Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog. II. Statistical Parallax Analysis
We present a statistical parallax analysis of low-mass dwarfs from theSloan Digital Sky Survey. We calculate absolute r-band magnitudes(Mr ) as a function of color and spectral type andinvestigate changes in Mr with location in the Milky Way. Wefind that magnetically active M dwarfs are intrinsically brighter inMr than their inactive counterparts at the same color orspectral type. Metallicity, as traced by the proxy ?, also affectsMr , with metal-poor stars having fainter absolute magnitudesthan higher metallicity M dwarfs at the same color or spectral type.Additionally, we measure the velocity ellipsoid and solar reflex motionfor each subsample of M dwarfs. We find good agreement between ourmeasured solar peculiar motion and previous results for similarpopulations, as well as some evidence for differing motions of early andlate M-type populations in U and W velocities that cannot be attributedto asymmetric drift. The reflex solar motion and the velocitydispersions both show that younger populations, as traced by magneticactivity and location near the Galactic plane, have experienced lessdynamical heating. We introduce a new parameter, the independentposition altitude (IPA), to investigate populations as a function ofvertical height from the Galactic plane. M dwarfs at all types exhibitan increase in velocity dispersion when analyzed in comparable IPAsubgroups.
| The Age, Stellar Content, and Star Formation Timescale of the B59 Dense Core
We have investigated the stellar content of Barnard 59 (B59), the mostactive star-forming core in the Pipe Nebula. Using the SpeX spectrographon the NASA Infrared Telescope Facility, we obtained moderateresolution, near-infrared (NIR) spectra for 20 candidate young stellarobjects (YSOs) in B59 and a representative sample of NIR and mid-IRbright sources distributed throughout the Pipe. Measuring luminosity andtemperature sensitive features in these spectra, we identified likelybackground giant stars and measured each star's spectral type,extinction, and NIR continuum excess. To measure B59's age, we place itscandidate YSOs in the Hertzsprung-Russell diagram and compare theirlocation to YSOs in several well-studied star-forming regions, as wellas predictions of pre-main-sequence (PMS) evolutionary models. We findthat B59 is composed of late-type (K4-M6) low-mass (0.9-0.1 Msun) YSOs whose median stellar age is comparable to, if notslightly older than, that of YSOs within the ? Oph, Taurus, andChameleon star-forming regions. Deriving absolute age estimates from PMSmodels computed by D'Antona et al., and accounting only for statisticaluncertainties, we measure B59's median stellar age to be 2.6 ±0.8 Myr. Including potential systematic effects increases the errorbudget for B59's median (DM98) stellar age to 2.6+4.1-2.6 Myr. We also find that the relative age orderingsimplied by PMS evolutionary tracks depend on the range of stellar massessampled, as model isochrones possess significantly different massdependences. The maximum likelihood median stellar age we measure forB59, and the region's observed gas properties, suggests that the B59dense core has been stable against global collapse for roughly sixdynamical timescales and is actively forming stars with a star formationefficiency per dynamical time of ~6%. While the ~150% uncertaintiesassociated with our age measurement propagate directly into thesederived star formation timescales, the maximum likelihood valuesnonetheless agree well with recent star formation simulations thatincorporate various forms of support against collapse, such assubcritical magnetic fields, outflows, and radiative feedback fromprotostellar heating.
| Observation and modelling of main-sequence star chromospheres - XIV. Rotation of dM1 stars
We have measured v sin i for a selected sample of dM1-typestars. We give 114 measurements of v sin i for 88 different stars, andsix upper detection limits. These are the first measurements of v sin ifor most of the stars studied here. This represents the largest sampleof v sin i measurements for M dwarfs at a given spectral type. For thesemeasurements, we used four different spectrographs: HARPS (ESO), SOPHIE(OHP), ÉLODIE (OHP) and UVES (ESO). Two of these spectrographs(HARPS and SOPHIE) are particularly stable in wavelength since they weredesigned for exoplanet searches.We measured v sin i down to an accuracy of 0.3kms-1 for thehighest resolution spectrographs and a detection limit of about1kms-1. We show that this unprecedented accuracy for M dwarfsin our data set is possible because all the targets have the samespectral type. This is an advantage and it facilitates the determinationof the narrowest line profiles for v sin i ~ 0. Although it is possibleto derive the zero-point profiles using several spectral types at atime. These values were combined with other measurements taken from theliterature. The total sample represents detected rotation for 100 stars(10 dM1e and 90 dM1 stars). We confirm our finding of Paper VII that thedistribution of the projected rotation period is bimodal for dM1 starswith a much larger sample, i.e. there are two groups of stars: the fastrotators with P/sin i ~ 4.5d and the slow rotators with P/sin i ~ 14.4d.There is a gap between these two groups. We find that the distributionof stars as a function of P/sin i has two very abrupt cuts, below 10dand above 18d. There are very few stars observed out of this range10-18d. We also observe that the distribution increases slightly from 18to 10d.We find that the M1 subdwarfs (very low metallicity dwarfs) rotate withan average period of P/sin i ~ 7.2d, which is about twice faster as themain group of normal M1 dwarfs. We also find a correlation for P/sin ito decrease with stellar radius among dM1e stars. Such a trend is alsoobserved in dM1 stars.We also derive metallicity and radius for all our target stars using thesame method as in Paper VII. We notably found that 11 of our targetstars are subdwarfs with metallicities below -0.5dex.Based on observations available at Observatoire de Haute Provence andthe European Southern Observatory data bases and on Hipparcos parallaxmeasurements.E-mail: eric_houdebine@yahoo.fr
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| Rotation and Magnetic Activity in a Sample of M-Dwarfs
We have analyzed the rotational broadening and chromospheric activity ina sample of 123 M-dwarfs, using spectra taken at the W.M. KeckObservatory as part of the California Planet Search program. We findthat only seven of these stars are rotating more rapidly than ourdetection threshold of v sin i ? 2.5 km s-1.Rotation appears to be more common in stars later than M3 than in theM0-M2.5 mass range: we estimate that less than 10% of early-M stars aredetectably rotating, whereas roughly a third of those later than M4 showsigns of rotation. These findings lend support to the view thatrotational braking becomes less effective in fully convective stars. Bymeasuring the equivalent widths of the Ca II H and K lines for the starsin our sample, and converting these to approximate L Ca/Lbol measurements, we also provide constraints on theconnection between rotation and magnetic activity. Measurable rotationis a sufficient, but not necessary condition for activity in our sample:all the detectable rotators show strong Ca II emission, but so too do asmall number of non-rotating stars, which we presume may lie at highinclination angles relative to our line of sight. Our data areconsistent with a "saturation-type" rotation-activity relationship, withactivity roughly independent of rotation above a threshold velocity ofless than 6 km s-1. We also find weak evidence for a"gap" in L Ca/L bol between a highly activepopulation of stars, which typically are detected as rotators, andanother much less active group.
| Observation and modelling of main sequence star chromospheres - XIII. The NaI D1 and D2, and HeI D3 lines in dM1 stars
We investigate spectral lines of interest in dM1 stars, namely the NaID1 and D2 and HeI 5876 lines. We study in detail the line shapes of theNaI D1 and D2 lines. We find that these lines are strong and broad innormal dM1 stars and become weaker and narrower when metallicity is low,although our sample is insufficient in order to find out an empiricalcorrelation between these parameters.We find correlations between the CaII resonance line-mean equivalentwidth (EW) and vsini as well as between the NaI mean line core relativeflux and vsini. These correlations include low activity dM1 stars andshow that the NaI mean line core flux is a good chromosphericdiagnostic.We find a good correlation between the NaI D1 line core flux and the NaID2 line core flux. This correlation shows that the line core opticaldepths decrease with an increasing activity level, that is the oppositeof what was found for the CaII lines. The NaI D1 and D2 mean line coreflux also correlates well with the CaII mean EW and with the H?EW. We also compare these correlations to the available modelcomputations. We investigate in detail the shapes of the NaI D1 and D2lines through the full line widths at 85 per cent, 62 per cent and 35per cent of the continuum. The significant differences from one star toanother cannot be explained at this stage. Detailed modelling of thestellar photospheres will be necessary to interpret the observeddifferences.The HeI 5876 line is detected in only one dM1 star in our sample. Weobtain activity correlations between the HeI 5876 line EW and the CaIImean EW, and the H? EW.
| Rotational Velocities for M Dwarfs
We present spectroscopic rotation velocities (v sin i) for 56 M dwarfstars using high-resolution Hobby-Eberly Telescope High ResolutionSpectrograph red spectroscopy. In addition, we have also determinedphotometric effective temperatures, masses, and metallicities ([Fe/H])for some stars observed here and in the literature where we couldacquire accurate parallax measurements and relevant photometry. We haveincreased the number of known v sin i values for mid M stars by around80% and can confirm a weakly increasing rotation velocity withdecreasing effective temperature. Our sample of v sin is peak at lowvelocities (~3 km s-1). We find a change in therotational velocity distribution between early M and late M stars, whichis likely due to the changing field topology between partially and fullyconvective stars. There is also a possible further change in therotational distribution toward the late M dwarfs where dust begins toplay a role in the stellar atmospheres. We also link v sin i to age andshow how it can be used to provide mid-M star age limits. When allliterature velocities for M dwarfs are added to our sample, there are198 with v sin i <= 10 km s-1 and 124 in themid-to-late M star regime (M3.0-M9.5) where measuring precision opticalradial velocities is difficult. In addition, we also search the spectrafor any significant Hα emission or absorption. Forty three percentwere found to exhibit such emission and could represent young, activeobjects with high levels of radial-velocity noise. We acquired twoepochs of spectra for the star GJ1253 spread by almost one month and theHα profile changed from showing no clear signs of emission, toexhibiting a clear emission peak. Four stars in our sample appear to below-mass binaries (GJ1080, GJ3129, Gl802, and LHS3080), with both GJ3129and Gl802 exhibiting double Hα emission features. The tablespresented here will aid any future M star planet search target selectionto extract stars with low v sin i.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| Radio Interferometric Planet Search. I. First Constraints On Planetary Companions For Nearby, Low-Mass Stars From Radio Astrometry
Radio astrometry of nearby, low-mass stars has the potential to be apowerful tool for the discovery and characterization of planetarycompanions. We present a Very Large Array survey of 172 active M dwarfsat distances of less than 10 pc. Twenty-nine stars were detectedwith flux densities greater than 100 μJy. We observed seven ofthese stars with the Very Long Baseline Array at milliarcsecondresolution in three separate epochs. With a detection threshold of500 μJy in images of sensitivity 1σ ~ 100 μJy, wedetected three stars three times (GJ 65B, GJ 896A, GJ 4247), one startwice (GJ 285), and one star once (GJ 803). Two stars were undetected(GJ 412B and GJ 1224). For the four stars detected in multiple epochs,residuals from the optically determined apparent motions have anroot-mean-square deviation of ~0.2 milliarcseconds, consistent withstatistical noise limits. Combined with previous optical astrometry,these residuals provide acceleration upper limits that allow us toexclude planetary companions more massive than 3-6 M Jup at adistance of ~1 AU with a 99% confidence level.
| Observation and modelling of main-sequence stellar chromospheres - VII. Rotation and metallicity of dM1 stars
We have measured v sini and metallicity from high-resolutionspectroscopic observations of a selected sample of dM1-type stars.To measure v sini, we first selected three template stars known fortheir slow rotation or their very low activity levels and thencross-correlated their spectra with those of our target stars. Theexcess broadening of the cross-correlation peaks gives v sini. Formetallicity, we compiled all available measurements from the literatureand correlated them with the stellar radius. Provided the parallax isknown, this new method allows us to derive metallicities for all ourtarget stars.We measured v sini to an accuracy of 2 kms-1. These valueswere combined with other measurements taken from the literature. We havedetected rotation in seven dM1e stars and 11 dM1 stars and upper limitsfor 20 other dM1 stars. Our results show that the distribution of therotation period may be bimodal for dM1 stars, i.e. there are two groupsof stars: the fast rotators with Prot ~ 6 d and the slowrotators with Prot ~ 24 d. There is a gap between these twogroups.We obtained a correlation between metallicity and stellar radius whichallows us to derive metallicities for all stars in our sample and moregenerally for all dM1 stars with [M/H] in the range -1.5 to 0.5 dex,with a reasonable accuracy. We compare this correlation to models andfind a significant disagreement in radii. However, the observed shape ofthe correlation is globally reproduced by the models. We derive themetallicity for 87 M1 dwarfs and subdwarfs.Based on observations collected at Observatoire de Haute Provence andthe European Southern Observatory and on Hipparcos parallaxmeasurements.E-mail: eric_houdebine@yahoo.fr
| Dynamical Mass of GJ 802B: A Brown Dwarf in a Triple System
We report a dynamical measurement of the mass of the brown dwarf GJ 802Busing aperture-masking interferometry and astrometry. In addition, wereport the discovery that GJ 802A is itself a close spectroscopicnoneclipsing binary with a 19 hr period. We find the mass of GJ 802B tobe 0.063+/-0.005 Msolar. GJ 802 has kinematics inconsistentwith a young star and more consistent with the thick-disk population,implying a system age of ~10 Gyr. However, model evolutionary tracks forGJ 802B predict system ages of ~2 Gyr, suggesting that brown dwarfevolutionary models may be underestimating luminosity for old browndwarfs.
| Characterizing the Near-UV Environment of M Dwarfs
We report the results of our Hubble Space Telescope (HST) snapshotsurvey with the ACS HRC PR200L prism, designed to measure the near-UVemission in a sample of nearby M dwarfs. Thirty-three stars wereobserved, spanning the mass range from 0.1 to 0.6 solar masses(Teff~2200-4000 K) where the UV energy distributions varywidely between active and inactive stars. These observations providemuch needed constraints on models of the habitability zone and theatmospheres of possible terrestrial planets orbiting M dwarf hosts andwill be useful in refining the target selection for future spacemissions such as Terrestrial Planet Finder (TPF). We compare our datawith a new generation of M dwarf atmospheric models and discuss theirimplications for the chromospheric energy budget. These NUV data willalso be valuable in conjunction with existing optical, FUV, and X-raydata to explore unanswered questions regarding the dynamo generation andmagnetic heating in low-mass stars.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| The χ Factor: Determining the Strength of Activity in Low-Mass Dwarfs
We describe a new, distance-independent method for calculating themagnetic activity strength in low-mass dwarfs,LHα/Lbol. Using a well-observed sample ofnearby stars and cool standards spanning spectral type M0.5 to L0, wecompute χ, the ratio between the continuum flux near Hα andthe bolometric flux, fλ6560/fbol. Thisratio can be multiplied by the measured equivalent width of the Hαemission line to yield LHα/Lbol. We provideχ values for all objects in our sample, and also fits to χ as afunction of color and average values by spectral type. This method wasused by West et al. to examine trends in magnetic activity strength inlow-mass stars.
| 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.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| L Dwarfs and the Substellar Mass Function
Analysis of initial observations sky surveys has shown that theresulting photometric catalogs, combined with far-red optical data,provide an extremely effective method of finding isolated, verylow-temperature objects in the general field. Follow-up observationshave already identified more than 25 sources with temperatures coolerthan the latest M dwarfs. A comparison with detailed model predictions(Burrows & Sharp 1999) indicates that these L dwarfs have effectivetemperatures between ~2000+/-100 K and 1500+/-100 K, while the availabletrigonometric parallax data place their luminosities at between 10^-3.5and 10. Those properties, together with the detection of lithium inone-third of the objects, are consistent with the majority havingsubstellar masses. The mass function cannot be derived directly, sinceonly near-infrared photometry and spectral types are available for mostsources, but we can incorporate VLM/brown dwarf models in simulations ofthe solar neighborhood population and constrain Psi(M) by comparing thepredicted L dwarf surface densities and temperature distributionsagainst observations from the Deep Near-Infrared Survey (DENIS) and 2Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can berepresented by a power-law mass function, Psi(M)~M^-alpha, with1M/M_solar>0.01brown dwarfs is 0.10 systems pc^-3. In that case, brown dwarfs are twiceas common as main-sequence stars but contribute no more than ~15% of thetotal mass of the disk.
| Rotation and chromospheric activity in field M dwarfs
We have obtained high resolution spectra for a volume-limited sample of118 field M dwarfs. From these observations we derive projectedrotational velocities and fluxes in the H_alpha and H_beta lines. 8stars are double-lined spectroscopic binaries with measured or probableperiods short enough for rotation to be tidally synchronized with theorbit, and another 11 are visual binaries where we cannot yet separatethe lines of the two stars. Of the remaining 99 stars, 24 haverotational velocities above our detection limit of ~ 2 km.s(-1) , andsome are quite fast rotators, including two with v sin i\ =~ 30 km.s(-1)and one with v sin i\ =~ 50 km.s(-1) . Given the small radii of Mdwarfs, these moderate rotational velocities correspond to rather shortmaximum rotational periods, of only 7-8 hours. These three stars aregood candidates for Doppler imaging. We find that rotation is stronglycorrelated with both spectral type and kinematic population: all starswith measurable rotation are later than M3.5, and all but one havekinematic properties typical of the young disk, or intermediate betweenthe young disk and the the old disk. We interpret this correlation asevidence for a spin-down timescale that increases with decreasing mass.At the age of the old disk or halo, all stars earlier than M5-M6(0.1-0.15Msun) have spun-down to below our detection limit,while at the age of the young disk this has only happened for starsearlier than M3.5. The one star with measurable rotation and akinematics intermediate between old disk and population II has spectraltype M6. It is probably an old star whose mass is low enough that it hasretained significant rotation up to present, still consistently withlonger spin-down times for lower mass stars. We observe, on the otherhand, no conspicuous change in the v sin i\ distribution or activitypattern at the mass (M ~ 0.35 Msun) below which stars remainfully convective down to the main sequence. These new data areconsistent with a saturated correlation between rotation and activity,similar to the one observed for younger or more massive stars:L_X/Lbol and L_{H_alpha }/Lbol both correlate withv sin i\ for v sin i\ -5km.s^{-1} and then saturate at respectively10^{-2.5} and 10^{-3.5}$. Based on observations made at the Observatoirede Haute-Provence (CNRS), France Tables 2 and 4 are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| Low-Mass Binaries and the Stellar Luminosity Function
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.2246R
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Геркулес |
| Прямое восхождение: | 17h37m53.35s |
| Склонение: | +18°35'30.1" |
| Видимая звёздная величина: | 9.581 |
| Расстояние: | 8.129 парсек |
| Собственное движение RA: | 926.6 |
| Собственное движение Dec: | 982.1 |
| B-T magnitude: | 11.461 |
| V-T magnitude: | 9.737 |
Каталоги и обозначения:
|
