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Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Hubble Space Telescope and United Kingdom Infrared Telescope Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by bands of obscuring dust lanes and excited by an O7.5 star,HD 164492A. Previous near-IR, mid-IR, and radio continuum observationsof the cluster of stars at the center of the Trifid Nebula indicated thepresence of circumstellar disks associated with hot stars with envelopesthat are photoionized externally by the UV radiation from the hotcentral star, HD 164492A. Using the WFPC2 on the Hubble Space Telescope,we present evidence of a resolved proplyd in Hα and [S II] lineemission from a stellar source emitting cool dust emission. Using theUnited Kingdom Infrared Telescope, an infrared observation of thestellar source with a proplyd indicates a late F to mid-G spectral type.We also note a remarkable complex of filamentary and sheetlikestructures that appear to arise from the edge of a protostellarcondensation. These observations are consistent with a picture in whichthe bright massive star HD 164492A is responsible for thephotoevaporation of protoplanetary disks of other less massive membersof the cluster, as well as the closest protostellar condensation facingthe central cluster. Using the evidence for a proplyd, we argue that themassive and intermediate-mass members of the cluster, HD 164492C (B6star) and HD 164492 (Herbig Be star), have disks associated with them.
| Massive Stars in the SGR 1806-20 Cluster
We report the discovery of additional hot and massive stars in thecluster surrounding the soft gamma repeater SGR 1806-20, based on UnitedKingdom Infrared Telescope and Keck near-infrared spectroscopy. Of thenewly identified stars, three are Wolf-Rayet stars of types WC8, WN6,and WN7, and a fourth star is an OB supergiant. These three stars, alongwith four previously discovered, imply a cluster age of ~3.0-4.5 Myr, onthe basis of the presence of WC stars and the absence of redsupergiants. Assuming coevality, this age suggests that the progenitorof SGR 1806-20 had an initial mass greater than ~50 Msolar.This is consistent with the suggestion that SGRs are postsupernova endstates of massive progenitors and may suggest that only massive starsevolve into magnetars that produce SGRs. It also suggests that verymassive stars can evolve into neutron stars, not just black holes, asrecently predicted by theory. The cluster age also provides constraintson the very high mass object LBV 1806-20.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are 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/418/989
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| Takýmyýldýz: | Yay |
| Sað Açýklýk: | 17h44m19.67s |
| Yükselim: | -16°51'53.3" |
| Görünürdeki Parlaklýk: | 7.5 |
| Uzaklýk: | 55.897 parsek |
| özdevim Sað Açýklýk: | -6.6 |
| özdevim Yükselim: | -46.5 |
| B-T magnitude: | 8.091 |
| V-T magnitude: | 7.549 |
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