Contents
Images
Upload your image
DSS Images Other Images
Related articles
Disk evaporation in a planetary nebula
Aims: Binary interactions are believed to be importantcontributors to the structures seen in planetary nebulae (PN), and thesole cause of the newly discovered compact dust disks. The evolution ofthese disks is not clear, nor are the binary parameters required fortheir creation. Methods: Using HST imaging and VLT spectroscopy,both long-slit and integral field, we study the Galactic bulge planetarynebula M 2-29 for which a 3-year eclipse event of the central star hasbeen attributed to a dust disk. Results: The central PN cavity ofM 2-29 is being filled with a decreasingly, slow wind. An inner highdensity core is detected, of radius smaller than 250 AU, interpreted asa rotating gas/dust disk with a bipolar disk wind. The evaporating diskis argued to be the source of the slow wind. The central star is thesource of a very fast wind (~103 km s-1). Anouter, partial ring is seen in the equatorial plane, expanding at 12 kms-1. The azimuthal asymmetry is attributed to mass-lossmodulation by an eccentric binary. A crucial point in disk evolution isrepresented by M 2-29 where ionization causes the gas to be lost,leaving a low-mass dust disk behind.Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile (proposals075.D-0104, 077.D-0652, 081.D-0130) and HST (program 9356).
| Wall Emission in Circumbinary Disks: the Case of Coku Tau/4
A few years ago, the mid-IR spectrum of a Weak Line T Tauri Star, CoKuTau/4, was explained as emission from the inner wall of a circumstellardisk, with the inner disk truncated at ~10 AU. Based on the spectralenergy distribution (SED) shape and the assumption that it was producedby a single star and its disk, CoKu Tau/4 was classified as aprototypical transitional disk, with a clean inner hole possibly carvedout by a planet, some other orbiting body, or by photodissociation.However, recently it has been discovered that CoKu Tau/4 is a closebinary system. This implies that the observed mid-IR SED is probablyproduced by the circumbinary disk. The aim of the present paper is tomodel the SED of CoKu Tau/4 as arising from the inner wall of acircumbinary disk, with parameters constrained by what is known aboutthe central stars and by a dynamical model for the interaction betweenthese stars and their surrounding disk. We lack a physical prescriptionfor the shape of the wall, thus, here we use a simplified andunrealistic assumption: the wall is vertical. In order to fit theSpitzer IRS SED, the binary orbit should be almost circular, implying asmall mid-IR variability (10%) related to the variable distances of thestars to the inner wall of the circumbinary disk. In the context of thepresent model, higher eccentricities would imply that the stars arefarther from the wall, the latter being too cold to explain the observedSED. Our models suggest that the inner wall of CoKu Tau/4 is located at1.7a, where a is the semi-major axis of the binary system (a ~ 8 AU). Asmall amount of optically thin dust in the hole (lsim0.01 lunar masses)helps to improve the fit to the 10 ?m silicate band. Also, we findthat water ice should be absent or have a very small abundance (a dustto gas mass ratio lsim5.6 × 10-5). In general,for a binary system with eccentricity e>0, the model predicts mid-IRvariability with periods similar to orbital timescales, assuming thatthermal equilibrium is reached instantaneously.
| Analysis of the infrared spectra of the peculiar post-AGB stars EP Lyrae and HD 52961
Aims: We aim to study in detail the peculiar mineralogy and structure ofthe circumstellar environment of two binary post-AGB stars, EP Lyr andHD 52961. Both stars were selected from a larger sample of evolved discsources observed with Spitzer and show unique solid-state and gasfeatures in their infrared spectra. Moreover, they show a very smallinfrared excess in comparison with the other sample stars. Methods: The different dust and gas species are identified on thebasis of high-resolution Spitzer-IRS spectra. We fit the full spectrumto constrain grain sizes and temperature distributions in the discs.This, combined with our broad-band spectral energy distribution andinterferometric measurements, allows us to study the physical structureof the disc, using a self-consistent 2D radiative-transfer disc model.Results: We find that both stars have strong emission features dueto CO2 gas, dominated by 12C16O2, but with clear13C16O2 and even16O12C18O isotopic signatures.Crystalline silicates are apparent in both sources but proved very hardto model. EP Lyr also shows evidence of mixed chemistry, with emissionfeatures of the rare class-C PAHs. Whether these PAHs reside in theoxygen-rich disc or in a carbon-rich outflow is still unclear. With thestrongly processed silicates, the mixed chemistry and the low12C/13C ratio, EP Lyr resembles some silicateJ-type stars, although the depleted photosphere makes nucleosyntheticsignatures difficult to probe. We find that the disc environment of bothsources is, to a first approximation, well modelled with a passive disc,but additional physics such as grain settling, radial dustdistributions, and an outflow component must be included to explain thedetails of the observed spectral energy distributions in both stars.Based on observations made with the 1.2 m Flemish Mercator telescope atRoque de los Muchachos, Spain, the 1.2 m Swiss Euler telescope at LaSilla, Chile and on observations made with the Spitzer Space Telescope(program id 3274), which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract with NASA.
| Ubiquitous H?-Polarized Line Profiles: Absorptive Spectropolarimetric Effects and Temporal Variability in Post-AGB, Herbig Ae/Be, and Other Stellar Types
We show here that the absorptive H? polarized line profilepreviously seen in many Herbig Ae/Be (HAeBe) stars is a nearlyubiquitous feature of other types of embedded or obscured stars. Thischaracteristic 1% linear polarization variation across the absorptivepart of the H? line is seen in post-AGB stars as well as RV Tau,? Scuti, and other types. Each of these stars shows evidence ofobscuration by intervening circumstellar hydrogen gas and thepolarization effect is in the absorptive component, consistent with anoptical pumping model. We present ESPaDOnS spectropolarimetricobservations of nine post-AGB and RV Tau types in addition to manymulti-epoch HiVIS observations of these targets. We find significantpolarization changes across the H? line in 8/9 stars withpolarization amplitudes of 0.5% to over 3% (5/6 post-AGB and 3/3 RVTau). In all but one of these, the polarization change is dominated bythe absorptive component of the line profile. There is no evidence thatsubclasses of obscured stars showing stellar pulsations (RV Tau forpost-AGB stars and ? Scuti for Herbig Ae/Be stars) showsignificant spectropolarimetric differences from the main class.Significant temporal variability is evident from both HiVIS and ESPaDOnSdata for several stars presented here: 89 Her, AC Her, SS Lep, MWC 120,AB Aurigae, and HD144668. The morphologies and temporal variability arecomparable to existing large samples of Herbig Ae/Be and Be type stars.Since post-AGB stars have circumstellar gas that is very different fromBe stars, we discuss these observations in the context of theirdiffering environments.
| 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
| Hubble Space Telescope Snapshot Survey of Post-AGB Objects
The results of a Hubble Space Telescope (HST) snapshot survey ofpost-AGB objects are shown. The aim of the survey is to complementexisting HST images of protoplanetary nebulae and to connect varioustypes of nebulosities with the physical and chemical properties of theircentral stars. Nebulosities are detected in 15 of 33 sources. Images andphotometric and geometric measurements are presented. For sources withnebulosities we see a morphological bifurcation into two groups, DUPLEXand SOLE, as previous studies have found. We find further support forthe previous results, suggesting that this dichotomy is caused by adifference in the optical thickness of the dust shell. The remaining 18sources are classified as stellar post-AGB objects, because ourobservations indicate a lack of nebulosity. We show that some stellarsources may in fact be DUPLEX or SOLE objects based on their infraredcolors. The causes of the differences among the groups are investigated.We discuss some evidence suggesting that high progenitor mass AGB starstend to become DUPLEX post-AGB objects and intermediate progenitor massAGB stars tend to become SOLE post-AGB objects. Most of the stellarsources probably have low-mass progenitors and do not seem to developnebulosities during the post-AGB phase; therefore, they do not becomeplanetary nebulae.
| Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Centauri and AC Herculis
Context: We study the structure and evolution of the circumstellar discsaround evolved binaries and their impact on the evolution of the centralsystem. Aims: By combining a wide range of observational data andtechniques, we aim to study in detail the binary nature of RU Cen and ACHer, as well as the structure and mineralogy of the circumstellarenvironment. Methods: We combine a multi-wavelength observationalprogram with a detailed 2D radiative transfer study. Our radial velocityprogram is instrumental in the study of the nature of the central stars,while our Spitzer spectra complimented with the broad-band spectralenergy distribution (SED) are used to constrain mineralogy, grain sizesand physical structure of the circumstellar environment. Results:We determine the orbital elements of RU Cen showing that the orbit ishighly eccentric with a large velocity amplitude despite the rather longperiod of 1500 days. The infrared spectra of both objects are verysimilar and the spectral dust features are dominated by magnesium-richcrystalline silicates. The small peak-to-continuum ratios areinterpreted as being due to large grains. Our model contains twocomponents with a cold midplain dominated by large grains, and the near-and mid-IR which is dominated by the emission of smaller silicates. Theinfrared excess is well modelled assuming a hydrostatic passiveirradiated disc. The profile-fitting of the dust resonances shows thatthe grains must be very irregular. Conclusions: These twoprototypical RV Tauri pulsators with circumstellar dust are binarieswhere the dust is trapped in a stable disc. The mineralogy and grainsizes show that the dust is highly processed, both in crystallinity andgrain size. The cool crystals show that either radial mixing is veryefficient and/or that the thermal history at grain formation has beenvery different from that in outflows. The physical processes governingthe structure of these discs are very similar to those observed inprotoplanetary discs around young stellar objects.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.
| 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
| Do Yellow Semiregular (SRd) Variables Show Long Secondary Periods?
Semiregular pulsating variable supergiants of spectral types F, G, and K(SRd variables in the General Catalogue of Variable Stars), and RV Tauri(RV) stars share many similarities in their physical properties,evolutionary status, and light curve properties. A significant fractionof RV variables (called RVb stars) show long secondary photometricperiods, an order of magnitude longer than the primary (pulsation)period. We have searched for long secondary periods in 8 well-studiedSRd variables in the AAVSO visual observing program, using light curveanalysis, and especially self-correlation analysis. We first tested theprocedure on 4 RVa variables, and on 4 RVb variables, and derived newvalues for the long secondary periods in the latter. We find nocompelling evidence for long secondary photometric periods in the RVa orSRd variables. Since the RVb phenomenon is believed to be due tobinarity, it is not clear why SRd variables could not also be binarymembers which show long secondary periods
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS point source catalogue
Aims.We study the optical spectral properties of a sample of starsshowing far infrared colours similar to those of well-known planetarynebulae. The large majority of them were unidentified sources or poorlyknown in the literature at the time when this spectroscopic surveystarted, some 15 years ago. Methods: .We present low-resolutionoptical spectroscopy, finding charts and improved astrometriccoordinates of a sample of 253 IRAS sources. Results: .We haveidentified 103 sources as post-AGB stars, 21 as "transition sources",and 36 as planetary nebulae, some of them strongly reddened. Among therest of sources in the sample, we were also able to identify 38 youngstellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49sources in our spectroscopic sample do not show any optical counterpart,and most of them are suggested to be heavily obscured post-AGB stars,rapidly evolving on their way to becoming planetary nebulae.Conclusions: .An analysis of the galactic distribution of the sourcesidentified as evolved stars in the sample is presented together with astudy of the distribution of these stars in the IRAS two-colour diagram.Finally, the spectral type distribution and other properties of thesources identified as post-AGB in this spectroscopic survey arediscussed in the framework of stellar evolution.
| 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.
| Binary post-AGB stars and their Keplerian discs .
In this contribution we give a progress report on our systematic studyof a large sample of post-AGB stars. The sample stars were selected onthe basis of their infrared colours and the selection criteria weretuned to discover objects with hot dust in the system. We started a veryextensive, multi-wavelength programme which includes the analysis of ourradial velocity monitoring; our optical high-resolution spectra; ourgroundbased N-band spectral data as well as the Spitzer full spectralscans; the broad-band SED and the high spatial-resolutioninterferometric experiments with the VLTI. In this contribution wehighlight the main results obtained so far and argue that all systems inour sample are indeed binaries, which are surrounded by dusty Kepleriancircumbinary discs. The discs play a lead role in the evolution of thesystems.
| 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.
| Strong dust processing in circumstellar discs around 6 RV Tauri stars. Are dusty RV Tauri stars all binaries?
We present extended Spectral Energy Distributions (SEDs) of sevenclassical RV Tauri stars, using newly obtained submillimetre continuummeasurements and Geneva optical photometry supplemented with literaturedata. The broad-band SEDs show a large IR excess with a black-body slopeat long wavelengths in six of the seven stars, R Sct being thenoticeable exception. This long wavelength slope is best explainedassuming the presence of a dust component of large grains in thecircumstellar material. We show that the most likely distribution of thecircumstellar dust around the six systems is that the dust resides in adisc. Moreover, very small outflow velocities are needed to explain thepresence of dust near the sublimation temperature and we speculate thatthe discs are Keplerian. The structure and evolution of these compactdiscs are as yet not understood but a likely prerequisite for theirformation is that the dusty RV Tauri stars are binaries.
| 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
| Post-Agb Stars
In this contribution, a review is presented on the ample data obtainedon post-AGB stars, both on the central stars and their circumstellarmaterial. The fast evolutionary phase is characterized by a rapid changein the properties of the objects, but the variety is so large that thereis yet no clear consensus on how the detailed studies of individualobjects are linked together by evolutionary channels. The absence ofstrong molecular veiling in the photospheres of the central stars,together with a spread in intrinsic metallicity make post-AGB stars veryuseful in constraining AGB chemical evolutionary models. We discuss thesurprisingly wide variety of chemical signatures observed. The onset inthe creation process of the panoply of structures and shapes observed inplanetary nebulae occurs during the short post-AGB evolution, but thephysical nature of the processes involved is still badly understood. Inthe rapidly growing field of circumstellar mineralogy, post-AGB starshave their story to tell and also the molecular envelope changessignificantly due to dilution and hardening of the stellar radiation.The real-time evolution of some objects suffering a late thermal flashis reviewed and their possible link to other hydrogen-deficient objectsis discussed. Any review on stellar evolution has a section on binariesand this contribution is no exception because binaries make up asignificant fraction of the post-AGB stars known to date.
| 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).
| Near-IR spectroscopy of planetary nebulae precursors
We present near-IR spectroscopy of a sample of 30 IRAS sources recentlyidentified as late AGB stars, post-AGB stars or early PNe. The spectraobtained are centered at various wavelengths covering the molecularhydrogen v=1-> 0 S(1) 2.122 mu m and v=2->1 S(1) 2.248 mu memission lines, the recombination lines of hydrogen Brgamma 2.166 mu m,Pfgamma 3.741 mu m and Bralpha 4.052 mu m, and the CO[v=2->0] firstovertone bandhead at 2.294 mu m. As a result of these observations wehave detected molecular hydrogen emission for the first time in 9 ofthese sources and confirmed a previous detection by Weintraub et al.(\cite{Weintraub98}). This increases from 4 to 13 the total number ofproto-PNe detected in molecular hydrogen. In most cases, the positivedetections also show emission in the recombination lines of hydrogen(with the exception of IRAS 17150-3224) indicating that the onset ofmolecular hydrogen emission takes place in the post-AGB phase, veryshortly before the nebula becomes ionized. When the molecular hydrogenis fluorescence-excited the detection rate is found to be directlycorrelated with the evolutionary stage of the central star, rather thanwith the nebular morphology. When the temperature of the central star ishot enough, fluorescence excitation can be induced by the absorption ofUV photons escaping from the rapidly evolving central post-AGB star. Incontrast, shock-excited molecular hydrogen is detected only in stronglybipolar proto-PNe, sometimes even at an early stage in the post-AGBphase. Shock excitation is the consequence of the interaction of thefast post-AGB wind with the slow wind material ejected during the AGB.The strong correlation of shock-excited molecular hydrogen emission withbipolarity found confirms the result previously reported by Kastner etal. (\cite{Kastner96}) in evolved PNe. However, our results show thatthis correlation does not exist in the case of fluorescence-excitedmolecular hydrogen. Based on observations collected at the EuropeanSouthern Observatory (La Silla, Chile).
| RU Cen and SX Cen: Two strongly depleted RV Tauri stars in binary systems. The RV Tauri photometric b phenomenon and binarity
We present a chemical abundance analysis on the basis of highsignal-to-noise and high-resolution (lambda /Delta lambda ~ 48 000)optical spectra of two RV Tauri stars RU Cen and SX Cen. With an [Fe/H]= -1.9 and a [Zn/Fe] = +0.9 for RU Cen and a [Fe/H] = -1.1 and a [Zn/Fe]= +0.6 for SX Cen, both stars of spectroscopic class B display strongdepletion of refractory elements in their photospheres. Our CORALIEradial velocity measurements prove the stars to be members of binarysystems and a detailed construction of the spectral energy distributionindicate the presence of a large amount of hot circumstellar dust.Moreover, the orbital period of SX Cen of around 600 days is similar tothe published period of mean magnitude variation in the light curve (RVTauri phomometric class b phenomenon). All these observations indicatethe presence of a stable circumbinary disk in the objects and strengthenthe model that this is a necessary condition for the depletion processto take place. Based on observations collected at the European SouthernObservatory in Chile (64.L-0117(A), 67.D-0054(A)), on the 1.2 m SwissEuler telescope on La Silla and on the 70 cm Swiss photometric telescopealso on La Silla.
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
| 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
| 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.
| 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.
| The nature of RV Tauri stars.
The infrared colours of the RV Tauri stars fall in a well-defined andthinly-populated region of the IRAS [12]-[25],[25]-[60] diagram. Thisprovides an alternative method of searching for such stars. Stars withthese IRAS colours as well as the K and L excesses which are alsotypical of RV Tauri stars have spectral types from B to M. Thecharacteristic variability of RV Tauri-type has been found for severalstars with spectral types typical of known RV Tauri stars, while starsof different spectral types do not vary in this way. RV Tauri stars arethe stars with these colours, probably indicative of a dustycircumstellar disk, which lie within the Cepheid instability strip.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Zentaur |
| Right ascension: | 12h09m23.81s |
| Declination: | -45°25'34.8" |
| Apparent magnitude: | 9.056 |
| Proper motion RA: | -5.8 |
| Proper motion Dec: | -4.1 |
| B-T magnitude: | 9.992 |
| V-T magnitude: | 9.134 |
Catalogs and designations:
|
