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Identifying the Young Low-mass Stars within 25 pc. II. Distances, Kinematics, and Group Membership
We have conducted a kinematic study of 165 young M dwarfs with ages oflsim300 Myr. Our sample is composed of stars and brown dwarfs withspectral types ranging from K7 to L0, detected by ROSAT and withphotometric distances of lsim25 pc assuming that the stars are singleand on the main sequence. In order to find stars kinematically linked toknown young moving groups (YMGs), we measured radial velocities for thecomplete sample with Keck and CFHT optical spectroscopy andtrigonometric parallaxes for 75 of the M dwarfs with the CAPSCaminstrument on the du Pont 2.5 m Telescope. Due to their youthfuloverluminosity and unresolved binarity, the original photometricdistances for our sample underestimated the distances by 70% on average,excluding two extremely young (lsim3 Myr) objects found to havedistances beyond a few hundred parsecs. We searched for kinematicmatches to 14 reported YMGs and identified 10 new members of the AB DorYMG and 2 of the Ursa Majoris group. Additional possible candidatesinclude six Castor, four Ursa Majoris, two AB Dor members, and onemember each of the Her-Lyr and ? Pic groups. Our sample alsocontains 27 young low-mass stars and 4 brown dwarfs with ages lsim150Myr that are not associated with any known YMG. We identified anadditional 15 stars that are kinematic matches to one of the YMGs, butthe ages from spectroscopic diagnostics and/or the positions on the skydo not match. These warn against grouping stars together based only onkinematics and that a confluence of evidence is required to claim that agroup of stars originated from the same star-forming event.Based on observations collected at the W. M. Keck Observatory, theCanada-France-Hawaii Telescope, the du Pont Telescope at Las CampanasObservatory, and the Subaru Telescope. The Keck Observatory is operatedas a scientific partnership between the California Institute ofTechnology, the University of California, and NASA, and was madepossible by the generous financial support of the W. M. Keck Foundation.The CFHT is operated by the National Research Council of Canada, theCentre National de la Recherche Scientifique of France, and theUniversity of Hawaii.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| Nearby Stars from the LSPM-North Proper-Motion Catalog. I. Main-Sequence Dwarfs and Giants within 33 Parsecs of the Sun
A list of 4131 dwarfs, subgiants, and giants located or suspected to belocated within 33 pc of the Sun is presented. All the stars are drawnfrom the new Lépine Shara Proper Motion (LSPM)-North catalog of61,976 stars with annual proper motions larger than 0.15"yr-1. Trigonometric parallax measurements are found in theliterature for 1676 of the stars in the sample; photometric andspectroscopic distance moduli are found for another 783 objects. Theremaining 1672 objects are reported here as nearby star candidates forthe first time. Photometric distance moduli are calculated for the newstars based on the (MV, V-J) relationship, calibrated withthe subsample of stars that have trigonometric parallaxes. The list ofnew candidates includes 539 stars that are suspected to be within 25 pcof the Sun, including 63 stars estimated to be within only 15 pc. Thecurrent completeness of the census of nearby stars in the northern skyis discussed in light of the new candidates presented here. It isestimated that ~32% (~18%) of nuclear-burning stars within 33 pc (25 pc)of the Sun remain to be located. The missing systems are expected tohave proper motions below the 0.15" yr-1 limit of theLSPM-North catalog.
| 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.
| Identification of selected sources from the ROSAT Galactic Plane Survey. I.
We report on optical searches in the error circles of 93 \ ROSAT surveysources located at low galactic latitudes (|b| < 20°e ). Thesesources were extracted from the ROSAT Galactic Plane Survey usingvarious selection criteria on hardness ratio, X-ray and opticalbrightness and integrated galactic absorption in the direction of thesource. We find optical identifications in 76 \ cases, among which are25 new AGN, 6 new CVs and a new Be/X-ray binary. In order to illustratethe relevance of the source selections applied here, we cross-correlatedthe ROSAT all-sky survey bright source list with SIMBAD. Differentclasses of X-ray emitters populate distinct regions of a multidimensional parameter space involving flux ratios, galactic latitude andN_H. This relatively good segregation offers the possibility to buildsource samples with enhanced probability of identification with a givenclass. Complete optical identification of such subsamples couldeventually be used to compute meaningful probabilities of identificationfor all sources using as basis a restricted set of multi-wavelengthinformation. Partly based on observations obtained at the EuropeanSouthern Observatory, La Silla (Chile) with the 2.2~m telescope of theMax-Planck-Society, with the ESO 1.5~m telescope and at the Observatoirede Haute-Provence, CNRS, France.}\fnmsep\thanks{Figures 8, 9 and 10 areonly available in the electronic version. Tables 4 and 5 are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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Observation and Astrometry data
| Constellation: | Cygnus |
| Right ascension: | 19h35m29.19s |
| Declination: | +37°46'08.0" |
| Apparent magnitude: | 11.613 |
| Proper motion RA: | -138 |
| Proper motion Dec: | -102.3 |
| B-T magnitude: | 13.642 |
| V-T magnitude: | 11.781 |
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