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Mid-Infrared Photometry of Mass-losing Asymptotic Giant Branch Stars
We present ground-based mid-IR imaging for 27 M-, S-, and C-typeasymptotic giant branch (AGB) stars. The data are compared with those ofthe database available thanks to the IRAS, Infrared Space Observatory,Midcourse Space Experiment, and Two Micron All Sky Survey catalogs. Ourgoal is to establish relations between the IR colors, the effectivetemperature Teff, the luminosity L, and the mass-loss rateM˙, for improving the effectiveness of AGB modeling. Bolometric(absolute) magnitudes are obtained through distance compilations and byapplying previously derived bolometric corrections; the variability isalso studied, using data accumulated since the IRAS epoch. The mainresults are as follows: (1) Values of L and M˙ for C stars fitrelations previously established by us, with Mira variables being onaverage more evolved and mass-losing than semiregular variables. (2)Moderate IR excesses (as compared to evolutionary tracks) are found forS and M stars in our sample: they are confirmed to originate from thedusty circumstellar environment. (3) A larger reddening characterizesC-rich Mira variables and post-AGB stars. In this case, part of theexcess is due to AGB models overestimating Teff for C stars,as a consequence of the lack of suitable molecular opacities. This has alarge effect on the colors of C-rich sources, and sometimesdisentangling the photospheric and circumstellar contributions isdifficult; better model atmospheres should be used in stellarevolutionary codes for C stars. (4) The presence of a long-termvariability at mid-IR wavelengths seems to be limited to sources withmaximum emission in the 8-20 μm region, usually Mira variables(one-third of our sample). Most of the semiregular and post-AGB starsstudied here have remained remarkably constant in the mid-IR over thelast 20 years.
| Correlations between Lithium and Technetium Absorption Lines in the Spectra of Galactic S Stars
Correlations between the presence of the 6707 Å line of lithiumand the resonance lines of technetium (4238 and 4262 Å) in a largesample of Galactic S stars are analyzed. Half of the sample stars areintrinsic S stars (those exhibiting technetium in their spectra), and1/3 of these stars also have strong lithium lines in their spectra.Stars having both lithium and technetium in their spectra areinterpreted as intermediate-mass thermally pulsating asymptotic giantbranch (TP-AGB) stars in which lithium is produced by the Cameron-Fowlermechanism. The production of lithium is predicted to occur inhigh-luminosity (Mbol<=-6) TP-AGB stars by the hot-bottomburning (HBB) mechanism. Data on the carbon isotope ratios of stars inour sample agree with the predictions of HBB; however, oxygen isotoperatios in these stars do not agree with the predictions of HBB.Furthermore, the available luminosities for our sample stars are belowthe minimum value necessary for HBB to occur in available models.Cool-bottom processing (CBP) is one possible explanation for thepresence of lithium in the spectra of these stars. Intrinsic S starshaving technetium but no lithium in their spectra are interpreted aslower mass (1.5-3 Msolar) thermally pulsating AGB stars thathave not undergone CBP. Extrinsic S stars constitute the remaining halfof the sample. Carbon and oxygen isotope ratios, as well as the lack oftechnetium and lithium in the spectra of these stars, are consistentwith these being low-mass red giant branch stars (1-2Msolar), with mass transfer from a now extinct thermallypulsating AGB star being responsible for the enhanced abundance ofs-process elements.
| Analysis of ISO SWS01 spectra of S stars
The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer(SWS01) plays an important role in studying properties of S stars. Wereduce and analyze the SWS01 spectra of 17 S stars, and identify thecandidate carriers of molecular absorption features. The ISO SpectralAnalysis Package (ISAP) developed by the LWS and SWS Instrument Teamsand Data Centers is used to process and analyze the SWS01 spectra of 17S stars. The ISO archives of 17 S stars are obtained from the ISOdatabase. Of 17 S stars, two stars are extrinsic S stars, the others areintrinsic S stars. The 15 intrinsic S stars can be divided into threegroups (6 stars for Group I, 7 stars for Group II, and 2 stars for GroupIII) according to their Infrared Astronomical Satellite (IRAS)low-resolution spectra (LRS) and dust mass-loss rate dot{M}_dust, wheredot{M}_dust increases from Group I to II and III gradually. 17 S starsshow the following properties: 1. two extrinsic S stars and 15 intrinsicS stars among different groups have different infrared properties; 2.two extrinsic S stars and 6 intrinsic S stars in Group I have similarISO SWS01 spectra and their continua can be approximately described by asingle blackbody representing the stellar photosphere, while someintrinsic S stars in Group I have 60 μm infrared excess; 3. forintrinsic S stars in Groups I, II, and III, their continua peak shiftstoward longer wavelength from Groups I to II to III; 4. S stars inGroups II and III show obvious dust emission features in which the 10μm dust features seem to display two different shapes. Moreover, twoS stars (IRAS 00192-2020 & IRAS 15492+4837) present the 13 μmfeature; 5. molecules H{2}O, CO, and CO{2} greatly affect the ISO SWS01spectra of S stars. It is noted that the absorption features ofmolecules CS and HCN usually thought to exist only in C stars may appearin the S stars.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands, and the United Kingdom) with the participation of ISAS andNASA.
| 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.
| Two Micron All Sky Survey, Infrared Astronomical Satellite, and Midcourse Space Experiment Color Properties of Intrinsic and Extrinsic S Stars
We attempt to select new candidate intrinsic and extrinsic S stars inthe General Catalogue of Galactic S Stars (GCGSS) by combining data fromthe Two Micron All Sky Survey, the Infrared Astronomical Satellite, andthe Midcourse Space Experiment. Catalog entries are cross-identified,yielding 528 objects, out of which 29 are known extrinsic S stars and 31are known intrinsic S stars. Their color-color diagrams,(H-[12])-(K-[12]) and (K-[12])-(J-[25]), are drawn and used to identifya new sample of 147 extrinsic and 256 intrinsic S star candidates, whilethe nature of 65 stars remains identified. We infer that about 38%+/-10%of the GCGSS objects are of extrinsic type. Moreover, we think thatcolors such as J-[25] can be used to split off the two categories of Sstars, while single colors are not appropriate. The color-colordiagrams, such as (H-[12])-(K-[12]) and (K-[12])-(J-[25]), are proven tobe powerful tools for distinguishing the two kinds of S stars.
| Mass-loss properties of S-stars on the AGB
We have used a detailed non-LTE radiative transfer code to model newAPEX CO(J = 3 → 2) data, and existing CO radio line data, on asample of 40 AGB S-stars. The derived mass-loss-rate distribution has amedian value of 2 × 10-7~Mȯyr-1, and resembles values obtained for similar samples ofM-stars and carbon stars. Possibly, there is a scarcity ofhigh-mass-loss-rate (≥10-5~Mȯyr-1) S-stars. The distribution of envelope gas expansionvelocities is similar to that of the M-stars, the median is 7.5 kms-1, while the carbon stars, in general, have higher gasexpansion velocities. The mass-loss rate correlates well with the gasexpansion velocity, in accordance with results for M-stars and carbonstars.
| 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
| A study of bright Southern long period variables
In this paper we present radial velocity curves of AGB variables thatexhibit various kinds of anomalies: semiregular variables (SRVs) withtypical mira periods, SRVs exceeding the mira 2.5 mag amplitude limit,miras with secondary maxima in their light curves, and a SRV with a longsecondary period. The stars with reliable Hipparcos parallaxes from thisand from previous studies are plotted in a log P-MK-diagram.Our objects nicely follow the log P-MK-relations determinedfor the LMC. This allows the pulsation mode to be identified. While allmiras fall on the fundamental mode sequence, the SRVs fall on both thefirst overtone and fundamental mode sequences. The SRVs on thefundamental mode sequence occur at both high and low luminosities, someof them being more luminous than larger amplitude miras. Thisdemonstrates observationally that some parameter other than luminosityaffects the stability of long period variables, probably mass. Firstovertone pulsators all show velocity amplitudes around 4 kms-1. For the fundamental mode pulsators, the velocityamplitude shows a correlation with light amplitude. The two miras R Cenand R Nor, known for their double-peaked light curves, have velocitycurves that are quite different. The R Nor velocity curve shows noevidence of the double peaks, meaning that the true pulsation period isthe time between alternate minima or maxima. There is slight evidencefor a double bump in the R Cen velocity curve. It is suggested thatthese stars are relatively massive (3-5 Mȯ).
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} 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/430/165}
| The S-Type Asymptotic Giant Branch Stars RS Cnc, ST Her, OP Her And HR Peg
We present FCAPT differential Johnson l BV and Cousins l RI photometryand radial velocities of the S-type AGB stars RS Cnc, ST Her, OP Her andHR Peg. We performed multiperiodic analyses of this photometry to learnabout the frequencies of variability. The l B, V, R, and l I lightcurves are generally in phase. RS Cnc has periods of order 122 and 248days. Repeating the analyses using our data with the extensive l V and lR photometry of Percy et al. (2001) shows several periods near 248 daysand a period near 135 days. The primary period of ST Her appears to beabout 144 days although it is manifested most readily by its 103 dayalias. OP Her and HR Peg exhibit periods, respectively, of about 416 and116 days and of 54 and 74 days. The differences between our data set andthe larger one of Percy et al. (2001) for RS Cnc raises importantquestions about multiperiodic analyses for AGB stars.Tables 2, 4 and 6 are 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/Abstract.html.
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected bystellar luminosity derived from Hipparcos parallaxes. Tc, as an unstables-process element, is a good indicator for the evolutionary status ofstars on the asymptotic giant branch (AGB). In this paper we study theoccurrence of Tc as a function of luminosity to provide constraints onthe minimum luminosity for the third dredge up as estimated from recentstellar evolution models.A large number of AGB stars above the estimated theoretical limit forthe third dredge up are found not to show Tc. We confirm previousfindings that only a small fraction of the semiregular variables show Tclines in their spectra. Contrary to earlier results by Little et al.(\cite{llmb87}) we find also a significant number of Miras without Tc.The presence and absence of Tc is discussed in relation to the massdistribution of AGB stars. We find that a large fraction of the stars ofour sample must have current masses of less than 1.5 Msun .Combining our findings with stellar evolution scenarios we conclude thatthe fraction of time a star is observed as a SRV or a Mira is dependenton its mass.Partly based on observations collected at the European SouthernObservatory, Paranal, Chile (ESO-Programme 65.L-0317(A)).
| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| 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.
| 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
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| Near-infrared observations of candidate extrinsic S stars
Photometric observations in the near infrared for 161 S stars, including18 Tc-rich (intrinsic) stars, 19 Tc-deficient (extrinsic) ones and 124candidates for Tc-deficient S stars, are presented in this paper. Basedon some further investigations into the infrared properties of bothTc-rich and Tc-deficient S stars, 104 candidates are identified as verylikely Tc-deficient S stars. The large number of infrared-selectedTc-deficient S stars provides a convenient way to study the physicalproperties and the evolutionary status of this species of S stars.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Envelope tomography of long-period variable stars III. Line-doubling frequency among Mira stars
This paper presents statistics of the line-doubling phenomenon in asample of 81 long-period variable (LPV) stars of various periods,spectral types and brightness ranges. The set of observations consistsof 315 high-resolution optical spectra collected with the spectrographELODIE at the Haute-Provence Observatory, during 27 observing nights atone-month intervals and spanning two years. When correlated with a maskmimicking a K0III spectrum, 54% of the sample stars clearly showed adouble-peaked cross-correlation profile around maximum light, reflectingdouble absorption lines. Several pieces of evidence are presented thatpoint towards the double absorption lines as being caused by thepropagation of a shock wave through the photosphere. The observation ofthe Balmer lines appearing in emission around maximum light in thesestars corroborates the presence of a shock wave. The observed velocitydiscontinuities, ranging between 10 and 25 km s-1, are notcorrelated with the brightness ranges. A comparison with thecenter-of-mass (COM) velocity obtained from submm CO lines originatingin the circumstellar envelope reveals that the median velocity betweenthe red and blue peaks is blueshifted with respect to the COM velocity,as expected if the shock moves upwards. The LPVs clearly exhibitingline-doubling around maximum light with the K0III mask appear to be themost compact ones, the stellar radius being estimated from theireffective temperatures (via the spectral type) and luminosities (via theperiod-luminosity relationship). It is not entirely clear whether or notthis segregation between compact and extended LPVs is an artefact of theuse of the K0III mask. Warmer masks (F0V and G2V) applied to the mostextended and coolest LPVs yield asymmetric cross-correlation functionswhich suggest that line doubling is occurring in those stars as well.Although a firm conclusion on this point is hampered by the largecorrelation noise present in the CCFs of cool LPVs obtained with warmmasks, the occurrence of line doubling in those stars is confirmed bythe double CO Delta v = 3 lines observed around 1.6 mu m by Hinkle etal. (1984, ApJS, 56, 1). Moreover, the Hdelta line in emission, which isanother signature of the presence of shocks, is observed as well in themost extended stars, although with a somewhat narrower profile. This isan indication that the shock is weaker in extended than in compact LPVs,which may also contribute to the difficulty of detecting line doublingin cool, extended LPVs. Based on observations made at Observatoire deHaute Provence, operated by the Centre National de la RechercheScientifique, France. Table 3 is only available in electronic form athttp://www.edpsciences.org and 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/379/305
| Reddening and age for 13 southern Galactic open clusters determined from integrated spectra
In this study we present flux-calibrated integrated spectra in the range3800-6800 Å for 13 concentrated open clusters with Galacticlongitudes between 219deg and 316deg, nine ofwhich have not been previously studied. Using the equivalent widths ofthe Balmer lines and comparing the cluster spectra with template spectraof Magellanic Clouds and Galactic star clusters with known parameters,we derive both foreground interstellar reddening values and age. Fornine clusters these two parameters have been determined for the firsttime, while for the rest of the sample the results show good agreementwith previous studies. The present analysis indicates four very young(Hogg 11, NGC 5606, vdB-RN 80 and Pismis 17), seven moderately young(ESO 429-SC13, Hogg 3, Hogg 12, Haffner 7, BH 87, NGC 2368 and Bochum12) and two intermediate-age (Berkeley 75 and NGC 2635) open clusters.The derived foreground interstellar reddening values are in the range0.00 <= E(B-V) <= 0.38. The age and reddening distributions of thepresent sample of relatively faint open clusters match those of openclusters with known parameters in a 90deg sector centered atl = 270deg. Based on observations made at ComplejoAstronómico El Leoncito, which is operated under agreementbetween the Consejo Nacional de Investigaciones Científicas yTécnicas de la República Argentina and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina.
| Multi-colour light variation of AGB stars observed with ISO
New visual light curves and infrared multi-epoch photometry arepresented for a sample of AGB-stars spectroscopically observed with ISO.While the ISO work is or will be presented elsewhere, the aim of thispaper is to give an overview of the properties of the light change ofthe objects. This information is crucial for the interpretation of thevaluable ISO material. Using the University of Vienna Twin AutomaticPhotoelectric Telescope (APT) we monitored the stars of our sample inthe photometric bands V and I_C. We present the light curve of eachobject and derive the parameters of the current light change, such asperiod(s) and amplitude. Furthermore we give V-I_C colours and colourvariations for these objects. Our results allow us to derive somegeneral results on semiregular and irregular variables. Only in lessthan 50% of the cases could we confirm the GCVS period. Moreover, we didnot find any pronounced difference between SRb and Lb variables in theregularity of the light curve. The existence of rapid oscillationsindicated by Hipparcos data could not be confirmed. In addition to thevisual light changes we present new near infrared photometry data.Although typically only few data points are available, they can beviewed relative to the better-monitored visual light curves providinginformation on possible phase shifts and differences in amplitude indifferent parts of the spectrum. Furthermore, multi-epoch photometryallows us to derive mean colours for these objects. Based onobservations made with the Carlos Sánchez Telescope operated onthe island of Tenerife by the Instituto de Astrofísica deCanarias (IAC) in the Observatorio del Teide, Izaña. Tables B2 toB5 are only 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/375/527
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| On the Relations between Infrared Colors and Mass Loss Rates for S Stars
A relation between the infrared color K-[12] and the mass loss rate,dM/dt, is found for the intrinsic S stars. This relation isqualitatively similar to the ones for carbon stars and oxygen-rich Mirastars, but quantitatively different. It could be useful in the contextof the near infrared surveys, DENIS and 2MASS, which are in progress.Correlations between J-K and [12]-[25] indices and dM/dt are found to beloose for S stars.
| Hipparcos parallaxes for Mira-like long-period variables
This paper concerns the calibration of the K period-luminosity relationfor Mira variables using Hipparcos parallaxes. K magnitudes areavailable for 255 Mira-like variables which were observed by Hipparcos.Period-luminosity zero-points are evaluated for various subgroups ofdata. The best solution for oxygen-rich Miras, which uses 180 stars,omitting the short-period red group (which had different kinematics fromthe short-period blue stars) and the low-amplitude variables, provides azero-point of σ2σ2π +(0.4605)2π2PL(K)σ2K + σ2PL(K),0.84+/-0.14mag, which implies a distance modulus for the LargeMagellanic Cloud of σK = 0.3ΔK√N,18.64+/-0.14mag, or perhaps slightly greater if a metallicity correctionis required, in good agreement with the value derived from Cepheids. Thezero-point of the period-luminosity relation for carbon stars is brieflydiscussed. Linear diameters are derived for red variables with measuredangular diameters and parallaxes, and are used to examine thelong-standing question of the pulsation mode(s) of these stars. Evidenceis presented to suggest that most of them are pulsating in the same modeand, if published model atmospheres are correct, this is probably thefirst overtone. Some discussion is given of sequences in theperiod-luminosity and period-colour diagrams and their bearing on thepulsation mode problem.
| A comparison of light and velocity variations in semiregular variables
NIR velocity variations are compared with simultaneous visual lightcurves for a sample of late-type semiregular variables (SRV). Preciseradial velocity measurements are also presented for the SRV V450 Aql.Our aim is to investigate the nature of the irregular light changesfound in these variables. Light and velocity variations are correlatedin all stars of our sample. Based on these results we discuss severalpossibilities to explain the observed behavior. We find that pulsationis responsible for large amplitude variations. In a recent paperLebzelter (\cite{Lebzelter99}) invoked large convective cells tounderstand observed velocity variations. This possibility is discussedwith respect to the observed correlation between light and velocitychanges. In the light of these results we investigate the origin of thesemiregular variations.
| Velocity variations of semiregular variables in the infrared
We obtained time series of high resolution spectra around 1.6 mu m for15 short period semiregular variables to study the velocity variationsin their atmospheres with the help of second overtonevibration-rotational lines of CO. All objects of our sample clearly showvelocity variations of a few km s-1. The amplitude of the`blue' SRVs (according to Kerschbaum & Hron te{KH92}) is on theaverage a factor of two smaller than that of the `red' SRVs. Periodicityof the variations differs from star to star. Some SRVs vary with theperiod listed in the General Catalogue of Variable Stars (GCVS4), otherswith a considerable longer period. But for all SRVs the measuredvelocities at 1.6 mu m are over a large part of the cycle smaller thanthe velocities found in the literature. Different explanations of thisfeature are discussed. We think that the observed difference in meanvelocity might be caused by a blueshift due to large convective cells onthe stellar surface.
| A search for Technetium in semiregular variables
We searched for the lines of Tc in the spectra of Semiregular variables(SRVs) in the wavelength region from 4180 to 4300 Å using highresolution spectroscopy. Tc as an s-process element is produced on thethermally pulsing AGB and is therefore a good indicator for theevolutionary status of Semiregular variables. Combining our results withprevious investigations we get a database large enough for a statisticalstudy. Tc is not found in SRVs with periods below 100 days, spectraltypes earlier than M5 and photospheric IRAS colours. These objects are`blue' SRVs in the classification system of Kerschbaum & Hron(\cite{KH94}). Among the `red' SRVs (periods longer than 100 days) thefraction of stars showing Tc in their spectra is about 15 % with aprobably lower fraction among the stars with periods above 150 days.This is significantly lower than for the typical Miras. Taking intoaccount the probable conditions for the occurence of the third dredge-upand the expected behavior of the Tc abundance along an evolutionarytrack on the AGB, our results support an evolutionary scenario from`blue' SRVs (early AGB) to `red' SRVs (early TP-AGB) and on to longperiod Miras. Only the most massive (masses above 2M_ȯ) stars showTc during the SRV stage. The luminosities of the Tc-rich SRVs and Mirasare compatible with theoretical estimates of the minimum core massrequired for the third dredge-up. Based on observations collected at theEuropean Southern Observatory, La Silla, Chile (ESO No.54.E-0350), theGerman Spanish Astronomical Centre, Calar Alto, operated by theMax-Planck-Institute for Astronomy, Heidelberg, jointly with the SpanishNational Commission for Astronomy, and Kitt Peak National Observatory,USA.
| Infrared spectra of evolved stars with unusual dust shells
New mid-infrared spectra are presented of a number of oxygen-richevolved stars which have IRAS LRS (Low Resolution Spectrometer) spectrathat were classified as showing SiC emission. Two of the sources,IRC-20445 and IRC-20461, show the unidentified infrared (UIR) bandssuperposed on silicate emission features. Both objects have beenclassified as M supergiants. Several other sources show three-componentspectra, with peaks at 10, 11 and 13μm. The 13-μm source FI Lyrshows a narrow emission feature at 19μm. Emission by oxide grains maybe responsible for the 11-, 13- and 19-μm features. One object,IRC-20455, shows a self-absorbed silicate feature. There is no clearevidence for SiC emission in any of the spectra: the LRS spectra wereerroneously classified as showing SiC emission because of the relativelystrong 11-μm emission.
| Multiperiodicity in semiregular variables. I. General properties
We present a detailed period analysis for 93 red semiregular variablesby means of Fourier and wavelet analyses of long-term visualobservations carried out by amateur astronomers. The results of thisanalysis yield insights into the mode structure of semiregular variablesand help to clarify the relationship between them and Mira variables.After collecting all available data from various international databases(AFOEV, VSOLJ, HAA/VSS and AAVSO) we test the accuracy and reliabilityof data. We compare the averaged and noise-filtered visual light curveswith simultaneous photoelectric V-measurements, the effect of the lengthversus the relatively low signal-to-noise ratio is illustrated by periodanalysis of artificial data, while binning effects are tested bycomparing results of frequency analyses of the unbinned and averagedlight curves. The overwhelming majority of the stars studied showmultiperiodic behaviour. We found two significant periods in 44variables, while there are definite signs of three periods in 12 stars.29 stars turned out to be monoperiodic with small instabilities in theperiod. Since this study deals with the general trends, we wanted tofind only the most dominant periods. The distribution of periods andperiod ratios is examined through the use of the (log P_0, log P_1) and(log P_1, log P_0/P_1) plots. Three significant and two less obvioussequences are present which could be explained as the direct consequenceof different pulsational modes. This hypothesis is supported by theresults for multiperiodic variables with three periods. Finally, thesespace methods are illustrated by several interesting case studies thatshow the best examples of different special phenomena such as long-termamplitude modulation, amplitude decrease and mode switching.
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