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Temporal variations of the outer atmosphere and the dust shell of the carbon-rich Mira variable V Ophiuchi probed with VLTI/MIDI
Aims.We present the first multi-epoch N-band spectro-interferometricobservations of the carbon-rich Mira variable V Ophusing MIDI at the ESO's Very Large Telescope Interferometer. Our aim isto study temporal variations of physical properties of the outeratmosphere and the circumstellar dust shell based onspectrally-dispersed N-band visibilities over the C2H2 (+HCN) featuresand the dust emission. Methods: Our MIDI observations werecarried out at three different phases 0.18, 0.49, and 0.65, with threedifferent baselines (projected baseline lengths of 42-124 m) using four8.2 m Unit Telescopes (UT2-UT4, UT1-UT4, and UT2-UT3 baselineconfigurations). Results: The wavelength dependence of theuniform-disk diameters obtained at all epochs is characterized by aroughly constant region between 8 and 10 μm with a slight dipcentered at 9.5 μm and a gradual increase longward of 10 μm.These N-band angular sizes are significantly larger than the estimatedphotospheric size of V Oph. The angular sizes observed at differentepochs reveal that the object appears smaller at phase 0.49 (minimumlight) with uniform-disk diameters of 5-12 mas than at phases 0.18 (12-20 mas) and 0.65 ( 9-15 mas). We interpret these results with a modelconsisting of optically thick C2H2 layers and an optically thin dustshell. Our modeling suggests that the C2H2 layers around V Oph are moreextended ( 1.7-1.8 Rstar) at phases 0.18 and 0.65 than atphase 0.49 ( 1.4 Rstar) and that the C2H2 column densitiesappear to be the smallest at phase 0.49. We also find that the dustshell consists of amorphous carbon and SiC with an inner radius of 2.5Rstar, and the total optical depths of τV ≈0.6-0.9 (τ11.3 μm ≈ 0.003 and 0.004 for amorphouscarbon and SiC, respectively) found at phases 0.18 and 0.65 are higherthan the value obtained at phase 0.49, τV ≈ 0.3(τ11.3 μm ≈ 0.001 and 0.002 for amorphous carbonand SiC, respectively). Conclusions: Our MIDI observations andmodeling indicate that carbon-rich Miras also have extended layers ofpolyatomic molecules as previously confirmed in oxygen-rich Miras. Thetemporal variation of the N-band angular size is largely governed by thevariations of the opacity and the geometrical extension of the C2H2layers and the dust shell, and consequently, this masks the sizevariation of the photosphere. Also, the observed weakness of themid-infrared C2H2 absorption in carbon-rich Miras can be explained bythe emission from the extended C2H2 layers and the dust shell.Based on observations made with the Very Large Telescope Interferometerof the European Southern Observatory. Program ID: 075.D-0607. Table 4 isonly available 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/466/1099
| 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.
| Mid-Infrared Spectroscopy of Carbon Stars in the Small Magellanic Cloud
We have observed a sample of 36 objects in the Small Magellanic Cloud(SMC) with the Infrared Spectrometer on the Spitzer Space Telescope.Nineteen of these sources are carbon stars. An examination of the near-and mid-infrared photometry shows that the carbon-rich and oxygen-richdust sources follow two easily separated sequences. A comparison of thespectra of the 19 carbon stars in the SMC to spectra from the InfraredSpace Observatory (ISO) of carbon stars in the Galaxy revealssignificant differences. The absorption bands at 7.5 and 13.7 μm dueto C2H2 are stronger in the SMC sample, and theSiC dust emission feature at 11.3 μm is weaker. Our measurements ofthe MgS dust emission feature at 26-30 μm are less conclusive, butthis feature appears to be weaker in the SMC sample as well. All ofthese results are consistent with the lower metallicity in the SMC. Thelower abundance of SiC grains in the SMC may result in less efficientcarbon-rich dust production, which could explain the excessC2H2 gas seen in the spectra. The sources in theSMC with the strongest SiC dust emission tend to have redder infraredcolors than the other sources in the sample, which implies moreamorphous carbon, and they also tend to show stronger MgS dust emission.The weakest SiC emission features tend to be shifted to the blue; thesespectra may arise from low-density shells with large SiC grains.
| 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.
| Carbon-rich Mira variables: radial velocities and distances
Optical radial velocities have been measured for 38 C-type Miravariables (C-Miras). These data together with others in the literatureare used to study the differences between optical and CO millimetre (mm)observations for C-Miras and the necessary corrections to the opticalvelocities are derived in order to obtain the true radial velocities ofthe variables. The difference between absorption and emission-linevelocities is also examined. A particularly large difference (+30kms-1) is found in the case of the Hα line. A catalogue isgiven of 177 C-Miras with estimated distances and radial velocities. Thedistances are based on bolometric magnitudes derived in Paper I usingSouth African Astronomical Observatory (SAAO) observations or (for 60 ofthe stars) using non-SAAO photometry. In the latter case, the necessarytransformations to the SAAO system are derived. These data will be usedin Paper III to study the kinematics of the C-Miras.
| Proposal of observational program of CCD monitoring of 30 carbon Miras in V, R and I
We present a new long-term observational program of CCD monitoring of 30selected carbon Miras (V 374 Aql, S Aur, UV Aur, AU Aur, AZ Aur, S Cam,R CMi, W Cas, X Cas, HV Cas, S Cep, V CrB, U Cyg, V Cyg, RS Cyg, WX Cyg,T Dra, R For, VX Gem, ZZ Gem, V Hya, CZ Hya, R Lep, U Lyr, CL Mon, VOph, RZ Peg, SY Per, RU Vir, SS Vir) in the standard V, RC and I Johnsonian filters. The aim of the program is to reveal and describephotometric behavior of above mentioned objects and to ascertain thereliability of visual observations of these variable stars. The authorswill send interested persons the pertinent finding charts and they willsee to process rough CCD exposures taken in desired colors themselves.
| 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.
| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| Atmospheric dynamics in carbon-rich Miras
Originating in different depths of the very extended atmospheres of AGBstars, various molecular spectral lines observable in the near-infraredshow diverse behaviours and can be used to probe atmospheric dynamicsthroughout the outer layers of these pulsating red giants. In Nowotny etal. (2005b, A&A, 437, 273, Paper I) time series of synthetichigh-resolution spectra were presented, computed from a dynamic modelatmosphere for a typical carbon-rich Mira. In this work, line profileshapes, their variations during the lightcycle and radial velocitiesderived from wavelength shifts are analyzed and compared with resultsfrom observed FTS spectra of the C-rich Mira S Cep and other Miras. Itis found that the global velocity structure of the model is inqualitative agreement with observations. Radial velocities of molecularlines sampling different layers behave comparably, although somedifferences are apparant concerning absolute values. A correction factorof p ≈ 1.36 between measured RVs and actual gas velocities is derivedfor CO Δ v=3 lines. It is shown that dynamic model atmospheres arecapable of reproducing Mira spectra without introducing an additional"static layer" proposed by several authors.
| Atmospheric dynamics in carbon-rich Miras
Atmospheres of evolved AGB stars are heavily affected by pulsation, dustformation and mass loss, and they can become very extended. Time seriesof observed high-resolution spectra proved to be a useful tool to studyatmospheric dynamics throughout the outer layers of these pulsating redgiants. Originating at various depths, different molecular spectrallines observed in the near-infrared can be used to probe gas velocitiesthere for different phases during the lightcycle. Dynamic modelatmospheres are needed to represent the complicated structures of Miravariables properly. An important aspect which should be reproduced bythe models is the variation of line profiles due to the influence of gasvelocities. Based on a dynamic model, synthetic spectra (containing COand CN lines) were calculated, using an LTE radiative transfer code thatincludes velocity effects. It is shown that profiles of lines thatsample different depths qualitatively reproduce the behaviour expectedfrom observations.
| Atlas of Hα emission lines and V light curves of 30 carbon Miras
This paper presents an atlas of V light curves and Hα emissionlines of 30 carbon Miras observed in various photometrical phases. Thevisualization of both photometric and spectral variations allowed us toreveal a strong correlation between the equivalent widths of theHα emission of carbon Miras and their V brightnesses as a functionof the photometric phase.
| 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.
| 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
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| 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.
| 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.}
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| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| 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
| 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.
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| Multiepoch Interferometric Study of Mira Variables. I. Narrowband Diameters of RZ Pegasi and S Lacertae
As part of the long-term monitoring of Mira variables at the PalomarTestbed Interferometer, we report high-resolution narrowband angularsizes of the oxygen-rich Mira S Lac and the carbon-rich Mira RZ Peg inthe near-infrared. The data set spans three pulsation cycles for S Lacand two pulsation cycles for RZ Peg (a total of 1070 25 s observations)and represents the first study to correlate multiepoch narrowbandinterferometric data of Mira variables. When the calibrated visibilitydata are fitted using a uniform disk brightness model, differences areseen in their angular diameters as a function of wavelength within the Kband (2.0-2.4 μm), the source of which is believed to be molecularabsorptions in or above the photospheres of the two chemically differentMiras. Using visible photometric data provided by the AssociationFrancaise des Observateurs d'Etoiles Variables (AFOEV), the continuumminimum size of RZ Peg lags this by 0.28+/-0.02 in pulsation, similar tothe phase lag found in Correlation Radial Velocities (CORAVEL) data.However, for S Lac, the continuum minimum size tracks the visual maximumbrightness. Based on the mean of the continuum angular diametercycloids, basic stellar parameters are computed for both RZ Peg and SLac, with both showing maximum atmospheric extension with respect to the2.0 and 2.4 μm diameters near phase 0.9. Using the mean value of thefitted cycloids, RZ Peg has a radius Rmean=377+/-111Rsolar and a mean Teff=2706+/-36 K; S Lac has aradius Rmean=292+/-73 Rsolar and a meanTeff=2605+/-47 K. The dominant source of error in the radiiis the large uncertainty in the distances to these two stars.
| Probing the mass loss history of carbon stars using CO line and dust continuum emission
An extensive modelling of CO line emission from the circumstellarenvelopes around a number of carbon stars is performed. By combiningradio observations and infrared observations obtained by ISO thecircumstellar envelope characteristics are probed over a large radialrange. In the radiative transfer analysis the observational data areconsistently reproduced assuming a spherically symmetric and smooth windexpanding at a constant velocity. The combined data set gives betterdetermined envelope parameters, and puts constraints on the mass losshistory of these carbon stars. The importance of dust in the excitationof CO is addressed using a radiative transfer analysis of the observedcontinuum emission, and it is found to have only minor effects on thederived line intensities. The analysis of the dust emission also putsfurther constraints on the mass loss rate history. The stars presentedhere are not likely to have experienced any drastic long-term mass lossrate modulations, at least less than a factor of ~ 5, over the pastthousands of years. Only three, out of nine, carbon stars were observedlong enough by ISO to allow a detection of CO far-infrared rotationallines. Based on observations with ISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries: France,Germany, The Netherlands and the UK) and with the participation of ISASand NASA. Radio data collected with the OSO 20 m telescope, the SEST,and the JCMT, have also been used.
| 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
| Period-doubling events in the light curve of R Cygni: Evidence for chaotic behaviour
A detailed analysis of the century long visual light curve of thelong-period Mira star R Cygni is presented and discussed. The data werecollected from the publicly available databases of the AFOEV, the BAAVSSand the VSOLJ. The full light curve consists of 26655 individual pointsobtained between 1901 and 2001. The light curve and its periodicity wereanalysed with help of the O-C diagram, Fourier analysis andtime-frequency analysis. The results demonstrate the limitations ofthese linear methods. The next step was to investigate the possiblepresence of low-dimensional chaos in the light curve. For this, asmoothed and noise-filtered signal was created from the averaged dataand with the help of time delay embedding, we have tried to reconstructthe attractor of the system. The main result is that R Cygni shows suchperiod-doubling events that can be interpreted as being caused by arepetitive bifurcation of the chaotic attractor between a period 2Torbit and chaos. The switch between these two states occurs in a certaincompact region of the phase space, where the light curve ischaracterized by ~ 1500-day long transients. The Lyapunov spectrum wascomputed for various embedding parameters confirming the chaoticattractor, although the exponents suffer from quite high uncertaintybecause of the applied approximation. Finally, the light curve iscompared with a simple one zone model generated by a third-orderdifferential equation which exhibits well-expressed period-doublingbifurcation. The strong resemblance is another argument for chaoticbehaviour. Further studies should address the problem of global flowreconstruction, including the determination of the accurate Lyapunovexponents and dimension.
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