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Detached shells as tracers of asymptotic giant branch-interstellar medium bow shocks
New Spitzer imaging observations have revealed the structure around theMira variable star R Hya to be a one-sided parabolic arc 100 arcsec tothe west, stretching from north to south. We successfully model R Hyaand its surroundings in terms of an interaction of the stellar wind froman asymptotic giant branch (AGB) star with the interstellar medium (ISM)the star moves through. Our three-dimensional hydrodynamic simulationreproduces the structure as a bow shock into the oncoming ISM. Wepropose this as another explanation of detached shells around suchstars, which should be considered alongside current theories of internalorigin. The simulation predicts the existence of a tail ofram-pressure-stripped AGB material stretching downstream. Indicationsfor such a tail behind R Hya are seen in IRAS maps.
| A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud
We present a Spitzer Space Telescope spectroscopic survey of mass-losingcarbon stars (and one oxygen-rich star) in the Large Magellanic Cloud(LMC). The stars represent the superwind phase on the asymptotic giantbranch (AGB), which forms a major source of dust for the interstellarmedium (ISM) in galaxies. Bolometric magnitudes indicate progenitormasses of 1.5-2.5Msolar. The spectra cover the wavelengthrange 5-38μm. They show varying combinations of dust continuum, dustemission features (SiC, MgS) and molecular absorption bands(C2H2, HCN). A 10-μm absorption feature isattributed to C3. A weak band at 5.8μm is suggestive ofcarbonyl. The circumstellar 7.5-μm C2H2 band isfound to be stronger at lower metallicity, explained by higher C/Oratios at low metallicity. The J - K versus K - A colours, used toselect the sample, are shown to be relatively insensitive in separatingcarbon versus oxygen-rich AGB stars. The predominance of carbon starstherefore indicates that in the range 1.5-2.5Msolar, LMC AGBstars become carbon-rich before onset of the superwind. A set of fournarrow bands, dubbed the Manchester system, is used to define theinfrared continuum for dusty carbon stars. We investigate the strengthand central wavelength of the SiC and MgS dust bands as a function ofcolour and metallicity. The line-to-continuum ratio of these bands showssome indication of being lower at low metallicity. The MgS band is onlyseen at dust temperatures below 600K. Metal-poor carbon stars can formamorphous carbon dust from self-produced carbon. The formationefficiency of oxygen-rich dust depends more strongly on metallicity. Inlower-metallicity environments, the dust input into the ISM by AGB starsmay be strongly biased towards carbonaceous dust.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderliche Sterne e.V.
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| Evolution of the Circumstellar Molecular Envelope. I. A BIMA CO Survey of Evolved Stars
This paper reports the results of a small imaging survey of eightevolved stars including two AGB stars (IRC +10216 and Mira), fiveproto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027).We present high-resolution 12CO J=1-->0 maps of their fullmolecular envelopes made by combining BIMA Millimeter Array and NRAO 12m telescope observations. For the PPNe and PN, the neutral molecularenvelopes are compared with images taken at optical, near-IR, and mid-IRwavelengths. Drawing from the literature, we augmented our BIMA surveysample to 38 well-studied sources with CO emission maps. We classifiedthis sample of sources based on the kinematics and morphologies of theCO emission into three types: spherical/elliptical/shell sources, disksources, and structured outflow sources. Confirming previous studies, wefind strong evidence for the photodissociation of the molecular envelopeas an object evolves from the AGB to PN stages. While the spherical AGBstars follow theoretical expectations for mass-loss rate versus envelopesize, the post-AGB structured outflow sources have significantly highermass-loss rates than expected probably because of their recentsuperwinds. We find evidence that the structured outflows are clearlyyounger than the AGB wind. The disk sources have little correlationbetween mass-loss rate and envelope size because their properties aredetermined more by the properties of the central stars and diskevolution than by the mass-loss rate history that shapes the sphericaland structured-outflow sources.
| Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisited
As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) starsat infrared (IR) wavelengths, we discuss a sample (357) of carbon starsfor which mass loss rates, near-IR photometry and distance estimatesexist. For 252 sources we collected mid-IR fluxes from the MSX (6C) andthe ISO-SWS catalogues. Most stars have spectral energy distributions upto 21 μm, and some (1/3) up to 45 μm. This wide wavelengthcoverage allows us to obtain reliable bolometric magnitudes. Theproperties of our sample are discussed with emphasis on ~70 stars withastrometric distances. We show that mid-IR fluxes are crucial toestimate the magnitude of stars with dusty envelopes. We construct HRdiagrams and show that the luminosities agree fairly well with modelpredictions based on the Schwarzschild's criterion, contrary to what iswidely argued in the literature. A problem with the brightness of Cstars does not appear to exist. From the relative number of Mira andSemiregular C-variables, we argue that the switch between these classesis unlikely to be connected to thermal pulses. The relevance of the twopopulations varies with the evolution, with Miras dominating the finalstages. We also analyze mass loss rates, which increase for increasingluminosity, but with a spread that probably results from a dependence ona number of parameters (like e.g. different stellar masses and differentmechanisms powering stellar winds). Instead, mass loss rates are wellmonitored by IR colours, especially if extended to 20 μm and beyond,where AGB envelopes behave like black bodies. From these colours theevolutionary status of various classes of C stars is discussed.
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
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| Properties of detached shells around carbon stars. Evidence of interacting winds
The nature of the mechanism responsible for producing the spectacular,geometrically thin, spherical shells found around some carbon stars hasbeen an enigma for some time. Based on extensive radiative transfermodelling of both CO line emission and dust continuum radiation for allobjects with known detached molecular shells, we present compellingevidence that these shells show clear signs of interaction with asurrounding medium. The derived masses of the shells increase withradial distance from the central star while their velocities decrease. Asimple model for interacting winds indicates that the mass-loss rateproducing the faster moving wind has to be almost two orders ofmagnitudes higher (~10-5 Mȯ yr-1)than the slower AGB wind (a few 10-7 Mȯyr-1) preceding this violent event. At the same time, thepresent-day mass-loss rates are very low indicating that the epoch ofhigh mass-loss rate was relatively short, on the order of a few hundredyears. This, together with the number of sources exhibiting thisphenomenon, suggests a connection with He-shell flashes (thermalpulses). We report the detection of a detached molecular shell aroundthe carbon star DR Ser, as revealed from newsingle-dish CO (sub-)millimetre line observations. The properties of theshell are similar to those characterising the young shell aroundU Cam.
| Dust cloud formation in stellar environments. II. Two-dimensional models for structure formation around AGB stars
This paper reports on computational evidence for the formation ofcloud-like dust structures around C-rich AGB stars. This spatio-temporalstructure formation process is caused by a radiative/thermal instabilityof dust-forming gases as identified by Woitke et al. (2000, A&A,358, 665). Our 2D (axisymmetric) models combine a time-dependentdescription of the dust formation process according to Gail &Sedlmayr (1988, A&A, 206, 153) with detailed, frequency-dependentcontinuum radiative transfer by means of a Monte Carlo method (Niccoliniet al. 2003, A&A, 399, 703) in an otherwise static medium (v = 0).These models show that the formation of dust behind already condensedregions, which shield the stellar radiation field, is strongly favoured.In the shadow of these clouds the temperature decreases by severalhundred Kelvin, which triggers the subsequent formation of dust andensures its thermal stability. Considering an initially dust-free gaswith small density inhomogeneities, we find that finger-like duststructures develop which are cooler than the surroundings and pointtowards the centre of the radiant emission, similar to the“cometary knots” observed in planetary nebulae and starformation regions. Compared to a spherical symmetric reference model,the clumpy dust distribution has little effect on the spectral energydistribution, but dominates the optical appearance in near IRmonochromatic images.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is 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/431/773
| The mass loss of C-rich giants
The mass loss rates, expansion velocities and dust-to-gas density ratiosfrom millimetric observations of 119 carbon-rich giants are compared, asfunctions of stellar parameters, to the predictions of recenthydrodynamical models. Distances and luminosities previously estimatedfrom HIPPARCOS data, masses from pulsations and C/O abundance ratiosfrom spectroscopy, and effective temperatures from a new homogeneousscale, are used. Predicted and observed mass loss rates agree fairlywell, as functions of effective temperature. The signature of the massrange M≤4 Mȯ of most carbon-rich AGB stars is seenas a flat portion in the diagram of mass loss rate vs. effectivetemperature. It is flanked by two regions of mass loss rates increasingwith decreasing effective temperature at nearly constant stellar mass.Four stars with detached shells, i.e. episodic strong mass loss, andfive cool infrared carbon-rich stars with optically-thick dust shells,have mass loss rates much larger than predicted values. The latter(including CW Leo) could be stars of smaller masses (M≃ 1.5-2.5Mȯ) while M≃ 4 Mȯ is indicated formost of the coolest objects. Among the carbon stars with detachedshells, R Scl returned to a predicted level (16 times lower) accordingto recent measurements of the central source. The observed expansionvelocities are in agreement with the predicted velocities at infinity ina diagram of velocities vs. effective temperature, provided the carbonto oxygen abundance ratio is 1≤ɛ C/ɛO≤2, i.e. the range deduced from spectra and modelatmospheres of those cool variables. Five stars with detached shellsdisplay expansion velocities about twice that predicted at theireffective temperature. Miras and non-Miras do populate the same locus inboth diagrams at the present accuracy. The predicted dust-to-gas densityratios are however about 2.2 times smaller than the values estimatedfrom observations. Recent drift models can contribute to minimize thediscrepancy since they include more dust. Simple approximate formulaeare proposed.This research has made use of the Simbad database operated at CDS.Partially based on data from the ESA HIPPARCOS astrometry satellite.Table 3 is only available in electronic form 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/429/235
| Australia Telescope Compact Array imaging of circumstellar HCN line emission from R Scl
We present radio-interferometric observations of HCN J=1->0 lineemission from the carbon star R Scl, obtained with the interim 3-mmreceivers of the Australia Telescope Compact Array. The emission isresolved into a central source with a Gaussian FWHM of 1 arcsec,which we identify as the present mass loss envelope. Using a simplephotodissociation model and constraints from single-dish HCN spectra, weargue that the present mass-loss rate is low, 2 ×10-7 Mȯ yr-1, supporting the ideathat R Scl had to experience a brief episode of intense mass loss inorder to produce the detached CO shell at 10 arcsec radius inferredfrom single-dish observations. Detailed radiative transfer modellingyields an abundance of HCN relative to H2, fHCN,of 10-5 in the present-day wind. There appears to be adiscrepancy between model results obtained with higher transitionsingle-dish data included and those from the J=1->0 interferometerdata alone, in that the interferometer data suggest a smaller envelopesize and larger HCN abundance than the single-dish data. The lack of HCNin the detached shell, fHCN 2× 10-7, isconsistent with the rapid photodissociation of HCN into CN as it expandsaway from the star.
| Preliminary analysis of light curves of seven carbon stars
We present a preliminary analysis of the light curves of the followingcarbon stars: WZ Cas, VY UMa, Y CVn, RY Dra, T Lyr, HK Lyr and TT Cyg,constructed on the basis of our own BV photoelectric observationsobtained at the Brno Observatory in 1979-94 and Hipparcos observations.The analysis suggests that "semiregular" light curves of all studiedstars can faithfully be expressed by a superposition of long-termchanges and a set of medium-term harmonic variations (possiblypulsations) with periods from 50 to 500 days.
| New Laboratory Spectra of Isolated β-SiC Nanoparticles: Comparison with Spectra Taken by the Infrared Space Observatory
We present new laboratory infrared spectra of matrix-isolated β-SiCnanoparticles, which perfectly match the band profile of the 11+ μmfeature observed in carbon stars. The new laboratory spectra differ inthe shape of the band profile from former measurements of SiCnanoparticles thanks to the matrix-isolation technique which allows toobtain spectra of nonagglomerated particles. The final spectra arecorrected for the influence of the surrounding medium (argon-matrix) bya computational technique proposed by Papoular et al. (1998).Furthermore, we study the influence of nitrogen incorporation into theSiC lattice, which introduces a strong near-infrared absorption owing tosurface-plasmon excitation (Mutschke et al. 1999). Our laboratoryspectra are compared with Infrared Space Observatory observations ofseveral carbon stars showing an 11 μm feature either in emission orin absorption. We discuss the implications of the new laboratory resultsfor the interpretation of the spectra of carbon stars.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the UK) and with the participation of ISAS and NASA.The SWS is a joint project of SRON and MPE.
| Discovery of Multiple Molecular Shells in the Outer Envelope of IRC +10216
We present 12CO J=1-->0 maps of the extended envelope ofIRC +10216 made by combining BIMA millimeter array and NRAO 12 mtelescope observations. CO emission was detected beyond the radius of140", which traces the history of mass loss for the past ~7000 yr. Wediscover superposed on the CO envelope clumpy arcs of enhanced emission,circumscribing the star, at multiple radii ranging from about 26" to120". The arcs are found to be coherent across a range of velocities,thus forming sections of multiple shells that are participating in thecircumstellar expansion. The intershell timescales range from 1300 to2900 yr, which are roughly consistent with simulations where windmodulations from a single thermal pulse event can lead to the formationof multiple shells. The multiple enhanced shells within the continuousenvelope of IRC +10216 may be the high mass-loss rate counterpart to thedetached shells detected around several low mass-loss rate carbon stars,such as TT Cyg.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also available in electronic form 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/399/1167
| Imaging polarimetry of stellar light scattered in detached shells around the carbon stars R Scl and U Ant
Imaging polarimetry has been used to study the extended, detachedcircumstellar shells around the bright carbon stars RScl and U Ant. The observations were donein two narrow band filters centred on the resonance lines of neutral Kand Na, but much broader than the expected line widths. The polarimetricdata reveal brightness distributions, in both cases, which are inperfect agreement with previous observations of scattered light obtainedthrough direct imaging techniques. The total intensity images towardsR Scl outline, in both filters, circular disk-likedistributions out to a radius of ~21arcsec , where the intensity dropssharply. The polarised intensity images reveal, however, that thescattering occurs in a geometrically thin shell. The degree ofpolarisation reaches values of ~35% in both filters. The imagingpolarimetry observations of U Ant reveal a somewhatmore complex structure, where the existence of several shells can bediscerned. The polarised scattered light comes from a component, at aradius of ~50arcsec from the star, which lies outside the region wherethe bulk of the light is scattered. The latter comes from a dominatingshell at ~43arcsec , which coincides spatially with the detached gasshell inferred from CO radio line data, and there may be another twoshells inside this. The polarisation degree reaches ~50% in the outercomponent. We model, with a code based on the Monte Carlo method, thescattered emission under the assumption of dust scattering, using theobserved polarised brightness distributions as constraints. In the caseof R Scl we found that the polarised, as well as thetotal, light distributions can be explained by scattering in a 2arcsecwide shell of radius 20arcsec containing a dust mass of~2*E-6 Msun. This dust shell is also responsiblefor the thermal dust emission measured by IRAS. There is room, up to 30%of the total scattered flux, for other scattering agents. Comparisonwith CO radio line data shows that this dust shell probably lies outsidethe detached CO gas shell. In the case of U Ant themodelling explains the outer component in terms of a 5arcsec wide shellat a radius of about 52arcsec with a dust mass of ~4*E-6Msun. This is also the dust shell responsible for theemission measured by IRAS. However, the bulk of the scattered lightcannot in this case be due to scattering by dust. In accordance with adiscussion in a previous paper we attribute the remaining, unpolarised,scattering to the KI and Na D resonance lines. In both cases we foundevidence that a dust shell has separated from the rest of thecircumstellar medium. This may be due to gas-grain drift, or tohydrodynamical effects, which may also explain the complexmultiple-shell structure seen towards U Ant. Themodel results are very dependent on the grain size distribution, and theobservational data can only be reconciled with a very steep decline ingrain size.Based on observations using the 3.6 m telescope of the European SouthernObservatory, La Silla, Chile.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Stellar masers: a review
Not Available
| Shapes and Shaping of Planetary Nebulae
We review the state of observational and theoretical studies of theshaping of planetary nebulae (PNe) and protoplanetary nebulae (pPNe). Inthe past decade, high-resolution studies of PNe have revealed abewildering array of morphologies with elaborate symmetries. Recentimaging studies of pPNe exhibit an even richer array of shapes. Thevariety of shapes, sometimes multiaxial symmetries, carefully arrangedsystems of low-ionization knots and jets, and the often Hubble-flowkinematics of PNe and pPNe indicate that there remains much tounderstand about the last stages of stellar evolution. In many cases,the basic symmetries and shapes of these objects develop on extremelyshort timescales, seemingly at the end of AGB evolution when the mode ofmass loss abruptly and radically changes. No single explanation fits allof the observations. The shaping process may be related to externaltorques of a close or merging binary companion or the emergence ofmagnetic fields embedded in dense outflowing stellar winds. We suspectthat a number of shaping processes may operate with different strengthsand at different stages of the evolution of any individual object.
| s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-elementcontent in Galactic carbon stars of N type. Abundances of Sr, Y, Zr(low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-masss-elements, or hs) are derived using the spectral synthesis techniquefrom high-resolution spectra. The N stars analyzed are of nearly solarmetallicity and show moderate s-element enhancements, similar to thosefound in S stars, but smaller than those found in the only previoussimilar study (Utsumi 1985), and also smaller than those found insupergiant post-asymptotic giant branch (post-AGB) stars. This is inagreement with the present understanding of the envelope s-elementenrichment in giant stars, which is increasing along the spectralsequence M-->MS-->S-->SC-->C during the AGB phase. Wecompare the observational data with recent s-process nucleosynthesismodels for different metallicities and stellar masses. Good agreement isobtained between low-mass AGB star models (M<~3 Msolar)and s-element observations. In low-mass AGB stars, the13C(α, n)16O reaction is the main source ofneutrons for the s-process a moderate spread, however, must exist in theabundance of 13C that is burnt in different stars. Bycombining information deriving from the detection of Tc, the infraredcolors, and the theoretical relations between stellar mass, metallicity,and the final C/O ratio, we conclude that most (or maybe all) of the Nstars studied in this work are intrinsic, thermally pulsing AGB stars;their abundances are the consequence of the operation of third dredge-upand are not to be ascribed to mass transfer in binary systems.
| A Wall of Dust around a Proto-Mira?
We present the discovery of a huge (19'×16')dust ring surrounding a bright (V=10.60) red star. The dust ring has, atD=700 pc, a diameter of 4 pc and a central hole of ~1.5 pc across. Partof the shell is also seen as an absorption nebulosity. The star isclassified as an M3 III asymptotic giant branch (AGB) star. Among AGBstars, its detached shell is of unrivaled size. Detached shells aroundAGB stars are normally interpreted in terms of thermal pulses. However,in this case, a significant fraction of the shell may consist ofswept-up ISM; the detached appearance can be explained with wind-ISMinteraction. We present a model in which the AGB wind has been stoppedby the surrounding ISM and the swept-up shell is now expanding at thesound speed. The model predicts that the ring will disperse over a fewtimes 105 yr and eventually will leave a large hole in theISM surrounding the AGB star or its future planetary nebula.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the 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/390/967
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is 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/386/492, and from theauthors on CD-Rom.
| An improved mass-loss description for dust-driven superwinds and tip-AGB evolution models
We derive an improved description of dust-driven stellar mass-loss forthe cool winds of carbon-rich tip-AGB stars. We use pulsating windmodels in which the mass loss is driven by radiation pressure on dustgrains, for C-rich chemistry. From a larger set of these models,selected for representative dynamical (pulsational velocity amplitudeDelta v, period P) and chemical (theepsilonC/epsilonO abundance ratio) inputparameters, an improved approximative mass-loss formula has been derivedwhich depends only on the stellar parameters (effective temperatureTeff, luminosity L and mass M). Due to the detailedconsideration of the chemistry and the physics of the dust nucleationand growth processes, there is a particularly strong dependence of themass-loss rate dot {M} (in Msun/yr) on Teff: log{dot {M}} = 8.86 - 1.95 * log {M/{Msun}} - 6.81 * log {T/K} +2.47 * log {L/{Lsun}}. The dependence of the model mass-losson the pulsational period has explicitly been accounted for inconnection with the luminosity dependence, by applying an observedperiod-luminosity relation for C-rich Miras. We also apply the improvedmass-loss description to our evolution models, and we revisit theirtip-AGB mass-loss histories and the total masses lost, in comparison toour earlier work with a preliminary mass-loss description. While thereis virtually no difference for the models in the lower mass range ofconsideration (Mi = 1.0 to ~ 1.3 Msun), we nowfind more realistic, larger superwind mass-loss rates for larger stellarmasses: i.e., dot {M} between ~ 0.4 and 1.0 x 10-4Msun/yr for Mi between 1.85 and 2.65Msun, removing between 0.6 and 1.2 Msun,respectively, during the final 30 000 yrs on the tip-AGB.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| 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.
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
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Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Cygnus |
| Ascensió Recta: | 19h40m57.01s |
| Declinació: | +32°37'05.7" |
| Magnitud Aparent: | 7.548 |
| Distancia: | 510.204 parsecs |
| Moviment propi RA: | -6.1 |
| Moviment propi Dec: | -3.5 |
| B-T magnitude: | 11.588 |
| V-T magnitude: | 7.882 |
Catàlegs i designacions:
|
INHABITED SKY
