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Ýlgili Makaleler
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.
| Changes in the Velocity Centroid of SiO Maser Emission
The v=1, J=1-0 silicon monoxide maser emission from 85 stars wasresurveyed in 2005 and 2006. The emission from these stars hadpreviously been measured in 1979 and/or 1983. For the detected stars thevelocity centroids were determined and compared with the previousvalues. If supergiant stars, known binary stars, and S Virginis wereremoved from the data set, 76 sources remained. Seven of these sourceswere not of sufficient flux density to be claimed as detections. Themean difference in the velocity centroids of the remaining sources was0.065 km s-1, and the standard deviation was 2.00 km s-1. It is important to determine the standard deviation of SiOmaser time variations in order to identify the level of the timevariability of the sources, possible binary stars, unusual emissionpatterns, or perhaps planets. The velocity centroid of S Virginisdiffered by 13.5 km s-1 from its 1983 value. This differenceindicates that S Virginis is probably part of a multiple-star system. Anaive analysis of the present data combined with earlier publishedobservations indicates a revolution period of 14.8 yr.
| 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.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| Observational constraints on s-process nucleosynthesis in massive galactic O-rich AGB stars.
Using high resolution optical spectroscopy (R40,000-50,000) we havederived the Li and Zr abundances of a large sample of galactic O-richAGB stars. Our chemical analysis shows that some stars are Lioverabundant while others are not. The observed Li overabundances areattributed to the activation of the so-called ``hot bottom burning''(HBB) process, confirming that they are actually massive AGB stars.However, these stars do not show the zirconium enhancement (taken as arepresentative for the s-process element enrichment) usually associatedto the third dredge-up. Our study reveals that the s-process elementabundances of the more massive O-rich AGB stars in our Galaxy aredramatically different from those found in the equivalent population ofAGB stars in the Magellanic Clouds. We conclude that probably thedifferent metallicity environment can explain the differences observed.
| 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
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} 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/430/165}
| AGB and post-AGB stars .
Intermediate mass stars (1-8 Mȯ) evolve along theAsymptotic Giant Branch after completion of hydrogen and helium coreburning. At the tip they lose for several ten to hundred thousand yearscopious amounts of mass and exhibit various forms of variability, as forexample large-amplitude variations with periods of one to five years. Inthe oxygen-rich circumstellar envelopes powerful OH, H_2O and SiO masersmay operate. Part of the AGB population is converted to carbon stars.During the latest phases of AGB evolution the mass loss rates approachseveral 10-5 Mȯ yr-1, so that thestars become invisible in the optical and in part in the near infrared.On departure from the AGB a fundamental transition in the mass lossprocess is taking place changing from a spherically symmetric outflow onthe AGB to axi-symmetric or point-symmetric geometries. While the massloss rates decrease to ≈10-8 Mȯyr-1, the velocities are strongly increasing. Stars are nowin their "post-AGB" phase. Mounting evidence is gathered that during thevery latest phase of AGB evolution and during the post-AGB phase,evolution proceeds on very short timescales, which in extreme cases iscomparable to the working life of an astronomer.
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| Letter to the Editor: Brighter Maxima of 30 Selected Mira-Type Variable Stars for the Period 1978-1977
Letter to the Editor
| 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.
| VLBA observations of SiO masers towards Mira variable stars
We present new total intensity and linear polarization VLBA observationsof the ν=2 and ν=1 J=1-0 maser transitions of SiO at 42.8 and 43.1GHz in a number of Mira variable stars over a substantial fraction oftheir pulsation periods. These observations were part of an observingprogram that also includes interferometric measurements at 2.2 and 3.6micron \citep{Mennesson2002}; comparison of the results from differentwavelengths allows studying the envelope independently of the poorlyknown distances to these stars. Nine stars were observed at from one tofour epochs during 2001. The SiO emission is largely confined to ringswhich are smaller than the inner radius of the dust shells reported by\citet{Danchi1994}. Two stars (U Orionis, R Aquarii) have maser ringswith diameters corresponding to the size of the hot molecular layer asmeasured at 3.6 micron; in the other cases, the SiO rings aresubstantially larger. Variations of ring diameter for most, but not allstars, had an rms amplitude in agreement with the models of\citet{Humphreys2002} although the expected relationship between thediameter and pulsation phase was not seen. The ring diameter in UOrionis shows remarkably small variation. A correlation between the2.2/3.6 μm diameter ratio with that of the SiO/3.6 μm diameterratio is likely due to differences in the opacities at 2.2 and 3.6 μmin a molecular layer. A further correlation with the inner size of thedust shell reported by \citet{Danchi1994} suggest some differences inthe temperature structure. Clear evidence is seen in R Aquarii for anequatorial disk similar to that reported by \citet{Hollis2001}; rotationis possibly also detected in S Coronae Boralis.
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| 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.
| VLBI astrometry of circumstellar OH masers: Proper motions and parallaxes of four AGB stars
The main-line OH masers around 4 AGB stars have been observed with theNRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5yrs. Using a phase referencing technique, the position of the mostcompact maser spot of each star was monitored with respect to twoextragalactic reference sources. For U Her and W Hya, we observe themost blue-shifted maser spot, while for R Cas and S CrB we only detect acompact red-shifted maser spot. We managed to determine an accurateproper motion and parallax for U Her, R Cas and S CrB, while additionalmotion of the compact blue-shifted maser of W Hya is shown to possiblybe related to the stellar pulsation. The motion and radio position arecompared with the stellar trajectory and absolute optical positiondetermined by the Hipparcos satellite. For U Her and W Hya, the mostblue-shifted maser is consistent with the amplified stellar image. Thenew distances are compared with several published P-L relations, and inthis respect the VLBI distances seem an improvement upon the Hipparcosdistances.
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| 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
| A Polarization Survey of SiO Maser Variability in Evolved Stars
We have monitored the SiO (v=1, J=2-->1) maser polarization in 17variable stars (Miras, OH-IR stars, and supergiants) to investigate thelong-term persistence of masers. The eight epochs of observations span2.5 yr, thereby sampling multiple cycles for these stars with typicalperiods of ~1 yr. The average polarization was 23% with a typicaldispersion of 7%, although the variability differed substantially fromstar to star. In the Stokes q and u spectra of individual stars, a fewstrong maser features tended to dominate the polarization, with themaser features persisting for less than one stellar cycle for somestars, and for multiple cycles for a few stars. Because individualmasers are not resolved in our beam-averaged total intensity spectra, wecorrelated the polarization spectra between epochs to measure thecharacteristic lifetimes of the features, rather than attempting totrace the evolution of separate line components. We found thatindividual maser feature lifetimes ranged from a few months or less tomore than 2 yr. These data indicate that for the sample of stars as awhole, the masers are not reset at a particular stellar phase.
| 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.
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| 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.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is 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/386/492, and from theauthors on CD-Rom.
| Molecular Masers in Variable Stars
When a star with a mass of one to a few solar masses enters the redgiant stage of its evolution, the radius of its atmosphere reachesseveral astronomical units. Pulsational instability is typical for thisstage. Most stars become Mira-type or semiregular variables with lightcycles of a few hundred days. Red giants lose mass at a rateM=10-7-10-5Msolar yr-1.Extensive gas-dust circumstellar envelopes form. These envelopes containvarious molecular species. Some of these molecules (OH, H2O,SiO, HCN) manifest themselves in maser radio emission. Data on theH2O maser variability and its connection with the stellarbrightness variations are discussed. In the H2O linecircumstellar masers can be divided into `stable' (showing persistentemission - R Aql, U Her, S CrB, X Hya) and `transient' (appearing in theH2O line once per 10-15 stellar light cycles - R Leo, R Cas,U Aur). Physical mechanisms of the maser variability are discussed. Themost probable process explaining the observed visual-H2Ocorrelation is the influence of shock waves on the masing region.Usually it is assumed that shocks in Mira atmospheres are driven bystellar pulsations. Here an alternative explanation is proposed. If astar during its main sequence life possessed a planetary system, similarto the solar system, the planets will be embedded in a rather dense andhot medium. Effects of a planet revolving around a red giant at a shortdistance (inside its circumstellar envelope) are discussed. A shockproduced by the supersonic motion of a planet can account for thecorrelated variability of the Hα line emission and H2Omaser. If the planetary orbit is highly eccentric, then the connectedHα-H2O flare episodes may be explained by theperiastron passage of the planet. New tasks for the upgraded ATCA arediscussed.
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| Takýmyýldýz: | Kuzey Taci |
| Sað Açýklýk: | 15h21m23.95s |
| Yükselim: | +31°22'02.5" |
| Görünürdeki Parlaklýk: | 9.22 |
| Uzaklýk: | 526.316 parsek |
| özdevim Sað Açýklýk: | -9.7 |
| özdevim Yükselim: | -15.9 |
| B-T magnitude: | 10.281 |
| V-T magnitude: | 9.308 |
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