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First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars
We have measured nonzero closure phases for about 29% of our sample of56 nearby asymptotic giant branch (AGB) stars, using the three-telescopeInfrared Optical Telescope Array (IOTA) interferometer at near-infraredwavelengths (H band) and with angular resolutions in the range 5-10 mas.These nonzero closure phases can only be generated by asymmetricbrightness distributions of the target stars or their surroundings. Wediscuss how these results were obtained and how they might beinterpreted in terms of structures on or near the target stars. We alsoreport measured angular sizes and hypothesize that most Mira stars wouldshow detectable asymmetry if observed with adequate angular resolution.
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
| Diameters of Mira Stars Measured Simultaneously in the J, H, and K' Near-Infrared Bands
We present the first spatially resolved observations of a sample of 23Mira stars simultaneously measured in the near-infrared J, H, and K'bands. The technique used was optical long-baseline interferometry, andwe present for each star visibility amplitude measurements as a functionof wavelength. We also present characteristic sizes at each spectralband, obtained by fitting the measured visibilities to a simple uniformdisk model. This approach reveals the general relation J diameter < Hdiameter < K' diameter.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| The volatile OH Mainline Masers of R Leo
R Leo was observed soon after the Arecibo upgrade, in mid-1999, when itsprincipal 1665 & 1667 MHz masers respectively had intensities of 2.2& 3.2 Jy: these had completely disappeared by mid-2002. The 1665 MHzmaser has never exceeded 10 mJy since, while the 1667 has never exceeded70 mJy. Moreover the initial decline of both, by factors of ≥ 25,occurred in 10 months. Though both masers recovered by a factor of ≥3 soon after, this was immediately followed by a sharp decline fromwhich they have never rallied. The 1667 MHz maser declined from 1000 to27 mJy over the first 9 months of 2001, and even more quickly, from 60to 24 mJy, in 36 days through September 2004.This behaviour contrasts sharply with that of 1612 MHz masers of normalOH/IR stars, which commonly vary by a factor of three around thepulsation cycle, and which have on occasion been observed to disappearwith an e-folding time of 1 yr (ApJ 548, L77). Nor is thisdifference simply the usual pulsational variation of OH mainlines, asthose of R LMi only vary in intensity by a factor of three. We concludethat the mainline masers of R Leo are unusually volatile, and prone toan order of magnitude more variability than most such masers.The NIR colors of OH/IR stars follow a well-defined locus, whose entireextent is traversed ≤ 60 yr after the cessation of mass-loss. Lewis(BAAS 35, 1358) therefore argues that the rapid loss of 1612 MHz masersis set by changes in the maser pump following on changes in thereprocessing of stellar radiation by dust in the inner environs of itscircumstellar shell. But the pumping of OH mainline masers is even moresensitive to the details of IR-line overlap than the 1612. The amplitudeand brevity of changes in the OH mainline masers of R Leo thereforesuggest that these are generated by small changes in IR line overlap.
| 28SiO v = 1 and v = 2, J = 1-0 maser variability in evolved stars. Eleven years of short spaced monitoring
This paper presents and discusses the final data set of a long-term andshort-spaced monitoring of 21 SiO maser sources, mostly evolved stars,carried out in two SiO maser lines at 43 GHz with the ObservatorioAstronómico Nacional 13.7 m telescope at the CentroAstronómico de Yebes (Guadalajara, Spain). In most objects, morethan 80 spectra per transition over a period of 11 years have beenrecorded. The new data presented here, previously unpublished, representnearly 50% of the total SiO data collected in the project. In addition,the availability of optical light curves from the AAVSO for most of theobjects during the whole period of the SiO monitoring, ground-basednear-IR data for four sources overlapping with 3 to 5 observed SiOperiods, and DIRBE near-IR data covering a significant portion of an SiOperiod in 10 sources, make this data set a unique reference forcomparing optical, NIR and SiO variability in order to elucidate thephysical mechanisms that pump SiO masers in evolved stars. The basis forthe conclusions obtained in this work comes from a numerical time seriesanalysis of the suitable SiO, optical and NIR light curves in regularvariables to obtain precise values of the periods and phase lags betweenthe different curves. This analysis shows evidence that in regularvariable evolved stars the three types of emission have the same periodand that the SiO maxima happen in phase with NIR maxima and with a phaselag typically between 0.05 and 0.20 with respect to optical maxima. Weconclude that in these objects the observational evidence presented inthis work favors the radiative pumping of SiO masers against thecollisional pumping.Figures 1-4 and 11-21 are only available in electronic form athttp://www.edpsciences.org
| First mm-VLBI Observations between the TRAO 14-m and the NRO 45-m Telescopes: Observations of 86 GHz SiO Masers in VY Canis Majoris
We have made VLBI observations at 86GHz using a 1000-km baseline betweenKorea and Japan with successful detections of SiO v = 1, J = 2 ‑ 1maser emissions from VY CMa and Orion KL in 2001 June. This was thefirst VLBI result for this baseline and the first astronomical VLBIobservation for the Korean telescope. Since then, we observed SiO v = 1,J = 2 ‑ 1 maser emission in VY CMa in 2002 January and 2003February and derived the distributions of the maser emissions. Ourresults show that the maser emissions extend over 2-4 stellar radii, andwere within the inner radius of the dust shell. We observed other SiOmaser sources and continuum sources, and 86-GHz continuum emissions weredetected from three continuum sources. It was verified that thisbaseline has a performance comparable to the most sensitive baseline inthe VLBA and the CMVA, and is capable of investigating the propermotions of maser features in circumstellar envelopes using monitoringobservations.
| Observations of Late-Type Variable Stars in the Water Vapor Radio Line. The Long-Period Variable R Cassiopeia
Observations of circumstellar maser emission from the long-periodvariable R Cas in the 1.35-cm water-vapor line are reported. Theobservations were carried out on the 22-m radio telescope of thePushchino Radio Astronomy Observatory in 1980 2003 (JD=2444409 2452724).Over the 23 years of observations, strong flares in the H2O line profilewere recorded in 1982 (with a peak flux density up to 400 Jy) and 19861989 (up to 750 Jy). Subsequently, from 1990 to March 2003, the H2O lineflux was usually below the detection threshold of the radio telescope(<5 10 Jy). Episodic small increases of the emission with peak fluxdensities of 20 60 Jy were observed. The variations of the H2O line fluxF are correlated with variations in the visual brightness of the star.The phase delay Δγ of the F variations relative to theoptical light curve of R Cas ranged from 0.2 0.3P during theobservations (P=430.46d is the star's period). A model for thevariability of the H2O maser in R Cas is discussed. If the variationsare due to periodic impacts by shock waves driven by the stellarpulsations, the time for the shock to travel from the photosphere to theinner boundary of the H2O-masing shell may reach 2 4P. The flares couldbe due to transient episodes of enhanced mass loss by the star or to thepropagation of an exceptionally strong shock from the stellar surface.
| 2MASS Counterparts for OH/IR Stars. I. The Arecibo Sample
The positions of the IRAS counterparts to the 420 OH/IR stars in theArecibo sky (0°<δ<+38°) are usually accurate tobetter than 10". But every star has recently been observed by the TwoMicron All Sky Survey (2MASS), which provides ~0.2" quality positions,while those with |b|<=4.5d have also been observed by the MidcourseSpace Experiment (MSX), which provides ~2" quality positions. We use theMSX and/or IRAS coordinates to guide us to 2MASS counterparts for the134 Arecibo OH/IR stars with images in the second release of the 2MASSPoint Source Catalog. An unexpected by-product of having the J-H versusH-Ks plot generated from the 2MASS fluxes is the realizationthat most (~85%) of the redder OH/IR stars have detached circumstellarshells. We identify five objects that probably, by contrast, have``normal'' shells, and we confirm the status of AU Vul as aprotoplanetary nebula.
| 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 and rotational CO emission from Asymptotic Giant Branch stars
We present submillimeter observations of rotational transitions ofcarbon monoxide from J = 2 -> 1 up to 7 -> 6 for a sample ofAsymptotic Giant Branch stars and red supergiants. It is the first timethat the high transitions J=6 -> 5 and 7 -> 6 are included in sucha study. With line radiative transfer calculations, we aim to determinethe mass-loss history of these stars by fitting the CO line intensities.We find that the observed line intensities of the high transitions,including the J=4 -> 3 transition, are significantly lower than thepredicted values. We conclude that the physical structure of the outflowof Asymptotic Giant Branch stars is more complex than previouslythought. In order to understand the observed line intensities andprofiles, a physical structure with a variable mass-loss rate and/or agradient in stochastic gas velocity is required. A case study of the AGBstar WX Psc is performed. We find that the CO linestrengths may be explained by variations in mass-loss on time scalessimilar to those observed in the separated arc-like structures observedaround post-AGB stars. In addition, a gradient in the stochasticvelocity may play a role. Until this has been sorted out fully, any massloss determinations based upon single CO lines will remain suspect.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also available in electronic form at the CDS, via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1167
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| 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.
| Evidence for Very Extended Gaseous Layers around O-rich Mira Variables and M Giants
Nine bright O-rich Mira stars and five semiregular variable cool Mgiants have been observed with the Infrared and Optical Telescope Array(IOTA) interferometer in both K' (~2.15 μm) and L' (~3.8 μm)broadband filters, in most cases at very close variability phases. Allof the sample Mira stars and four of the semiregular M giants showstrong increases, from ~=20% to ~=100%, in measured uniform-disk (UD)diameters between the K' and L' bands. (A selection of hotter M starsdoes not show such a large increase.) There is no evidence that K' andL' broadband visibility measurements should be dominated by strongmolecular bands, and cool expanding dust shells already detected aroundsome of these objects are also found to be poor candidates for producingthese large apparent diameter increases. Therefore, we propose that thismust be a continuum or pseudocontinuum opacity effect. Such an apparentenlargement can be reproduced using a simple two-component modelconsisting of a warm (1500-2000 K), extended (up to ~=3 stellar radii),optically thin (τ~=0.5) layer located above the classicalphotosphere. The Planck weighting of the continuum emission from the twolayers will suffice to make the L' UD diameter appear larger than the K'UD diameter. This two-layer scenario could also explain the observedvariation of Mira UD diameters versus infrared wavelength-outside ofstrong absorption bands-as already measured inside the H, K, L, and Natmospheric windows. This interpretation is consistent with the extendedmolecular gas layers (H2O, CO, etc.) inferred around some ofthese objects from previous IOTA K'-band interferometric observationsobtained with the Fiber Linked Unit for Optical Recombination (FLUOR)and from Infrared Space Observatory and high-resolution ground-based FTSinfrared spectra. The two-component model has immediate implications.For example, the Mira photosphere diameters are smaller than previouslyrecognized-this certainly implies higher effective temperatures, and itmay favor fundamental mode pulsation. Also, the UD model fails generallyto represent the brightness distribution and has very limitedapplicability for Mira stars. The presence of a very extended gas layerextending up to ~=3 stellar radii seems now well established on a fairsample of asymptotic giant branch stars ranging from late-type giants tolong-period variables, with some probable impact on stellar modelatmospheres and mass-loss mechanisms.
| L'-Band Interferometric Observations of Evolved Stars
Ten bright Miras, six semiregular variable giants, and two semiregularvariable supergiants have been observed with the Infrared-OpticalTelescope Array interferometer in the L' band (from 3.4 to 4.1 μm).Observations were carried out in 2000 March and November with theFLUOR/TISIS instrument, using optimized single-mode waveguides foroptical recombination and a dedicated chopping system for accuratesubtraction of slow thermal background drifts. Four of the sources (theMira stars R Leo and R Cnc, α Ori, and RS Cnc) were observed inboth runs. We report on visibility measurements and derive L' broadbanduniform disk (UD) diameter best fits for all 18 stars in our sample. Wealso detect strong departures from UD models in some peculiar cases.
| A high-sensitivity OH 5-cm line survey in late-type stars
We have undertaken a comprehensive search for 5-cm excited OH maseremission from evolved stars representative of various stages of latestellar evolution. Observed sources were selected from known 18-cm OHsources. This survey was conducted with the 100-m Effelsberg telescopeto achieve high signal to noise ratio observations and a sensitivitylimit of about 0.05 to 0.1 Jy. A total of 65 stellar sources weresearched for both main line and satellite line emission. We confirm theprevious detection of 5 cm OH in Vy 2-2, do not confirm emission fromNML-Cyg and do not report any other new detection within the abovesensitivity limit. Implications of these results on the pumpingmechanism of the OH radical in circumstellar envelopes are brieflydiscussed. Table \ref{table1} is also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/975
| Angular Size Measurements of Mira Variable Stars at 2.2 Microns. II.
We present angular size measurements of 22 oxygen-rich Mira variablestars. These data are part of a long-term observational program usingthe Infrared Optical Telescope Array to characterize the observablebehavior of these stars. Complementing the infrared angular sizemeasurements, values for variable star phase, spectral type, bolometricflux, and distance were established for stars in the sample; flux anddistance led to values for effective temperature (Teff) andlinear radius, respectively. Additionally, values for the K-[12] colorexcess were established for these stars, which is indicative of dustymass loss. Stars with higher color excess are shown to be systematically120 Rsolar larger than their low color excess counterparts,regardless of period. This analysis appears to present a solution to along-standing question presented by the evidence that some Mira angulardiameters are indicative of first-overtone pulsation, while otherdiameters are more consistent with fundamental pulsation. A simpleexamination of the resultant sizes of these stars in the context ofpulsation mode is consistent with at least some of these objectspulsating in the fundamental mode.
| Numerical simulations of stellar SiO maser variability. Investigation of the effect of shocks
A stellar hydrodynamic pulsation model has been combined with a SiOmaser model in an attempt to calculate the temporal variability of SiOmaser emission in the circumstellar envelope (CE) of a model AGB star.This study investigates whether the variations in local physicalconditions brought about by shocks are the predominant contributingfactor to SiO maser variability because, in this work, the radiativepart of the pump is constant. We find that some aspects of thevariability are not consistent with a pump provided by shock-enhancedcollisions alone. In these simulations, gas parcels of relativelyenhanced SiO abundance are distributed in a model CE by a Monte Carlomethod, at a single epoch of the stellar cycle. From this epoch on,Lagrangian motions of individual parcels are calculated according to thevelocity fields encountered in the model CE during the stellar pulsationcycle. The potentially masing gas parcels therefore experience differentdensities and temperatures, and have varying line-of-sight velocitygradients throughout the stellar cycle, which may or may not be suitableto produce maser emission. At each epoch (separated by 16.6 days),emission lines from the parcels are combined to produce syntheticspectra and VLBI-type images. We report here the results for v=1, J=1-0(43-GHz) and J=2-1 (86-GHz) masers and compare synthetic lineshapes andimages with those observed. Strong SiO maser emission is calculated toform in an unfilled ring within a few stellar radii of the photosphere,indicating a tangential amplification process. The diameter of thesynthetic maser ring is dependent upon stellar phase, as clearlyobserved for TX Cam, and upon maser transition. Proper motions ofbrightly masing parcels are comparable to measurements for some masercomponents in R Aqr and TX Cam, although we are unable to reproduce allof the observed motions. Synthetic lineshapes peak at the stellarvelocity, have typical Mira linewidths and vary in intensity withstellar phase. However, the model fails quantitatively in severalrespects. We attribute these failings to (i) lack of an accurate,time-varying stellar IR field (ii) post-shock kinetic temperatures whichare too high, due to the cooling function included in our model and(iii) the lack of a detailed treatment of the chemistry of the inner CE.We expect the use of oxygen-rich hydrodynamical stellar models which arecurrently under development to alleviate these problems.
| Infrared Light Curves of Mira Variable Stars from COBE DIRBE Data
We have used the COBE DIRBE database to derive near- and mid-infraredlight curves for a well-defined sample of 38 infrared-bright Miravariable stars and compared with optical data from the AAVSO. In generalthe 3.5 and 4.9 μm DIRBE bandpasses provide the light curves with thebest signal-to-noise ratio (S/N), with S/N decreasing with wavelength atlonger wavelengths. At 25 μm good light curves are only available for~10% of our stars, and at wavelengths >=60 μm extracting highquality light curves is not possible. The amplitude of variability istypically less in the near-infrared than in the optical and less in themid-infrared than in the near-infrared, with decreasing amplitude withincreasing wavelength. On average there are 0.20+/-0.01 mag variation at1.25 μm and 0.14+/-0.01 mag variation at 4.9 μm for each magnitudevariation in V. The observed amplitudes are consistent with results ofrecent theoretical models of circumstellar dust shells around Miravariables. For a few stars in our sample we find clear evidence of timelags between the optical and near-infrared maxima of phase ~0.05-0.13,with no lags in the minima. For three stars mid-infrared maximum appearsto occur slightly before that in the near-infrared, but after opticalmaximum. We find three examples of secondary maxima in the risingportions of the DIRBE light curves, all of which have opticalcounterparts in the AAVSO data, supporting the hypothesis that they aredue to shocks rather than newly formed dust layers. We find noconclusive evidence for rapid (hours to days) variations in the infraredbrightnesses of these stars.
| The region of Fe II emission line formation of the symbiotic nova RR Telescopii
In our investigation of the region of formation of the Fe II emissionlines in RR Tel, we have applied the SAC method to the optical Fe IIemission line fluxes measured by Crawford et al. (1999). It is possibleto determine physical parameters of the line emitting region, because ofthe presence both of lines with large and much smaller opticalthicknesses. The values we obtained, given in Tables 3 and 4, are limitsin most cases. The upper limit to the excitation temperature of themetastable levels in the permitted line region of 6600 K, is close tothe value of this temperature in the forbidden line region. Otherexcitation temperature limits corresponding to level population ratioswere also found. The permitted lines are formed in a region with aradius between 1.8 x 1012 cm and 1.6 x 1014 cm,while the forbidden lines are formed in a region with a minimum radiusof 5.3 x 1012 cm. The populations of the lower levels of thepermitted lines indicate a minimum column density of Fe+ of 3.0 x1018 atoms cm-2 and, by assuming cosmicabundances, to a minimum column density of 7.5 x 1022 atomscm-2 for H. If we assume a minimum radius of 1.0 x1014 cm, which is supposed to be that of dust condensation ofthe cool Mira component of this symbiotic binary, a minimum mass lossrate of 3.5 x 10-6 M_o yr-1 is found. Our resultssupport a model according to which both the permitted and the forbiddenlines are formed in the wind of the cool Mira component above theregions where absorption by dust is important. The forbidden and thepermitted lines are not formed in quite the same region, the formerbeing most probably formed further out in the part of an acceleratedwind which is ionized by the hot component. Tables 1 and 2 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/507
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
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| 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.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool stars.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| 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.
| Snapshot VLBI Observations of SiO v=1 and v=2 (J=1 - 0) Maser Sources
We have made snapshot VLBI observations of SiO masers of the J=1-0transition at two vibrational levels v=1 and v=2 using theKashima-Nobeyama Interferometer (KNIFE). Out of 37 sources observed, wedetected both of the lines from 16 sources with a typical detectionlimit of 10 Jy. Most of the SiO masers were significantly spatiallyresolved with a KNIFE baseline (200 km), indicating extended/complicatedstructures of the SiO masers with a typical angular size of a fewmilliarcseconds. Similar fringe-phase profiles along a radial velocityspanning larger than 1.0km s-1 between the two lines indicatea similar spatial structure along the velocity spacings. This tendency,shown from 13 SiO maser sources except Orion KL, supports spatialcoincidence of the two lines found previously for M-type stars VY CMaand W Hya. The spatial coincidence should enable us to measuregroup-delays of the SiO masers for stellar astrometry.
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