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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.
| A coronagraphic search for brown dwarfs and planets around nearby stars
We have started a corongraphic search for brown dwarfs and planetsaround young nearby stars within 20 pc of the Sun, using the adaptiveoptics coronagraph, CIAO, on Subaru. The dynamic range we have achievedis \Delta K = 13 at 2.5'' from the central star. For atypical target with K=7 at 10 pc, the limiting absolute magnitude isM_K=20. We apply two kinematical age criteria to select M and K dwarfsstatistically younger than 350Myr. The first criterion is a smallvelocity deviation from the velocity of LSR. The second is a (U,V,W)velocity vector similar to a particular young moving group. Thecombination of the age and magnitude limits implies that the mass limitfor giant planet detection is about 2M_J. We show a sample image of atarget field at 3 pc of the Sun with faint companion candidates, to befollowed up for the common proper motion test. We give a briefdescription of our procedures for data acquisition, reduction, andanalysis.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Evolution of Cold Circumstellar Dust around Solar-type Stars
We present submillimeter (Caltech Submillimeter Observatory 350 μm)and millimeter (Swedish-ESO Submillimetre Telescope [SEST] 1.2 mm, OwensValley Radio Observatory [OVRO] 3 mm) photometry for 127 solar-typestars from the Formation and Evolution of Planetary Systems SpitzerLegacy program that have masses between ~0.5 and 2.0 Msolarand ages from ~3 Myr to 3 Gyr. Continuum emission was detected towardfour stars with a signal-to-noise ratio>=3: the classical T Tauristars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST, and thedebris-disk system HD 107146 with OVRO. RX J1842.9-3532 and RXJ1852.3-3700 are located in projection near the R CrA molecular cloud,with estimated ages of ~10 Myr (Neuhäuser et al.), whereas PDS 66is a probable member of the ~20 Myr old Lower Centaurus-Crux subgroup ofthe Scorpius-Centaurus OB association (Mamajek et al.). The continuumemission toward these three sources is unresolved at the 24" SESTresolution and likely originates from circumstellar accretion disks,each with estimated dust masses of ~5×10-5Msolar. Analysis of the visibility data toward HD 107146(age~80-200 Myr) indicates that the 3 mm continuum emission is centeredon the star within the astrometric uncertainties and resolved with aGaussian-fit FWHM size of (6.5"+/-1.4")×(4.2"+/-1.3"), or185AU×120 AU. The results from our continuum survey are combinedwith published observations to quantify the evolution of dust mass withtime by comparing the mass distributions for samples with differentstellar ages. The frequency distribution of circumstellar dust massesaround solar-type stars in the Taurus molecular cloud (age~2 Myr) isdistinguished from that around 3-10 Myr and 10-30 Myr old stars at asignificance level of ~1.5 and ~3 σ, respectively. These resultssuggest a decrease in the mass of dust contained in small dust grainsand/or changes in the grain properties by stellar ages of 10-30 Myr,consistent with previous conclusions. Further observations are needed todetermine if the evolution in the amount of cold dust occurs on evenshorter timescales.
| Low-mass companions to solar-type stars
We present preliminary results from a coronagraphic survey of youngnearby Sun-like stars using the Palomar and Keck adaptive opticssystems. We have targeted 251 solar analogs (F5-K5) at 20-160 pc fromthe Sun, spanning the 3-3000 Myr age range. The youngest (<500 Myr)≈100 of these have been imaged with deeper exposures to search forsub-stellar companions. The deep survey is sensitive to brown-dwarfcompanions at separations >0.5arcsec from their host stars, withsensitivity extending to planetary-mass (5-15M_Jup) objects at wider(>3arcsec ) separations. Based on the discovery of a number of newlow-mass (<0.2M_ȯ) stellar companions, we infer that theirfrequency at >20 AU separations (probed via direct imaging) may begreater (≈12%) than that found from radial velocity surveys probing<4 AU separations \citep[≈6%;][]{mazeh_etal03}. We also report theastrometric confirmation of the first sub-stellar companion from thesurvey -- an L4 brown dwarf at a projected distance of 44 AU from the≈500 Myr-old star HD 49197. Based on this detection, we estimate thatfrequency of sub-stellar companions to solar-type stars is at least 1%,and possibly of order a few per cent.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| 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
| UVBY beta photometry of peculiar B and A stars, discovered at Abastumani
Stromgren uvby photometry for 70 peculiar Bp, Ap, or Am type stars ispresented. The model grid of Relyea and Kurucz (1978) is used to obtaineffective temperatures and log g values, confirming the dwarf-nature ofAm stars. Good agreement is found between effective temperatures andequivalent spectral classes derived from the (b-y)0 values, although theA4-A5 stars in this sample may be cooler than normal stars ofluminosity-class III-V in this spectral range.
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| קבוצת-כוכבים: | עגלון |
| התרוממות ימנית: | 06h49m21.33s |
| סירוב: | +43°45'32.8" |
| גודל גלוי: | 7.331 |
| מרחק: | 44.623 פארסק |
| תנועה נכונה: | -36.6 |
| תנועה נכונה: | -50.5 |
| B-T magnitude: | 7.916 |
| V-T magnitude: | 7.38 |
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