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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}
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| Hubble Space Telescope WFPC2 Study of the Trapezium Cluster: The Influence of Circumstellar Disks on the Initial Mass Function
We have performed the first measures of mass accretion rates in the coreof the Orion Nebula Cluster. Four adjacent fields centered on theTrapezium stars have been imaged in the U and B bands using the WideField Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. Weobtained photometry for 91 stars in the U band (F336W) and 71 stars inthe B band (F439W). The WFPC2 archive was also searched to obtaincomplementary V-band (F547M) and I-band (F791W) photometry. In thispaper we focus our attention on a group of 40 stars with known spectraltypes and complete UBVI WFPC2 photometry. We locate each star on the H-Rdiagram, considering both the standard ISM reddening law withRV=3.1 and the ``anomalous'' reddening law withRV=5.5 more appropriate for the Orion Nebula. Then we derivethe stellar masses and ages by comparing with the evolutionary tracksand isochrones calculated by D'Antona & Mazzitelli and Palla &Stahler. Approximately three-quarters of the sources show excessluminosity in the U band, which we attribute to mass accretion. Theknown correlation between the U-band excess and the total accretionluminosity, recalibrated for our photometric system, allows us toestimate the accretion rates, which are all found to be in the range10-8 to 10-12 Msolar yr-1.For stars older than 1 Myr, there is some evidence of a relation betweenmass accretion rates and stellar age. Overall, mass accretion ratesappear lower than those measured by other authors in the Orion flankingfields or in Taurus-Auriga. Mass accretion rates remain low even in thevicinity of the 10-5 Msolar yr-1 birthline of Palla & Stahler, suggesting that in the core of theTrapezium cluster, disk accretion has been recently depressed by anexternal mechanism. We suggest that the UV radiation generated by theTrapezium OB stars, responsible for the disk evaporation, may also causethe drop of the mass accretion rate. In this scenario, low-mass starsmay terminate their pre-main-sequence evolution with masses lower thanthose they would have reached if disk accretion could have proceededundisturbed until the final disk consumption. In OB associations thelow-mass end of the initial mass function (IMF) may therefore beaffected by the rapid evolution of the most massive cluster's stars,causing a surplus of ``accretion-aborted,'' very low mass stars andbrown dwarfs and a deficit of intermediate-mass stars. This trend is inagreement with recent observations of the IMF in the Trapezium cluster.Based on observations taken with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS5-26555.
| Theoretical colours and isochrones for some Hubble Space Telescope colour systems. II
A grid of synthetic surface brightness magnitudes for 14 bandpasses ofthe Hubble Space Telescope Faint Object Camera is presented, as well asa grid of UBV, uvby, and Faint Object Camera surface brightnessmagnitudes derived from the Gunn-Stryker spectrophotometric atlas. Thesynthetic colors are used to examine the transformations between theground-based Johnson UBV and Stromgren uvby systems and the Faint ObjectCamera UBV and uvby. Two new four-color systems, similar to theStromgren system, are proposed for the determination of abundance,temperature, and surface gravity. The synthetic colors are also used tocalculate color-magnitude isochrones from the list of theoretical tracksprovided by VandenBerg and Bell (1990). It is shown that by using theappropriate filters it is possible to minimize the dependence of thiscolor difference on metallicity. The effects of interstellar reddeningon various Faint Object Camera colors are analyzed as well as theobservational requirements for obtaining data of a given signal-to-noisefor each of the 14 bandpasses.
| Stellar spectrophotometric atlas, wavelengths from 3130 to 10800 A
Over the course of the past 10 years, a large body of homogeneousspectrophotometric data on relatively bright stars has been gathered toserve as a library with which to synthesize galaxy spectra in populationstudies. As the data might be generally useful, they are presented inthree tables. One hundred seventy-five (175) stars were selected, mostfrom the Navy photometric catalog. Covered are complete ranges ofspectral type and luminosity class. Normalized spectral energydistributions of these stars in pictorial form are provided along withtables of calculated colors, scan line and continuum indices, and someinformation on the relation of the scan colors to real photometricsystems.
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