Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
The abundance of SiS in circumstellar envelopes around AGB stars
Aims.Given their photospheric origin and refractive nature, SiSmolecules can provide major constraints on the relative roles of dustcondensation and non-equilibrium processes in regulating the chemistryin circumstellar envelopes around evolved stars. Methods: New SiSmulti-transition (sub-)millimetre line observations of a sample of AGBstars with varying photospheric C/O-ratios and mass-loss rates arepresented. A combination of low- and high-energy lines are important inconstraining the circumstellar distribution of SiS molecules. A detailedradiative transfer modelling of the observed SiS line emission isperformed, including assessment of the effect of thermal dust grains inthe excitation analysis. Results: We find that the circumstellarfractional abundance of SiS in these environments has a strongdependence on the photospheric C/O-ratio as expected from chemicalmodels. The carbon stars (C/O > 1) have a mean fractional abundanceof 3.1 × 10-6, about one order of magnitude higher thanthat found for the M-type AGB stars (C/O < 1) where the mean value is2.7 × 10-7. These numbers are in reasonable agreementwith photospheric LTE chemical models. SiS appears to behave similarlyto SiO in terms of photodissociation in the outer part of thecircumstellar envelope. In contrast to previous results for the relatedmolecule SiO, there is no strong correlation of the fractional abundancewith density in the CSE, as would be the case if freeze-out onto dustgrains were important. However, possible time-variability of the lineemission in the lower J transitions and the sensitivity of the lineemission to abundance gradients in the inner part of the CSE may mask acorrelation with the density of the wind. There are indications that theSiS fractional abundance could be significantly higher closer to thestar which, at least in the case of M-type AGB stars, would requirenon-equilibrium chemical processes.
| Correlations between Lithium and Technetium Absorption Lines in the Spectra of Galactic S Stars
Correlations between the presence of the 6707 Å line of lithiumand the resonance lines of technetium (4238 and 4262 Å) in a largesample of Galactic S stars are analyzed. Half of the sample stars areintrinsic S stars (those exhibiting technetium in their spectra), and1/3 of these stars also have strong lithium lines in their spectra.Stars having both lithium and technetium in their spectra areinterpreted as intermediate-mass thermally pulsating asymptotic giantbranch (TP-AGB) stars in which lithium is produced by the Cameron-Fowlermechanism. The production of lithium is predicted to occur inhigh-luminosity (Mbol<=-6) TP-AGB stars by the hot-bottomburning (HBB) mechanism. Data on the carbon isotope ratios of stars inour sample agree with the predictions of HBB; however, oxygen isotoperatios in these stars do not agree with the predictions of HBB.Furthermore, the available luminosities for our sample stars are belowthe minimum value necessary for HBB to occur in available models.Cool-bottom processing (CBP) is one possible explanation for thepresence of lithium in the spectra of these stars. Intrinsic S starshaving technetium but no lithium in their spectra are interpreted aslower mass (1.5-3 Msolar) thermally pulsating AGB stars thathave not undergone CBP. Extrinsic S stars constitute the remaining halfof the sample. Carbon and oxygen isotope ratios, as well as the lack oftechnetium and lithium in the spectra of these stars, are consistentwith these being low-mass red giant branch stars (1-2Msolar), with mass transfer from a now extinct thermallypulsating AGB star being responsible for the enhanced abundance ofs-process elements.
| Challenging the Carbon Star Dust Condensation Sequence: Anarchist C Stars
There have been several investigations of the evolution of themid-infrared (IR) dust features in carbon star spectra based on IRAS LRSdata, but these studies are somewhat contradictory. In order tounderstand these differences in interpretations and to develop anunderstanding of the carbon star dust sequence, we have reexamined 26IRAS LRS spectra of carbon stars that have also been observedspectroscopically by ISO SWS. The low resolution and narrow wavelengthcoverage of the IRAS LRS data hinder determination of the effect ofmolecular absorptions in these spectra. This has led to incorrectestimations of the continuum levels in these spectra, which has a hugeeffect on the continuum-divided and continuum-subtracted spectra used toanalyze trends in the shape, strength, and position of the mid-IRfeatures. The higher resolution and broader wavelength coverage of theISO data allow more accurate fitting of the underlying continuum. Wehave reassessed the trends in shape, strength, and position of the ~11μm silicon carbide (SiC) feature and the apparent emergence of the ~9μm feature. We find that there are no correlations between thespectral parameters. We also investigate whether any of these parameterscorrelate with the strength of the molecular bands; no correlation wasfound. Moreover, we show that the apparent 9 μm feature is probablyan artifact. We discuss the implications of this study in terms of botha carbon star condensation sequence and the application of this study tothe larger IRAS data set.
| 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.
| 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.
| SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption
Aims.New SiO multi-transition millimetre line observations of a sampleof carbon stars, including J = 8→7 observations with the APEXtelescope, are used to probe the role of non-equilibrium chemistry andthe influence of grains in circumstellar envelopes of carbon stars. Methods: .A detailed radiative transfer modelling, including theeffect of dust emission in the excitation analysis, of the observed SiOline emission is performed. A combination of low- and high-energy linesare important in constraining the abundance distribution. Results:.It is found that the fractional abundance of SiO in these C-richenvironments can be several orders of magnitude higher than predicted byequilibrium stellar atmosphere chemistry. In fact, the SiO abundancedistribution of carbon stars closely mimic that of M-type (O-rich) AGBstars. A possible explanation for this behaviour is a shock-inducedchemistry, but also the influence of dust grains, both as a source fordepletion as well as production of SiO, needs to be furtherinvestigated. As observed for M-type AGB stars, a clear trend that theSiO fractional abundance decreases as the mass-loss rate of the starincreases is found for the carbon stars. This indicates that SiO isaccreted onto dust grains in the circumstellar envelopes.
| Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
| 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.
| 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.
| Carbon Stars in the Infrared Telescope in Space Survey
We have identified 139 cool carbon stars in the near-infraredspectrophotometric survey of the Infrared Telescope in Space (IRTS) fromthe conspicuous presence of molecular absorption bands at 1.8, 3.1, and3.8 μm. Among them, 14 are new bright (K~4.0-7.0) carbon stars. Wefind a trend relating the 3.1 μm band strength to the K-L'color index, which is known to correlate with mass-loss rate. This couldbe an effect of a relation between the depth of the 3.1 μm featureand the degree of development of the extended stellar atmosphere wheredust starts to form.
| 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
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Molecular and dust features of 29 SiC carbon AGB stars
We have reduced and analyzed the Infrared Space Observatory (ISO)Short-Wavelength Spectrometer (SWS) spectra of 29 infrared carbon starswith a silicon carbide feature at 11.30 μm, 17 of which have not beenpreviously published. Absorption or emission features of C2,HCN, C2H2, C3 and silicon carbide (SiC)have been identified in all 17 unpublished carbon stars. In addition,two unidentified absorption features at 3.50 and 3.65 μm are listedfor the first time in this paper. We classify these 29 carbon stars intogroups A, B, C and D according to the shapes of their spectral energydistribution, and this classification seems to show an evolutionarysequence of carbon stars with an SiC feature. Moreover we have found thefollowing results for the different groups: on average, the relativeintegrated flux of the 3.05 μm C2H2+HCNabsorption feature increases gradually from group A to B and C; that ofthe 5.20 μm C3 absorption feature becomes gradually weakerfrom group A to B and C; that of the 11.30 μm SiC emission featureincreases gradually from group A to B and C but weakens in group D; andin contrast, that of the 13.70 μm C2H2absorption feature weakens gradually from group A to B and C but becomesstronger in group D. We suggest that the evolution of the IR spectra ofcarbon stars along the sequence A to D is a result of the followingphenomena: as the near-IR black-body temperature (Tnir)decreases, the circumstellar envelope becomes thicker; also theeffective temperature (Teff) of the photosphere of thecentral star decreases gradually and the C/O ratio increases from A toB.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) with the participation of ISAS and NASA.
| MgS in detached shells around carbon stars. Mining the mass-loss history
We investigate the dust composition of detached shells around carbonstars, with a focus to understand the origin of the coolmagnesium-sulfide (MgS) material around warm carbon stars, which hasbeen detected around several of these objects\citep{2002A&A...390..533H}. We build a radiative transfer model ofa carbon star surrounded by an expanding detached shell of dust. Theshell contains amorphous carbon grains and MgS grains. We find that asmall fraction of MgS dust (2% of the dust mass) can give a significantcontribution to the IRAS 25 μm flux. However, the presence of MgS inthe detached shell cannot be inferred from the IRAS broadband photometryalone but requires infrared spectroscopy.We apply the model to the detached-shell sources R Scl and U Cam, bothexhibiting a cool MgS feature in their ISO/SWS spectra. We use the shellparameters derived for the molecular shell, using the CO submillimetremaps \citep{1999A&A...351L...1L,2001A&A...368..969S}. Themodels, with MgS grains located in the detached shell, explain the MgSgrain temperature, as derived from their ISO spectra, very well. Thisdemonstrates that the MgS grains are located at the distance of thedetached shell, which is a direct indication that these shells originatefrom a time when the stellar photosphere was already carbon-rich. In thecase of R Scl, the IRAS photometry is simultaneously explained by thesingle shell model. In the case of U Cam, the IRAS photometry is underpredicted, pointing to a contribution from cooler dust located evenfarther away from the star than the molecular shell.We present a simple diagnostic to constrain the distance of the shellusing the profile of the MgS emission feature. The emission featureshifts to longer wavelength with decreasing grain temperature. One cantherefore infer a temperature and a corresponding distance to the starfrom the observed profile. Such a diagnostic might prove useful forfuture studies of such systems with SIRTF or SOFIA.based on observations obtained with ISO, an ESA project with instrumentsfunded by ESA member states (especially the PI countries: France,Germany, the Netherlands and UK) with the participation of ISAS andNASA.}
| 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.
| 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
| Galactic mass-losing AGB stars probed with the IRTS. II.
We are using the 2002 data-release from the Japanese space experimentIRTS to investigate the spatial distribution of galactic mass-losing(>2x 10-8 Msund) AGB stars and the relativecontribution of C-rich and O-rich ones to the replenishment of the ISM.Our sample contains 126 C-rich and 563 O-rich sources which are sortedon the basis of the molecular bands observed in the range 1.4-4.0 mu m,and for which we estimate distances and mass loss rates fromnear-infrared photometry (K and L'). There is a clear dependence ongalactocentric distance, with O-rich sources outnumbering C-rich onesfor rGC< 8 kpc, and the reverse for rGC> 10kpc. The contribution to the replenishment of the ISM by O-rich AGBstars relative to C-rich ones follows the same trend. Although they arerare ( ~ 10% in our sample), sources with 10-6Msund < dot {M} < 10-5 Msunddominate the replenishment of the ISM by contributing to ~ 50% of thetotal of the complete sample. We find 2 carbon stars at more than 1 kpcfrom the Galactic Plane, that probably belong to the halo of our Galaxy.The complete Tables \ref{tab_C-rich} and \ref{tab_O-rich} are availablein electronic 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/403/943}
| 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
| Infrared investigation from earth and space on the evolutionary state of a sample of LPV
We selected a sample of highly reddened AGB stars among the sourcesobserved with the SWS instrument on the ISO satellite. These SWS dataallow us to compute the source's photometry in the mid-IR filters of thecamera TIRCAM at the TIRGO telescope. Our photometric data, supplementedwith other measurements taken from the literature, permit to select thecarbon-rich sources in the sample. For these stars, a linear relationholds between dust mass loss and the color index [8.8]-[12.5]. One maythen, from photometric data alone, evaluate the total mass loss (forwhich we used the estimate of \citet{loup}, based on radio data). Theoxygen-rich sources, on the other hand, are distributed in two branches,of which the upper one appears superimposed with carbon stars; the starsin this group have both high luminosity and high wind velocity andtherefore higher masses. Finally S stars lie between the carbon-starbranch and the low-mass oxygen-rich stars, in agreement with theirintermediate evolutionary status.
| Medium-Resolution Stellar Spectra in the L Band from 2400 to 3000 cm-1 (3.3 to 4.2 Microns)
We present a brief atlas of L-band (3.3-4.2 μm) spectra for 42 starsplus the Sun and a sunspot observed at a resolving power of R~3000. Thiscontribution is intended to supplement our previous K-band, H-band, andJ-band spectral atlases. The L-band data, which cover some or all of the2400 to 3000 cm-1 (3.3-4.2 μm) region, are mainly forluminous late-type stars. In reducing these data, special care has beentaken to remove telluric features, especially water vapor. We identifytemperature- and luminosity-sensitive atomic and molecular indices toaid in the classification of stellar spectra in the L band. The data areavailable electronically.
| 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.
| 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
| The carrier of the ``30'' mu m emission feature in evolved stars. A simple model using magnesium sulfide
We present 2-45 mu m spectra of a large sample of carbon-rich evolvedstars in order to study the ``30'' mu m feature. We find the ``30'' mu mfeature in a wide range of sources: low mass loss carbon stars, extremecarbon-stars, post-AGB objects and planetary nebulae. We extract theprofiles from the sources by using a simple systematic approach to modelthe continuum. We find large variations in the wavelength and width ofthe extracted profiles of the ``30'' mu m feature. We modelled the wholerange of profiles in a simple way by using magnesium sulfide (MgS) dustgrains with a MgS grain temperature different from the continuumtemperature. The systematic change in peak positions can be explained bycooling of MgS grains as the star evolves off the AGB. In severalsources we find that a residual emission excess at ~ 26 mu m can also befitted using MgS grains but with a different grains shape distribution.The profiles of the ``30'' mu m feature in planetary nebulae arenarrower than our simple MgS model predicts. We discuss the possiblereasons for this difference. We find a sample of warm carbon-stars withvery cold MgS grains. We discuss possible causes for this phenomenon. Wefind no evidence for rapid destruction of MgS during the planetarynebula phase and conclude that the MgS may survive to be incorporated inthe ISM. Based on observations obtained with ISO, an ESA project withinstruments funded by ESA Member states (especially the PI countries:France, Germany, The Netherlands and the United Kingdom) with theparticipation of ISAS and NASA. Appendix A (Figs. A.1 and A.2) is onlyavailable in electronic form at http://www.edpsciences.org
| High Angular Resolution Observations of Late-Type Stars
This paper presents speckle observations of Mira (o Cet) and late-typestars with the PISCO speckle camera of Pic du Midi during the period1995-1998. A survey for binarity among a sample of late-type stars wasperformed, which led to seven positive detections out of 36 objects.Photometric and color variations of the companion of Mira were searchedfor, but no significant brightness variations could be found over atimescale of ~5-10 minutes. The position and photometry measurements,the restored images with high angular resolution of the binary systemMira A-B (ADS 1778) are in full agreement with Hubble Space Telescopedata obtained at the same epoch. A new orbit has been derived for MiraA-B.
| On the Transience of High-Latitude OH/IR Stars. II. Thermal Pulse Link
High-latitude, |b|>=10deg, asymptotic (AGB) stars in theirsuperwind phase have distinctive IR colors, which make them easy toidentify and count. But the expansion velocity (Ve) versuscolor plots of these stars are a puzzle, with features implying (1) astep increase in dM/dt and (2) short-term changes in the circumstellarshell that are enhanced by increased metallicity. The net duration oftheir superwind phase is ~3700 yr when estimated from the relativefrequency of associated proto-planetary nebulae and the ~103 yrexpansion age of one, IRAS 18095+2704. Since these stars lose on average~0.04 Msolar during the superwind phase and have progenitormasses circa 1 Msolar, they must lose more than 0.3Msolar prior to the AGB. A single 3700 yr superwind episode,however, cannot explain the 2-18 km s-1 range of expansionvelocities exhibited by these stars. This range is best understood asresulting from a cyclical superwind, wherein stars lose most of theirenvelope mass in three to four discrete episodes of ever largerVe, with each episode being triggered by a sudden onset inthe coupling of photon momentum by dust to gas near a thermal pulse. Theresulting newly accelerated shell thereafter quickly expands beyond theprotective dust shroud of the prior shell, which allows interstellar UVto degrade its molecules. This turns the shell into an OH/IR star colormimic (a shell with the dM/dt of an OH/IR star without 1612 MHz masers),until enough dust is again in place beyond the shell to allow moleculesthe longevity needed to support masers. Two by-products of this modelare the natural explanation it offers for why Ve(CO)>Ve(OH), as well as for the occurrence and integration ofmimics into the normal development of a shell. The circumstellar shellsof high-latitude OH/IR stars are thus cyclically transient phenomena.
| The 85Kr s-Process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type redgiants. These objects belong to the class of asymptotic giant branchstars, experiencing thermal instabilities in the He-burning shell(thermal pulses). Mixing episodes called third dredge-up enrich thephotosphere with newly synthesized 12C in the He-rich zone,and this is the source of the high observed ratio between carbon andoxygen (C/O>=1 by number). Our spectroscopic abundance estimatesconfirm that, in agreement with the general understanding of the lateevolutionary stages of low- and intermediate-mass stars, carbonenrichment is accompanied by the appearance of s-process elements in thephotosphere. We discuss the details of the observations and of thederived abundances, focusing in particular on rubidium, a neutrondensity sensitive element, and on the s-elements Sr, Y, and Zr belongingto the first s-peak. The critical reaction branching at 85Kr,which determines the relative enrichment of the studied species, isdiscussed. Subsequently, we compare our data with recent models fors-processing in thermally pulsing asymptotic giant branch stars, atmetallicities relevant for our sample. A remarkable agreement betweenmodel predictions and observations is found. Thanks to the differentneutron density prevailing in low- and intermediate-mass stars,comparison with the models allows us to conclude that most C(N) starsare of low mass (M<~3 Msolar). We also analyze the12C/13C ratios measured, showing that most of themcannot be explained by canonical stellar models. We discuss how thisfact would require the operation of an ad hoc additional mixing,currently called cool bottom process, operating only in low-mass starsduring the first ascent of the red giant branch and, perhaps, alsoduring the asymptotic giant branch.
| Galactic mass-losing AGB stars probed with the IRTS. I.
AGB mass-losing sources are easy to identify and to characterize in thenear-infrared range (1-5 mu m). We make use of the near-infrared dataacquired by the Japanese space experiment IRTS to study a sample ofsources detected in the 2 celestial strips surveyed by the IRTS.Mass-loss rates and distances are estimated for 40 carbon-rich sourcesand 86 oxygen-rich sources of which 8 are probably of S-type. Althoughthe sample is small, one sees a dependence of the relative contributionof the two kinds of sources to the replenishment of the interstellarmedium (ISM) on the galactocentric distance. E.g. from 6 to 8 kpc,oxygen-rich sources in our sample contribute 10-12 times as much ascarbon rich sources, whereas from 10 to 12 kpc, the latters contribute3-4 times as much as the formers. Therefore, one would expect a gradientin the composition of the ISM between 6 and 12 kpc from the GalacticCentre, especially in its dust component. Most of the replenishment(>50%) by AGB stars is due to sources with mass-loss rate larger than10-6 Msun yr-1.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Camelopardalis |
| Ascensió Recta: | 03h41m48.18s |
| Declinació: | +62°38'54.4" |
| Magnitud Aparent: | 7.382 |
| Distancia: | 1265.823 parsecs |
| Moviment propi RA: | 7.9 |
| Moviment propi Dec: | -0.6 |
| B-T magnitude: | 12.449 |
| V-T magnitude: | 7.801 |
Catàlegs i designacions:
|
