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AGB variables and the Mira period-luminosity relation
Published data for large-amplitude asymptotic giant branch variables inthe Large Magellanic Cloud (LMC) are re-analysed to establish theconstants for an infrared (K) period-luminosity relation of the formMK = ρ[logP - 2.38] + δ. A slope of ρ = -3.51+/- 0.20 and a zero-point of δ = -7.15 +/- 0.06 are found foroxygen-rich Miras (if a distance modulus of 18.39 +/- 0.05 is used forthe LMC). Assuming this slope is applicable to Galactic Miras we discussthe zero-point for these stars using the revised Hipparcos parallaxestogether with published very long baseline interferometry (VLBI)parallaxes for OH masers and Miras in globular clusters. These result ina mean zero-point of δ = -7.25 +/- 0.07 for O-rich Galactic Miras.The zero-point for Miras in the Galactic bulge is not significantlydifferent from this value.Carbon-rich stars are also discussed and provide results that areconsistent with the above numbers, but with higher uncertainties. Withinthe uncertainties there is no evidence for a significant differencebetween the period-luminosity relation zero-points for systems withdifferent metallicity.
| Lithium and zirconium abundances in massive Galactic O-rich AGB stars
Lithium and zirconium abundances (the latter taken as representative ofs-process enrichment) are determined for a large sample of massiveGalactic O-rich AGB stars, for which high-resolution opticalspectroscopy has been obtained (R˜ 40 000{-}50 000). This was doneby computing synthetic spectra based on classical hydrostatic modelatmospheres for cool stars and using extensive line lists. The resultsare discussed in the framework of "hot bottom burning" (HBB) andnucleosynthesis models. The complete sample is studied for variousobservational properties such as the position of the stars in the IRAStwo-colour diagram ([ 12] - [25] vs. [ 25] - [60] ), Galacticdistribution, expansion velocity (derived from the OH maser emission),and period of variability (when available). We conclude that aconsiderable fraction of these sources are actually massive AGB stars(M>3{-}4 Mȯ) experiencing HBB, as deduced from thestrong Li overabundances we found. A comparison of our results withsimilar studies carried out in the past for the Magellanic Clouds (MCs)reveals that, in contrast to MC AGB stars, our Galactic sample does notshow any indication of s-process element enrichment. The differencesobserved are explained as a consequence of metallicity effects. Finally,we discuss the results obtained in the framework of stellar evolution bycomparing our results with the data available in the literature forGalactic post-AGB stars and PNe.Based on observations at the 4.2 m William Herschel Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto de Astrofisicade Canarias. Also based on observations with the ESO 3.6 m telescope atLa Silla Observatory (Chile). Tables [see full textsee full text]-[seefull textsee full text] are only available in electronic form athttp://www.aanda.org
| Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars
To search for phase lags in the optical-infrared light curves ofasymptotic giant branch stars, we have compared infrared data from theCOBE DIRBE satellite with optical light curves from the AAVSO and othersources. We found 17 examples of phase lags between the times ofinfrared and optical maximum, and 4 stars with no observed lags. Thereis a clear difference between the Mira variables and the semiregularvariables in the sample, with the maximum in the optical preceding thatin the near-infrared in the Miras, while in most of the semiregularvariables no lags are observed. Comparison to published theoreticalmodels indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggeststhat Miras pulsate in the fundamental mode, while at least somesemiregular variables are first-overtone pulsators. There is a clearoptical-near-infrared phase lag in the carbon-rich Mira V CrB; this islikely due to C2 and CN absorption variations in the optical.
| 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 ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 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?/A+A/390/1033
| On the Transience of High-Latitude OH/IR Stars. II. Thermal Pulse Link
High-latitude, |b|>=10deg, asymptotic (AGB) stars in theirsuperwind phase have distinctive IR colors, which make them easy toidentify and count. But the expansion velocity (Ve) versuscolor plots of these stars are a puzzle, with features implying (1) astep increase in dM/dt and (2) short-term changes in the circumstellarshell that are enhanced by increased metallicity. The net duration oftheir superwind phase is ~3700 yr when estimated from the relativefrequency of associated proto-planetary nebulae and the ~103 yrexpansion age of one, IRAS 18095+2704. Since these stars lose on average~0.04 Msolar during the superwind phase and have progenitormasses circa 1 Msolar, they must lose more than 0.3Msolar prior to the AGB. A single 3700 yr superwind episode,however, cannot explain the 2-18 km s-1 range of expansionvelocities exhibited by these stars. This range is best understood asresulting from a cyclical superwind, wherein stars lose most of theirenvelope mass in three to four discrete episodes of ever largerVe, with each episode being triggered by a sudden onset inthe coupling of photon momentum by dust to gas near a thermal pulse. Theresulting newly accelerated shell thereafter quickly expands beyond theprotective dust shroud of the prior shell, which allows interstellar UVto degrade its molecules. This turns the shell into an OH/IR star colormimic (a shell with the dM/dt of an OH/IR star without 1612 MHz masers),until enough dust is again in place beyond the shell to allow moleculesthe longevity needed to support masers. Two by-products of this modelare the natural explanation it offers for why Ve(CO)>Ve(OH), as well as for the occurrence and integration ofmimics into the normal development of a shell. The circumstellar shellsof high-latitude OH/IR stars are thus cyclically transient phenomena.
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| 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).
| On the Difference between Type E and Type A OH/IR Stars
The observed spectral energy distributions of a sample of 60 OH/IR starsare fitted using a radiative transfer model of a dusty envelope. Amongthe whole sample, 21 stars have reliable phase-lag distances, while theothers have less accurate distances. L*-P, M-P, andM-L* relations have been plotted for these stars. It is foundthat type E (with an emission feature at 10 μm) and type A (with anabsorption feature at 10 μm) OH/IR stars have differentL*-P and M-L* relations, while both follow asingle M-P relation. The type E stars are shown to be located in an areawithout large-scale dense interstellar medium, while the type A starsare probably located in dense interstellar medium. It is argued herethat this may indicate that the two types of OH/IR stars do not sharethe same chemical composition or zero-age main-sequence mass and soevolve in different ways. This conclusion has reinforced the argument byChen et al., who reached a similar conclusion from the Galacticdistribution of about 1000 OH/IR stars, based on IRAS low-resolutionspectrometer spectra.
| SiO Maser Survey of Southern IRAS Sources
We have surveyed 163 southern IRAS sources in the SiO J = 2 - 1 v = 1maser line at 86 GHz, obtaining 38 (22 new and 16 previously known)detections. These detections include 6 stars not previously detected inOH 1612 MHz surveys, and 4 sources assigned incorrect OH radialvelocities. The distance reachable in this survey turned out to be about4 kpc. The local velocity dispersion near the Sun were found to beapproximately 30 km s-1 for these IRAS/SiO sources. Arelatively high concentration of the SiO detected sources were found atthe area 320° < l < 340° (toward ``Norma tangent''). Theestimated distances for these sources suggest that this concentration isspread between 2 and 6 kpc from the Sun, peaking on the Scutum-Crux arm.
| 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.
| Infrared colours for Mira-like long-period variables found in the (Mȯ<~10-7 Msolar yr-1) Hipparcos Catalogue
Near-infrared, JHKL, photometry is presented for 193 Mira andsemi-regular variables that were observed by Hipparcos; periods,bolometric magnitudes and amplitudes are derived for 92 of them. Becauseof the way in which the Hipparcos targets were selected, this group ofstars provides a useful data base of Miras with low mass-loss rates(Mȯ<~10-7Msolaryr-1).Various period-colour relationships are discussed in detail. The colour,particularly BCK = 10.86 - 38.10 K (J - K)0 +64.16(J - K)20 - 50.72(J -K)30 + 19, K-L, at a given period is found todepend on the pulsation amplitude of the star. A comparison with modelssuggests that this is a consequence of atmospheric extension, in thesense that large-amplitude pulsators have very extended atmospheres andredder Mȯ<10-7Msolaryr-1, K-L and H-K but bluerJ-H than their lower amplitude counterparts. The stars with veryextended atmospheres also have higher values of K-[12] and hence highermass-loss rates. This finding provides further evidence for the causalconnection between pulsation and mass loss. Two sequences are identifiedin the Hp-K versus logP diagram (where Hp is the Hipparcos broad-bandmagnitude) at short periods (logP<2.35). At a given period these twogroups have, on average, the same pulsation amplitude, but differentJHKL colours and spectral types. The short-period stars in the bluersequence have similar near-infrared colours to the Miras found inglobular clusters. Long-term trends in the infrared light curves arediscussed for stars that have sufficient data.
| 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.
| Millimeter and some near infra-red observations of short-period Miras and other AGB stars
Millimeter observations of 48 oxygen- and 20 carbon-rich AGB Miras withperiods shorter than 400 days are presented. In addition, observationsof 14 O-rich and 15 C-stars with longer, or no known, periods have alsobeen obtained. The detection statistics is as follows: in12CO J=1-0 and 2-1 we observed 97 stars, and detected 66 inat least one line. We find 24 new detections in the 1-0 line, 38 newdetections in the 2-1 line, and 29 stars have been detected for thefirst time in one or both lines. In 12CO J=3-2 we observed 14stars and detected 11, with 4 new detections. In 13CO J=2-1,3-2 we observed 2 stars and had one new detection. In HCN(1-0) weobserved 5 carbon stars and detected 3, one new. In SO(6_5-5_4) weobserved the same 5 stars and detected none. In CS(3-2) we observed 8carbon stars and detected 3, all new. In SiO(3-2, v=0) we observed 34O-rich stars and detected 25, all new except one. Near-infrared JHKphotometry is presented for seven stars. For four stars it is the firstNIR data published. The luminosity and dust mass loss rate are obtainedfor seven very red stars with unknown pulsation period from modellingthe spectral energy distribution (SED) and IRAS LRS spectra. Thereby, anew IR supergiant is confirmed (AFGL 2968). For the rest of the sample,luminosity and distance are obtained in a variety of ways: usinghipparcos parallaxes, period-luminosity and period-M_K-relationscombined with apparent K magnitudes, and kinematic distances. The dustmass loss rate is obtained from model fitting of the SED (either fromthe literature, or presented in the present paper), or from the observedIRAS 60 mu m flux, corrected for the photospheric contribution. The gasmass loss rate is derived from the observed CO line intensities, aspresented here, combined with existing literature data, if any. Thisallows the derivation of the dust-to-gas ratio. Our and literature CO J= 3-2 data has been used to calibrate the relation between mass lossrate and peak intensity of the CO(3-2) line. Diagrams showing mass lossrate, dust-to-gas ratio and expansion velocity versus pulsation periodare presented. Our observations confirm the existence of an upper limitfor the expansion velocity of C- and O-rich stars, and that this maximumis larger for C-stars, as predicted by the theory of radiation pressureon dust particles. The James Clerk Maxwell Telescope is operated by theObservatories on behalf of the Science and Engineering Council of theUK, the Netherlands Organization for Scientific Research, and theNational Research Council of Canada. Based on observations made with theCarlos Sanchez telescope operated on the island of Tenerife in theSpanish Observatorio del Teide of the Instituto de Astrof\'\i sica deCanarias. Based on observations collected at the European SouthernObservatory, La Silla, Chile within programs ESO 57.E-0105, 59.E-0198and 61.E-0254. Based on data from the ESA Hipparcos astrometrysatellite.
| 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
| 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.
| 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.
| Semiregular variables of types SRa and SRb. Silicate dust emission features.
We have analysed the IRAS-LRS spectra of representative samples ofO-rich Semiregular (SR) variables of types SRa and SRb and of Miravariables. The silicate features were extracted by fitting the energydistribution with two blackbodies, approximating the continuous emissionfrom the photosphere and the circumstellar dust. The shape and strengthof the silicate features in the LRS range were then studied by computingthe residual fluxes in 5 selected wavelength regions covering the whole10μm and 18μm features and parts of the 10μm feature assignedto emission from olivine and possibly corundum. We compare our approachwith previous investigations and argue that a quantitative study ofdetails in the feature shape requires subtraction of the stellar and thedust continuum and the use of flux ratios rather than a discreteclassification system. The Miras form an extension of the SRb's towardslower stellar temperatures and higher dust shell opacities and they haveslightly higher average dust temperatures. The SRa's seem to be moresimilar to the Miras in their dust shell properties. The average 10μmfeature shapes of the three groups of variables agree, but taking intoaccount the photospheric and dust shell parameters, systematicdifferences show up. For stars hotter than about 2900K, the 10μmfeature width shows a wide range of values but no clear trend with thestellar temperature or the optical depth of the dust shell. These starsare generally SRb variables and have the thinnest dust envelopes. Atcooler stellar temperatures, where mostly Miras are found, the opticaldepth of the dust shell determines the feature width in the sense thatthicker shells have narrower features. It appears that the 13μmfeature is obvious only in a narrow range of effective temperature andoptical depth of the dust shell. We discuss our results in terms ofradiative transfer effects, differences in the average grain size,annealing and hydration of amorphous silicates and contributions fromother dust components. Of these possibilities the last one seems to bemost plausible with regard to the behavior of the 10μm feature width.The observations can be interpreted in terms of changing contributionsfrom olivine and corundum possibly caused by an increasing amount ofdust processing (Miras) and the influence of the atmospheric structureon the formation of these dust components (SRb's).
| AGB stars: densities and formation rates obtained from OH/IR stars.
We determine densities and star formation rates of AGB stars from ananalysis of OH/IR stars in the solar neighbourhood. The stars aredivided into three distinct mass ranges corresponding to the precursorsof type I, II and III planetary nebulae, according to the Peimbertclassification scheme. The adopted distance scale is based on theperiod-luminosity relationship by Feast et al. (1989). The formation (ordeath) rates are calculated using stellar evolution models for AGBstars, and are compared with the corresponding rates for planetarynebulae and white dwarfs. The results indicate that both surface andvolume densities of AGB stars are in good agreement with previousdeterminations in the literature. The formation rates are found to bevery sensitive to the evolution time at the AGB, and depend also on theadopted model.
| IRAS low resolution spectra with ? Pictoris-type silicate emission.
We present a list of 88 IRAS low resolution spectra that exhibit 10?msilicate emission features similar to the one in the spectrum of ?Pictoris. Unlike the majority of LRS silicate emission spectra, whichshow a relatively narrow, single peak at ~10?m, the ones in thepresent list show a wide feature, with two peaks at 9 and 11?m. Thespectral characteristics indicate the emission originates fromoxygen-rich circumstellar dust. Many of the sources have no knownassociations; those that do are typically low mass asymptotic giantbranch Miras or massive supergiants. The sources show a preferentialdistribution towards the galactic center.
| Circumstellar dust envelopes of oxygen-rich AGB stars.
Using IRAS observational data, the best-fitting models are searched foroxygen-rich AGB stars in various evolutionary stages from opticallyvisible red giant stars to OH/IR stars with heavy mass loss. The modelinfrared spectral energy distributions of the red giant stars withcircumstellar dust envelopes are calculated with the use of a radiativetransfer code. It is found that the oxygen-rich IRAS AGB starssurrounded by circumstellar dust envelopes are divided into threegroups, which can be clearly distinguished by their mass loss rate andthe grain temperature at the inner boundary of the circumstellar dustenvelopes. One group corresponds to Mira type objects with a moderatemass loss rate and a grain formation temperature of about 500 K, thesecond to OH/IR star type objects with a large mass loss rate and a highgrain formation temperature, and the third to M stars with a very coldgrain temperature at the inner boundary, indicating that the moderatemass loss stopped a considerable time ago in these objects. The highergrain formation temperature of the OH/IR star type objects indicates agrain formation point closer to the stellar surface, suggesting aneffective acceleration of dust grains by radiation pressure. Such aneffective acceleration can play an important role in the mass lossphenomena of OH/IR stars with large mass loss rates. From the modelfitting results, it is suggested that an AGB star undergoes severaldozen phases of moderate mass loss of˙(M)~10^-7^-10^-6^Msun_/yr for a period of10^3^-10^4^years with interruptions of 10^3^-10^4^years during theordinary Mira variable phase before becoming an OH/IR star with a massloss rate exceeding 10^-5^Msun_/yr and a lifetime of about10^3^-10^4^years.
| High Mass Agb-Stars in the South Galactic CAP
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.267..711W&db_key=AST
| A search for variability in the IRAS-LRS spectra of long period variables
Time-averaged IRAS-low resolution spectrometer (LRS) spectra have beenextensively used in the past for the study of mass loss in cool stars.Using the GEISHA system available at Groningen, we have extracted theraw LRS data for a sample of 40 mass losing asymptotic giant branch(AGB) stars. The selection was made on the basis of the number ofindividual spectra obtained by IRAS, the flux at 12 micrometers, an LRSclassification of 2n or 4n and a known association with an optical longperiod variable (Miras, Semiregulars). The spectra were analyzed inthree different ways, one of them being similar to the original LRSclassification scheme. Using spectra taken within 24 hours we estimatedthe internal accuracy of a single spectrum as a function of the 12micrometer flux. The spectra were then searched for variability in thesilicate and SiC dust features. Only one possible case of variabilitywas found. The results are discussed in terms of the pulsational andmass loss characteristics of our sample and other observational data oncircumstellar dust features.
| Circumstellar shells resolved in the IRAS survey data. I - Data processing procedure, results, and confidence tests
We have examined the IRAS 60 and 100 micron survey data covering 512evolved stars and young planetary nebulae for evidence of spatiallyresolved structure. A simple model, consisting of a central unresolvedsource surrounded by a resolved isothermal shell, was fitted to the datafor each star. Seventy-six stars were found to be resolved in the 60micron data. Tests have been performed to verify that the extendedstructure seen is not an artifact of the data-processing algorithm.
| CO and HCN observations of circumstellar envelopes. A catalogue - Mass loss rates and distributions
We have searched the literature for all observations of the (C-12)O(1-0), (C-12)O (2-1), and HCN (1-0) lines in circumstellar envelopes oflate type stars published between January 1985 and September 1992. Wereport data for 1361 observations (stellar velocity, expansion velocity,peak intensity, integrated area, noise level). This CO-HCN sample nowcontains 444 sources; 184 are identified as O-rich, 205 as C-rich, andthere are 9 S stars. About 85 percent of the sources are AGB stars.There are 32 planetary nebulae and about thirty post-AGB starscandidates. Besides results of mm-observations, we also listidentifications, coordinates, IRAS data, and chemical and spectral typesfor every source. For AGB stars, we have estimated (or compiled)bolometric fluxes and distances for 349 sources, and mass loss ratesdeduced from CO results for 324 sources, taking into account theinfluence of the CO photodissociation radius. We also list mass lossrates derived from detailed models of CO emission in the literature.
| Near-infrared photometry of a sample of IRAS point sources
This paper presents the J, H, K, L, M photometry of 516 sourcespertaining to a sample of 787 sources which has been extracted from theIRAS Point Source Catalog in order to study the late stages of stellarevolution and the concomitant phenomena of mass loss. Three differentclssifications of these sources based on broad-band photometry and IRASlow-resolution spectra are given, and the distributions of the sourcesin terms of these classifications are presented. A subsample of peculiarsources, believed to have recently undergone a helium flash, has beenisolated using the K - L, (12-micron) color diagram. Some objectsmeriting further study are also mentioned.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
| Short- and intermediate-period oxygen-rich Miras
Available IR photometry and the period-IR luminosity relationship areused to analyze a nearly complete sample of oxygen-rich Miras atGalactic latitudes with /b/ greater than 30 deg. In agreement withprevious kinematic studies, a marked difference is found in the spatialdistributions between the stars with periods less than and those withperiods greater than 300 d. For stars with periods in the range 300-400d, the exponential scale height from the Galactic plane is close to 240pc, the projected surface density is about 100/sq kpc, and the localspace density is about 210/cu kpc. The progenitors of the Miras withperiods greater than 300 d appear to be disk dwarfs with typicalmain-sequence masses of between 1.0 and 1.2 solar masses. The masses ofthe main-sequence progenitors of the short-period Miras are 1.1 solarmasses or less. The duration of the Mira phase for theintermediate-period stars is estimated to be about 20,000 yr, somewhatlonger than other recent estimates. Both short- and intermediate-periodoxygen-rich Miras characteristically lose mass at about 10 exp -7 solarmass/yr.
| 1612 MHz OH survey of IRAS point sources. I - Observations made at Dwingeloo, Effelsberg and Parkes
The data from a large sky survey are presented including a northernpilot study and a detailed southern study in which detections are biasedtoward the most evolved sources and distant sources. Both areinvestigated at the 1612-MHz transition of OH to take advantage of thestrongest line for AGB stars with optically thick dust shells. The IRASsources are chosen by considering their IR colors related to fluxes at12, 25, and 60 microns. Observations are reported for 2703 IRAS pointsources at the 1612-MHz transition, and 738 OH/IR stars are detected.The survey identifies 597 of the sources as previously unidentified, and95 percent of the OH profiles observed have twin-peak masercharacteristics which are related to emission from expandingcircumstellar shells. The other 5 percent of the sources are concludedto be transition objects between OH/IR stars and planetary nebulae.
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