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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.
| High resolution optical spectroscopy of IRAS 09425-6040 (=GLMP 260)
We present high resolution optical spectroscopic observations of IRAS09425-6040, a peculiar, extremely red, C-rich AGB star showing prominentO-rich dust features in its ISO infrared spectrum attributed tocrystalline silicates. Our analysis shows that IRAS 09425-6040 is indeeda C-rich star slightly enriched in lithium ({log (Li/H) + 12} 0.7)with a low 12C/13C = 15 ± 6 ratio. We alsofound some evidence that it may be enriched in s-elements. Combining ourresults with other observational data taken from the literature weconclude that the star is possibly an intermediate-mass TP-AGB star (M 3 M_ȯ) close to the end of its AGB evolution which may haveonly very recently experienced a radical change in its chemistry,turning into a carbon-rich AGB star.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| The cool Galactic R Coronae Borealis variable DY Persei
Results of first CCD photometry during the recent deep light decline,and high-resolution spectroscopy, are presented for DY Persei. Thespectra show variable blueshifted features in the sodium D lines. The Ci lines are strong whereas neutron-capture elements are not enhanced.The isotopic 13CN(2, 0) lines relative to 12CN areof similar strength with those for the carbon star U Hya. All theseconfirm the RCB nature of DY Per and the existence of ejected clouds. Atleast two clouds are revealed at -197.3 and -143.0 km s-1. Astar was detected about 0.4 arcsec to the west and 2.5 arcsec to thenorth from DY Per. This anonymous companion, with observed colourindices (B-V) = 0.68 and (V-R) ≃ 1.1, may be a foreground star.
| 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.
| 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
| Carbon Stars in the Uves Paranal Observatory Project
The high resolution spectra of five cool carbon stars (X TrA, TW Hor, WOri, U Ant and U Hya) obtained in the UVES Paranal Observatory Projectwere used to examine the quality of R. A. Bell's line-list forsynthesizing the spectra of cool carbon stars. We propose theimprovements of this line-list and estimate the abundances of severalchemical elements.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| On the Origin of Long Secondary Periods in Semiregular Variables
The presence of a long secondary period (LSP) in the light curves ofsome local semiregular variables has been known for many years.Furthermore, the LSPs have recently been found in the light curves ofapproximately 25% of the semiregular variables in the LMC. Theytypically have a length of ~500-4000 days, some 5-15 times longer thanthe primary period. Binarity, pulsation, periodic dust ejection, androtation have been suggested as the origin of the LSPs. Here we analyzeechelle spectra of a group of local semiregular variables with LSPs(hereafter LSPVs) in order to try to distinguish between thesesuggestions. In general, we find that LSPVs do not have broader spectralfeatures than semiregulars without a long secondary period (hereafternon-LSPVs). The general upper limit on the equatorial rotation velocityof 3 km s-1 rules out rotating spot and similar models. OneLSPV, V Hya, does have broader spectral lines than similar carbon stars,but it is shown here that rotation alone is not a good model forexplaining the broad lines. Mid-infrared colors of LSPs and non-LSPVsare similar and there are no LSPVs showing the large (60-25) μm IRAScolor exhibited by some R Coronae Borealis (RCB) stars. Thus, there isno evidence for periodic dust ejection from LSPVs. Finally, we find thatthe LSPVs show larger radial velocity variations than non-LSPVs, whichsuggests that LSPs are caused either by binarity or by pulsation. Asimilar conclusion was derived by Hinkle and co-workers.
| 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
| 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
| The evolution of the Mira variable R Hydrae
The Mira variable R Hydrae is well known for its declining period, whichWood & Zarro attributed to a possible recent thermal pulse. Here weinvestigate the long-term period evolution, covering 340 years, goingback to its discovery in AD 1662. The data include photometricmonitoring by amateur and other astronomers over the last century, andrecorded dates of maximum for earlier times. Wavelets are used todetermine both the period and the semi-amplitude. We show that theperiod decreased linearly between 1770 and 1950; since 1950 the periodhas stabilized at 385d. The semi-amplitude is shown to follow the periodevolution closely. Analysis of the oldest data shows that before 1770the period was about 495d. We find no evidence for an increasing periodduring this time as found by Wood & Zarro. We discuss the mass-losshistory of R Hya: the IRAS data show that the mass loss droppeddramatically around AD 1750. The evolution of the mass loss as functionof period agrees with the mass-loss formalism from Vassiliadis &Wood; it is much larger than predicted by the Blöcker law. An outerdetached IRAS shell suggests that R Hya has experienced mass-lossinterruptions before. The period evolution can be explained by twomodels: a thermal pulse occurring around AD 1600, or a non-linearinstability leading to an internal relaxation of the stellar structure.The elapsed time between the mass-loss decline giving rise to the outerdetached shell and the recent event, of approximately 5000yr, suggeststhat only one of these events could be due to a thermal pulse. Furthermonitoring of R Hya is recommended, as both models make strongpredictions for the future period evolution. We argue that R Hya-typeevents could provide part of the explanation for the rings seen aroundsome asymptotic giant branch (AGB) and post-AGB stars. Changes in Miraproperties were already known on a cycle-to-cycle basis, and on thethermal pulse time-scale of ~104yr. R Hya shows thatsignificant evolution can also occur on intermediate time-scales of theorder of 102-103yr.
| 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
| Millimetre observations of infrared carbon stars. II. Mass loss rates and expansion velocities
Dust- and gas mass loss rates and distances are determined for a sampleof about 330 infra-red carbon stars that probe a distance up to about5.5 kpc. The dependence of the dust- and gas mass loss rates, and theexpansion velocity upon galactic longitude (l) are studied. It is foundthat the expansion velocity significantly depends on l, but that theabsolute bolometric magnitude, the dust mass loss rate and thegas-to-dust ratio depend on l marginally, if at all, and the gas massloss rate does not depend on l. Beyond the solar circle, the expansionvelocity (as well as the luminosity, dust-to-gas ratio, dust mass lossrate) is lower than inside the solar circle, as expected from theoverall gradient in metallicity content of the Galaxy. Combining theaverage expansion velocity inside and beyond the solar circle with thetheoretically predicted relation between expansion velocity andgas-to-dust ratio, we find that the metallicity gradient in the solarneighbourhood is about -0.034 dex/kpc, well within the quoted range ofvalues in the literature.
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| Discovery of Two New HCN Maser Lines in Five Carbon Stars
A survey with the Heinrich Hertz Submillimeter Telescope of HCN emissionfrom mass-losing carbon stars has revealed masers in the J=3-2 and 4-3transitions of the (011c0) vibrational bending mode. Theselines have not previously been known to show maser action. Five stars-RScl, V384 Per, R Lep, Y CVn, and V Cyg-out of 12 observed were detectedas masers. Allowing for evidence of variability, this detection ratesuggests that these HCN lines are masers at least some of the time inthe majority of mass-losing carbon stars. The line widths and velocitiesimply that the maser action occurs in gas close to the star, where thecircumstellar envelope is just being accelerated outward.
| 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).
| Imaging of detached shells around the carbon stars R Scl and U Ant through scattered stellar light
We present the first optical images of scattered light from large,detached gas/dust shells around two carbon stars, RScl and U Ant, obtained in narrow bandfilters centred on the resonance lines of neutral K and Na, and in aStröm}gren b filter (only U Ant). They confirmresults obtained in CO radio line observations, but also reveal new andinteresting structures. Towards R Scl the scatteringappears optically thick in both the K and Na filters, and both imagesoutline almost perfectly circular disks with essentially uniformintensity out to a sharp outer radius of ~21arcsec . These disks arelarger - by about a factor of two - than the radius of the detachedshell which has been marginally resolved in CO radio line data. InU Ant the scattering in the K filter appears to be,at least partially, optically thin, and the image is consistent withscattering in a geometrically thin (~3arcsec ) shell (radius ~43arcsec )with an overall spherical symmetry. The size of this shell agrees verywell with that of the detached shell seen in CO radio line emission. Thescattering in the Na filter appears more optically thick, and the imagesuggests the presence of at least one, possibly two, shells inside the43arcsec shell. There is no evidence for such a multiple-shell structurein the CO data, but this can be due to considerably lower masses forthese inner shells. Weak scattering appears also in a shell which islocated outside the 43arcsec shell. The present data do not allow us toconclusively identify the scattering agent, but we argue that most ofthe emission in the K and Na filter images is to due to resonance linescattering, and that there is also a weaker contribution from dustscattering in the U Ant data. Awaiting newobservational data, our interpretation must be regarded as tentative.Based on observations using the 3.6 m telescope of the European SouthernObservatory, La Silla, Chile.
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| Models of circumstellar molecular radio line emission. Mass loss rates for a sample of bright carbon stars
Using a detailed radiative transfer analysis, combined with an energybalance equation for the gas, we have performed extensive modelling ofcircumstellar CO radio line emission from a large sample of opticallybright carbon stars, originally observed by Olofsson et al. (ApJS, 87,267). Some new observational results are presented here. We determinesome of the basic parameters that characterize circumstellar envelopes(CSEs), e.g., the stellar mass loss rate, the gas expansion velocity,and the kinetic temperature structure of the gas. Assuming a sphericallysymmetric CSE with a smooth gas density distribution, created by acontinuous mass loss, which expands with a constant velocity we are ableto model reasonably well 61 of our 69 sample stars. The derived massloss rates depend crucially on the assumptions in the circumstellarmodel, of which some can be constrained if enough observational dataexist. Therefore, a reliable mass loss rate determination for anindividual star requires, in addition to a detailed radiative transferanalysis, good observational constraints in the form of multi-lineobservations and radial brightness distributions. In our analysis we usethe results of a model for the photodissociation of circumstellar CO byMamon et al. (1988). This leads to model fits to observed radialbrightness profiles that are, in general, very good, but there are alsoa few cases with clear deviations, which suggest departures from asimple r-2 density law. The derived mass loss rates spanalmost four orders of magnitude, from ~ 5 10-9Msun yr-1 up to ~ 2 10-5Msun yr-1, with the median mass loss rate being ~3 10-7 Msun yr-1. We estimate that themass loss rates are typically accurate to ~ 50% within the adoptedcircumstellar model. The physical conditions prevailing in the CSEs varyconsiderably over such a large range of mass loss rates. Among otherthings, it appears that the dust-to-gas mass ratio and/or the dustproperties change with the mass loss rate. We find that the mass lossrate and the gas expansion velocity are well correlated, and that bothof them clearly depend on the pulsational period and (with largerscatter) the stellar luminosity. Moreover, the mass loss rate correlatesweakly with the stellar effective temperature, in the sense that thecooler stars tend to have higher mass loss rates, but there seems to beno correlation with the stellar C/O-ratio. We conclude that the massloss rate increases with increased regular pulsation and/or luminosity,and that the expansion velocity increases as an effect of increasingmass loss rate (for low mass loss rates) and luminosity. Five, of theremaining eight, sample stars have detached CSEs in the form ofgeometrically thin CO shells. The present mass loss rates and shellmasses of these sources are estimated. Finally, in three cases weencounter problems using our model. For two of these sources there areindications of significant departures from overall spherical symmetry ofthe CSEs. Carbon stars on the AGB are probably important in returningprocessed gas to the ISM. We estimate that carbon stars of the typeconsidered here annually return ~ 0.05 Msun of gas to theGalaxy, but more extreme carbon stars may contribute an order ofmagnitude more. However, as for the total carbon budget of the Galaxy,carbon stars appear to be of only minor importance. Presented in thispaper is observational data collected using the Swedish-ESOsubmillimetre telescope, La Silla, Chile, the 20\,m telescope at OnsalaSpace Observatory, Chalmers Tekniska Högskola, Sweden, and the NRAO12\,m telescope located at Kitt Peak, USA.}
| Modeling of C stars with core/mantle grains: Amorphous carbon + SiC
A set of 45 dust envelopes of carbon stars has been modeled. Among them,34 were selected according to their dust envelope class (as suggested bySloan et al. \cite{Sloan98}) and 11 are extreme carbon stars. The modelswere performed using a code that describes the radiative transfer indust envelopes considering core/mantle grains composed by an alpha -SiCcore and an amorphous carbon (A.C.) mantle. In addition, we have alsocomputed models with a code that considers two kinds of grains - alpha-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dustenvelopes of evolved carbon stars, while two homogeneous grains are moreable to reproduce thinner dust envelopes. Our results suggest that thereexists an evolution of dust grains in the carbon star sequence. In thebeginning of the sequence, grains are mainly composed of SiC andamorphous carbon; with dust envelope evolution, carbon grains are coatedin SiC. This phenomena could perhaps explain the small quantity of SiCgrains observed in the interstellar medium. However, in this work weconsider only alpha -SiC grains, and the inclusion of beta -SiC grainscan perhaps change some of these results.
| The Milton Bureau Revisited
Under the direction of Cecilia Payne-Gaposchkin and Sergei Gaposchkin, aprogram was subsidized by the Milton Fund of Harvard Observatory in 1937for the study of all variable stars then known to be brighter than tenthphotographic magnitude at maximum. This included some 1512 stars forwhich a grand total of 1,263,562 estimates of magnitude were made,ranging from a low of 16 (except for a few novae) to 4084 observationsper star. The sky had been divided into 54 fields, and the results ofthe measurements presented field by field in two volumes of the Annalsof Harvard Observatory. Then, in another volume, the results werediscussed in four sections, each dealing with a particular class ofvariable: 1, those of RV Tauri type; 2, the eclipsing variables; 3,Cepheids and RR Lyrae variables, and 4, the red variables, especiallyMira-type and semiregular variables.For the present paper, many of these results have been compared withmodern determinations in the 1985-87 version of the "General Catalogueof Variable Stars (GCVS)". In particular, there are numerous instancesof disagreement as to whether a star should be classified RV or SR.Although there are many instances where the Milton Bureau determinationsof types of variability differ from the types given in moderncatalogues, the reasons for the differences are generallyunderstandable.For 17 RV Tauri type stars in this survey multiple periods have now beendetermined. Many of these still deserve continued observations in orderto ascertain the constance of the periods and improve the accuracy oftheir longest reported periods.
| Horizontal-Branch Models and the Second-Parameter Effect. III. The Impact of Mass Loss on the Red Giant Branch and the Case of M5 and Palomar 4/Eridanus
Deep Hubble Space Telescope (HST) photometry has recently been presentedfor the outer halo globular clusters Palomar 4 and Eridanus. The newhigh-precision color-magnitude diagrams obtained for these globularshave allowed a measurement of their ages relative to M5 (NGC 5904),which is a well-observed, much closer cluster. Assuming that theglobular clusters share the same chemical composition, Pal 4/Eridanushave been reported to be younger than M5 by ~1-2 Gyr, based on both themagnitude difference between the horizontal branch (HB) and the turnoffand the difference in color between the turnoff and the lower subgiantbranch. In the present article, we address the following question: Whatage difference would be required to account for the difference in HBtypes between M5 and Pal 4/Eridanus, assuming age to be the ``secondparameter''? We find that, unless all these clusters (including M5) areyounger than 10 Gyr, such an age difference is substantially larger thanthat based on an analysis of the cluster turnoffs. To reach such aconclusion, six different analytical mass-loss rate formulae (reportedin an Appendix), all implying a dependence of mass loss on the red giantbranch on age, were employed. Our results appear to be in conflict withclaims that age can be the only second parameter in the Galacticglobular cluster system.
| Moderate-Resolution Near-Infrared Spectroscopy of Cool Stars: A New K-Band Library
I present an atlas of near-infrared K-band spectra of 31 late-typegiants and supergiants and two carbon stars. The spectra were obtainedat resolving powers of 830 and 2000, and have a signal-to-noise ratio>~100. These data are complemented with results from similar existinglibraries in both K and H band, and they are used to identify varioustools useful for stellar population studies at moderate resolution. Ifocus on several of the most prominent absorption features and (1)investigate the effects of spectral resolution on measurements of theirequivalent width (EW), (2) examine the variations with stellarparameters of the EWs, and (3) construct composite indices as indicatorsof stellar parameters and of the contribution from excess continuumsources commonly found in star-forming and AGN galaxies. Among thefeatures considered, the 12CO (2,0) and 12CO (6,3)bandheads together with the Si I 1.59 μm feature, first proposed byOliva, Origlia, and coworkers, constitute the best diagnostic set forstellar spectral classification and for constraining the excesscontinuum emission. The Ca I 2.26 μm and Mg I 2.28 μm featuresoffer alternatives in the K band to the 12CO (6,3) bandheadand Si I feature.
| The 12C/13C-ratio in cool carbon stars
We present observations of circumstellar millimetre-wave 13COline emission towards a sample of 20 cool carbon stars. Using a detailedradiative transfer model we estimate the circumstellar12CO/13CO-ratios, which we believe accuratelymeasure the important stellar 12C/13C-ratios. Forthose optically bright carbon stars where it is possible, our derived12C/13C-ratios are compared with the photosphericresults, obtained with different methods. Our estimates agree well withthose of Lambert et al. \citeyearpar{Lambert86}. It is shown that astraightforward determination of the12CO/13CO-ratio from observed line intensityratios is often hampered by optical depth effects, and that a detailedradiative transfer analysis is needed in order to determine reliableisotope ratios. Presented in this paper is observational data collectedusing the Swedish-ESO submillimetre telescope, La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Chalmers Tekniska Högskola,Sweden, and the NRAO 12 m telescope located at Kitt Peak, USA.
| Distance Determination of Mass-Losing Stars
Based on the Principal Component Analysis on IRAS colors and the radiodata, the distances to 183 mass-losing red giant stars were determinedusing the radial velocity and Oort's galactic rotation model for azero-point calibration in the distance modulus. Also, based on therequirement of higher accuracy of the distance determination, themass-losing red giant stars were divided into two groups by means of thefirst-principal component representing an intrinsic photometric propertyof the expanding shell; then, the distances were estimated to be log{d(kpc)}=0.458 p_2+0.09+/-0.13 for group 1 and log {d(kpc)}=0.325p_2+0.45+/-0.15 for group 2, where p_2 is the principal componentcorresponding to the distance, as obtained from the IRAS flux, which wasassumed to be inversely proportional to the square of the distance.Thus,these two groups differ from each other not only by theirphotometric properties, but also by their average distances, by a factorof about 2. Systematic differences exist between the two groups in theirpopulation characteristics and in their evolutionary stages.
| "Real-time" evolution in Mira variables.
After a brief review of our current understanding of Miras and theirevolutionary status, three aspects of ``real-time'' evolution in theseand related stars are examined: # Chemical changes (O-rich to C-rich)due to third dredge-up # Period changes due to the effects of thehelium-shell flash # The existence of ``fossil'' dust and gas shells.Studies of resolved gas and dust shells are highlighted as particularlyinteresting. They will enable us to examine the mass-loss histories ofmany late-type stars over the last ten thousand years or so. Suchobservations have only recently become technically feasible and they areexpected to provide important new insights into the late stages ofstellar evolution.
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Dades d'Observació i Astrometria
| Constel·lació: | Hydra |
| Ascensió Recta: | 10h37m33.20s |
| Declinació: | -13°23'04.0" |
| Magnitud Aparent: | 4.82 |
| Distancia: | 161.812 parsecs |
| Moviment propi RA: | 40.9 |
| Moviment propi Dec: | -38.4 |
| B-T magnitude: | 8.505 |
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