Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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
| Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields
We present a sample of 1777 bright (91.0) metal-poor([Fe/H]<-2.0) giants of 9%+/-2%, which is lower than previouslyreported. However, the frequency rises to similar (>20%) and highervalues with increasing distance from the Galactic plane. Although thenumbers of stars at low metallicity are falling rapidly at the lowestmetallicities, there is evidence that the fraction of carbon-enhancedmetal-poor stars is increasing rapidly as a function of decliningmetallicity. For ~60 objects, high-resolution data have already beenobtained; one of these, HE 1327-2326, is the new record holder for themost iron-deficient star known.
| 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.
| V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity
This paper reports the detection of a heavy element enriched RV Taurivariable with an abundance pattern that differs significantly from astandard s-process enriched object: V453 Oph. Basedon optical high-resolution spectra, we determined that this object oflow intrinsic metallicity ([Fe/H]=-2.2) has a mild, but significant,enrichment ([s/Fe]˜ + 0.5) of heavy elements for which thedistribution points to slow neutron capture nucleosynthesis. This resultis strengthened by a comparative analysis to the non-enriched RV Tauristar DS Aqr ([s/Fe]= 0.0). Although V453 Oph is thefirst RV Tauri star showing a strong s-process signature, it is notaccompanied by C enhancement, challenging our current nucleosyntheticmodels of post-AGB stars that predict a simultaneous enrichment in C ands-process elements. The low N abundance excludes CN cycling as beingresponsible for the low C abundance. We explore three differentscenarios to explain the heavy element distribution in this evolvedobject: an enrichment of the parental cloud, an accretion scenario inwhich the chemical patterns were acquired by mass transfer in a binarysystem and an intrinsic enrichment by dredge-up.
| Abundance Analyses of Field RV Tauri Stars. VI. An Extended Sample
An abundance analysis is presented and discussed for a sample of 14 RVTauri stars. The present abundance data and those from our previouspapers and by other workers are combined in an attempt to furtherunderstanding of the dust-gas separation process that afflicts many RVTauri variables. We propose that a star's intrinsic (i.e., initial)metallicity is given by the photospheric zinc abundance. Variableswarmer than about 5000 K and with an initial metallicity [Fe/H]>=-1are affected by dust-gas separation. Variables of all metallicities andcooler than about Teff~=5000 K are unaffected by dust-gasseparation. The RV Tauri variables show a spread in their C abundances,with the lower boundary of the points in the C versus Zn plane fallingclose to the predicted trend for giants after the first dredge-up. Theupper boundary is inhabited by a few stars that are carbon-rich. The Oabundances in the mean follow the predicted trend from unevolved stars,in line with the expectation that photospheric O abundance is unaffectedby the first dredge-up. An evolutionary scenario involving mass loss bya first-ascent or early-AGB red giant, the primary star of a binary, issketched.
| The evolutionary status of the bright high-latitude supergiant HD 190390
Despite its mean apparent magnitude of mV = 6.39, theevolutionary status of HD 190390 (HR7671), a luminous F-type supergiant at high galactic latitude,is still not very clear, but in most papers a post-AGB classification isassumed. New observational material has been obtained with fourdifferent instruments and is presented here. An extensive abundanceanalysis based on high resolution, high signal-to-noise NTT+EMMI spectraconfirms the metal deficiency of this object ([Fe/H] = -1.6), togetherwith a high lithium content (log ɛ(Li) = 1.9). A variabilityanalysis based on Geneva photometry over seven years reveals beatingwith a period of ~3000 days. It is, however, not clear whether thisbeating is caused by a stable triplet, or it is the consequence of smallchanges in the main frequency. More recent data obtained with theHIPPARCOS satellite and the Mercator telescope not only confirm the mainperiod, but also support the presence of a second periodicity of 11days, which was also found in the Geneva photometry. A conclusiveevolutionary status of this object is not given, but alternative to theUU Her (i.e. post-AGB) status, a W Vir classification is discussed.Based on observations collected at the European Southern Observatory, LaSilla, Chile (programme 61.E-0426), and at the Observatorio del Roque delos Muchachos, La Palma, Spain.
| s-process nucleosynthesis in very metal-deficient post-AGB stars
In this contribution, we present a detailed abundance study of a starthat challenges our current understanding of the s-process: V453 Oph.This RV Tauri star of low intrinsic metallicity displays simultaneouslymild s-process overabundances and a low C content. The remarkablepattern is confirmed and strengthened in an analysis relative toanother, very similar RV Tauri star without s-process enrichment: DSAqr. Two nucleosynthetic models have been explored to explain theabundances of V453 Oph. In a radiative model, s-process elements aregenerated in the radiative zone in the He-rich intershell during theinterpulse phase, while in the convective model, the s-process elementsare produced in convective conditions during the thermal pulse only.Both models succeed in explaining the mild s-process overabundances, butonly the convective model is able to produce s-process elements withoutenhancing the carbon content significantly.Based on observations collected at ESO, La Silla, Chile (programme67.D-0054)
| 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
| 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).
| 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.
| 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.
| Accurate Positions Of Variable Stars Near The South Galactic Pole
Not Available
| The metallicity and luminosity of RV Tauri variables from medium-resolution spectra
Results are reported from spectroscopic observations of members of theRV Tauri class of pulsating variable stars at moderate resolution, andfrom photometric observations of these stars with the B and V filters ofthe Johnson UBV system and the first four filters of the Wingeight-color narrowband near-IR system. Of the 22 variables observed, TiOabsorption was detected in eight, and 10 variables displayedhydrogen-line emission at some phase. Spectral classifications wereassigned relative to the MK process. Color excesses were determined forthose variables for which spectroscopic and photometric data wereobtained within a few days of each other. The spectra obtained at aresolution of 2.5 A were fitted with synthetic spectra to determine thestellar atmospheric parameters. The range in metallicity among thevariables was found to be quite extensive, -0.3 to -0.7, and may reflecta mixture of stellar populations or masses.
| Optical and infrared observations of RV Tauri stars
Optical and IR photometry of RV Tauri stars, much of which was obtainednearly simultaneously over the wavelength range 0.36-10 microns, ispresented. From the dereddened optical-IR flux distributions, stellarand dust shell parameters are deduced. The deduced stellar colors are ingood agreement with those inferred from optical spectroscopy. The Planckmean optical depths of the dusty RV Tauri stars range from 0.07 to 0.63,the corresponding circumstellar contributions to E(B-V) ranging fromabout 0.02 to about 0.2. The observations show that the fluxdistributions of oxygen- and carbon-rich types are indistinguishablephotometrically, that some of these objects may have multiple dustshells, and hence that dust production in RV Tauri stars probably occurssporadically rather than continuously.
| Circumstellar material and the light variations of RV Tauri stars
Infrared photometry on the JHKL system is given for 35 stars of the RVTauri and related types. The RV Tauri stars show infrared excessesindicating the presence of circumstellar dust with a temperature ofabout 900 K. These dust shells can be equally prominent in oxygen-richor carbon-rich stars; the high carbon content of the latter appears tocounteract the inhibiting effect of their generally lower metalabundance in promoting dust formation. The wide range of energydistributions of RV Tauri stars is attributed in part to differences inthe radial distribution of dust. The RVb stars (whose mean light variescyclically) are redder than the RVa stars (which have constant meanlight), indicating that their dust shells are denser (especially in theinner regions, which radiate strongly in the 1-3 micron band).Far-infrared data reveal cooler dust around some of the stars of groupa. Three W Virginis stars, two of which were already known to showaffinities to the RV Tauri stars, have dust shells. The dust shells forwhich IRAS infrared spectra are available show the 10-micron silicateemission feature regardless of whether the optical spectra are carbon-or oxygen-rich. Some considerations regarding possible models are given.A comparison with the R Coronae Borealis variables has revealedsimilarities.
| Stellar-population samples at the galactic poles. I. Proper-motion stars, blue objects, and eclipsing binaries near the south pole
Abstract image available at:http://adsabs.harvard.edu/abs/1968ApJ...153..723E
| Le type de population des céphéides galactiques; critères de distinction des types et catalogue
Not Available
| Notes on Eclipsing Binaries
Not Available
| 71 südlicheneue Veränderliche
Not Available
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Водолија |
| Ректацензија: | 22h53m17.04s |
| Deклинација: | -18°35'31.0" |
| Apparent магнитуда: | 10.711 |
| Proper motion RA: | 5.3 |
| Proper motion Dec: | -6.4 |
| B-T magnitude: | 11.263 |
| V-T magnitude: | 10.757 |
Каталог и designations:
|
