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VLTI observations of AGB stars
We report on recent observations of AGB stars obtained with the VLTInterferometer (VLTI). We illustrate in general the potential ofinterferometric measurements to study stellar atmospheres andcircumstellar envelopes, and demonstrate in particular the advantages ofa coordinated multi-wavelength approach including near/mid-infrared aswell as radio interferometry. We report on studies of the atmosphericstructure of non-Mira and Mira variable giants. We have used VLTIobservations of the near- and mid-infrared stellar sizes and concurrentVLBA observations of the SiO maser emission. So far, this projectincludes studies of the Mira stars S Ori and RR Aql as well as of thesupergiant AH Sco. The results from our first epochs of S Orimeasurements have recently been published and the main results arereviewed here. The S Ori maser ring is found to lie at a mean distanceof approximately 2 stellar radii, a result that is virtually free of theusual uncertainty inherent in comparing observations of variable starswidely separated in time and stellar phase. We discuss the status of ourmore recent S Ori, RR Aql, and AH Sco observations, and present anoutlook on the continuation of our project.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z = 0.02
We present new synthetic broad-band photometric colors for late-typegiants based on synthetic spectra calculated with the PHOENIX modelatmosphere code. The grid covers effective temperatures T_eff=3000dots5000 K, gravities log g=-0.5dots{+3.5}, and metallicities[M/H]=+0.5dots{-4.0}. We show that individual broad-band photometriccolors are strongly affected by model parameters such as molecularopacities, gravity, microturbulent velocity, and stellar mass. Ourexploratory 3D modeling of a prototypical late-type giant shows thatconvection has a noticeable effect on the photometric colors too, as italters significantly both the vertical and horizontal thermal structuresin the outer atmosphere. The differences between colors calculated withfull 3D hydrodynamical and 1D model atmospheres are significant (e.g.,Δ(V-K)0.2 mag), translating into offsets in effectivetemperature of up to 70 K. For a sample of 74 late-type giants in theSolar neighborhood, with interferometric effective temperatures andbroad-band photometry available in the literature, we compare observedcolors with a new PHOENIX grid of synthetic photometric colors, as wellas with photometric colors calculated with the MARCS and ATLAS modelatmosphere codes. We find good agreement of the new synthetic colorswith observations and published T_eff-color and color-color relations,especially in the T_eff-(V-K), T_eff-(J-K) and (J-K)-(V-K) planes.Deviations from the observed trends in the T_eff-color planes aregenerally within ±100 K for T_eff=3500 to 4800 K. Syntheticcolors calculated with different stellar atmosphere models agree to±100 K, within a large range of effective temperatures andgravities. The comparison of the observed and synthetic spectra oflate-type giants shows that discrepancies result from the differencesboth in the strengths of various spectral lines/bands (especially thoseof molecular bands, such as TiO, H2O, CO) and the continuum level.Finally, we derive several new T_eff-log g-color relations for late-typegiants at solar-metallicity (valid for T_eff=3500 to 4800 K), based bothon the observed effective temperatures and colors of the nearby giants,and synthetic colors produced with PHOENIX, MARCS and ATLAS modelatmospheres.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| 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}
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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.
| Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch
We present the results of our radial velocity (RV) measurements of G andK giants, concentrating on the presence of multiple systems in oursample. Eighty-three giants have been observed for 2.5 years with thefiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in LaSilla, Chile. Seventy-seven stars (93%) of the targets have beenanalyzed for RV variability using simultaneous Th-Ar calibration and across-correlation technique. We estimate the long-term precision of ourmeasurement as better than 25 m s-1. Projected rotationalvelocities have been measured for most stars of the sample. Within ourtime-base only 21 stars (or 27%) show variability below 2\sigma, whilethe others show RV variability with amplitudes up to several kms-1. The large amplitude (several km s-1) andshape (high eccentricity) of the RV variations for 11 of the programstars are consistent with stellar companions, and possibly brown dwarfcompanions for two of the program stars. In those systems for which afull orbit could be derived, the companions have minimum masses from0.6 M\sun down to 0.1 M\sun. To thesemultiple systems we add the two candidates of giant planets alreadydiscovered in the sample. This analysis shows that multiple systemscontribute substantially to the long-term RV variability of giant stars,with about 20% of the sample being composed of multiple systems despitescreening our sample for known binary stars. After removing binaries,the range of RV variability in the whole sample clearly decreases, butthe remaining stars retain a statistical trend of RV variability withluminosity: luminous cool giants with B-V≥1.2 show RV variationswith \sigma_{/lineRV} > 60 m s-1, while giants with B-V< 1.2 including those in the clump region exhibit less variability orthey are constant within our accuracy. The same trend is observed withrespect to absolute visual magnitudes: brighter stars show a largerdegree of variability and, when plotted in the RV variability vs.magnitude diagram a trend of increasing RV scatter with luminosity isseen. The amplitude of RV variability does not increase dramatically, aspredicted, for instance, by simple scaling laws. At least two luminousand cooler stars of the sample show a correlation between RV andchromospheric activity and bisector asymmetry, indicating that in thesetwo objects RV variability is likely induced by the presence of(chromospheric) surface structures.Based on observations collected at the 1.52 m-ESO telescope at the LaSilla Observatory from Oct 1999 to Feb. 2002 under ESO programs and theESO-Observatório Nacional, Brazil, agreement and in part onobservations collected on the Alfred Jensch 2 m telescope of theThüringer Landessternwarte Tautenburg.
| High-Precision Stellar Radial Velocities in the Galactic Center
We present radial velocities for 85 cool stars projected onto thecentral parsec of the Galaxy. The majority of these velocities haverelative errors of ~1 km s-1, or a factor of ~30-100 smallerthan those previously obtained with proper-motion or other radialvelocity measurements for a similar stellar sample. The error in atypical individual stellar velocity, including all sources ofuncertainty, is 1.7 km s-1. Two similar data sets wereobtained 1 month apart, and the total error in the relative velocitiesis 0.80 km s-1 in the case where an object is common to bothdata sets. The data are used to characterize the velocity distributionof the old population in the Galactic center. We find that the starshave a Gaussian velocity distribution with a mean heliocentric velocityof -10.1+/-11.0 km s-1 (blueshifted) and a standard deviationof 100.9+/-7.7 km s-1 the mean velocity of the sample isconsistent with no bulk line-of-sight motion with respect to the localstandard of rest. At the 1 σ level, the data are consistent with asymmetric velocity distribution about any arbitrary axis in the plane ofthe sky. We find evidence for a flattening in the distribution oflate-type stars within a radius of ~0.4 pc and infer a volume densitydistribution of r-1/4 in this region. Finally, we establish afirst epoch of radial velocity measurements that can be compared withsubsequent epochs to measure small accelerations (1 km s-1yr-1), corresponding to the magnitude expected over a timespan of several years for stars nearest to Sgr A*.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| K3-50A: An Ultracompact H II Region Ionized by a Massive Stellar Cluster
We have made imaging and spectroscopic observations of the ultracompactH II region K3-50A with a spatial resolution of 0.4" using a newmid-infrared instrument, the Cooled Mid-Infrared Camera andSpectrometer, on the 8.2 m Subaru Telescope. The spectra show thermaldust emission, 9.7 μm silicate absorption, and fine-structure lineemissions of [Ne II] at 12.8 μm, [Ar III] at 8.99 μm, and [S IV]at 10.51 μm. From the maps of the continuum, line emissions, and/orderived dust parameters, we identify eight mid-infrared sources inK3-50A. Especially the central [Ne II] emission has been resolved intotwo peaks clearly. The ionization condition is investigated with theline flux ratios I([Ar III])/I([Ne II]) and I([S IV])/I([Ne II]). It issuggested that the spectral types of the ionizing stars in K3-50Acorrespond to B0-O8 V, which is much later than O5.5 V, the typeestimated from radio continuum observations under a single-starassumption. The three line fluxes suggest a number of Ne+ions greater than what is ionized by a single star of any spectral type,but the numbers of Ar2+ and S3+ are similar tothat formed by a single O8-O9 V star and that by a single O7-O9 V star,respectively. From these features as well as the dust temperature andthe correspondence of each identified source with the near-infraredsource, we propose that K3-50A is excited by at least two (possiblythree) ionizing stars. This is the first convincing example that amassive stellar cluster is ionizing an ultracompact H II region.Based on data collected with the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Angular Diameters of Stars from the Mark III Optical Interferometer
Observations of 85 stars were obtained at wavelengths between 451 and800 nm with the Mark III Stellar Interferometer on Mount Wilson, nearPasadena, California. Angular diameters were determined by fitting auniform-disk model to the visibility amplitude versus projected baselinelength. Half the angular diameters determined at 800 nm have formalerrors smaller than 1%. Limb-darkened angular diameters, effectivetemperatures, and surface brightnesses were determined for these stars,and relationships between these parameters are presented. Scatter inthese relationships is larger than would be expected from themeasurement uncertainties. We argue that this scatter is not due to anunderestimate of the angular diameter errors; whether it is due tophotometric errors or is intrinsic to the relationship is unresolved.The agreement with other observations of the same stars at the samewavelengths is good; the width of the difference distribution iscomparable to that estimated from the error bars, but the wings of thedistribution are larger than Gaussian. Comparison with infraredmeasurements is more problematic; in disagreement with models, coolerstars appear systematically smaller in the near-infrared than expected,warmer stars larger.
| The Evolutionary State of Stars in the NGC 1333S Star Formation Region
We present 2 μm near-IR spectroscopic observations of a sample of 33objects in the NGC 1333S active star-forming cluster centered on thepre-main-sequence star SSV 13. We have previously studied this regionphotometrically in the optical and near-IR, and with the addition ofthese near-IR spectra, we further probe the pre-main-sequence clustermembership and evolutionary state. From the atomic and molecularabsorption features observed, together with the earlier photometry, wederive spectral types, effective temperatures, masses, and ages of thestars and conclude that almost all (90%) the stars observed in thissample are pre-main-sequence objects. This result significantly refinesthe evolutionary information obtained from photometric evidence alone.Comparison with theoretical evolutionary tracks and isochrones suggeststhat our survey has sampled sources with masses in the range 0.2-2Msolar and stellar ages between 7×104 and1×108 yr with a preponderance of sources around3×106 yr. This implies the presence of low- tointermediate-mass T Tauri stars of evolutionary designation Class I toClass III. We conclude that star formation seems to have occurred inlikely several bursts rather than occurring coevally. Star formation insuch a region as NGC 1333S is likely significantly affected by the largenumber of active molecular outflows in the region, which could provide amechanism for cloud turbulence and the onset of subsequent starformation.
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K/'-band observations of five Mira stars with the IOTAinterferometer. The interferograms were obtained with the FLUOR fiberoptics beam combiner, which provides high-accuracy visibilitymeasurements in spite of time-variable atmospheric conditions. For theM-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB wederived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas,and 7.9 mas (/+/-0.3 mas), respectively. Simultaneous photometricobservations yielded the bolometric fluxes. The derived angularRosseland radii and the bolometric fluxes allowed the determination ofeffective temperatures. For instance, the effective temperature of R Aqlwas determined to be 2970/+/-110 K. A linear Rosseland radius for R Aqlof 250+100-60Rsolar was derived fromthe angular Rosseland radius of 5.5+/-0.2mas and the HIPPARCOS parallaxof 4.73+/-1.19mas. The observations were compared with theoretical Mirastar models of Bessel et al. [A&A 307 (1996) 481] and Hofmann et al.[A&A 339 (1998) 846]. The effective temperatures of the M-type Mirasand the linear radius of R Aql indicate fundamental mode pulsation.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The Effect of TiO Absorption on Optical and Infrared Angular Diameters of Cool Stars
We review the systematic variation between optical- andinfrared-wavelength angular diameters reported for stars in theapproximate range of spectral types K0-M6. We show that there is acorrelation between the ratio of angular diameters and the depth of TiOabsorption, in the sense that the optical diameters are larger. We arguethat this validates a recent proposal by Houdashelt et al. that TiOabsorption affects certain, but not all, optical-wavelength angulardiameters significantly. Those authors pointed out that the infraredangular diameters appear to yield better effective temperatures than dothe optical diameters, even though the latter are of higher precision.The observed angular diameter differences may arise either from limbdarkening, atmospheric extension, or a combination of these twoprocesses. Model atmosphere calculations of limb-darkening coefficientsare needed to see whether the diameter discrepancy may be resolved.These models need to contain the correct opacity sources and a realisticestimate of the atmospheric geometry and dynamics. A comparison withobservations such as those described in this paper will be useful fortesting the validity of atmosphere models.
| A catalogue of calibrator stars for long baseline stellar interferometry
Long baseline stellar interferometry shares with other techniques theneed for calibrator stars in order to correct for instrumental andatmospheric effects. We present a catalogue of 374 stars carefullyselected to be used for that purpose in the near infrared. Owing toseveral convergent criteria with the work of Cohen et al.(\cite{cohen99}), this catalogue is in essence a subset of theirself-consistent all-sky network of spectro-photometric calibrator stars.For every star, we provide the angular limb-darkened diameter, uniformdisc angular diameters in the J, H and K bands, the Johnson photometryand other useful parameters. Most stars are type III giants withspectral types K or M0, magnitudes V=3-7 and K=0-3. Their angularlimb-darkened diameters range from 1 to 3 mas with a median uncertaintyas low as 1.2%. The median distance from a given point on the sky to theclosest reference is 5.2degr , whereas this distance never exceeds16.4degr for any celestial location. The catalogue is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/183
| 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.
| Absolute spectrophotometry of late-type stars.
Not Available
| Direct multi-wavelength limb-darkening measurements of three late-type giants with the Navy Prototype Optical Interferometer
We present direct measurements of the limb-darkened intensity profilesof the late-type giant stars HR 5299, HR 7635, and HR 8621 obtained withthe Navy Prototype Optical Interferometer (NPOI) at the LowellObservatory. A triangle of baselines with lengths of 18.9 m, 22.2 m, and37.5 m was used. We utilized squared visibility amplitudes beyond thefirst minimum, as well as triple amplitudes and phases in up to 10spectral channels covering a wavelength range of { ~ }650 nm to { ~ }850nm. We find that our data can best be described by featureless symmetriclimb-darkened disk models while uniform disk and fully darkened diskmodels can be rejected. We derive high-precision angular limb-darkeneddiameters for the three stars of 7.44 mas +/- 0.11 mas, 6.18 mas+/-0.07mas, and 6.94 mas+/-0.12 mas, respectively. Using the HIPPARCOSparallaxes, we determine linear limb-darkened radii of 114Rsun+/- 13 Rsun, 56 Rsun+/- 4Rsun, and 98 Rsun+/- 9 Rsun,respectively. We compare our data to a grid of Kurucz stellar modelatmospheres, with them derive the effective temperatures and surfacegravities without additional information, and find agreement withindependent estimates derived from empirical calibrations and bolometricfluxes. This confirms the consistency of model predictions and directobservations of the limb-darkening effect.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| The effective temperature scale of giant stars (F0-K5). III. Stellar radii and the calibration of convection
We present an analysis of radii of giant stars with 6200 K>= T_eff>= 3800 K based on angular diameters obtained by means of the IRFMand distances computed from Hipparcos parallaxes. In order to asses thereliability of IRFM diameters we have considered a selected sample ofstars whose diameters have been directly measured by interferometrictechniques with internal errors below 5%. The intercomparison shows afairly good consistency and no systematic differences againsttemperature are apparent in the analysis. By averaging the individualvalues obtained for a sample of approximately 300 stars, we present meanvalues of linear radii of giants of solar metallicity; the results aretentatively extended to metal-poor giants. We have also devised a methodto derive distance moduli of globular clusters complementary to thestandard Main Sequence (MS) and Horizontal Branch (HB) fitting. Thismethod is based on the fit of observed linear radii and effectivetemperatures of Red Giant Branch stars of a given globular cluster tothe yields of theoretical isochrones. A careful assessment of theuncertainty on the derived distances is provided. As expected, thedistances are critically dependent on the value of the mixing lengthparameter adopted in the stellar models. We have applied the method toprovide a homogeneous distance scale for a representative sample ofGalactic globular clusters. The comparison of these distances with thedistance scale obtained by means of the MS- or HB-fitting permits aconsistent calibration and/or test of the superadiabatic gradient instellar envelopes, independent of the use of colour-T_efftransformations.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| Spectral Irradiance Calibration in the Infrared. X. A Self-Consistent Radiometric All-Sky Network of Absolutely Calibrated Stellar Spectra
We start from our six absolutely calibrated continuous stellar spectrafrom 1.2 to 35 μm for K0, K1.5, K3, K5, and M0 giants. These wereconstructed as far as possible from actual observed spectral fragmentstaken from the ground, the Kuiper Airborne Observatory, and the IRAS LowResolution Spectrometer, and all have a common calibration pedigree.From these we spawn 422 calibrated ``spectral templates'' for stars withspectral types in the ranges G9.5-K3.5 III and K4.5-M0.5 III. Wenormalize each template by photometry for the individual stars usingpublished and/or newly secured near- and mid-infrared photometryobtained through fully characterized, absolutely calibrated,combinations of filter passband, detector radiance response, and meanterrestrial atmospheric transmission. These templates continue ourongoing effort to provide an all-sky network of absolutely calibrated,spectrally continuous, stellar standards for general infrared usage, allwith a common, traceable calibration heritage. The wavelength coverageis ideal for calibration of many existing and proposed ground-based,airborne, and satellite sensors, particularly low- tomoderate-resolution spectrometers. We analyze the statistics of probableuncertainties, in the normalization of these templates to actualphotometry, that quantify the confidence with which we can assert thatthese templates truly represent the individual stars. Each calibratedtemplate provides an angular diameter for that star. These radiometricangular diameters compare very favorably with those directly observedacross the range from 1.6 to 21 mas.
| The effective temperature scale of giant stars (F0-K5). I. The effective temperature determination by means of the IRFM
We have applied the InfraRed Flux Method (IRFM) to a sample ofapproximately 500 giant stars in order to derive their effectivetemperatures with an internal mean accuracy of about 1.5% and a maximumuncertainty in the zero point of the order of 0.9%. For the applicationof the IRFM, we have used a homogeneous grid of theoretical modelatmosphere flux distributions developed by \cite[Kurucz (1993)]{K93}.The atmospheric parameters of the stars roughly cover the ranges: 3500 K<= T_eff <= 8000 K; -3.0 <= [Fe/H] <= +0.5; 0.5 <= log(g) <= 3.5. The monochromatic infrared fluxes at the continuum arebased on recent photometry with errors that satisfy the accuracyrequirements of the work. We have derived the bolometric correction ofgiant stars by using a new calibration which takes the effect ofmetallicity into account. Direct spectroscopic determinations ofmetallicity have been adopted where available, although estimates basedon photometric calibrations have been considered for some stars lackingspectroscopic ones. The adopted infrared absolute flux calibration,based on direct optical measurements of stellar angular diameters, putsthe effective temperatures determined in this work in the same scale asthose obtained by direct methods. We have derived up to fourtemperatures, TJ, TH, TK and T_{L'},for each star using the monochromatic fluxes at different infraredwavelengths in the photometric bands J, H, K and L'. They show goodconsistency over 4000 K, and there is no appreciable trend withwavelength, metallicity and/or temperature. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of error and their effect on final temperatures. Wealso provide a comparison of the results with previous work.
| Catalogs of temperatures and [Fe/H] averages for evolved G and K stars
A catalog of mean values of [Fe/H] for evolved G and K stars isdescribed. The zero point for the catalog entries has been establishedby using differential analyses. Literature sources for those entries areincluded in the catalog. The mean values are given with rms errors andnumbers of degrees of freedom, and a simple example of the use of thesestatistical data is given. For a number of the stars with entries in thecatalog, temperatures have been determined. A separate catalogcontaining those data is briefly described. Catalog only available atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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Observation and Astrometry data
| Constellation: | Sagitta |
| Right ascension: | 19h58m45.40s |
| Declination: | +19°29'32.0" |
| Apparent magnitude: | 3.47 |
| Distance: | 84.034 parsecs |
| Proper motion RA: | 65.5 |
| Proper motion Dec: | 23.3 |
| B-T magnitude: | 5.574 |
| V-T magnitude: | 3.668 |
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INHABITED SKY
