirspectral types. Three stars, HD 101584, HD 179821 and HD 170756 have theCO first overtone bands in emission; in two of these the emission isvariable. It is suggested that the CO emission is the result of post-AGBmass loss. HD 52961 shows the rarely occurring 3.5μm emission featureand is only the fourth object in which this feature has been detected.
| Photometry of EP Lyrae and period changes in RV Tauri stars.
The RV Tauri variable EP Lyrae was observed between 1985 and 1992 in theUBV-system. The star, like AC Herculis, has a relatively regular lightcurve. Its period is rather stable during most of the observations, itscurrent value is 83.46d EP Lyrae is classified as an RVB variable, i.e.its mean brightness is supposed to vary. It is shown that thisclassification is not correct, the mean brightness of the star isconstant. The question of period changes in RV Tauri stars is discussedand a possible relation between colour and O-C amplitude is found. Thedifficulties with the usually assumed post-AGB evolutionary status ofthese variables are also highlighted.
| Photometry of yellow semiregular variables - AC Herculis, R Sagittae and V Vulpeculae
Three RV Tauri variables - AC Her, R Sge, and V Vul - have been observedphotoelectrically since 1984. Their current periods are 75.48 d for ACHer, 69.74 d for R Sge, and 75.35 d for V Vul. The light curves of ACHer and R Sge are quite similar and show the usual RV Tauri-like form.The secondary minima of V Vul, however, is very shallow. Period analysisrevealed the presence of the fundamental period and its harmonics (twoor three). All of the three stars show cyclic period changes, the causeof which is still unknown. The expected long-term period change is notdetected in R Sge and V Vul, but this is probably the result of the fewobserved cycles in the O-C diagram. Frequency clustering around one ortwo principal frequencies is found in all three stars. Possible causesfor this phenomenon, such as amplitude or period changes, are suggested.The question of the pulsation period of RV Tauri stars is alsodiscussed, the formal period being the preferred choice. Since alltheoretical models fail to reproduce the periods of RV Tauri variables,there is no possibility to compare the observed properties of thesestars to model predictions. Some discrepancies in the derived propertiesof AC Her are pointed out, too.
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Observation and Astrometry data
| Constellation: | Sagitta |
| Right ascension: | 20h14m03.74s |
| Declination: | +16°43'35.0" |
| Apparent magnitude: | 9.202 |
| Proper motion RA: | -2.4 |
| Proper motion Dec: | -2.7 |
| B-T magnitude: | 10.459 |
| V-T magnitude: | 9.306 |
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