Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Observation and modelling of main-sequence star chromospheres - XIX. FIES and FEROS observations of dM1 stars
We present 187 high-resolution spectra for 62 different M1 dwarfs fromobservations obtained with the FIbre-fed Echelle Spectrograph (FIES) onthe Nordic Optical Telescope (NOT) and from observations with theFibre-fed Extended Range Echelle Spectrograph (FEROS) from the EuropeanSouthern Observatory (ESO) data base. We also compiled othermeasurements available in the literature.We observed two stars, Gl 745A and Gl 745B, with no Ca II line coreemission and H? line equivalent widths (EWs) of only 0.171 and0.188 Å, respectively. We also observed another very low activityM1 dwarf, Gl 63, with an H? line EW of only 0.199 Å. Theseare the lowest activity M dwarfs ever observed and are of particularinterest for the non-local thermodynamic equilibrium radiative transfermodelling of M1 dwarfs.Thanks to the high signal-to-noise ratio of most of our spectra, we wereable to measure the Ca II H&K full width at half-maximum (FWHM) formost of our stars. We find good correlations between the FWHM values andthe mean Ca II line EW for dM1 stars. Then the FWHM seems to saturatefor dM1e stars. Our previous models of M1 dwarfs can reproduce the FWHMfor dM1e stars and the most active dM1 stars, but fail to reproduce theobservations of lower activity M1 dwarfs. We believe this is due to aneffect of metallicity. We also investigate the dependence of theH? line FWHM as a function of its EW. We find that the modelsglobally agree with the observations including subwarfs, but tend toproduce too narrow profiles for dM1e stars.We re-investigate the correlation between the Ca II line mean EW and theabsolute magnitude. With our new data that notably include several M1subdwarfs, we find a slightly different and better correlation with aslope of -0.779 instead of -0.909. We also re-investigatethe variations of the H? line EW as a function of radius and findthat the EW increases continuously with increasing radius. This confirmsour previous finding that the level of magnetic activity in M1 dwarfsincreases with the radius.For the first time, we investigate the Wilson-Bappu correlation for agiven spectral type. We find a rather linear correlation for stars ofabsolute magnitude greater than 9.6, but below this value the FWHM seemsto saturate. In fact, we show that these Wilson-Bappu type correlationsare activity-FWHM correlations and are due to the diminishing columnmass of the transition region with decreasing activity level. Based onobservations available at the European Southern Observatory data basesand on Hipparcos parallax measurements.
| Age Determinations of Early-M Type Pre-Main Sequence Stars Using a High-Resolution Near-Infrared Spectroscopic Method
We present a method for determining the age of early-M type pre-mainsequence (PMS) stars based on estimations of the surface gravity. Thesurface gravity was measured using high-resolution near-infrared K-bandspectroscopy. The age of the PMS stars can be determined from thesurface gravity, which correlates with the photospheric contraction. Toestimate the surface gravity while avoiding veiling contamination, wedeveloped a surface gravity indicator using equivalent width ratios(EWRs) of nearby absorption lines. We derived a relationship between theratios of the Sc (22057.8 Å and 22071.3 Å) and Na (22062.4Å and 22089.7 Å) absorption lines and the surface gravity byobserving giants and main-sequence stars. The surface gravities ofearly-M type stars were determined with an accuracy of 0.1 in logg. Theages of target PMS stars were estimated within a factor of 1.5 bycomparing the surface gravity with the evolution model of Baraffe et al.(1998, A&A, 337, 403). The ages of 4 PMS stars were estimated to beolder than indicated by previous age determinations made using thephotometric method. The EWR method allows estimating the age of PMSstars without contaminating the uncertainty of the distance, extinction,and veiling.
| Observation and modelling of main-sequence star chromospheres - XV. New constraints on the dynamo mechanisms for dM1 stars
With the help of measures of rotation, radius and metallicity for aselected sample of dM1 stars (with Teff= 3460 ± 60 K),we aim to set new constraints on the dynamo mechanisms.We recover 913 high-resolution spectra for 97 different M1 dwarfs fromthe European Southern Observatory and Observatoire de Haute Provencedata bases. We present 660 new measurements of the Ca II resonance linesand 913 new measurements of the H? line for dM1 stars. We alsocompile other measurements available in the literature. In total, weobtain 2216 measures of the Ca II lines for 113 different dM1 stars.This represents the largest compilation of chromospheric linemeasurements at a single spectral type.We cross-correlate these magnetic activity indicators with variousstellar parameters to set new constraints on the dynamo mechanisms andon the properties of the outer atmosphere.We find a correlation of the Ca II line mean equivalent width with theabsolute magnitude and the metallicity. We correct the Ca II linemeasures from the metallicity effect and find that the surface flux inthe Ca II lines grows roughly as the power of 3.6 of the stellar radius.This corrected flux is a direct measure of magnetic activity at thechromospheric level. We find that the total magnetic activity levelgrows roughly as the power of 5.6 of the stellar radius. This trend isconfirmed by the correlation between the H? line and absolutemagnitude and the H? line luminosity and stellar radius: theH? luminosity grows roughly as the volume of the star for lowactivity dM1 stars and as the power of roughly 5/2 of the stellar radiusfor dM1e stars. The advantage of the H? line is that its formationin not dependent on metallicity.In contrast to the Ca II line, we find no correlation betweenLX and the absolute magnitude. We find that LXroughly correlates with the Ca II luminosity although the correlation isnot very good. This correlation shows that LX grows as thepower of 3/2 of the Ca II luminosity, i.e. the coronal emission growsfaster than the chromospheric emission.We find a correlation between the corrected Ca II line equivalent widthand P/sin i, i.e. the Ca II surface flux grows as the power of -1.5 ofthe rotation period. We also find a correlation between FX,the X-ray surface flux, and P/sin i: FX? (P/sini)-3.7. In other words, the coronal emission is much moredependent on the rotation period than the chromospheric emission.We find that the level of magnetic activity in dM1 stars is moredependent on the stellar radius than on rotation at the chromosphericlevel. We discuss the implications of these results on the models ofstellar dynamos. Based on observations available at Observatoire deHaute Provence and the European Southern Observatory data bases and onHipparcos parallax measurements.
| 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
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| 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.
| Observation and modelling of main-sequence star chromospheres . IX. Two-component model chromospheres for nine M1 dwarfs
Aims: We aim to constrain the H?, CaII H andCaII K line profiles of quiescent and active regions of nine dM1 starsof near solar metallicity: Gl 2, GJ 1010A, Gl 49, Gl 150.1B, Gl 205, Gl229, Gl 526, G192-11A, and Gl 880. Methods: We propose a newmethod for building two-component model chromospheres for dM1stars-based on simple constraints and a grid of model atmospheresdeveloped by Houdebine & Stempels. This method is based on themeasurements of the equivalent width of H? and CaII H& K. Based on the peculiar relationship between these two equivalentwidths in the model atmospheres, our solutions provide an exact match ofthese equivalent widths. Results: We obtain two component(quiescent and active region) model chromospheres for our nine targetstars. We fit the H?, CaII H, and CaII K profiles forthese stars. These models show that seven of these stars lie in theintermediate activity range between H? maximumabsorption and emission. Two stars (Gl 49 and G192-11A) are quite activewith H? emission profiles in plages. As far as the CaIIemission is concerned, these two stars are almost as active as dM1estars. Two stars (GJ 1010A and Gl 526) have lower activity levels withnarrower and weaker H? profiles. The range of activitycovered by these stars is a factor of 13 in the CaII lines, from lowactivity to activity levels almost as high as those of dM1e stars. Ourmethod sometimes provides two solutions of the observedH? equivalent width as a function of the quiescentregion H? equivalent width. For Gl 205, one of thesolutions is shown to be impossible for the assumptions that we use. ForGl 49 and G192-11A, two solutions are possible; a low solution (low CaIIEW) and a high solution (high CaII EW). The difference between these twosolutions is mainly in the plage-filling factor. The two solutions givealmost identical H? and CaII profiles. We prefer thelow solutions because the filling factors are in closer agreement withthose of other stars. We find plage-filling factors typically in therange 20%-40%. We also find that it is the chromospheric pressure ratherthan the filling factor that increases with increasing activity. Wedefine a minimum theoretical H? equivalent width as afunction of the mean CaII H & K equivalent width. We show that ourobservations agree well with this lower limit. We also show that theproperties of the chromosphere in quiescent and active regions correlatewith the mean CaII H & K equivalent width. This could be useful infuture studies to derive an estimate of the chromospheric propertiesfrom the observed mean CaII H & K equivalent width.Based on observations collected at Observatoire de Haute Provence,France
| 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.
| 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
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| 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.
| Meeting the Cool Neighbors. VIII. A Preliminary 20 Parsec Census from the NLTT Catalogue
Continuing our census of late-type dwarfs in the solar neighborhood, wepresent BVRI photometry and optical spectroscopy of 800 mid-type Mdwarfs drawn from the NLTT proper-motion catalog. The targets are takenboth from our own cross-referencing of the NLTT Catalogue and the 2MASSSecond Incremental Data Release, and from the revised NLTT compiledrecently by Salim & Gould. All are identified as nearby-starcandidates based on their location in the(mr,mr-Ks) diagram. Three hundred starsdiscussed here have previous astrometric, photometric, or spectroscopicobservations. We present new BVRI photometry for 101 stars, togetherwith low-resolution spectroscopy of a further 400 dwarfs. In total, wefind that 241 stars are within 20 pc of the Sun, while a further 70 liewithin 1 σ of our distance limit. Combining the present resultswith previous analyses, we have quantitative observations for 1910 ofthe 1913 candidates in our NLTT nearby-star samples. Eight hundredfifteen of those stars have distance estimates of 20 pc or less,including 312 additions to the local census. With our NLTT follow-upobservations essentially complete, we have searched the literature for Kand early-type M dwarfs within the sampling volume covered by the 2MASSsecond release. Comparing the resultant 20 pc census against predictednumbers, derived from the 8 pc luminosity function, shows an overalldeficit of ~20% for stellar systems and ~35% for individual stars.Almost all are likely to be fainter than MJ=7, and at leasthalf are probably as yet undiscovered companions of known nearby stars.Our results suggest that there are relatively few missing systems at thelowest luminosities, MJ>8.5. We discuss possible means ofidentifying the missing stars.
| A Dedicated M Dwarf Planet Search Using The Hobby-Eberly Telescope
We present the first results from our planet-search program using the9.2 m Hobby-Eberly Telescope (HET) at McDonald Observatory to detectplanets around M-type dwarf stars by means of high-precision radialvelocity (RV) measurements. Although more than 100 extrasolar planetshave been found around solar-type stars of spectral type F-K, there isonly a single M dwarf (GJ 876) known to harbor a planetary system. Withthe current incompleteness of Doppler surveys with respect to M dwarfs,it is not yet possible to decide whether this is due to a fundamentaldifference in the formation history and overall frequency of planetarysystems in the low-mass regime of the Hertzsprung-Russell diagram, orsimply an observational bias. Our HET M dwarf survey plans to survey 100M dwarfs in the next 3 to 4 years, with the primary goal being to answerthis question. Here we present the results from the first year of thesurvey, which show that our routine RV precision for M dwarfs is 6 ms-1. We found that GJ 864 and GJ 913 are binary systems withas yet undetermined periods, while five out of 39 M dwarfs reveal a highRV scatter and represent candidates for having short-period planetarycompanions. For one of them, GJ 436 (rms=20.6 m s-1), we havealready obtained follow-up observations, but no periodic signal ispresent in the RV data.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.
| 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.
| 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.
| Observation and modelling of main sequence stellar chromospheres. VI. Halpha_ and CaII line observations of M1 dwarfs and comparison with models.
We compare hydrogen and calcium line calculations for dM1 (Teff=3500K)stellar chromospheres with high resolution observations of a selectedsample of stars with the same spectral type ((R-I)_K_=0.875+/-0.05). Webring evidence that grids of uniform model atmospheres in theplane-parallel and hydrostatic equilibrium approximations can reproducethe average spectral signatures throughout the entire activity range.Observations confirm that when magnetic activity level rises, theHα line is first weak, then increases in absorption strength,rapidly fills in and eventually goes into emission. We obtain acorrelation between the Hα line width and equivalent width that isin good agreement with our model calculations. Simultaneous Hα andCaII line observations allow to remove the degeneracy in Hαequivalent width for low activity (weak absorption) and intermediateactivity stars (filled in profiles). We show that the latter grouprepresents a significant proportion of the stellar population. Withinthe active stars group, we find an exclusion zone in the[0.25Å;-1Å] Hα equivalent width domain, that can besimply explained by the rapid change from the absorption to the emissionregimes when the chromospheric pressure increases. In our sample of 154stars, covering a large luminosity range, we found no ``zero-Hα''stars but instead a minimum (possibly ``basal'') Hα equivalentwidth of ~0.20Å which, with reference to our models, suggests atransition region column mass of log(M)~-5.5. This implies that for anoverwhelming majority of M1 type dwarfs the amount of non-thermal energyinput in the chromosphere is much higher than in the Sun, and byreference to acoustic heating calculations, that they are alsomagnetically much more active (per unit area). Our observations provideevidence for gradual and important changes in the integrated physicalproperties of the chromosphere throughout the activity range. Forexample, the equivalent widths of the H and K line cores are tightlycorrelated with their ratio, the later decreasing from low activity(~1.50 at -0.2Å) to high activity stars (~1.06 at -15Å). TheK lines are also typically 30% broader than the H lines and their widthsincrease with increasing activity level. This suggests that theiroptical depths in their region of formation also increase withincreasing activity level. Our calcium line calculations reproduce theobserved trends. We confirm a near UV and blue excess in active dMestars that increases with activity level; in average 0.12 magnitudes inU-B (and up to 0.26mag.) and 0.03 magnitudes in B-V. This excess isabout three times larger than expected from our calculations for a givenatmospheric pressure, and together with discrepancies between models andobservations for spectral lines, all converge to imply that highpressure plages with a filling factor of about 30% are present on thesestars. Low metallicity halo dwarfs in our sample also exhibit a U-Bexcess, but in the case of single dMe stars an effect of metallicity isexcluded. In our sample, single dMe stars are more luminous than theirless active absorption line counterparts. We present anactivity-luminosity relationship for the CaII lines; namely, the CaIIline fluxes rise as the power of 5.4 of the stellar radius. Hence, weexpect the stellar magnetic flux to rise approximately as the power of7.4 of the radius; an important constraint for the dynamo mechanism.Such a correlation is also found with Hα and L_X_, the X-rayluminosity. With our present understanding, these correlations are, atleast partly, activity-metallicity relationships. We emphasize theimportance of metallicity on stellar activity as a whole, i.e., metaldeficient stars are also activity deficient.
| The Palomar/MSU Nearby Star Spectroscopic Survey.II.The Southern M Dwarfs and Investigation of Magnetic Activity
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2799H&db_key=AST
| Photometry of Stars with Large Proper Motion
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2300W&db_key=AST
| The empirical scale of temperatures of the low main sequence (F0V-K5V).
We have calibrated the effective temperatures of the low main sequencestars ranging spectral types from F0 to K5 versus [Fe/H] and colours(B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and ubvy-β, using alarge sample of dwarfs and subdwarfs. The effective temperatures, scaledto direct T_eff_ determinations via reliable angular diametermeasurements, were derived applying the InfraRed Flux Method with thenew grid of atmosphere models developed by Kurucz (1993). We have fittedpolynomial functions of the form θ_eff_=P(colour,[Fe/H]) usingthe least squares method. The precision of the fits ranges from 30K for(V-K) to 154K for (J-H). The new relations have been compared toprevious calibrations. We also provide the empirical intrinsic colours(U-B), (B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and β, inthe ranges: 4000K[Fe/H]>-2.5.
| Determination of effective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5).
We have applied the InfraRed Flux Method (IRFM) to a sample of 475dwarfs and subdwarfs in order to derive their effective temperatureswith a mean accuracy of about 1.5%. We have used the new homogeneousgrid of theoretical model atmosphere flux distributions developed byKurucz (1991, 1993) for the application of the IRFM. The atmosphericparameters of the stars cover, roughly, the ranges:3500K<=T_eff_<=8000K -3.5<=[Fe/H]<=+0.53.5<=log(g)<=5. The monocromatic infrared fluxes at the continuum,and the bolometric fluxes are derived using recent results, whichsatisfy the accuracy requeriments of the work. Photometric calibrationshave been revised and applied to estimate metallicities, although directspectroscopic determinations were preferred when available. The adoptedinfrared absolute flux calibration, based on direct optical measurementsof angular stellar diameters, sets the effective temperatures determinedusing the IRFM on the same scale than those obtained by direct methods.We derive three temperatures, T_J_, T_H_ and T_K_, for each star usingthe monochromatic fluxes at different infrared wavelengths in thephotometric bands J, H, and K. They show good consistency over 4000 K,and no trend with wavelength may be appreciated. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of the errors associated to the different inputs ofthe method, and their transmission into the final temperatures. We alsoprovide comparison with previous works.
| The Palomar/MSU Nearby-Star Spectroscopic Survey. I. The Northern M Dwarfs -Bandstrengths and Kinematics
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.1838R&db_key=AST
| The general catalogue of trigonometric [stellar] paralaxes
Not Available
| Photographic astrometry of binar and proper-motion stars: 8.
300 trigonometric parallaxes, 15 revised binary-star orbits, and 24 massratios are listed and annotated.
| Broad band JHK infrared photometry of an extended sample of late type dwarfs and subdwarfs.
The results of a long term programme of broad band JHK photometry, for asample of 360 late type stars, made at the Observatorio del Teide(Tenerife, Spain) are presented. Transformations between thesemagnitudes and those of several currently used systems (CIT (Elias etal. 1982 and Carney 1983), Johnson (Johnson 1966, and Lee 1970), and ESO(Bouchet et al. 1991)) are proposed. A comparison to the narrow-bandsystem of Selby et al. (1988) has been made, in order to check theaccuracy of the photometric system. A mean internal accuracy better than0.02mag in the three bands can be inferred from the comparison to thelarge number of stars in common with Carney (1983), and from thedispersion of the multiple measured stars. The list of standards, thefilter passbands and effective wavelengths, together with correlationsbetween the extinction coefficients, ultimately characterize thephotometric IR system of the Observatorio del Teide (TCS). Data ofcomparable quality previously published have been added in order tocomplete the sample. This way the final sample consists of 550 stars.From the analysis of optical and IR colour:colour diagrams, we maydeduce that the range F0-K0 is properly sampled for0.1>[Fe/H]>-3.0. In the range K0-M4, no reliable photometricestimates of metallicity can be assigned, and only a small number ofstars have spectroscopic determination of the metallicity. Nevertheless,after kinematical considerations, the stars in this spectral range arealso expected to sample the galactic populations of dwarfs. Themetallicity effects on the IR and optical colour:colour diagrams arebriefly discussed.
| Long term variability in dwarf M stars
Broadband VRI observations for 43 dwarf M stars have been made over theperiod 1980-1991, and 21 of them, including eight UBV standard stars,show long term variability at the 95% confidence level. While itundoubtedly exists, periodic behavior cannot be demonstrated from thesedata except perhaps in the cases of GL 213 and GL 876 which appear toshow periods of 2.7 and 2.9 yr, respectively. Substantial short termvariations in the light of GL 277A (= VV Lyn) are also noted.
| Far infrared properties of late type dwarfs. Infrared fluxes of K & M dwarfs
IRAS fluxes/upper limits are presented for a large sample of K and Mdwarfs. Good agreement is found between the 12 micrometer fluxes andthose derived from the photospheric models of Mould (1976).Relationships between the optical and infrared colors are derived. Theactive dMe/dKe stars appear systematically brighter in the infraredcompared with the less active dM/dK stars, which could be attributed tomore efficient nonradiative heating in their atmosphere. Any systematicdifferences found in our results when compared with those obtained fromprevious studies are attributed to the different analysis packages used.
| Photometry of dwarf K and M stars
An observational program using UBVRI photometry is presented for 688stars from among the dwarf K and M stars already found spectroscopicallyby Vyssotsky (1958). Of these, 211 have not been observedphotometrically. These observations were obtained over a period ofseveral years at the Kitt Peak National Observatory using a GaAsphotomultiplier with an 0.9 m reflector. Based on night-to-nightvariations in the measures of individual stars, the internal errors maybe estimated to be roughly 0.01 mag for the colors and 0.015 for the Vmagnitudes. The photometric parallaxes reported for each star werecomputed in the manner discussed by Weis (1986).
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Близанци |
| Ректацензија: | 07h48m16.39s |
| Deклинација: | +20°22'05.4" |
| Apparent магнитуда: | 11.646 |
| Даљина: | 14.136 parsecs |
| Proper motion RA: | 1454.2 |
| Proper motion Dec: | -985.6 |
| B-T magnitude: | 13.361 |
| V-T magnitude: | 11.788 |
Каталог и designations:
|
