Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Functional relationships for T_eff and log g in F-G supergiants from uvby-beta photometry
From photoelectric uvby-beta data and recent accurate synthetic andspectroscopic values of T_eff and log g for 50 F-G supergiants, we havecalculated functional relationships that lead to initial estimates ofeffective temperature and gravities for these types of stars. It isshown that while the T_eff relationships are calculated using the dataon young massive supergiants, they are also valid for evolved stars ofsimilar temperatures like post-AGB and RV Tau stars. The gravity canalso be predicted from the Delta [c_1] index with an uncertainty ofabout 0.26 dex. Although a clear and significant trend between M_V andDelta [c_1] is seen, no calibration is found that predicts accuratevalues of M_V.
| 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
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| 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.
| 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.
| 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.
| 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.
| 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.
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| 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
| Self-Correlation Analysis of RV Tauri Stars and Related Objects
We have used self-correlation-a simple form of variogram analysis-tostudy 33 RV Tauri and related stars in the LMC, using MACHO data. Weconfirm the periods and classifications of Alcock et al. and discuss afew stars of special interest. We find that self-correlation is a usefuladjunct to Fourier analysis, especially for stars whose classificationis based on their cycle-to-cycle behavior. In particular, it canidentify stars whose behavior is more complicated than the standard``alternating deep and shallow minima'' and begin to investigate thequestion of whether the Population II Cepheids, the RV Tauri variables,and the SRd variables form a continuous sequence from periodicity toirregularity. Our results also emphasize that the RV Tauri phenomenonhas two dimensions: the relative depths of adjacent minima and thenumber of cycles over which the alternating minima persist.
| Fading of light maximum and linear polarization variation in the carbon Mira R Leporis
Polarimetry of R Lep obtained over the years 1991-2002 is presented.During this period the star underwent an episode of fading of thebrightness at light maximum, after an interval of about 35 yr. Ananalysis of the data shows that the percentage linear polarizationincreased as the fading progressed, attained a maximum of slightly over3 per cent in the V band close to the epoch of minimum, and remainedmore or less at the same level during and well after the recovery tonormal brightness. The polarization, apparently, originated from thecircumstellar envelope above the region where the dust that caused thefading in the star condensed. The physical mechanism that causes therather large polarization during fadings is perhaps selective extinctionby aligned foreground grains produced by the passage of shocks throughthe circumstellar envelope. The small-amplitude, short-term fluctuationsin polarization observed in R Lep, which appear to be superposed on thefading-related large-amplitude variation, are probably pulsation-relatedand arise from the inner zones of the circumstellar envelope.
| HP Lyr - Possibly the Hottest RV Tau Type Object
We report Johnson's UBVRI photometric and optical spectroscopicobservations of a long period variable HPLyr which up to now has beenconsidered to be an eclipsing binary with a period of 140 days. Itsspectral type changes continuously from A2-3 at maxima to A7-F2 atminima. We propose that the brightness changes are caused by pulsationof the star with two periods: P_1=69.35 days, and P_2=2 times P_1=138.7days. These periods decreased by more than 1% between 1960 and 1980. Thespectral luminosity class corresponds to an A type supergiant Iab. HPLyr is also the optical counterpart of the infrared source IRAS19199+3950. Relatively high galactic latitude (b=+11.7 arcd) and highradial velocity (-113 km/s) indicate that HP Lyr is an evolved, mostlikely post-AGB star. All these features suggest that this star is an RVTau type object.
| High-resolution spectroscopy of QY Sge: an obscured RV Tauri variable?
The first high-resolution optical spectra of QY Sge are presented anddiscussed. Menzies & Whitelock, on the basis of photometry andlow-resolution spectra, suggested that this G0I supergiant was obscuredby dust and seen only by scattered light from a circumstellar reflectionnebula. The new spectra confirm and extend this picture. Photosphericlines are unusually broad indicating scattering of photons from dust inthe stellar wind. The presence of very broad, NaD emission lines isconfirmed. Sharp emission lines from low levels of abundant neutralmetal atoms are reported for the first time. An abundance analysis ofphotospheric lines shows that the stellar atmosphere is of approximatelysolar composition but with highly condensable (e.g. Sc and Ti) elementsdepleted by factors of 5-10.
| 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.
| 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
Not Available
| Spectral Characteristics of RV Tauri Stars
Not Available
| 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.
| Linear polarization in the RV Tauri star AR Puppis
The UBVR polarimetry of AR Pup obtained on 14 nights during 1990-93shows a large variation in both the amount (~ 6 per cent in the U band)and the wavelength dependence of polarization. The simultaneousphotometry (20 nights) and polarimetry (30 nights) in the V band during1997 December-1998 April, which cover about one and a half pulsationalcycles, clearly establish the polarization-light curve connection in ARPup: both go through the maximum and minimum at about the same time. Weinterpret the observed near-straight line locus of polarization betweenany two consecutive minima in the Q, U plane as resulting from thesimultaneous presence of two independently varying polarizationcomponents, one slowly and the other relatively rapidly. We suggest thatthe mechanism that produces the exceptionally large polarization in ARPup is, most likely, selective absorption by non-spherical grains, whichcondense with a certain amount of alignment after each light minimumduring a pulsational cycle. BV photometry was also obtained on 44 nightsabout the times of UBVR polarimetry for information on thecontemporaneous light variation. The position angle of polarization inAR Pup does not show any correlation with its mean light level which isfound to vary nearly sinusoidally with a 1165 +/- 4 d period.
| 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
| 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.
| 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.
| The High-Resolution IRAS Galaxy Atlas
An atlas of the Galactic plane (-4.dg7 < b < 4.dg7), along withthe molecular clouds in Orion, rho Oph, and Taurus-Auriga, has beenproduced at 60 and 100 mu m from IRAS data. The atlas consists ofresolution-enhanced co-added images with 1'--2' resolution and co-addedimages at the native IRAS resolution. The IRAS Galaxy Atlas, togetherwith the Dominion Radio Astrophysical Observatory H I line/21 cmcontinuum and FCRAO CO (1--0) Galactic plane surveys, which both havesimilar (~1') resolution to the IRAS atlas, provides a powerful tool forstudying the interstellar medium, star formation, and large-scalestructure in our Galaxy. This paper documents the production andcharacteristics of the atlas.
| 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)
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Puppis |
| Ascensió Recta: | 08h03m01.65s |
| Declinació: | -36°35'47.9" |
| Magnitud Aparent: | 9.537 |
| Moviment propi RA: | -1.9 |
| Moviment propi Dec: | 16.2 |
| B-T magnitude: | 10.426 |
| V-T magnitude: | 9.611 |
Catàlegs i designacions:
|
