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Resim Yükleyin
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Ýlgili Makaleler
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.
| Carbon-rich Mira variables: kinematics and absolute magnitudes
The kinematics of Galactic C-Miras are discussed on the basis of thebolometric magnitudes and radial velocities of Papers I and II of thisseries. Differential Galactic rotation is used to derive a zero-pointfor the bolometric period-luminosity relation which is in satisfactoryagreement with that inferred from the Large Magellanic Cloud (LMC)C-Miras. We find for the Galactic Miras, Mbol = -2.54 logP +2.06(+/-0.24), where the slope is taken from the LMC. The mean velocitydispersion, together with the data of Nordström et al. and thePadova models, leads to a mean age for our sample of C-Miras of 1.8 +/-0.4Gyr and a mean initial mass of 1.8 +/- 0.2 Msolar.Evidence for a variation of velocity dispersion with period is found,indicating a dependence of period on age and initial mass, the longerperiod stars being younger. We discuss the relation between the O- andC-Miras and also their relative numbers in different systems.
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| 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.
| First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars
We have measured nonzero closure phases for about 29% of our sample of56 nearby asymptotic giant branch (AGB) stars, using the three-telescopeInfrared Optical Telescope Array (IOTA) interferometer at near-infraredwavelengths (H band) and with angular resolutions in the range 5-10 mas.These nonzero closure phases can only be generated by asymmetricbrightness distributions of the target stars or their surroundings. Wediscuss how these results were obtained and how they might beinterpreted in terms of structures on or near the target stars. We alsoreport measured angular sizes and hypothesize that most Mira stars wouldshow detectable asymmetry if observed with adequate angular resolution.
| 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.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars
To search for phase lags in the optical-infrared light curves ofasymptotic giant branch stars, we have compared infrared data from theCOBE DIRBE satellite with optical light curves from the AAVSO and othersources. We found 17 examples of phase lags between the times ofinfrared and optical maximum, and 4 stars with no observed lags. Thereis a clear difference between the Mira variables and the semiregularvariables in the sample, with the maximum in the optical preceding thatin the near-infrared in the Miras, while in most of the semiregularvariables no lags are observed. Comparison to published theoreticalmodels indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggeststhat Miras pulsate in the fundamental mode, while at least somesemiregular variables are first-overtone pulsators. There is a clearoptical-near-infrared phase lag in the carbon-rich Mira V CrB; this islikely due to C2 and CN absorption variations in the optical.
| 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.
| 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.
| 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.
| 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.
| 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
| JHKLM Photometry for Carbon Stars
We discuss our JHKLM photometry for nine carbon Mira stars, eighteencarbon semiregular variables, and two oxygen Mira stars. For fourteencarbon stars, we present and analyze their infrared light and colorcurves. For all of the observed objects, we have estimated the opticaldepths of the circumstellar dust envelopes, the angular diameters of thestars, and their temperatures.
| 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.
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| 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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| Post-Agb Stars
In this contribution, a review is presented on the ample data obtainedon post-AGB stars, both on the central stars and their circumstellarmaterial. The fast evolutionary phase is characterized by a rapid changein the properties of the objects, but the variety is so large that thereis yet no clear consensus on how the detailed studies of individualobjects are linked together by evolutionary channels. The absence ofstrong molecular veiling in the photospheres of the central stars,together with a spread in intrinsic metallicity make post-AGB stars veryuseful in constraining AGB chemical evolutionary models. We discuss thesurprisingly wide variety of chemical signatures observed. The onset inthe creation process of the panoply of structures and shapes observed inplanetary nebulae occurs during the short post-AGB evolution, but thephysical nature of the processes involved is still badly understood. Inthe rapidly growing field of circumstellar mineralogy, post-AGB starshave their story to tell and also the molecular envelope changessignificantly due to dilution and hardening of the stellar radiation.The real-time evolution of some objects suffering a late thermal flashis reviewed and their possible link to other hydrogen-deficient objectsis discussed. Any review on stellar evolution has a section on binariesand this contribution is no exception because binaries make up asignificant fraction of the post-AGB stars known to date.
| A Uniform Database of 2.4-45.4 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer
We present a complete set of all valid SWS full-scan 2.4-45.4 μmspectra processed and renormalized in as uniform a manner as possible.The processing produces a single spectrum for each observation from the288 individual spectral segments, which are the most processed formavailable from the ISO archive. The spectra, and the programs used tocreate them, are available to the community on-line.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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
| 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
| Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K/'-band observations of five Mira stars with the IOTAinterferometer. The interferograms were obtained with the FLUOR fiberoptics beam combiner, which provides high-accuracy visibilitymeasurements in spite of time-variable atmospheric conditions. For theM-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB wederived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas,and 7.9 mas (/+/-0.3 mas), respectively. Simultaneous photometricobservations yielded the bolometric fluxes. The derived angularRosseland radii and the bolometric fluxes allowed the determination ofeffective temperatures. For instance, the effective temperature of R Aqlwas determined to be 2970/+/-110 K. A linear Rosseland radius for R Aqlof 250+100-60Rsolar was derived fromthe angular Rosseland radius of 5.5+/-0.2mas and the HIPPARCOS parallaxof 4.73+/-1.19mas. The observations were compared with theoretical Mirastar models of Bessel et al. [A&A 307 (1996) 481] and Hofmann et al.[A&A 339 (1998) 846]. The effective temperatures of the M-type Mirasand the linear radius of R Aql indicate fundamental mode pulsation.
| Aus der Sektion Mirasterne.
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| Very Large Telescope Spectra of Carbon Stars in the Large Magellanic Cloud and Their Metallicity Dependence
Very Large Telescope (VLT) L-band spectra of six carbon stars in theLarge Magellanic Cloud are presented. The stars show absorption bands at3.1 μm (HCN and C2H2), and 3.8 μm, which isprobably due to C2H2. Two LMC stars show strong3.5 μm HCN absorption. The equivalent widths of the 3.1 μm and 3.8μm bands are systematically larger in LMC carbon stars than in carbonstars in the solar neighborhood. Moreover, the ratio of the equivalentwidths of the 3.8 and 3.1 μm bands is much larger in the LMC,suggesting a higher ratio of n(C2H2)/n(HCN). Thestronger absorption bands are in contrast to the assumption that if theelemental abundances are scaled from the carbon star's abundances in thesolar neighbor, the abundances of these molecules are less at lowermetallicity. We argue for a systematically larger C/O ratio in LMCcarbon stars. In the Galactic carbon stars n(C)/n(O)~1.05-1.1 onaverage; our chemical model shows that the stronger molecular bands inthe LMC carbon stars could be explained with n(C)/n(O)>1.2. Thehigher C/O ratio can also explain the higher ratio ofn(C2H2)/n(HCN) in LMC stars than in the solarneighborhood. Based on observations obtained at the European SouthernObservatory, Chile (proposal 68.D-0660).
| s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-elementcontent in Galactic carbon stars of N type. Abundances of Sr, Y, Zr(low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-masss-elements, or hs) are derived using the spectral synthesis techniquefrom high-resolution spectra. The N stars analyzed are of nearly solarmetallicity and show moderate s-element enhancements, similar to thosefound in S stars, but smaller than those found in the only previoussimilar study (Utsumi 1985), and also smaller than those found insupergiant post-asymptotic giant branch (post-AGB) stars. This is inagreement with the present understanding of the envelope s-elementenrichment in giant stars, which is increasing along the spectralsequence M-->MS-->S-->SC-->C during the AGB phase. Wecompare the observational data with recent s-process nucleosynthesismodels for different metallicities and stellar masses. Good agreement isobtained between low-mass AGB star models (M<~3 Msolar)and s-element observations. In low-mass AGB stars, the13C(α, n)16O reaction is the main source ofneutrons for the s-process a moderate spread, however, must exist in theabundance of 13C that is burnt in different stars. Bycombining information deriving from the detection of Tc, the infraredcolors, and the theoretical relations between stellar mass, metallicity,and the final C/O ratio, we conclude that most (or maybe all) of the Nstars studied in this work are intrinsic, thermally pulsing AGB stars;their abundances are the consequence of the operation of third dredge-upand are not to be ascribed to mass transfer in binary systems.
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| Takýmyýldýz: | Kuzey Taci |
| Sað Açýklýk: | 15h49m31.31s |
| Yükselim: | +39°34'18.0" |
| Görünürdeki Parlaklýk: | 9.552 |
| özdevim Sað Açýklýk: | 0 |
| özdevim Yükselim: | 0 |
| B-T magnitude: | 13.55 |
| V-T magnitude: | 9.883 |
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