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BVRI Photometry of nz Gem, HD 73017, HD 77247, RT Vir and 104 Her
We examined single channel differential BVRI photometry of the coolstars NZ Gem, HD 73017, HD 77247, RT Vir and 104 Her obtained by thefirst author with the Four College Automated Photoelectric Telescopewith of order 100 observations taken over two or more years. Four ofthese stars are Small-Amplitude Red Variables (SARVs). The primaryperiod of NZ Gem (M3 II-IIIs) is about 33.70 days. HD 77247, theshortest period barium star with spectral type K0, has a photometricperiod of about 82 days which is close to its binary period of 80.53days. Its check star HD 73017, a non-variable in B, V and R, is variablein I due most likely to a previously unknown cooler companion. RT Vir(M8 III) is found to be a multiperiodic star whose observations areconsistent with the 155 day primary period of Lebzelter & Hinkle(2002). 104 Her (M3 III) is also multiperiodic with a primary period of21.48 days.
| 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.
| Circumstellar Atomic Hydrogen in Evolved Stars
We present new results of a spectroscopic survey of circumstellar H I inthe direction of evolved stars made with the NançayRadiotelescope. The H I line at 21 cm has been detected in thecircumstellar shells of a variety of evolved stars: asymptotic giantbranch stars, oxygen-rich and carbon-rich stars, semiregular and Miravariables, and planetary nebulae. The emissions are generally spatiallyresolved, i.e., larger than 4', indicating shell sizes on the order of 1pc, which opens the possibility of tracing the history of mass loss overthe past ~104-105 yr. The line profiles aresometimes composite. The individual components generally have aquasi-Gaussian shape; in particular, they seldom show the double-hornprofile that would be expected from the spatially resolved opticallythin emission of a uniformly expanding shell. This probably implies thatthe expansion velocity decreases outward in the external shells (0.1-1pc) of these evolved stars. The H I line profiles do not necessarilymatch those of the CO rotational lines. Furthermore, the centroidvelocities do not always agree with those measured in the CO linesand/or the stellar radial velocities. The H I emissions may also beshifted in position with respect to the central stars. Without excludingthe possibility of asymmetric mass ejection, we suggest that these twoeffects could also be related to a nonisotropic interaction with thelocal interstellar medium. H I was detected in emission toward severalsources (ρ Per, α Her, δ2 Lyr, U CMi) thatotherwise have not been detected in any radio lines. Conversely, it wasnot detected in the two oxygen-rich stars with substantial mass-lossrate, NML Tau and WX Psc, possibly because these sources are young, withhydrogen in molecular form, and/or because the temperature of thecircumstellar H I gas is very low (<5 K).This paper is dedicated to the memory of Marie-Odile Mennessier(1940-2004).
| On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.
| 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.
| A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars
We study the conditions for operation of the 22 GHz ortho-water maser ina dusty medium near late-type stars. The main physical processes, suchas exchange of energy between dust and gas in the radiation field of astar, radiative cooling by water molecules and pumping of water masersare described self-consistently. We show that the presence of dustgrains of various types (or of one type with size distribution) stronglyaffects the maser action. The pumping mechanism based on the presence ofthe dust of different optical properties is able to explain water masersin the silicate carbon star V778 Cyg. However, the masers in the windsfrom asymptotic giant branch stars require an additional source ofheating, for instance due to the dust drift through the gas.
| Variability of the H2O maser associated with the M-supergiant S Persei
We present the results from observing the circumstellar maser emissionof the M-type supergiant S Per in the 6{16}-5{23} water-vapour line at1.35 cm. The observations were carried out in 1981-2002 (JD=2 444 900-2452 480) on the RT-22 radio telescope of the Pushchino Radio AstronomyObservatory, Astrospace Center of the Lebedev Institute of Physics,Russian Academy of Sciences. The H2O spectra obtained represent anunprecedented long, uniform dataset on this star. We discuss theproperties of the optical and maser variations of S Per, together withparticulars of the available VLBI maps. The close relation between maserand optical variations favors a model in which mass-loss is episodic.Changes observed in the total H2O line flux follow the visual lightcurve with a delay of 0.01 to 0.5P, where P≈ 800 d is the mean lightcycle for S Per. The feature at VLSR=-44 km s-1flared in July 1988, which seemed to be the response of the maser to anunusually bright optical maximum. The position of the -44-kms-1 feature on the VLBI maps coincides with the directiontoward the optical stellar disc, which can be explained by amplificationof enhanced stellar continuum by the H2O line.
| 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}
| High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer
We present first results of an experiment to combine data from Keckaperture masking and the Infrared-Optical Telescope Array to image thecircumstellar environments of evolved stars with ~20 mas resolution. Theunique combination of excellent Fourier coverage at short baselines andhigh-quality long-baseline fringe data allows us to determine thelocation and clumpiness of the innermost hot dust in the envelopes andto measure the diameters of the underlying stars themselves. We findevidence for large-scale inhomogeneities in some dust shells and alsosignificant deviations from uniform brightness for the photospheres ofthe most evolved M stars. Deviations from spherically symmetric massloss in the red supergiant NML Cyg could be related to recent evidencefor dynamically important magnetic fields and/or stellar rotation. Wepoint out that dust shell asymmetries, like those observed here, canqualitatively explain the difficulty recent workers have had insimultaneously fitting the broadband spectral energy distributions andhigh-resolution spatial information, without invoking unusual dustproperties or multiple distinct shells (from hypothetical``superwinds''). This paper is the first to combine opticalinterferometry data from multiple facilities for imaging, and we discussthe challenges and potential for the future of this method, givencurrent calibration and software limitations.
| Synthetic Lick Indices and Detection of α-Enhanced Stars
Synthetic Lick indices computed with solar scaled abundances and withα-element enhancement are presented and compared with predictionsfrom both theoretical computations (Tripicco & Bell; Thomas,Maraston, & Bender; Barbuy et al.) and empirical fitting functions(de Freitas Pacheco). We propose selected combinations of indicescapable of singling out α-enhanced stars without requiringprevious knowledge of their main atmospheric parameters. By applyingthis approach to the 460 stars in the Worthey et al. catalog, wedetected a list of 82 candidate α-enhanced stars. The confirmationof α-enhancement was obtained by searching the literature forindividual element abundance determinations from high-resolutionspectroscopy for a subsample of 34 stars. Preliminary discussion of theproperties of the detected α-enhanced stars with respect to their[Fe/H] values and kinematics is presented.
| 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
| 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.
| Sub-au imaging of water vapour clouds around four asymptotic giant branch stars
We present MERLIN maps of the 22-GHz H2O masers around fourlow-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with anangular resolution of ~15 milliarcsec and a velocity resolution of 0.1km s-1. The H2O masers are found in thickexpanding shells with inner radii ~6 to 16 au and outer radii four timeslarger. The expansion velocity increases radially through theH2O maser regions, with logarithmic velocity gradients of0.5-0.9. IK Tau and RT Vir have well-filled H2O maser shellswith a spatial offset between the near and far sides of the shell, whichsuggests that the masers are distributed in oblate spheroids inclined tothe line of sight. U Ori and U Her have elongated poorly filled shellswith indications that the masers at the inner edge have been compressedby shocks; these stars also show OH maser flares. MERLIN resolvesindividual maser clouds, which have diameters of 2-4 au and fillingfactors of only ~0.01 with respect to the whole H2O masershells. The circumstellar envelope velocity structure gives additionalevidence the maser clouds are density-bounded. Masing clouds can beidentified over a similar time-scale to their sound crossing time (~2yr) but not longer. The sizes and observed lifetimes of these clouds arean order of magnitude smaller than for those around red supergiants,similar to the ratio of low-mass:high-mass stellar masses and sizes.This suggests that cloud size is determined by stellar properties, notlocal physical phenomena in the wind.
| 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.
| Gravity Indicators in the Near-Infrared Spectra of Brown Dwarfs
We investigate the sensitivity to temperature and gravity of the strongabsorption features in the J- and K-band spectra of substellar objects.We compare the spectra of giants and young M dwarfs (of low gravity) tofield M and L dwarfs (of high gravity) and to model spectra from theLyon group. We find that low-resolution spectra of M4-M9 stars and youngbrown dwarfs at R~350 and signal-to-noise ratios greater than 70 candetermine the spectral type to a precision of +/-1 subtype using theH2O and CO bands and can measure the surface gravity to+/-0.5 dex using the atomic lines of K I and Na I. This result pointstoward the development of photometric spectral indices to separatelow-mass members from foreground and background objects in youngclusters and associations. We also emphasize the complexity of theinterpretation of the empirical quantities (e.g., spectral types) interms of the physical variables (e.g., temperature, opacities) in thecool atmospheres of young brown dwarfs.Based on observations obtained with the Multiple Mirror TelescopeObservatory, a joint facility of the University of Arizona and theSmithsonian Institution.
| The Three-dimensional Kinematics of Water Masers around the Semiregular Variable RT Virginis
We report observations of water masers around the semiregular variableRT Virginis, which have been made with the Very Long Baseline Array ofthe National Radio Astronomy Observatory at five epochs, each separatedby 3 weeks of time. We detected about 60 maser features at each epoch.Overall, 61 features, detected at least twice, were tracked by theirradial velocities and proper motions. The three-dimensional maserkinematics exhibited a circumstellar envelope that is expanding roughlyspherically with a velocity of ~=8 km s-1. Asymmetries inboth the spatial and velocity distributions of the maser features werefound in the envelope, but they were less significant than those foundin other semiregular variables. Systematic radial velocity drifts ofindividual maser features were found with amplitudes of <=2 kms-1 yr-1. For one maser feature, we found aquadratic position shift with time along a straight line on the sky.This apparent motion indicates an acceleration with an amplitude of 33km s-1 yr-1, implying the passage of a shock wavedriven by the stellar pulsation of RT Vir. The acceleration motion islikely seen only on the sky plane because of a large velocity gradientformed in the accelerating maser region. We estimated the distance to RTVir to be about 220 pc on the basis of both the statistical parallax andmodel-fitting methods for the maser kinematics.
| 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.
| ``Thermal'' SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics
An extensive radiative transfer analysis of circumstellar SiO``thermal'' radio line emission from a large sample of M-type AGB starshas been performed. The sample contains 18 irregulars of type Lb (IRV),7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12Miras. New observational data, which contain spectra of several groundvibrational state SiO rotational lines, are presented. The detectionrate was about 60% (44% for the IRVs, and 68% for the SRVs). SiOfractional abundances have been determined through radiative transfermodelling. The abundance distribution of the IRV/SRV sample has a medianvalue of 6*E-6, and a minimum of 2*E-6 and amaximum of 5*E-5. The high mass-loss rate Miras have a muchlower median abundance, la 10-6. The derived SiO abundancesare in all cases well below the abundance expected from stellaratmosphere equilibrium chemistry, on average by a factor of ten. Inaddition, there is a trend of decreasing SiO abundance with increasingmass-loss rate. This is interpreted in terms of depletion of SiOmolecules by the formation of silicate grains in the circumstellarenvelopes, with an efficiency which is high already at low mass-lossrates and which increases with the mass-loss rate. The high mass-lossrate Miras appear to have a bimodal SiO abundance distribution, a lowabundance group (on average 4*E-7) and a high abundance group(on average 5*E-6). The estimated SiO envelope sizes agreewell with the estimated SiO photodissociation radii using an unshieldedphotodissociation rate of 2.5*E-10 s-1. The SiOand CO radio line profiles differ in shape. In general, the SiO lineprofiles are narrower than the CO line profiles, but they havelow-intensity wings which cover the full velocity range of the CO lineprofile. This is interpreted as partly an effect of selfabsorption inthe SiO lines, and partly (as has been done also by others) as due tothe influence of gas acceleration in the region which produces asignificant fraction of the SiO line emission. Finally, a number ofsources which have peculiar CO line profiles are discussed from thepoint of view of their SiO line properties.Based on observations using the SEST at La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Sweden, the JCMT on Hawaii, andthe IRAM 30 m telescope at Pico Veleta, Spain.
| 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.
| Detection of 1612 MHz OH emission in the semiregular variable stars RT Vir, R Crt and W Hya
We present evidence of 1612 MHz emission in SR variable stars. The twoSRb, RT Vir and R Crt, as well as the SRa W Hya have been monitored withthe upgraded Nançay radio telescope since February 2001. Allthree objects have shown a weak 1612 MHz emission occuring in thevelocity range of the strongest emission observed in the main-lines.Such a detection is the second observational evidence for emission inthe 1612 MHz OH maser satellite line from SRb stars. It also confirmsthe presence of 1612 MHz emission in the SRa W Hya discovered by Etokaet al. (\cite{etoka01}). Such a finding strongly suggests that the shellproperties of those three objects are quite similar to those of the Mirastars with similar IR characteristics.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| 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
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
We have determined mass loss rates and gas expansion velocities for asample of 69 M-type irregular (IRV 22 objects) and semiregular (SRV; 47objects) AGB-variables using a radiative transfer code to model theircircumstellar CO radio line emission. We believe that this sample isrepresentative for the mass losing stars of this type. The (molecularhydrogen) mass loss rate distribution has a median value of 2.0 x10-7 Msun yr-1, and a minimum of 2.0 x10-8 Msun yr-1 and a maximum of 8 x10-7 Msun yr-1. M-type IRVs and SRVswith a mass loss rate in excess of 5 x 10-7 Msunyr-1 must be very rare, and among these mass losing stars thenumber of sources with mass loss rates below a few 10-8Msun yr-1 must be small. We find no significantdifference between the IRVs and the SRVs in terms of their mass losscharacteristics. Among the SRVs the mass loss rate shows no dependenceon the period. Likewise the mass loss rate shows no correlation with thestellar temperature. The gas expansion velocity distribution has amedian of 7.0 km s-1, and a minimum of 2.2 km s-1and a maximum of 14.4 km s-1. No doubt, these objects samplethe low gas expansion velocity end of AGB winds. The fraction of objectswith low gas expansion velocities is very high, about 30% havevelocities lower than 5 km s-1, and there are objects withvelocities lower than 3 km s-1: V584 Aql,T Ari, BI Car, RXLac, and L2 Pup. The mass loss rate and thegas expansion velocity correlate well, a result in line with theoreticalpredictions for an optically thin, dust-driven wind. In general, themodel produces line profiles which acceptably fit the observed ones. Anexceptional case is R Dor, where the high-quality,observed line profiles are essentially flat-topped, while the model onesare sharply double-peaked. The sample contains four sources withdistinctly double-component CO line profiles, i.e., a narrow featurecentered on a broader feature: EP Aqr, RVBoo, X Her, and SV Psc.We have modelled the two components separately for each star and derivemass loss rates and gas expansion velocities. We have compared theresults of this M-star sample with a similar C-star sample analysed inthe same way. The mass loss rate characteristics are very similar forthe two samples. On the contrary, the gas expansion velocitydistributions are clearly different. In particular, the number oflow-velocity sources is much higher in the M-star sample. We found noexample of the sharply double-peaked CO line profile, which is evidenceof a large, detached CO-shell, among the M-stars. About 10% of theC-stars show this phenomenon.
| VLBI Monitoring Observations of Water Masers around the Semi-Regular Variable Star R Crateris
We monitored water-vapor masers around the semi-regular variable star RCrateris with the Japanese VLBI Network (J-Net) at the 22 GHz bandduring four epochs with intervals of one month. The relative propermotions and Doppler-velocity drifts of twelve maser features weremeasured. Most of them existed for longer than 80 days. The 3-Dkinematics of the features indicates a bipolar expanding flow. The majoraxis of the asymmetric flow was estimated to be at P.A. = 136°. Theexistence of a bipolar outflow suggests that a Mira variable star hadalready formed a bipolar outflow. The water masers are in a region ofapparent minimum radii of 1.3 × 1012 m and maximumradii of 2.6 × 1012 m, between which the expansionvelocity ranges from 4.3 to 7.4 km s-1. These values suggestthat the water masers are radially accelerated, but stillgravitationally bound, in the water-maser region. The most positive andnegative velocity-drifting features were found relatively close to thesystemic velocity of the star. We found that the blue-shifted featuresare apparently accelerated and the red-shifted apparently decelerated.The acceleration of only the blue-shifted features seems to beconsistent with that of the expanding flow from the star.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Virgo |
| Ascensió Recta: | 13h02m37.98s |
| Declinació: | +05°11'08.4" |
| Magnitud Aparent: | 8.585 |
| Distancia: | 137.931 parsecs |
| Moviment propi RA: | 38.1 |
| Moviment propi Dec: | -15.7 |
| B-T magnitude: | 10.387 |
| V-T magnitude: | 8.734 |
Catàlegs i designacions:
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INHABITED SKY
