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מאמרים קשורים
A Slowly Expanding Disk and Fast Bipolar Outflow from the S Star π1 Gruis
We study the molecular outflow of the nearby evolved S starπ1 Gru. We imaged the outflow in CO J=2-1 and dustcontinuum with the Submillimeter Array. The CO emission was detectedover a very broad velocity width of ~90 km s-1. Ourhigh-resolution images show that the outflow at low velocities (<=15km s-1) is elongated east-west and at high velocities(>=25 km s-1) is displaced north (at redshiftedvelocities) and south (blueshifted velocities) of center as defined bythe dust continuum source. We model the spatial-kinematic structure ofthe low-velocity outflow as a flared disk with a central cavity ofradius 200 AU and an expansion velocity of 11 km s-1,inclined by 55° to our line of sight. We attribute the high-velocitycomponent to a bipolar outflow that emerges perpendicular to this diskwith a velocity of up to ~45 km s-1. This high-velocityoutflow may play an important role in shaping the gas envelopepreviously ejected by the AGB star and thus produce a bipolar morphologywhen the object evolves into a proto-planetary nebula.
| A new analytical thin-dusty-shell model to interpret interferometric data. Application to the S star π 1 Gruis
High angular resolution techniques allow the determination of physicaland geometrical parameters characterizing the circumstellar dustyenvironment of late-type stars. The resolution of the radiative transferequation under the thin-shell approximation provides an analyticalexpression for the interferometric visibility profile, which can then becompared to simple ad-hoc geometrical models and to physical modelsbased on radiative transfer. Using the DUSTY numerical code to calculatetheoretical visibilities of the S star π1 Gru, inagreement with spectrophotometric measurements from the near to the farinfrared, we compared the validity of the output parameters found by theχ2 least-square analysis of the visibility, given withthe thin-shell model and with a simple disk+ring geometrical model ofthe envelope. The results are discussed in terms of fitting the shape ofthe intensity profile and of the number of free parameters to beconsidered.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| 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.
| 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.
| 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
| Zirconium to Titanium Ratios in a Large Sample of Galactic S Stars
The [Zr/Ti] ratio for a large sample of Galactic S stars has beendetermined using high-quality, high-resolution spectra. The pattern ofZr enhancements in intrinsic and extrinsic S stars is found to differ,and the [Zr/Ti] ratio in the extrinsic S stars clearly links them to thestrong barium stars. In addition, the pattern of [Zr/Ti] ratios seems toindicate that the progression of spectral type M to MS to S to SC is duelargely to an increase in the abundance of s-process elements and notsolely to a changing C/O ratio as claimed by some investigators (such asScalo and Ross).
| 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
| Near-infrared observations of candidate extrinsic S stars
Photometric observations in the near infrared for 161 S stars, including18 Tc-rich (intrinsic) stars, 19 Tc-deficient (extrinsic) ones and 124candidates for Tc-deficient S stars, are presented in this paper. Basedon some further investigations into the infrared properties of bothTc-rich and Tc-deficient S stars, 104 candidates are identified as verylikely Tc-deficient S stars. The large number of infrared-selectedTc-deficient S stars provides a convenient way to study the physicalproperties and the evolutionary status of this species of S stars.
| 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.
| 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.
| High-Resolution Images of CO J=2-1 Emission from the Carbon Star V Cygni
This paper presents observations of the CO J=2-1 emission from thecircumstellar envelope of the mass-losing carbon star V Cyg. Theobservations were made with the Caltech Millimeter Array. A previouslypublished single-dish map was used to construct short-spacingvisibilities not sampled by the interferometer data, thereby recoveringmissing flux in extended low brightness emission. The images have anangular resolution of ~1.2" with a velocity resolution of 1 MHz (1.3 kms-1). The channel maps are consistent with an expandingenvelope that is roughly spherical, but they also show evidence forasymmetric structure, as well as small-scale clumping. We compare theseobservations, as well as other published spectra, with statisticalequilibrium models for CO in a circumstellar envelope. Models that fitthe spherically averaged data must invoke a mass-loss rate, M, that hasdecreased with time by a factor of ~2-3 over the past several hundredyears. The model kinetic temperature structure in radius,TK(r), decreases as r-0.8 out tor~6×1015 cm and levels off to a constant value atTK=23 K beyond. The secular change in M may be related tochanges in the stellar luminosity or temperature, as predicted by recentnumerical hydrodynamic models for mass loss. The inferred kinetictemperature structure suggests that heating by the photoelectric effecton dust grains is important in the outer envelope.
| Stellar and circumstellar evolution of long period variable stars
In a first paper, HIPPARCOS astrometric and kinematic data were used tocalibrate both infrared K and IRAS luminosities at the same time askinematic parameters of Long Period Variable stars (LPVs). Individualestimated absolute magnitudes and a probabilistic assignation togalactic populations were deduced from these calibrations for each LPVof our sample. Here we propose a scenario of simultaneous stellar andcircumstellar evolution according to the galactic populations. Thetransitory states of S and Tc stars allow us to confirm the location ofthe first dredge-up at Mbol=-3.5. There is also evidencesuggesting that a previous enrichment in s-elements from a more evolvedcompanion may accelerate the evolution along the AGB. The possibleevolution to OH LPVs is included in this scenario, and any of thesestars may have a mass at the limit of the capability for a C enrichmentup to C/O > 1. A list of bright massive LPVs with peculiar envelopeand luminosity properties is proposed as Hot Bottom Burning candidates.The He-shell flash star, R Cen, is found to be exceptionally bright andcould become, before leaving the AGB, a C-rich LPV brighter than theusual luminosity limit of carbon 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).
| Identification of Variable Stars in Grus
Not Available
| The Neutral Envelopes around AGB and Post-AGB Objects Their Structure and Kinematics
Not Available
| Barium Stars and Tc-Poor S Stars: Binary Masqueraders within the Carbon-Star Family
Not Available
| The Henize sample of S stars. II. Data
This paper presents data collected on the Henize sample of 205 S stars:(i) CORAVEL radial-velocity data; (ii) photometric data in the UBV bandsof the Geneva photometric system; (iii) photometric data in the JHKLbands of the SAAO photometric system; (iv) IRAS fluxes; (v)low-resolution spectra of 158 S stars. Close visual companions have beenfound for Hen 47, 94, 105 and 155. Spectroscopic orbital elements areprovided for Hen 2, 108, 121, 137 and 147. The analysis of these data ispresented in a companion paper. Based on observations carried out at theEuropean Southern Observatory (ESO, La Silla, Chile), at the 70~cm Swisstelescope at ESO and at the South African Astronomical Observatory.
| Strong asymmetries in the neutral envelope of Mira observations. CO and K I
We present high-resolution observations of the neutral envelope of oCeti. The molecular component has been mapped in the CO(2-1) radio lineand the atomic component is studied through long-slit spectroscopy ofthe optical Ki line. These observations reveal strong asymmetries in thegas distribution, which can be interpreted as a spherical envelopedisrupted by a bipolar outflow. These data, combined with otherobservations of this object as well as similar observations of other AGBstars, suggest that asymmetrical mass loss processes occur early duringthe AGB stage, well before the proto-planetary nebula phase. Based onobservations carried out with the IRAM Plateau de Bure Interferometer,the Canada-France-Hawaii Telescope, and at the Haute-ProvenceObservatory. IRAM is supported by INSU/CNRS (France), MPG (Germany) andIGN (Spain).
| The light curve and evolutionary status of the carbon star V Hya
V Hya, an evolved carbon star with a complex circumstellar envelope, hastwo variability periods, 530d and 6000d (17years). We analyze recent light curve data and show that both variationshave been present for at least 100 years and have been regular over thistime. The 530d period and its 1.5m-2mamplitude show that V Hya is a Mira variable. We suggest that the staris in a binary system (as also suspected from the structure of thecircumstellar envelope) and that the 17-year variation is due toextinction by circumstellar dust orbiting with the companion. Theproperties of the envelope found from molecular line observations: thefast molecular wind, the relatively small size of the densecircumstellar envelope, and the high mass loss rate, all suggest that VHya has entered its `superwind' phase. However, its spectral type,period, colors, and lack of ionizing radiation show that the star isstill on the AGB. These properties add to the evidence that the complexstructures of many planetary nebulae, including fast stellar winds,originate during the final phases of mass loss on the AGB.
| The circumstellar envelope of pi (1) Gru
CO(J = 2-1) and SiO(J = 5-4) emission has been observed from themolecular envelope around the nearby S star pi (1) Gru. The CO lineprofile differs from the usual parabolic shape seen inuniformly-expanding envelopes; it has a Voigt-like profile and twohorns. A model for line formation in the envelope shows that a tilted,expanding disk reproduces the observations well. The star also has afast molecular wind, with a projected outflow speed of at least 70 andperhaps as high as 90 km s(-1) . The fast wind is presumably ejectedfrom the poles of the disk. These observations show that the complexstructure seen in many planetary nebulae, including quadrupolarstructure and fast winds, may largely evolve from structure formed whilethe progenitor star is in the last stages of evolution on the AGB.
| Detection of warm SO_2 gas in oxygen-rich AGB stars
We report the discovery of the nu_3 band of SO_2 at 7.3 mu m in theISO/SWS spectra of oxygen-rich AGB stars. The band is clearly detectedin three stars, UX Cyg, o Cet and T Cep, and marginally detected in atleast four other stars. The band is seen in absorption in UX Cyg, whileit is in emission in o Cet. Seven spectra of T Cep taken at differentphases show that the feature changes from emission to absorption on atime scale of twice the pulsation period. Using an LTE model, we findthat the excitation temperature of SO_2 is typically 600 K, and that inT Cep the molecule occupies a region with dimensions of several stellarradii. The total number of molecules contained in this region is oforder 10(47) , which requires a local gas density of at least 10(9) H_2cm(-3) , and possibly up to 10(11) H_2 cm(-3) depending on the SO_2abundance. The variation with phase of the T Cep spectra can beexplained by photodissociation of SO_2 molecules by soft UV photons.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) with the participation of ISAS andNASA. The SWS is a joint project of SRON and MPE.
| Heavy-element abundances in seven SC stars and several related stars
We employ spectra of resolution 20-35,000 of seven SC stars, four Sstars, two Ba stars, and two K-M stars to derive abundances of a varietyof elements from Sr to Eu relative to iron. Special attention is paid toRb and Tc, and to the ratio of the heavy s-process species to the lights-process elements. Abundances are derived in LTE, both by using modelatmospheres in which the carbon and oxygen abundances are nearly equaland by using curves of growth. Spectrum synthesis is used for criticallines, such as the 5924-A line of Tc and the 7800-A line of Rb. For mostof the heavy-element stars, the enhancement of the s-process elements isabout a factor of 10. The ratio of the heavy to light s-process speciesis not far from solar, except for RR Her, for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we haveprobably detected the 5924-A line, we can only present abundances in theless-than-or-equal-to category.
| On the Variability of S Stars as Observed by the Hipparcos
The Hipparcos photometry of S type stars shows that they are allvariable. The intrinsic S stars show a larger range of amplitudes thando the extrinsic S stars.
| 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.
| Multiple Molecular Winds in Evolved Stars. I. A Survey of CO (2-1) and CO (3-2) Emission from 45 Nearby AGB Stars
This paper describes observations of a new phenomenon in evolvedmass-losing asymptotic giant branch (AGB) stars: the presence of twowinds with different expansion velocities. CO(2-1) and CO(3-2) lineemission was observed for 45 AGB stars at high velocity resolution anddouble winds found in 20% of the sample. Highly asymmetric lines werefound in six other stars. The data tentatively suggest that double windsoccur when the star undergoes a change (pulsational mode, chemicalcomposition) and that the very narrow components represent the onset ofa new phase of mass loss.
| Molecular line observations of southern S stars
We observed a sample of southern S stars with the SEST telescope, in theSiO (v=0, J=3-2) transition at 130.3 GHz, and the HCN (J=1-0) transitionat 88.6 GHz. SiO emission was detected in all seven stars observed,while HCN was detected in two. We employed a statisticalequilibrium/radiative transfer model to estimate the SiO abundance foran assumed molecular distribution. The inferred SiO abundances areconsistent with formation of the molecule under thermodynamicequilibrium (TE) conditions near the stellar photosphere, for reasonablephysical conditions. We also model the HCN emission by a similaranalysis, and find that if HCN is produced near the stellar photosphere,our model abundances are much higher than predicted by TE chemistry,unless the gas temperature is <1300 K and the gas density n(H_2) ~10(12) cm(-3) . Under such conditions, condensation of silicate grainsmay enhance production of HCN for C/O ~ 1. Alternatively, HCN may beformed by photochemical reactions in the outer circumstellar envelope,as has been proposed for O-rich giants.
| CO observations and mass loss of MS- and S-stars
We present (12) CO J = 1-0 and 2-1 observations of 14 S-stars, andreport 6 new detections. Two stars were observed in the (13) CO J = 1-0and 2-1 lines, and one tentative 2-1 detection is reported. Acompilation is presented of all CO observations of S-stars. The stars inthis sample are separated into ``intrinsic'' and ``extrinsic'' S-stars,based on direct observation of the Technetium line, or infraredproperties. The dust mass loss rate per unit distance is derived fromIRAS 60 mu m data taking into the fact that for small mass loss ratesthe observed flux is an overestimate of the excess emission due to dust.The gas mass loss rate per unit distance is derived from CO data.Distances and luminosities are estimated, partly from hipparcos parallaxdata. The largest mass loss rate derived is that for W Aql with(0.8-2.0) x 10(-5) {Msun} yr(-1) , and the lowest is that foro Ori with <1.2 x 10(-9) {Msun} yr(-1) . The S-starswithout Tc have smaller mass loss rates, than those with Tc. Diagramsshowing mass loss rate, dust-to-gas ratio and expansion velocity versuspulsation period are presented, and compared to similar data for carbon-and oxygen-rich Miras. The S-Miras stand not out in any way from the C-or O-Miras in these diagrams. In the diagram with expansion velocityversus pulsation period, the S-SRs span the same range in velocity asthe S-Miras, but they have periods which are about a factor of 2.5shorter. This was previously noted for O-rich SRs. As in that case, themost straightforward explanantion is that the SRs among the S-starspulsate in a higher order pulsation mode. Based on data from the ESAhipparcos astrometry satellite.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | עגור |
| התרוממות ימנית: | 22h22m43.90s |
| סירוב: | -45°56'52.0" |
| גודל גלוי: | 6.62 |
| מרחק: | 152.905 פארסק |
| תנועה נכונה: | 33.4 |
| תנועה נכונה: | -17.7 |
| B-T magnitude: | 9.159 |
| V-T magnitude: | 6.41 |
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INHABITED SKY
