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Lithium and zirconium abundances in massive Galactic O-rich AGB stars
Lithium and zirconium abundances (the latter taken as representative ofs-process enrichment) are determined for a large sample of massiveGalactic O-rich AGB stars, for which high-resolution opticalspectroscopy has been obtained (R˜ 40 000{-}50 000). This was doneby computing synthetic spectra based on classical hydrostatic modelatmospheres for cool stars and using extensive line lists. The resultsare discussed in the framework of "hot bottom burning" (HBB) andnucleosynthesis models. The complete sample is studied for variousobservational properties such as the position of the stars in the IRAStwo-colour diagram ([ 12] - [25] vs. [ 25] - [60] ), Galacticdistribution, expansion velocity (derived from the OH maser emission),and period of variability (when available). We conclude that aconsiderable fraction of these sources are actually massive AGB stars(M>3{-}4 Mȯ) experiencing HBB, as deduced from thestrong Li overabundances we found. A comparison of our results withsimilar studies carried out in the past for the Magellanic Clouds (MCs)reveals that, in contrast to MC AGB stars, our Galactic sample does notshow any indication of s-process element enrichment. The differencesobserved are explained as a consequence of metallicity effects. Finally,we discuss the results obtained in the framework of stellar evolution bycomparing our results with the data available in the literature forGalactic post-AGB stars and PNe.Based on observations at the 4.2 m William Herschel Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto de Astrofisicade Canarias. Also based on observations with the ESO 3.6 m telescope atLa Silla Observatory (Chile). Tables [see full textsee full text]-[seefull textsee full text] are only available in electronic form athttp://www.aanda.org
| WZ Cas - ein Halbregelmaessiger.
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| A chemical study of the inner winds of asymptotic giant branch stars
Aims.We investigate the non-equilibrium chemistry of the inner winds ofAGB stars for different stages of stellar evolution, choosing a standardAGB stellar model and changing photospheric C/O ratios, to describewinds of M, S, and C stars. Chemical formation pathways for severalimportant molecules and the chemistry of S stars and its implicationsfor the nature of the dust forming in these objects are discussed.Methods: .The inner wind standard model (≡gas density,temperature, and velocity) is derived from taking into account theeffect of shocks induced by stellar pulsation on the gas. The chemistryconsists of 68 elements and molecules and 752 chemical reactions.Molecular concentration profiles are derived by solving a system ofnon-linear, stiff, ordinary, coupled differential equations applied tothe wind model gas parameters. Results: .We find that unexpectedmolecules are present in the inner winds, as a result of non-equilibriumchemistry due to shock propagation. In particular, there exists a groupof molecules always formed in the inner wind of AGBs, whatever the stageof evolution of the star, i.e., CO, HCN, CS, and SiO, while other groupsof species are typical of a O-rich or C-rich chemistry. The shockedregions above the photosphere, where thermal equilibrium does not apply,act as true molecular factories.
| Improved HCN/HNC linelist, model atmospheres and synthetic spectra for WZ Cas
We build an accurate data base of 5200 HCN and HNC rotation-vibrationenergy levels, determined from existing laboratory data. 20000 energylevels in the Harris et al. linelist are assigned approximate quantumnumbers. These assignments, lab-determined energy levels and Harris etal. energy levels are incorporated in to a new energy level list. A newlinelist is presented, in which frequencies are computed using thelab-determined energy levels where available, and the ab initio energylevels otherwise.The new linelist is then used to compute new model atmospheres andsynthetic spectra for the carbon star WZ Cas. This results in better fitto the spectrum of WZ Cas in which the absorption feature at 3.56μmis reproduced to a higher degree of accuracy than has previously beenpossible. We improve the reproduction of HCN absorption features byreducing the abundance of Si to [Si/H]=-0.5dex, however, the strengthsof the Δv= 2 CS band heads are overpredicted.
| Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud
We present ESO/VLT spectra in the 2.9-4.1 μm range for a large sampleof infrared stars in the Large Magellanic Cloud (LMC), selected on thebasis of MSX and 2MASS colours to be extremely dust-enshrouded AGB starcandidates. Out of 30 targets, 28 are positively identified as carbonstars, significantly adding to the known population of opticallyinvisible carbon stars in the LMC. We also present spectra for sixIR-bright stars in or near three clusters in the LMC, identifying fourof them as carbon stars and two as oxygen-rich supergiants. We analysethe molecular bands of C2H2 at 3.1 and 3.8 μm, HCN at 3.57 μm, andsharp absorption features in the 3.70-3.78 μm region that weattribute to C2H2. There is evidence for a generally high abundance ofC2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundanceratios at the low metallicity in the LMC. The low initial metallicity isalso likely to have resulted in less abundant HCN and CS. The sample ofIR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do notfind strong correlations between the properties of the molecularatmosphere and circumstellar dust envelope, but the observed differencesin the strengths and shapes of the absorption bands can be explained bydifferences in excitation temperature. High mass-loss rates and strongpulsation would then be seen to be associated with a large scale heightof the molecular atmosphere.
| 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.
| The chemical composition of R-stars
The preliminary results of the chemical analysis of a sample of 22galactic R-stars with measured parallaxes are presented. We have derivedthe C/O and 12C/13C ratios, the averagemetallicity ([M/H]) and the Li abundances. We find that most of thestars have 12C/13C below 20 with nosignificant difference between cool and hot R-stars. The C/O ratioranges between 1 to 2. We obtain a clear separation in the Liabundance between hot and cool R stars, with a mean value of log epsilon(Li) = +0.90 and log epsilon (Li) =-0.50, respectively. Cool stars areof near solar metallicity whereas hot stars show a larger spread,-0.5< [Fe/H] < +0.1. Concerning the possibility of s-elementenhancements, we obtain [Rb/M] > 0 in seven hot stars and in three ofthem (R-hot) the analysis of the 5924 {Å} TcI line might becompatible with Tc detection. However, the detection of other s-elementenhancements is necessary to elucidate the evolutionary stage of thesecarbon stars of which origin is still unknown.
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| Status of the physics of substellar objects
A full understanding of the properties of substellar objects is one ofthe major challenges facing astrophysics. Since their discovery in 1995,we have discovered hundreds of brown dwarfs and extrasolar planets.While these discoveries have enabled important comparisons with theory,observational progress has been much more rapid than the theoreticalunderstanding of cool atmospheres. The reliable determination of mass,abundances, gravities and temperatures is not yet possible. The keyproblem is that substellar objects emit their observable radiation inthe infrared region of the spectrum where our knowledge of atomic,molecular and line broadening data is poor. Here we report on the statusof PoSSO (Physics of SubStellar Objects). In order to understand browndwarfs and extrasolar planets increasing more like those in our solarsystem, we are studying a wide range of processes. Here we give anupdate on the project and sketch an outline of atoms, molecules andprocesses requiring study.
| WZ Cas - variability on multiple time-scales
We present results on our long-term velocity monitoring of the C-richsemi-regular variable WZ Cas with a Coravel instrument. Our findings arecompared with light changes of the star. We suggest that the two mainperiods, 366 and 186 d, are due to radial pulsation. Furthermore, wefind an interesting variation of the width and depth of thecross-correlation profile over a time-scale of at least 1000 d. Severalpossible explanations for this behaviour are discussed by comparison oftime-scales and expected variability amplitudes. The influence ofmovements of large convective cells on the line profiles seems to be themost likely explanation of some of the observed phenomena.
| Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studiedusing infrared L-band spectra of AGB stars (both carbon-rich andoxygen-rich) and M-type supergiants in the Large and Small MagellanicClouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. Thespectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CHand HCN bands for carbon-rich AGB stars. The equivalent width ofacetylene is found to be high even at low metallicity. The high C2H2abundance can be explained with a high carbon-to-oxygen (C/O) ratio forlower metallicity carbon stars. In contrast, the HCN equivalent width islow: fewer than half of the extra-galactic carbon stars show the 3.5μm HCN band, and only a few LMC stars show high HCN equivalent width.HCN abundances are limited by both nitrogen and carbon elementalabundances. The amount of synthesized nitrogen depends on the initialmass, and stars with high luminosity (i.e. high initial mass) could havea high HCN abundance. CH bands are found in both the extra-galactic andGalactic carbon stars. One SMC post-AGB star, SMC-S2, shows the 3.3μm PAH band. This first detection of a PAH band from an SMC post-AGBstar confirms PAHs can form in these low-metallicity stars. None of theoxygen-rich LMC stars show SiO bands, except one possible detection in alow quality spectrum. The limits on the equivalent widths of the SiObands are below the expectation of up to 30 Å for LMC metallicity.Several possible explanations are discussed, mostly based on the effectof pulsation and circumstellar dust. The observations imply that LMC andSMC carbon stars could reach mass-loss rates as high as their Galacticcounterparts, because there are more carbon atoms available and morecarbonaceous dust can be formed. On the other hand, the lack of SiOsuggests less dust and lower mass-loss rates in low-metallicityoxygen-rich stars. The effect on the ISM dust enrichment is discussed.
| Detection of Silicon Nitride Particles in Extreme Carbon Stars
On the basis of spectra taken by the Infrared Space Observatory andlaboratory measurements, we present the first spectroscopic evidence forthe presence of silicon nitride (Si3N4) grains inthe circumstellar shells of a number of extreme carbon stars. Laboratorymeasurements of α, β, and amorphousSi3N4 are compared with spectra of the extremecarbon stars IRAS 21318+5631 and AFGL 2477. Both stars show anabsorption feature near the well-known 11 μm band of silicon carbide(SiC) but peaking at and extending to considerably shorter wavelengths.We demonstrate that the position and the band profile of the observedfeatures coincide well with the main features of laboratorySi3N4 spectra, especially in the case of IRAS21318+5631. While this band alternatively may be explained by SiC grainsplus interstellar silicate absorption, in the way worked out by Speckand coworkers, there is further strong evidence forSi3N4 grains in IRAS 21318+5631 and possibleevidence for Si3N4 grains in AFGL 2477 provided byweaker absorption features in the wavelength region longward of the mainfeature.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA. The SWS is a joint project of SRON and MPE.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| 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
| Period and chemical evolution of SC stars
The SC and CS stars are thermal-pulsing asymptotic giant branch starswith a C/O ratio close to unity. Within this small group, the Miravariable BH Cru recently evolved from spectral type SC (showing ZrObands) to CS (showing weak C2). Wavelet analysis shows thatthe spectral evolution was accompanied by a dramatic period increase,from 420 to 540 d, indicating an expanding radius. The pulsationamplitude also increased. Old photographic plates are used to establishthat the period before 1940 was around 490 d. Chemical models indicatethat the spectral changes were caused by a decrease in stellartemperature, related to the increasing radius. There is no evidence fora change in C/O ratio. The evolution in BH Cru is unlikely to be relatedto an ongoing thermal pulse. Periods of the other SC and CS stars,including nine new periods, are determined. A second SC star, LX Cyg,also shows evidence for a large increase in period, and one further starshows a period inconsistent with a previous determination. Mira periodsmay be intrinsically unstable for C/O ~ 1; possibly because of afeedback between the molecular opacities, pulsation amplitude, andperiod. LRS spectra of 6 SC stars suggest a feature at λ > 15μm, which resembles one recently attributed to the iron-sulphidetroilite. Chemical models predict a large abundance of FeS in SC stars,in agreement with the proposed association.
| Dynamic model atmospheres of AGB stars. IV. A comparison of synthetic carbon star spectra with observations
We have calculated synthetic opacity sampling spectra for carbon-richAsymptotic Giant Branch (AGB) stars based on dynamic model atmosphereswhich couple time-dependent dynamics and frequency-dependent radiativetransfer, as presented in the third paper of this series. We include themolecules CO, CH, CN, C2, CS, HCN, C2H2and C3 in our calculations, both when computing theatmospheric structures, and the synthetic spectra. A comparison of thesynthetic spectra with various observed colours and spectra in thewavelength range between 0.5 and 25 μm,of TX Psc, WZ Cas, V460 Cyg, TLyr and S Cep is presented. We obtain good agreement betweenobservations gathered at different phases and synthetic spectra of onesingle hydrodynamical model for each star in the wavelength regionbetween 0.5 and 5 μm. At longer wavelengths our models showing massloss offer a first self-consistent qualitative explanation of why astrong feature around 14 μm, which is predicted by all hydrostaticmodels as well as dynamical models showing no mass loss, is missing inobserved AGB carbon star spectra.
| An attempt to derive Mg isotopic ratios in carbon stars
We discuss the use of the spectral range near 7570 Å todetermine the Mg isotopic ratios in carbon stars using theB'2 Sigma +-X 2 Sigma + system of the MgH molecule. We also compare with thespectral range near 5140 Å that has been commonly used in normalstars (oxygen-rich stars). The region near 5140 Å is not thebetter choice for carbon stars because it is very crowded with CN andC_2 molecular bands and the localization of the continuum iscomplicated. The range near 7570 Å is less blended withmolecular bands. The continuum is easy to locate and the isotopicsplitting between MgH isotopic absorptions is larger. Unfortunately, weare not able to reproduce accurately the observed spectrum in thisregion and, moreover, the synthetic spectrum is not sensitive to largevariations in the isotopic ratios.
| Lithium in the symbiotic Mira V407 Cyg
We report an identification of the lithium resonance doublet Li I 6708Å in the spectrum of V407 Cyg, a symbiotic Mira with a pulsationperiod of about 745 d. The resolution of the spectra used wasR=λ/Δλ~ 18 500 and the measured equivalent width ofthe line is ~0.34 Å. It is suggested that the lithium enrichmentis due to hot bottom burning in the intermediate-mass asymptotic giantbranch (AGB) variable, although other possible origins cannot be totallyruled out. In contrast to lithium-rich AGB stars in the MagellanicClouds, ZrO λλ5551, 6474 absorption bands were not foundin the spectrum of V407 Cyg. These are the bands used to classify theS-type stars at low-resolution. Although we identified weak ZrOλλ5718, 6412 these are not visible in the low-resolutionspectra, and we therefore classify the Mira in V407 Cyg as an M type.This, together with other published work, suggests lithium enrichmentcan precede the third dredge up of s-process enriched material ingalactic AGB stars.
| The identification of HCN and HNC in carbon stars: model atmospheres, synthetic spectra and fits to observations in the 2.7-4.0 μm region
Model carbon star atmospheres and synthetic spectra have been calculatedusing the recent HCN/HNC vibration rotation line list of Harris et al.The calculations are repeated using only HCN lines and show that HNC hasa significant effect upon the temperature, density and optical depth ofa stellar atmosphere.We fit synthetic spectra in the 2.7-4.0 μm region to observed ISOspectra of the carbon stars WZ Cas and TX Psc obtained by Aoki et al.These fits allow us to identify absorption by HNC in the spectrum of WZCas at 2.8-2.9 μm, and to determine new independent estimates ofeffective temperature and log (NC/NO). Thefindings reported here indicate that absorption by both HCN and HNC isneeded to fully explain the observed stellar spectra and represent thefirst identification of HNC in a star. Q-branch absorption by the HCNΔv2= 1, Δv3= 1 andΔv1= 1, Δv2=-1 bands at 3.55 and 3.86μm, respectively, is identified in the spectrum of WZ Cas.
| Lithium in Large Magellanic Cloud carbon stars
19 carbon stars that show lithium enrichment in their atmospheres havebeen discovered among a sample of 674 carbon stars in the LargeMagellanic Cloud. Six of the Li-rich carbon stars are of the J type,i.e. they show strong 13C isotopic features. No super-Li-richcarbon stars were found. The incidence of lithium enrichment amongcarbon stars in the LMC is much rarer than in the Galaxy, and about fivetimes more frequent among J-type than among N-type carbon stars. Thebolometric magnitudes of the Li-rich carbon stars range between -3.3 and-5.7. Existing models of Li-enrichment via the hot bottom burningprocess fail to account for all of the observed properties of theLi-enriched stars studied here.
| 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.
| Detection of Lithium in the Spectrum of the Symbiotic Mira Star V407 Cygni
We detected the resonance lithium doublet, 7LiI lambda 6708,in the spectra of the symbiotic Mira V407 Cyg (P = 763 days) obtainedwith a resolution R = lambda/Delta lambda = 18500. The line equivalentwidth is 0.34 A. The presence of lithium in the atmosphere of the Mira,which, judging by its period, appreciably ascended the asymptotic giantbranch (AGB), can be explained by the penetration of its convectiveenvelope into the hot-bottom-burning (HBB) hydrogen-shell source. At thesame time, the spectrum of V407 Cyg does not reveal the ZrO lambda 5551,6474 absorption bands, which are used to classify S-type stars fromlow-resolution spectra. We found only weak ZrO lambda 5718, 6412 bands,which are, however, invisible in low-resolution spectra. Thus, the Mirastar V407 Cyg should be classified as being of the spectral type M. Ingeneral, the spectrum of V407 Cyg is similar to the spectrum of the redgiant in the symbiotic star CH Cyg, but the latter exhibits no lithiumlines. The switch-on of the HBB process without any significantenrichment of the atmosphere of an AGB star with s-process elementsimplies that the third dredge-up is not efficient for some of thesestars.
| 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
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 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/399/1083
| 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
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| Very Large Telescope Spectra of Carbon Stars in the Large Magellanic Cloud and Their Metallicity Dependence
Very Large Telescope (VLT) L-band spectra of six carbon stars in theLarge Magellanic Cloud are presented. The stars show absorption bands at3.1 μm (HCN and C2H2), and 3.8 μm, which isprobably due to C2H2. Two LMC stars show strong3.5 μm HCN absorption. The equivalent widths of the 3.1 μm and 3.8μm bands are systematically larger in LMC carbon stars than in carbonstars in the solar neighborhood. Moreover, the ratio of the equivalentwidths of the 3.8 and 3.1 μm bands is much larger in the LMC,suggesting a higher ratio of n(C2H2)/n(HCN). Thestronger absorption bands are in contrast to the assumption that if theelemental abundances are scaled from the carbon star's abundances in thesolar neighbor, the abundances of these molecules are less at lowermetallicity. We argue for a systematically larger C/O ratio in LMCcarbon stars. In the Galactic carbon stars n(C)/n(O)~1.05-1.1 onaverage; our chemical model shows that the stronger molecular bands inthe LMC carbon stars could be explained with n(C)/n(O)>1.2. Thehigher C/O ratio can also explain the higher ratio ofn(C2H2)/n(HCN) in LMC stars than in the solarneighborhood. Based on observations obtained at the European SouthernObservatory, Chile (proposal 68.D-0660).
| 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.
| Opacity Data for HCN and HNC from a New Ab Initio Line List
A new extensive ab initio rotation-vibration HCN/HNC line list ispresented. The line list contains rotation-vibration energy levels, linefrequencies, and line strengths for transitions between states withenergy less than 18,000 cm-1 and with J<=60. This linelist greatly improves the quality and range of HCN/HNC data available.It is presently the most extensive and most accurate ab initio HCN/HNCline list in existence. It is hoped that this data set will be used inmodels of C star atmospheres and elsewhere.
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| Takýmyýldýz: | Zincirli Prenses |
| Sað Açýklýk: | 00h01m15.86s |
| Yükselim: | +60°21'19.0" |
| Görünürdeki Parlaklýk: | 7.066 |
| Uzaklýk: | 787.402 parsek |
| özdevim Sað Açýklýk: | 9.3 |
| özdevim Yükselim: | -4.3 |
| B-T magnitude: | 11.073 |
| V-T magnitude: | 7.397 |
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