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Very Large Array Observations of H I in the Circumstellar Envelopes of Asymptotic Giant Branch Stars
We have used the Very Large Array to search for neutral atomic hydrogen(H I) in the circumstellar envelopes of five asymptotic giant branchstars. We have detected H I 21 cm emission coincident in both positionand velocity with the S-type semiregular variable star RS Cnc. Theemission comprises a compact, slightly elongated feature centered on thestar with a mean diameter of ~82" (1.5×1017 cm), plusan additional filament extending ~6' to the northwest. If this filamentis associated with RS Cnc, it would imply that a portion of its massloss is highly asymmetric. We estimateMHI~1.5×10-3 Msolar and amass-loss rate M˙~1.7×10-7 Msolaryr-1. Toward three other stars (IRC+10216, EP Aqr, R Cas) wehave detected arcminute-scale H I emission features at velocitiesconsistent with the circumstellar envelopes, but spatially offset fromthe stellar positions. Toward R Cas, the emission is weak but peaks atthe stellar systemic velocity and overlaps with the location of itscircumstellar dust shell and thus is probably related to the star. Inthe case of IRC+10216, we were unable to confirm the detection of H I inabsorption against the cosmic background previously reported by LeBertre & Gérard. However, we detect arcs of emission atprojected distances of r~14'-18'(~2×1018 cm) to the northwest of the star. The largeseparation of the emission from the star is plausible, given itsadvanced evolutionary status, although it is unclear if the asymmetricdistribution and complex velocity structure are consistent with acircumstellar origin. For EP Aqr, the detected H I emission comprisesmultiple clumps redward of the systemic velocity, but we are unable todetermine unambiguously whether the emission arises from thecircumstellar envelope or from interstellar clouds along the line ofsight. Regardless of the adopted distance for the H I clumps, theirinferred H I masses are at least an order of magnitude smaller thantheir individual gravitational binding masses. We did not detect any H Iemission from our fifth target, R Aqr (a symbiotic binary), but measureda 1.4 GHz continuum flux density of 18.8+/-0.7 mJy. R Aqr is apreviously known radio source, and the 1.4 GHz emission likely arisesprimarily from free-free emission from an ionized circumbinary envelope.
| Circumstellar Atomic Hydrogen in Evolved Stars
We present new results of a spectroscopic survey of circumstellar H I inthe direction of evolved stars made with the NançayRadiotelescope. The H I line at 21 cm has been detected in thecircumstellar shells of a variety of evolved stars: asymptotic giantbranch stars, oxygen-rich and carbon-rich stars, semiregular and Miravariables, and planetary nebulae. The emissions are generally spatiallyresolved, i.e., larger than 4', indicating shell sizes on the order of 1pc, which opens the possibility of tracing the history of mass loss overthe past ~104-105 yr. The line profiles aresometimes composite. The individual components generally have aquasi-Gaussian shape; in particular, they seldom show the double-hornprofile that would be expected from the spatially resolved opticallythin emission of a uniformly expanding shell. This probably implies thatthe expansion velocity decreases outward in the external shells (0.1-1pc) of these evolved stars. The H I line profiles do not necessarilymatch those of the CO rotational lines. Furthermore, the centroidvelocities do not always agree with those measured in the CO linesand/or the stellar radial velocities. The H I emissions may also beshifted in position with respect to the central stars. Without excludingthe possibility of asymmetric mass ejection, we suggest that these twoeffects could also be related to a nonisotropic interaction with thelocal interstellar medium. H I was detected in emission toward severalsources (ρ Per, α Her, δ2 Lyr, U CMi) thatotherwise have not been detected in any radio lines. Conversely, it wasnot detected in the two oxygen-rich stars with substantial mass-lossrate, NML Tau and WX Psc, possibly because these sources are young, withhydrogen in molecular form, and/or because the temperature of thecircumstellar H I gas is very low (<5 K).This paper is dedicated to the memory of Marie-Odile Mennessier(1940-2004).
| BIMA CO Observation of EP Aquarii: The Semiregular Pulsating Star with a Double-Component Line Profile
This paper reports the results of a Berkeley-Illinois-MarylandAssociation array interferometric observation of EP Aqr, a semiregularpulsating star with a double-component line profile in the CO J=1-0line. The broad component shows a flat-topped profile, and the narrowcomponent shows a spiky strong peak. Although previous single-dishobservations suggested that the CO J=2-1 line exhibits a Gaussian-likeprofile, the CO J=1-0 line does not. The spatial distributions of boththe narrow and the broad components appear to be roughly round with thesame peak positions. No significant velocity gradient is seen. Thespatial-kinetic properties of the molecular envelope of EP Aqr arereminiscent of a multiple-shell structure model rather than of a bipolarflow and disk model. A problem with this interpretation is that noevidence of interaction between the narrow- and broad-component regionsis seen. A Gaussian-like feature seen in the CO J=2-1 line might play akey role in understanding the spatiokinetic properties of the molecularenvelope of EP Aqr.
| Atomic hydrogen in asymptotic giant branch circumstellar environments. A case study: X Her
We report the detection of the HI line at 21 cm from the circumstellarshell around the asymptotic giant branch star X Her using theposition-switching technique with the Nançay Radio Telescope. Atthe star position, the line shows two components: (i) a broad one [fullwidth at half-maximum (FWHM) ~ 13 kms-1] centred at -72.2kms-1 and (ii) a narrow one (FWHM ~ 4 kms-1)centred at ~-70.6kms -1. Our map shows that the sourceassociated to the broad component is asymmetric with material flowingpreferentially towards the north-east. This source extends to ~10 arcmin(~0.4pc) from the star in that direction. On the other hand, the narrowcomponent is detected only at the star position and indicates materialflowing away from the observer. The total mass of atomic hydrogen is~6.5 × 10-3Msolar which, within a factor of2, agrees with the estimate obtained from IRAS data at 60 μm.
| BIMA Array Observations of the Highly Unusual SiO Maser Source with a Bipolar Nebulosity IRAS 19312+1950
We report the results of mapping observations of the bipolar nebula withSiO maser emission, IRAS 19312+1950, in the CO (J=1-0 and 2-1),13CO (J=1-0 and 2-1), C18O (J=1-0), CS (J=2-1), SO(JK=32-21), and HCO+ (J=3-2)lines with the Berkeley-Illinois-Maryland Association array. Theevolutional status of this source has been evoking a controversy sinceits discovery, although SiO maser sources are usually identified aslate-type stars with active mass loss. In line profiles, two kinematicalcomponents are found, as reported in previous single-dish observations:a broad pedestal component and a narrow component. Spatiokineticproperties of a broad-component region traced by 12CO linesare roughly explained by a simple spherical outflow model with anexpanding velocity typical of an AGB star, although some properties ofthe broad-component region still conflict with properties of a typicalAGB spherical outflow. A narrow-component region apparently exhibits abipolar flow. The angular size of the narrow-component region isspatially larger than that of the broad-component region. The intensitydistribution of the CS emission avoids the central region of the source,and that of the SO broad-component emission exhibits a small featurepeaked exactly at the mapping center. According to the present results,if the broad component really originates in a spherical outflow, anoxygen-rich evolved stellar object seems to be a natural interpretationfor the central star of IRAS 19312+1950.
| 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.
| High-Resolution Mid-Infrared Imaging of the Asymptotic Giant Branch Star RV Bootis with the Steward Observatory Adaptive Optics System
We present high-resolution (~0.1"), very high Strehl ratio (0.97+/-0.03)mid-IR adaptive optics (AO) images of the asymptotic giant branch (AGB)star RV Boo utilizing the MMT adaptive secondary AO system. RV Boo wasobserved at a number of wavelengths over two epochs (9.8 μm in 2003May and 8.8, 9.8, and 11.7 μm in 2004 February) and appeared slightlyextended at all wavelengths. While the extension is very slight at 8.8and 11.7 μm, the extension is somewhat more pronounced at 9.8 μm.With such high Strehl ratios, we can achieve superresolutions of 0.1" bydeconvolving RV Boo with a point-spread function (PSF) derived from anunresolved star. We tentatively resolve RV Boo into a 0.16" FWHMextension at a position angle of 120°. At a distance of390+250-100 pc, this corresponds to a FWHM of60+40-15 AU. We measure a total flux at 9.8 μmof 145+/-24 Jy for the disk and star. Based on a dust thermal emissionmodel for the observed IR spectral energy distribution and the 9.8 μmAO image, we derive a disk dust mass of 1.6×10-6Msolar and an inclination of 30°-45° from edge-on. Wediscuss whether the dust disk observed around RV Boo is an example ofthe early stages in the formation of asymmetric structure in planetarynebulae.
| The circumstellar environments of EP Aqr and Y CVn probed by the H I emission at 21 cm
The H I line at 21 cm has been detected in the circumstellar shells ofthe two semi-regular variables, EP Aqr and Y CVn, with the NançayRadio-Telescope (NRT). In both cases the line shape is composite and theemission is spatially extended compared to the NRT beam size (4 arcmin).The total H I masses of the two envelopes are respectively 0.047Mȯ and 0.044 Mȯ. For EP Aqr, we findthree components: a narrow one centered on the star and spatiallyunresolved, and two broad, Gaussian components, offset w.r.t. the starand spatially extended. The narrow component traces the present windfrom EP Aqr whereas the two others seem to trace one or severalmass-loss episodes of long duration. For Y CVn, we find two components:a narrow, spatially very extended, feature, and a broad one, lessextended. We argue that the second component traces the outflow and thefirst one, its interaction with the ISM. These observations show thatthe H I emission can be used not only to probe the mass loss history oflong-period variables, but also to investigate the interaction betweenstellar outflows and the surrounding ISM.
| The phase of H2O ice and the librational band in OH231.8+4.2: new interpretations
The phase of H2O ice and the librational ice band detectedtoward the post-asymptotic giant branch (post-AGB) star OH231.8+4.2 havebeen examined using radiative transfer modelling. The results indicatethat the ice is largely crystalline and not amorphous as previouslyreported in the literature. The earlier result is shown to be due to theassumption of unrealistically thick ice mantles. It has also been shownthat, in comparison with radiative transfer modelling, Mie theorymodelling of the 3-μm ice band detected towards moderately opticallythick shells leads to an underestimate of the ice mantle thickness. Themodelling results also suggest that the mid-infrared (MIR) featurepreviously identified with the librational band of H2O icecould be the result of amorphous Al2O3. This dustcomponent can also account for the observed enhanced opacity between the10- and 18-μm silicate bands.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Synthetic Lick Indices and Detection of α-Enhanced Stars
Synthetic Lick indices computed with solar scaled abundances and withα-element enhancement are presented and compared with predictionsfrom both theoretical computations (Tripicco & Bell; Thomas,Maraston, & Bender; Barbuy et al.) and empirical fitting functions(de Freitas Pacheco). We propose selected combinations of indicescapable of singling out α-enhanced stars without requiringprevious knowledge of their main atmospheric parameters. By applyingthis approach to the 460 stars in the Worthey et al. catalog, wedetected a list of 82 candidate α-enhanced stars. The confirmationof α-enhancement was obtained by searching the literature forindividual element abundance determinations from high-resolutionspectroscopy for a subsample of 34 stars. Preliminary discussion of theproperties of the detected α-enhanced stars with respect to their[Fe/H] values and kinematics is presented.
| Really Cool Stars and the Star Formation History at the Galactic Center
We present λ/Δλ=550-1200 near-infrared H and Kspectra for a magnitude-limited sample of 79 asymptotic giant branch andcool supergiant stars in the central ~5 pc (diameter) of the Galaxy. Weuse a set of similar spectra obtained for solar neighborhood stars withknown Teff and Mbol that is in the same range asthe Galactic center (GC) sample to derive Teff andMbol for the GC sample. We then construct the H-R diagram forthe GC sample. Using an automated maximum likelihood routine, we derivea coarse star formation history of the GC. We find that (1) roughly 75%of the stars formed in the central few parsecs are older than 5 Gyr; (2)the star formation rate (SFR) is variable over time, with a roughly 4times higher SFR in the last 100 Myr compared to the average SFR; (3)our model can match dynamical limits on the total mass of stars formedonly by limiting the initial mass function to masses above 0.7Msolar (this could be a signature of mass segregation or ofthe bias toward massive star formation from the unique star formationconditions in the GC); (4) blue supergiants account for 12% of the totalsample observed, and the ratio of red to blue supergiants is roughly1.5; and (5) models with isochrones with [Fe/H]=0.0 over all ages fitthe stars in our H-R diagram better than models with lower [Fe/H] in theoldest age bins, consistent with the finding of Ramírez et al.that stars with ages between 10 Myr and 1 Gyr have solar [Fe/H].
| ``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.
| 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
| Infrared investigation from earth and space on the evolutionary state of a sample of LPV
We selected a sample of highly reddened AGB stars among the sourcesobserved with the SWS instrument on the ISO satellite. These SWS dataallow us to compute the source's photometry in the mid-IR filters of thecamera TIRCAM at the TIRGO telescope. Our photometric data, supplementedwith other measurements taken from the literature, permit to select thecarbon-rich sources in the sample. For these stars, a linear relationholds between dust mass loss and the color index [8.8]-[12.5]. One maythen, from photometric data alone, evaluate the total mass loss (forwhich we used the estimate of \citet{loup}, based on radio data). Theoxygen-rich sources, on the other hand, are distributed in two branches,of which the upper one appears superimposed with carbon stars; the starsin this group have both high luminosity and high wind velocity andtherefore higher masses. Finally S stars lie between the carbon-starbranch and the low-mass oxygen-rich stars, in agreement with theirintermediate evolutionary status.
| On the origin of the 19.5 μ m feature. Identifying circumstellar Mg-Fe-oxides
We report the detection of a broad, prominent 19.5 mu m dust emissionfeature in ISO-SWS spectra. It is especially conspicuous in the spectraof low mass-loss AGB stars belonging to the variability classes SRb andLb with relatively high photospheric temperatures. The feature carrieris proposed to be Mg0.1Fe0.9O, a solid solution ofMgO (periclase) and FeO (wustite). This dust species has cubic crystalsymmetry like spinel, the carrier of the 13, 16.8 and 31.8 mu m spectralfeatures, together with which the 19.5 mu m feature is being observed. Abroad emission plateau between 11 and 15 mu m, which is attributable toamorphous Al2O3, is also detected together withthe 19.5 mu m and the spinel features. As a consequence of ourdiscovery, we postulate the existence of a distinct class ofcircumstellar shells, dominated by oxide and not by silicate dust. Basedon observations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| 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.
| 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.
| 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
| Monitoring of LPVs with an automatic telescope.. II: A comparison of APT data and visual observations
We discuss the possibilities of investigating the semiregular andirregular light change found in evolved late type giant stars withphotometric and visually obtained (amateur) data. Period analysis hasbeen done on light curve data from both sources for a sample ofsemiregular variables. The results are compared to test the ability ofvisual data to show the frequency contents present in these variables.We find that both sources of data complete each other, as fitting thefine structure visible only in the photometric data needs long-termvisual monitoring to guarantee the uniqueness of the fit.
| 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).
| Discovery of anomalous oxygen isotopic ratios in HR 4049
We report the discovery in the ISO/SWS spectrum of the post-AGB starHR 4049 of emission bands due to 17O and18O isotopes locked up in CO2 molecules. It is thefirst time these isotopomers are detected outside the solar system.Isotopic ratios derived in the optically thin limit are as low as16O/17O = 8.3+/- 2.3 and16O/18O = 6.9 +/- 0.9. These values are at leastone order of magnitude lower than any previously determined isotopicratio in any type of evolved star. Based on observations with ISO, anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, the Netherlands and the UK) with theparticipation of ISAS and NASA. The SWS is a joint project of SRON andMPE.
| 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.
| Dust features in the 10-mu m infrared spectra of oxygen-rich evolved stars
We have analyzed the 8-13.5 mu m UKIRT CGS3 spectra of 142 M-type starsincluding 80 oxygen-rich AGB stars and 62 red supergiants, with a viewto understanding the differences and similarities between the dustfeatures of these stars. We have classified the spectra into groupsaccording to the observed appearance of the infrared features. In eachcase the normalized continuum-subtracted spectrum has been compared tothose of the other stars to find similarities and form groups. The dustfeatures of the AGB stars are classified into six groups: broad AGB,where the feature extends from 8 mu m to about 12.5 mu m with littlestructure; broad+sil AGB, which consists of a broad feature with anemerging 9.7 mu m silicate bump; and four silicate AGB groups in which a``classic'' 9.7 mu m silicate feature gets progressively narrower.Likewise, the supergiant spectra have also been classified into groups,however these do not all coincide with the AGB star groups. In thesupergiant case we again have six groups: featureless, where there islittle or no emission above the continuum; broad Super, where thefeature extends from about 9 mu m to about 13 mu m; and four silicateSuper groups, which again show a progression towards the narrowest``classic'' 9.7 mu m silicate feature. We compare the mean spectrum foreach group, which yields two main results. Firstly, while the``classic'' silicate feature is essentially identical for both AGB starsand red supergiants, the broad features observed for these two stellartypes are quite different. We suggest that the dust in these twoenvironments follows different evolutionary paths, with the dust aroundMira stars, whose broad feature spectra can be fit by a combination ofalumina (Al2O3) and magnesium silicate,progressing from this composition to dust dominated by magnesiumsilicate only, while the dust around supergiants, whose broad featurecan be fit by a combination of Ca-Al-rich silicate andAl2O3, progresses from this initial composition toone eventually also dominated by magnesium silicate. The reason for thedifference in the respective broad features is not clear as yet, butcould be influenced by lower C/O ratios and chromospheric UV radiationfields in supergiant outflow environments. The second result concernsthe 12.5 - 13.0 mu m feature discovered in IRAS LRS spectra and widelyattributed to Al2O3. This feature is seenpredominantly in the spectra of semiregular variables, sometime in Mirasand only once (so far) in supergiant spectra. We argue that it isunlikely that this feature is due to Al2O3 or, ashas more recently been suggested, spinel(MgAl2O4), but could be associated with silicondioxide or highly polymerized silicates (not pyroxenes or olivines).
| CO2 emission in EP Aqr: Probing the extended atmosphere
We present an analysis of the ISO/SWS full resolution scan between 12.5and 16.5 μm of the O-rich AGB star EP Aqr, exhibiting a number ofstrong CO2 emission bands. We have developed a simple LTEmodel to calculate theoretical CO2 spectra assuming asingle-layer slab geometry and compared the SWS observations to thismodel in order to infer the physical properties of the extendedatmosphere. The single layer slab model is able to reproduce theindividual band profiles quite well with optically thick bands (columndensities typically 1018.5 cm-2). The derivedexcitation temperatures for the different bands are in the range T ~350-700 K in a region which extends from ~ 4 - 9R*. Thefundamental CO2 bending mode at 14.98 μm furthermore showsevidence for an optically thin component arising from a much cooler (T ~100 K) and more extended (Rem ~ 100R*) layer. Thestrong spectral signature of 13CO2 in the spectrumallows an (uncertain) determination of the 12C/13Cratio ~ 10. The parameters derived for the CO2 bands allow usto estimate the local temperature and density structure of the extendedatmosphere. We find that the derived local gas temperatures are somewhatlower than predicted by hydrodynamical model calculations whereas thelocal gas densities are in good agreement with these models when usingCO2 abundances derived from chemical network calculations.The CO2 layer extends from close to the stellar photosphereto the inner part of the dust forming region which makes it a unique newprobe of the whole extended atmosphere and the region where dustformation takes place. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA. The SWS is a joint project of SRONand MPE.
| ISO-SWS Observations of CO_2 and H_2O in R Cassiopeiae
We present ISO-SWS spectra of the O-rich Mira variable R Cas, showingCO_2 in absorption and emission, and H_2O in absorption. The CO_2absorption feature is the 0110-0000 ro-vibrationalband at 14.97 μm. The emission features are the1000-0110 and 1110-0220ro-vibrational transitions at 13.87 and 13.48 μm respectively. Thewater absorption spectrum shows the nu_1 and nu_3 ro-vibrational bandsin the 2.75-3 μm region. Using LTE models, we derive physicalparameters for the features. We find the CO_2 emission temperature to be~ 1100 K. We discuss the nature of the CO_2 feature at 15 μm and showthat it can be modeled as an emission/absorption band by deviating fromthermal equilibrium for the population of the 0110vibrational level. The H_2O absorption spectrum is shown to arise fromgas at different temperatures, but can be fit reasonably well with twocomponents at T=950 K and T=250 K. The CO_2 emission and hot H_2Oabsorption temperatures are similar, suggesting that these featuresprobe the same region of the inner envelope. We discuss the innerenvelope chemistry using molecular equilibrium calculations and recentmodeling work by Duari et al. (1999), and find our observationsconsistent with the results. We also report the detection of the CO_20110-0000 ro-vibrational band in absorptiontowards another oxygen-rich Mira, IRC+10011. Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, the Netherlands and theUnited Kingdom) with the participation of ISAS and NASA. The SWS is ajoint project of SRON and MPE.
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