תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
On the Correlation between the Magnetic Activity Levels, Metallicities, and Radii of Low-Mass Stars
The recent increase in the number of radius measurements of very lowmass stars from eclipsing binaries and interferometry of single starshas raised more questions about what could be causing the discrepancybetween the observed radii and those predicted by models. The two mainexplanations being proposed are a correlation between the radii of thestars and either their activity levels or their metallicities. Thispaper presents a study of such correlations using all the data publishedto date. The study also investigates correlations between the radiusdeviations from the models and the masses of the stars. There is noclear correlation between activity level and radius for the single starsin the sample. These single stars are slow rotators, with typicalvelocities vrotsini<3.0 km s-1. A clearcorrelation however exists in the case of the faster rotating members ofbinaries. This result is based on the X-ray emission levels of thestars. There also appears to be an increase in the deviation of theradii of single stars from the models as a function of metallicity, aspreviously indicated by Berger et al. The stars in binaries do not seemto follow the same trend. Finally, the Baraffe et al. models reproducewell the radius observations below 0.30-0.35 Msolar, wherethe stars become fully convective, although this result is preliminarysince almost all the sample stars in that mass range are slow rotatorsand metallicities have not been measured for most of them. The resultsindicate that stellar activity and metallicity play an important role indetermining the radius of very low mass stars, at least above 0.35Msolar.
| A Survey for Young Spectroscopic Binary K7-M4 Stars in Ophiuchus
This paper describes a high-resolution, infrared spectroscopic survey ofyoung, low-mass stars that is designed to identify and characterizepre-main-sequence spectroscopic binaries. This is the first largeinfrared radial velocity survey of very young stars to date. Thefrequency and mass ratio distribution of the closest, low-mass binariesbear directly on models of stellar, brown dwarf, and planetary masscompanion formation. Furthermore, spectroscopic binaries can providemass ratios and ultimately masses, independent of assumptions, needed tocalibrate models of young star evolution. I present the initial resultsfrom observations of a uniform sample of 33 T Tauri M stars in theOphiuchus molecular cloud. The average mass of this sample is less thanthat of other young star radial velocity surveys of similar scope by afactor of ~2. Almost every star was observed at 3-4 epochs over 3 yrwith the 10 m Keck II telescope and the facility infrared spectrometerNIRSPEC. An internal precision of 0.43 km s-1 was obtainedwith standard cross-correlation calibration techniques. Four of thetargets are newly discovered spectroscopic binaries, one of which islocated in a subarcsecond, hierarchical quadruple system. Three othersubarcsecond visual binaries were also serendipitously identified duringtarget acquisition. The spectroscopic multiplicity of the sample iscomparable to that of earlier type, pre-main-sequence objects.Therefore, there is no dearth of young, low-mass spectroscopic binarystars, at least in the Ophiuchus region.
| Resolving the faces of stars.
Not Available
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| The Variability and Rotation of Pre-Main-Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?
A variability study of the young cluster IC 348 at Van Vleck Observatoryhas been extended to a total of 7 yr. Twelve new periodic stars havebeen found in the last 2 yr, bringing the total discovered by thisprogram to 40. In addition, we confirm 16 of the periods reported byothers and resolve some discrepancies. The total number of knownrotation periods in the cluster from all studies has now reached 70.This is sufficient to demonstrate that the parent population of K5-M2stars is rotationally indistinguishable from that in the Orion NebulaCluster, even though their radii are 20% smaller and they would beexpected to spin about twice as fast if angular momentum were conserved.The median radius and therefore the inferred age of the IC 348 starsactually closely matches that of NGC 2264, but the stars spinsignificantly more slowly. This suggests that another factor besidesmass and age plays a role in establishing the rotation properties withina cluster, and we suggest that it is environment. If disk locking wereto persist for longer times in less harsh environments because the disksthemselves persist for longer times, it could explain the generallyslower rotation rates observed for stars in this cluster, whose earliesttype star is of class B5. We have also obtained radial velocities, thefirst for pre-main-sequence stars in IC 348, and vsini measurements for30 cluster stars to assist in the study of rotation and as anindependent check on stellar radii. Several unusual variable stars arediscussed; in some or all cases their behavior may be linked tooccultations by circumstellar material. A strong correlation existsbetween the range of photometric variability and the slope of thespectral energy distribution in the infrared. Nineteen of the 21 starswith I ranges exceeding 0.4 mag show infrared evidence for circumstellardisks.
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| Exploring the Frequency of Close-in Jovian Planets around M Dwarfs
We discuss our high-precision radial velocity results of a sample of 90M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J.Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLTand the Keck I telescopes, within the context of the overall frequencyof Jupiter-mass planetary companions to main-sequence stars. None of thestars in our sample show variability indicative of a giant planet in ashort-period orbit, with a<=1 AU. We estimate an upper limit of thefrequency f of close-in Jovian planets around M dwarfs as <1.27% (atthe 1 σ confidence level). Furthermore, we determine that theefficiency of our survey in noticing planets in circular orbits is 98%for companions with msini>3.8MJ and a<=0.7 AU. Foreccentric orbits (e=0.6) the survey completeness is 95% for all planetswith msini>3.5MJ and a<=0.7 AU. Our results pointtoward a generally lower frequency of close-in Jovian planets for Mdwarfs as compared to FGK-type stars. This is an important piece ofinformation for our understanding of the process of planet formation asa function of stellar mass.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen. Also based on observationscollected at the European Southern Observatory, Chile (ESO programs65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044,71.C-0498, 072.C-0495, 173.C-0606). Additional data were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration(NASA), and with the McDonald Observatory Harlan J. Smith 2.7 mtelescope.
| MK Classification and Dynamical Masses for Late-Type Visual Binaries
On the basis of slit spectra obtained with the SCORPIO spectral cameraattached to the 2.6 m telescope of the V. Ambartsumian ByurakanAstrophysical Observatory (Armenia), MK classifications for 30 visualbinaries comprising mostly late K and M type stars are presented.Comparison with other determinations shows that this configurationprovides a reliable MK classification. Dynamical masses for 25 systemsare computed. Using standard mass-luminosity calibrations, individualmass sums for 11 pairs consisting of virtually single, nonvariabledwarfs are calculated, showing a good agreement with correspondingdynamical masses. The dynamical parallax of HIP 112354 is closer to thetrigonometric parallax given in the Yale General Catalogue ofTrigonometric Stellar Parallaxes (van Altena et al.) than to theHipparcos parallax.
| First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars
We have measured the angular diameters of six M dwarfs with the CHARAArray, a long-baseline optical interferometer located at Mount WilsonObservatory. Spectral types range from M1.0 V to M3.0 V and linear radiifrom 0.38 to 0.69 Rsolar. These results are consistent withthe seven other M dwarf radii measurements from optical interferometryand with those for 14 stars in eclipsing binary systems. We compare alldirectly measured M dwarf radii to model predictions and find thatcurrent models underestimate the true stellar radii by up to 15%-20%.The differences are small among the metal-poor stars but becomesignificantly larger with increasing metallicity. This suggests thattheoretical models for low-mass stars may be missing some opacity sourcethat alters the computed stellar radii.
| 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.
| Speckle interferometry of nearby multiple stars. III.
Not Available
| 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.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| The Physical Natures of Class I and Flat-Spectrum Protostellar Photospheres: A Near-Infrared Spectroscopic Study
We present high-resolution (R~=18,000), high signal-to-noise ratio, 2μm spectra of 52 IR-selected Class I and flat-spectrum young stellarobjects in the Taurus-Auriga, ρ Ophiuchi, Serpens, Perseus, andCorona Australis dark clouds. We detect key absorption lines in 41objects and fit synthetic spectra generated from pre-main-sequencemodels to deduce the effective temperatures, surface gravities, near-IRveilings, rotation velocities, and radial velocities of each of these 41sources. We find these objects to span ranges in effective temperature,surface gravity, and stellar luminosity that appear similar to those oflate spectral type Class II sources and classical T Tauri stars.However, because of significant but uncertain corrections for scatteringand extinction, the derived luminosities for the embedded protostellarobjects must be regarded as being highly uncertain. We determine thatthe mean 2 μm veiling of Class I and flat-spectrum objects issignificantly higher than that of Class II objects in the same regionwhere both types of objects are extensively observed (ρ Oph). Wefind that a significant fraction of our protostellar sample alsoexhibits emission lines. Twenty-three objects show H2emission, which is usually indicative of the presence of energeticoutflows. Thirty-four sources show H I Brγ emission, and a numberof these exhibit profile asymmetries consistent with infall. Eightsources show significant Δv=2 CO emission suggestive of emissionfrom a circumstellar disk. Overall, these observations indicate thatClass I and flat-spectrum objects are self-embedded protostarsundergoing significant mass accretion, although the objects appear tospan a broad range of mass accretion activity.Data presented herein were obtained at the W. M. Keck Observatory fromtelescope time allocated to the National Aeronautics and SpaceAdministration through the agency's scientific partnership with theCalifornia Institute of Technology and the University of California. TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| A study of Kapteyn's star
We present a review of the current knowledge of Kapteyn's Star (KS) - anearby, low-metallicity M-dwarf, with an eccentric and retrogradeGalactic orbit. A brief survey of its spectroscopic properties isprovided, together with an analysis of its Galactic orbit in a Galaxymodel that incorporates resonances. We propose that KS may have oncebelonged to a dwarf spheroidal galaxy that merged with the Galaxy, andwhose present remnant, if it still exists, is a globular cluster similarto ω Cen.
| 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
| 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.
| The Double-lined Spectroscopic Binary Haro 1-14c
We report detection of the low-mass secondary in the spectroscopicbinary Haro 1-14c in the Ophiuchus star-forming region. Thesecondary/primary mass ratio is 0.310+/-0.014. With an estimatedphotometric primary mass of 1.2 Msolar, the secondary mass is~0.4 Msolar and the projected semimajor axis is ~1.5 AU. Thesystem is well suited for astrometric mapping of its orbit with thecurrent generation of ground-based IR interferometers. This could yieldprecision values of the system's component masses and distance.
| Near-Ultraviolet Spectra of Nine M Dwarf Stars, or a Second Effort to Find Optical Coronal Lines in M Dwarf Stars
We have searched for optical coronal lines in the 3100-3700 Åregion of eight M dwarf stars with rather low levels of activity. Thisbrief survey supplements a similar search in 15 active stars publishedin 1991. No coronal lines could be identified. However, the emissionspectra including lines of H I, He I, Ca II, Ca I, Si I, and Fe I aredescribed and illustrated. Radial velocities of the emission lines showno systematic differences from the stellar absorption lines. Coronaewith temperatures similar to those in the solar corona seem to be rareamong the M dwarfs, although at least one example has been found bySchmitt & Wichmann.
| 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
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
We present a final summary of all ROSAT X-ray observations of nearbystars. All available ROSAT observations with the ROSAT PSPC, HRI and WFChave been matched with the CNS4 catalog of nearby stars and the resultsgathered in the Nearby X-ray and XUV-emitting Stars data base, availablevia www from the Home Page of the Hamburger Sternwarte at the URLhttp://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. Newvolume-limited samples of F/G-stars (dlim = 14 pc), K-stars(dlim = 12 pc), and M-stars (dlim = 6 pc) areconstructed within which detection rates of more than 90% are obtained;only one star (GJ 1002) remains undetected in a pointed follow-upobservation. F/G-stars, K-stars and M-stars have indistinguishablesurface X-ray flux distributions, and the lower envelope of the observeddistribution at FX ≈ 104 erg/cm2/sis the X-ray flux level observed in solar coronal holes. Large amplitudevariations in X-ray flux are uncommon for solar-like stars, but maybemore common for stars near the bottom of the main sequence; a largeamplitude flare is reported for the M star LHS 288. Long term X-raylight curves are presented for α Cen A/B and Gl 86, showingvariations on time scales of weeks and demonstrating that α Cen Bis a flare star.Tables 1-3 are also 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/417/651
| Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars
We present the full target list and prioritization algorithm developedfor use by the microwave search for technological signals at the SETIInstitute. We have included the Catalog of Nearby Habitable StellarSystems (HabCat, described in Paper I), all of the nearest 100 stars and14 old open clusters. This is further augmented by a subset of theTycho-2 catalog based on reduced proper motions, and this larger catalogshould routinely provide at least three target stars within the largeprimary field of view of the Allen Telescope Array. The algorithm forprioritizing objects in the full target list includes scoring based onthe subset category of each target (i.e., HabCat, cluster, Tycho-2, ornearest 100), its distance (if known), and its proximity to the Sun onthe color-magnitude diagram.
| A Spectroscopic Technique for Measuring Stellar Properties of Pre-Main-Sequence Stars
We describe a technique for deriving effective temperatures, surfacegravities, rotation velocities, and radial velocities fromhigh-resolution near-IR spectra. The technique matches the observednear-IR spectra to spectra synthesized from model atmospheres. Ouranalysis is geared toward characterizing heavily reddenedpre-main-sequence stars, but the technique also has potentialapplications in characterizing main-sequence and post-main-sequencestars when these lie behind thick clouds of interstellar dust. For thepre-main-sequence stars, we use the same matching process to measure theamount of excess near-IR emission (which may arise in the protostellardisks) in addition to the other stellar parameters. The informationderived from high-resolution spectra comes from line shapes and therelative line strengths of closely spaced lines. The values for thestellar parameters we derive are therefore independent of those derivedfrom low-resolution spectroscopy and photometry. The new method offersthe promise of improved accuracy in placing young stellar objects onevolutionary model tracks. Tests with an artificial noisy spectrum withtypical stellar parameters and a signal-to-noise ratio of 50 indicate a1 σ error of 100 K in Teff, 2 km s-1 invsini, and 0.13 in continuum veiling for an input veiling of 1. If theflux ratio between the sum of the Na, Sc, and Si lines at 2.2 μm andthe (2-0) 12CO band head at 2.3 μm is known to an accuracyof 10%, the errors in our best-fit value for logg will beΔlogg=0.1-0.2. We discuss the possible systematic effects on ourdetermination of the stellar parameters and evaluate the accuracy of theresults derivable from high-resolution spectra. In the context of thisevaluation, we quantitatively explore the degeneracy between temperatureand gravity that has bedeviled efforts to type young stellar objectsusing low-resolution spectra. The analysis of high-resolution near-IRspectra of MK standards shows that the technique yields very accuratevalues for the effective temperature. The greatest uncertainty incomparing our results with optical spectral typing of MK standards is inthe spectral type-to-effective temperature conversion for the standardsthemselves. Even including this uncertainty, the 1 σ differencebetween the optical and infrared temperatures for dwarfs at 3000-5800 Kis only 140 K. In a companion paper, we present an analysis of heavilyextincted young stellar objects in the ρ Ophiuchi molecular cloud.
| The Galactic Disk Mass Function: Reconciliation of the Hubble Space Telescope and Nearby Determinations
We derive and parameterize the Galactic mass function (MF) below 1Msolar characteristic of both single objects and binarysystems. We resolve the long-standing discrepancy between the MFsderived from the Hubble Space Telescope (HST) and from the nearbyluminosity functions, respectively. We show that this discrepancystemmed from two cumulative effects, namely, (1) incorrectcolor-magnitude-determined distances, due to a substantial fraction of Mdwarfs in the HST sample belonging to the metal-depleted thick-diskpopulation, as corrected recently by Zheng et al., and (2) unresolvedbinaries. We show that both the nearby and HST MF for unresolved systemsare consistent with a fraction ~50% of M dwarf binaries, with the massof both the primaries and the companions originating from the sameunderlying single MF. This implies that ~30% of M dwarfs should have anM dwarf companion and ~20% should have a brown dwarf companion, inagreement with recent determinations. The present calculations show thatthe so-called ``brown dwarf desert'' should be reinterpreted as a lackof high mass ratio (m2/m1<~0.1) systems anddoes not preclude a substantial fraction of brown dwarfs as companionsof M dwarfs or for other brown dwarfs.
| Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high-resolution optical spectra obtained with the HIRESspectrograph on the W. M. Keck I Telescope of seven low-mass T Tauristars (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708Å absorption, low surface gravity signatures, and radialvelocities confirm that all are members of the Taurus star-formingregion; no new spectroscopic binaries are identified. Four of the seventargets observed appear to be T Tauri brown dwarfs. Of particularinterest is the previously classified ``continuum T Tauri star'' GM Tau,which has a spectral type of M6.5 and possibly a substellar mass. Thesespectra, in combination with previous high-resolution spectra of LMTTs,are used to understand the formation and early evolution of objects inTaurus-Auriga with masses near and below the stellar/substellarboundary. None of the LMTTs in Taurus are rapidly rotating (vsini<30km s-1), unlike low-mass objects in Orion. Many of the slowlyrotating, nonaccreting stars and brown dwarfs exhibit prominent Hαemission (equivalent widths of 3-36 Å), indicative of activechromospheres. We demonstrate empirically that the full width at 10% ofthe Hα emission profile peak is a more practical and possibly moreaccurate indicator of accretion than either the equivalent width ofHα or optical veiling: 10% widths >270 km s-1 areclassical T Tauri stars (i.e., accreting), independent of stellarspectral type. Although LMTTs can have accretion rates comparable tothat of more typical, higher mass T Tauri stars (e.g., K7-M0 spectraltypes), the average mass accretion rate appears to decrease withdecreasing mass. A functional form of M~M is consistent with theavailable data, but the dependence is difficult to establish because ofboth selection biases in observed samples and the decreasing frequencyof active accretion disks at low masses (M<0.2 Msolar).The diminished frequency of accretion disks for LMTTs, in conjunctionwith their lower, on average, mass accretion rates, implies that theyare formed with less massive disks than higher mass T Tauri stars. Theradial velocities, circumstellar properties, and known binaries do notsupport the suggestion that many of the lowest mass members of Taurushave been ejected from higher stellar density regions within the cloud.Instead, LMTTs appear to have formed and are evolving in the same way ashigher mass T Tauri stars, but with smaller disks and shorter disklifetimes.
| 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.
| First radius measurements of very low mass stars with the VLTI
We present 4 very low mass stars radii measured with the VLTI using the2.2 mu m VINCI test instrument. The observations were carried out duringthe commissioning of the 104-meter-baseline with two 8-meter-telescopes.We measure angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11mas, and for spectral type ranging from M0V to M5.5V. We determine anempirical mass-radius relation for M dwarfs based on all availableradius measurements. The observed relation agrees well with theoreticalmodels at the present accuracy level, with possible discrepancy around0.5-0.8 Msun that needs to be confirmed. In the near future,dozens of M dwarfs radii will be measured with 0.1-1% accuracy, with theVLTI, thanks to the improvements expected from the near infraredinstrument AMBER. This will bring strong observational constraints onboth atmosphere and interior physics.Based on observations made with the European Southern Observatorytelescopes and obtained from the ESO/ST-ECF Science Archive Facility.
| 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
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | אנדרומדה |
| התרוממות ימנית: | 00h18m22.89s |
| סירוב: | +44°01'22.6" |
| גודל גלוי: | 8.149 |
| מרחק: | 3.568 פארסק |
| תנועה נכונה: | 2887.5 |
| תנועה נכונה: | 408.9 |
| B-T magnitude: | 10.085 |
| V-T magnitude: | 8.309 |
קטלוגים וכינוים:
|
