Contents
Images
Upload your image
DSS Images Other Images
Related articles
Fitting of dust spectra with genetic algorithms. I. Perspectives and limitations
Aims: We present an automatised fitting procedure for the IRrange of AGB star spectra. Furthermore we explore the possibilities andboundaries of this method. Methods: We combine the radiativetransfer code DUSTY with the genetic algorithm PIKAIA in order toimprove an existing spectral fit significantly. Results: In orderto test the routine we carried out a performance test by feeding anartificially generated input spectrum into the program. Indeed theroutine performed as expected, so, as a more realistic test set-up, wetried to create model fits for ISO spectra of selected AGB stars. Herewe were not only able to improve existing fits, but also to show that aslightly altered dust composition may give a better fit for someobjects. Conclusions: The use of a genetic algorithm in order toautomatise the process of fitting stellar spectra seems to be verypromising. We were able to improve existing fits and further offer aquantitative method to compare different models with each other.Nevertheless this method still needs to be studied and tested in moredetail.
| Morphological effects on IR band profiles. Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars
Aims: To trace the source of the unique 13, 19.5, and 28 ?m emissionfeatures in the spectra of oxygen-rich circumstellar shells around AGBstars, we have compared dust extinction spectra obtained by aerosolmeasurements. Methods: We have measured the extinction spectrafor 19 oxide powder samples of eight different types, such asTi-compounds (TiO, TiO2, Ti2O3, Ti3O5, Al2TiO5, CaTiO3), ?-,?-, ?-?-?-Al2O3, and MgAl2O4 in the infraredregion (10-50 ?m) paying special attention to the morphological(size, shape, and agglomeration) effects and the differences in crystalstructure. Results: Anatase (TiO2) particles with rounded edgesare the possible 13, 19.5 and 28 ?m band carriers as the maincontributor in the spectra of AGB stars, and spherically shapednano-sized spinel and Al2TiO5 dust grains are possibly associated withthe anatase, enhancing the prominence of the 13 ?m feature andproviding additional features at 28 ?m. The extinction data setsobtained by the aerosol and CsI pellet measurements have been madeavailable for public use at http://elbe.astro.uni-jena.de.
| The dust condensation sequence in red supergiant stars
Context: Red supergiant (RSG) stars exhibit significant mass loss bymeans of a slow, dense wind. They are often considered to be the moremassive counterparts of Asymptotic Giant Branch (AGB) stars. While AGBmass loss is related to their strong pulsations, the RSG are often onlyweakly variable. This raises the question of whether their wind-drivingmechanism and the dust composition of the wind are the same. Aims: We study the conditions at the base of the wind by determiningthe dust composition of a sample of RSG. The dust composition is assumedto be sensitive to the density, temperature, and acceleration at thebase of the wind. We compare the derived dust composition with thecomposition measured in AGB star winds. Methods: We compile asample of 27 RSG infrared spectra (ISO-SWS) and supplement these withphotometric measurements to derive the full spectral energy distribution(SED). These data are modelled using a dust radiative-transfer code,taking into account the optical properties of the relevant candidatematerials to search for correlations between mass-loss rate, density atthe inner edge of the dust shell, and stellar parameters. Results: We find strong correlations between the dust composition,mass-loss rate, and the stellar luminosity, roughly in agreement withthe theoretical dust condensation sequence. We identify the need for acontinuous (near-)IR dust opacity and tentatively propose amorphouscarbon, and we note significant differences with AGB star winds in termsof the presence of PAHs, absence of “the” 13 μm band, anda lack of strong water bands. Conclusions: Dust condensation inRSG is found to experience a freeze-out process that is similar to thatin AGB stars. Together with the positive effect of the stellarluminosity on the mass-loss rate, this suggests that radiation pressureon dust grains is an important ingredient in the driving mechanism.Still, differences with AGB stars are manifold and thus the winds of RSGshould be studied individually in further detail.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Oxygen-rich AGB stars with optically thin dust envelopes
The dust composition and dynamics of the circumstellar envelopes ofoxygen-rich AGB stars with low mass-loss rates (5 ×10-8-10-5 Mȯ yr-1) havebeen investigated. We have analyzed the ISO-SWS spectra of twenty-eightoxygen-rich AGB stars with optically thin shells, and modelled theobservations with the radiative transfer code DUSTY using the opticalconstants from laboratory dust analogues. This has allowed us todetermine the composition of the dust and the physical conditions at theinner edge of the shell. Moreover, by comparing with CO observationsavailable in the literature, we have determined the gas-to-dust massratios and the mass-loss rates of these sources, and analyzed thewind-driving mechanism. The results show that the small amounts of dustpresent in these envelopes, characterized by visual optical depths inthe 0.03-0.6 range, are enough to drive the wind by radiation pressureon the grains. In some sources there are indications of circumstellardust that does not contribute to the wind-driving, and that maydistributed in a disk or clumps. Other sources show signs of variablemass-loss rates. A grain mixture in the shell consisting of aluminiumoxide, melilite, olivine, spinel and Mg{0.1}Fe{0.9}O fit the observedspectra well. From these species, only melilite is required to have afractional abundance greater than 25% in all cases. Although spinelreproduces the 13 μm feature, the absence of the 16.8 μm peak inour SWS spectra casts doubts on this identification. The outcome of themodelling reveals that the olivine content in these CSEs increases withpressure and temperature at the inner edge. Moreover, the aluminiumoxide percentage in the dust of the envelopes shows a positivecorrelation with the gas-to-dust mass ratio. These results, togetherwith the derived dust compositions, are consistent with thethermodynamic dust condensation sequence scenario and its freezing-outdue to kinetics. However, the temperatures at the inner edge of theshell are substantially lower than those predicted by theory.
| 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)).
| ``Thermal'' SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics
An extensive radiative transfer analysis of circumstellar SiO``thermal'' radio line emission from a large sample of M-type AGB starshas been performed. The sample contains 18 irregulars of type Lb (IRV),7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12Miras. New observational data, which contain spectra of several groundvibrational state SiO rotational lines, are presented. The detectionrate was about 60% (44% for the IRVs, and 68% for the SRVs). SiOfractional abundances have been determined through radiative transfermodelling. The abundance distribution of the IRV/SRV sample has a medianvalue of 6*E-6, and a minimum of 2*E-6 and amaximum of 5*E-5. The high mass-loss rate Miras have a muchlower median abundance, la 10-6. The derived SiO abundancesare in all cases well below the abundance expected from stellaratmosphere equilibrium chemistry, on average by a factor of ten. Inaddition, there is a trend of decreasing SiO abundance with increasingmass-loss rate. This is interpreted in terms of depletion of SiOmolecules by the formation of silicate grains in the circumstellarenvelopes, with an efficiency which is high already at low mass-lossrates and which increases with the mass-loss rate. The high mass-lossrate Miras appear to have a bimodal SiO abundance distribution, a lowabundance group (on average 4*E-7) and a high abundance group(on average 5*E-6). The estimated SiO envelope sizes agreewell with the estimated SiO photodissociation radii using an unshieldedphotodissociation rate of 2.5*E-10 s-1. The SiOand CO radio line profiles differ in shape. In general, the SiO lineprofiles are narrower than the CO line profiles, but they havelow-intensity wings which cover the full velocity range of the CO lineprofile. This is interpreted as partly an effect of selfabsorption inthe SiO lines, and partly (as has been done also by others) as due tothe influence of gas acceleration in the region which produces asignificant fraction of the SiO line emission. Finally, a number ofsources which have peculiar CO line profiles are discussed from thepoint of view of their SiO line properties.Based on observations using the SEST at La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Sweden, the JCMT on Hawaii, andthe IRAM 30 m telescope at Pico Veleta, Spain.
| Mass-loss from dusty, low outflow-velocity AGB stars. I. Wind structure and mass-loss rates
We present the first results of a CO(2-1), (1-0), and 86 GHz SiO masersurvey of AGB stars, selected by their weak near-infrared excess. Amongthe 65 sources of the present sample we find 10 objects with low COoutflow velocities, vexp <~ 5 km s-1.Typically, these sources show (much) wider SiO maser features.Additionally, we get 5 sources with composite CO line profiles, i.e. anarrow feature is superimposed on a broader one, where both componentsare centered at the same stellar velocity. The gas mass-loss rates,outflow velocities and velocity structures suggested by these lineprofiles are compared with the results of hydrodynamical modelcalculations for dust forming molecular winds of pulsating AGB stars.The observations presented here give support to our recent lowoutflow-velocity models, in which only small amounts of dust are formed.Therefore, the wind generation in these models is dominated by stellarpulsation. We interpret the composite line profiles in terms ofsuccessive winds with different characteristics. Our hydrodynamicalmodels, which show that the wind properties may be extremely sensitiveto the stellar parameters, support such a scenario.Based on observations obtained at the European Southern Observatory, LaSilla, Chile and at the IRAM, Pico Veleta, Spain.
| 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.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also available in electronic form at the CDS, via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1167
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
We have determined mass loss rates and gas expansion velocities for asample of 69 M-type irregular (IRV 22 objects) and semiregular (SRV; 47objects) AGB-variables using a radiative transfer code to model theircircumstellar CO radio line emission. We believe that this sample isrepresentative for the mass losing stars of this type. The (molecularhydrogen) mass loss rate distribution has a median value of 2.0 x10-7 Msun yr-1, and a minimum of 2.0 x10-8 Msun yr-1 and a maximum of 8 x10-7 Msun yr-1. M-type IRVs and SRVswith a mass loss rate in excess of 5 x 10-7 Msunyr-1 must be very rare, and among these mass losing stars thenumber of sources with mass loss rates below a few 10-8Msun yr-1 must be small. We find no significantdifference between the IRVs and the SRVs in terms of their mass losscharacteristics. Among the SRVs the mass loss rate shows no dependenceon the period. Likewise the mass loss rate shows no correlation with thestellar temperature. The gas expansion velocity distribution has amedian of 7.0 km s-1, and a minimum of 2.2 km s-1and a maximum of 14.4 km s-1. No doubt, these objects samplethe low gas expansion velocity end of AGB winds. The fraction of objectswith low gas expansion velocities is very high, about 30% havevelocities lower than 5 km s-1, and there are objects withvelocities lower than 3 km s-1: V584 Aql,T Ari, BI Car, RXLac, and L2 Pup. The mass loss rate and thegas expansion velocity correlate well, a result in line with theoreticalpredictions for an optically thin, dust-driven wind. In general, themodel produces line profiles which acceptably fit the observed ones. Anexceptional case is R Dor, where the high-quality,observed line profiles are essentially flat-topped, while the model onesare sharply double-peaked. The sample contains four sources withdistinctly double-component CO line profiles, i.e., a narrow featurecentered on a broader feature: EP Aqr, RVBoo, X Her, and SV Psc.We have modelled the two components separately for each star and derivemass loss rates and gas expansion velocities. We have compared theresults of this M-star sample with a similar C-star sample analysed inthe same way. The mass loss rate characteristics are very similar forthe two samples. On the contrary, the gas expansion velocitydistributions are clearly different. In particular, the number oflow-velocity sources is much higher in the M-star sample. We found noexample of the sharply double-peaked CO line profile, which is evidenceof a large, detached CO-shell, among the M-stars. About 10% of theC-stars show this phenomenon.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool stars.
| 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).
| Infrared optical properties of spinels. A study of the carrier of the 13, 17 and 32 mu m emission features observed in ISO-SWS spectra of oxygen-rich AGB stars
In a previous paper, we have proposed magnesium aluminium spinel to bethe carrier of the 13 and 17 mu m band features observed in the ISOspectra of some red giants. The IR optical properties of spinel stronglydepend on its chemistry and its internal structure. To study thedependence of spinel's IR-spectra on its aluminium content, we havesynthesized a number of crystals with different Al/Mg-ratios.Additionally, we performed an annealing experiment to investigate thephase transition between ordered and partially disordered spinel takingplace at about 1200 K. We derived sets of optical constants of ournatural, annealed and synthetic spinels in order to calculate theabsorption efficiencies of small (sub-mu m-sized) spherical particles.Thereby, it turned out that natural as well as near-stoichiometricsynthetic spinel can indeed be considered as a suitable candidate forthe carrier of the 13 mu m feature observed in the spectra of someoxygen rich circumstellar shells. To illustrate this, we reinvestigatedthe mean profile of the residual dust emission in the 12-18 mu mwavelength range (i.e., in the so-called trough region between the twosilicate bands). The reality of the emission feature at 16.8 mu m isconfirmed by our new investigation. We demonstrated that it is not aninstrumental artifact since it is not present in ISO spectra of K-stars.In the course of our laboratory work, we found a third prominentemissivity maximum of spinel at 32 mu m. This feature could also bedetected in the spectra of the brightest 13 mu m band emission sources;we derived its mean band profile, too. The new sets of optical constantshere presented have been made available for public access in theelectronic database http://www.astro.uni-jena.de. Based on observationswith ISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.
| 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.
| Mira kinematics from Hipparcos data: a Galactic bar to beyond the Solar circle
The space motions of Mira variables are derived from radial velocities,Hipparcos proper motions and a period-luminosity relation. Thepreviously known dependence of Mira kinematics on the period ofpulsation is confirmed and refined. In addition, it is found that Miraswith periods in the range 145-200d in the general Solar neighbourhoodhave a net radial outward motion from the Galactic Centre of75+/-18kms-1. This, together with a lag behind the circularvelocity of Galactic rotation of 98+/-19kms-1, is interpretedas evidence for an elongation of their orbits, with their major axesaligned at an angle of ~17° with the Sun-Galactic Centre line,towards positive Galactic longitudes. This concentration seems to be acontinuation to the Solar circle and beyond of the bar-like structure ofthe Galactic bulge, with the orbits of some local Miras probablypenetrating into the bulge. These conclusions are not sensitive to thedistance scale adopted. A further analysis is given of the short-period(SP) red group of Miras discussed in companion papers in this series. InAppendix A the mean radial velocities and other data for 842 oxygen-richMira-like variables are tabulated. These velocities were derived frompublished optical and radio observations.
| Modelling of oxygen-rich envelopes using corundum and silicate grains
A set of 31 oxygen-rich stars has been modelled using corundum andsilicate grains. These stars were selected according to theirdust-envelope class, as suggested by Little-Marenin and Little in 1990.Then 16 stars classified as Sil were modelled using silicate grains; 10Broad class stars using corundum (Al2O3) grains;and 5 `Intermediate' class stars using two kinds of grainsimultaneously: corundum and silicate. The temperature of the centralstars and some characteristics of their circumstellar envelopes such astheir extinction opacities and extensions were determined by fitting theflux curves. The corundum/silicate ratios as well as the energydistributions and temperature laws have been obtained. Based on theauthors' results they suggest the existence of chemical and structuralevolution of the modelled circumstellar dust shells. The temperature ofthe central stars and the temperature of the hottest grains decreasefrom Broad to Intermediate to Sil classes, while the inner radii andoptical depths increase in this sequence.
| Period-Luminosity-Colour distribution and classification of Galactic oxygen-rich LPVs. I. Luminosity calibrations
The absolute K magnitudes and kinematic parameters of about 350oxygen-rich Long-Period Variable stars are calibrated, by means of anup-to-date maximum-likelihood method, using Hipparcos parallaxes andproper motions together with radial velocities and, as additional data,periods and V-K colour indices. Four groups, differing by theirkinematics and mean magnitudes, are found. For each of them, we alsoobtain the distributions of magnitude, period and de-reddened colour ofthe base population, as well as de-biased period-luminosity-colourrelations and their two-dimensional projections. The SRa semiregulars donot seem to constitute a separate class of LPVs. The SRb appear tobelong to two populations of different ages. In a PL diagram, theyconstitute two evolutionary sequences towards the Mira stage. The Mirasof the disk appear to pulsate on a lower-order mode. The slopes of theirde-biased PL and PC relations are found to be very different from theones of the Oxygen Miras of the LMC. This suggests that a significantnumber of so-called Miras of the LMC are misclassified. This alsosuggests that the Miras of the LMC do not constitute a homogeneousgroup, but include a significant proportion of metal-deficient stars,suggesting a relatively smooth star formation history. As a consequence,one may not trivially transpose the LMC period-luminosity relation fromone galaxy to the other Based on data from the Hipparcos astrometrysatellite. Appendix B 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/Abstract.html
| 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.
| Oxygen-rich semiregular and irregular variables. A catalogue of circumstellar CO observations
Using the SEST, the Onsala 20 m telescope, the JCMT, and the IRAM 30 mtelescope we have carried out a survey of circumstellar CO(J=1-0, 2-1,3-2, and 4-3) emission on a large sample of oxygen-rich semiregular (SRaand SRb) and irregular variables (Lb). A total of 109 stars wereobserved in at least one CO line: 66 were shown to have circumstellar COline emission (7 SRa, 36 SRb, and 23 Lb variables), ~ 60% of thesemiregulars and all but one of the irregulars were detected for thefirst time. Most stars were observed in at least two transitions. Thereis a total of 138 detected CO lines. For twelve stars stronginterference from interstellar CO emission precluded detection. Wepresent here a catalogue of all observational data and the spectra ofall detections, as well as brief discussions on detection statistics(including its dependence on variability type, period, IRAS-colour, IRASLRS-class, and M-subclass), line profiles (including line shapeasymmetry, multi-component line shapes, and line intensity ratios), gasexpansion velocity distributions, and correlations between CO line andIR continuum fluxes (including implications for the mass-lossmechanism). Based on observations collected using at the EuropeanSouthern Observatory, La Silla, Chile, the Onsala Space Observatory,Chalmers Tekniska Högskola, Sweden, the James Clerk MaxwellTelescope, Hawaii and the IRAM 30~m telescope, Pico Veleta, Spain.
| SiO rotation-vibration bands in cool giants II. The behaviour of SiO bands in AGB stars
The first overtone rotation-vibration transitions of SiO give rise toprominent bandheads in the wavelength range between 4.0 and 4.5 mu m. Inorder to study the behaviour of these features in AGB stars we observedthe 3.94 to 4.12 mu m spectra for a sample of 23 oxygen-rich late-typevariables. In contrast to the SRb objects, the Miras show a very largescatter of the equivalent widths of the SiO bands. Despite their cooltemperatures some of them have only weak or no SiO absorption, whichseems to be related to their strong pulsations producing a largevariability of the features. When comparing the band intensities withphotometric data, we found a general decrease with bluer IRAS (12-25)colors. However, this trend may only reflect the different behaviour ofthe Miras and SRb stars in our sample. We did not discover anycorrelation of the equivalent widths with the effective temperaturesderived from (J-K), or with the (K-12) color and the IRAS-LRS class,both of which can be regarded as a rough measure for the thickness ofthe circumstellar shell. In Paper I of this series (Aringer et al.\cite{siop}) we have shown that synthetic spectra calculated fromhydrostatic MARCS atmospheres are successful in reproducing the observedband intensities of giants with spectral types earlier than about M5 IIIand M2 II\@. However, they generally predict too strong features forvery cool and extended objects, as they are discussed in this work. Andthey fail completely when it comes to Miras with weak or no SiOabsorption. These stars are dominated by dynamical phenomena and, notsurprisingly, they can therefore not be described by hydrostaticstructures. Thus, we have also computed synthetic spectra based onexperimental dynamical models. Although they still have someshortcomings, we demonstrate that, in principle, they are able toexplain the whole range of equivalent widths of the observed SiObandheads and their variations. Based on observations made at theEuropean Southern Observatory (ESO), La Silla (Chile)}
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Infrared giants vs. supergiants. II. CO observations
We report systematic observations of millimeter CO emission from asample of 109 oxygen-rich evolved stars (AGB and supergiants),colour-selected from the IRAS Point Source Catalog (0.69 200). In most cases, the observed spread in the valuesof this ratio can be explained by a large range of luminosities. Thisleads to a new criterion to identify AGB stars: an object with {cal R}< 150 must have a low mass progenitor. Here we study the correlationsbetween {cal R} and various physical properties of the sources. Mostsources with high values of {cal R} also have low galactic latitudes,small IRAS variability indices, and early spectral types (typicallyM1-M5). Conversely, there is no dependence on the IRAS colours, nor onthe intensity of silicate 10;microns emission. However, a few AGB starsexhibit large {cal R}; other factors than luminosity are required toexplain these values. Different hypotheses, such as the possiblepresence of a chromosphere, a low (12) C abundance or a variablemass-loss rate, are examined. Considering the global high OH detectionrate ( ~ 67%), we studied the correlations with CO and OH emission. Thedetection of OH seems to be a useful discriminator of mechanisms thatenhance {cal R}.
| Circumstellar Shells of the Mass-Losing Asymptotic Giant Branch Stars: Limits for the Dust-Driven Winds
We investigated whether the radiation pressure on dust grains alone mayaccount for driving winds in semiregular, irregular and non-variable AGBstars. A simple theoretical model is employed to calculate the limitsfor the radiatively driven winds and to compare theoretical predictionswith observations. Present analysis indicates, that for most of theobjects in the studied sample of 67 oxygen-rich and 40 carbon-richmass-losing AGB stars radiation pressure on dust grains alone is noteffective enough to drive the observed circumstellar outflows.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Mean light curves of long-period variables and discrimination between carbon- and oxygen-rich stars
Using 75 years of AAVSO data, mean light curve parameters of a sample of355 long period M, S, and C mira and semi-regular variable stars areinvestigated. We present a classification of the light curves of LPVsinto 6 distinct groups. Combining this classification with IRAS colorsmakes it possible to distinguish oxygen-rich from carbon-rich miras.Table 2 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/Abstract.html
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Ταώς |
| Right ascension: | 19h55m13.97s |
| Declination: | -59°11'44.4" |
| Apparent magnitude: | 8.213 |
| Distance: | 343.643 parsecs |
| Proper motion RA: | 14.4 |
| Proper motion Dec: | -46.7 |
| B-T magnitude: | 9.538 |
| V-T magnitude: | 8.323 |
Catalogs and designations:
|
