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Accepted:26-03-2004Meeting the Cool Neighbors VIII:A preliminary 20-parc census from the NLTT catalogue I.Neill Reid 1Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218;and Department of Physics and Astronomy,University of Pennsylvania,209South 33rd Street,Philadelphia,PA 19104;inr@stsci.edu Kelle L.Cruz 1,Peter R.Allen 1,F.Mungall 1Department of Physics and Astronomy,University of Pennsylvania,209South 33rd Street,Philadelphia,PA 19104D.Kilkenny South African Astronomical Obrvatory,P.O.Box 9,Obrvatory 7935,South Africa James Liebert 1Department of Astronomy and Steward Obrvatory,University of Arizona,Tucson,AZ 85721Suzanne L.Hawley,Oliver Frar,Kevin Covey Department of Astronomy,University of Washington,Seattle,WA 28195Patrick Lowrance 1,J.Davy Kirkpatrick IPAC,California Institute of Technology,Pasadena,CA 91125Adam J.Burgasr
University of California,Los Angeles,
ABSTRACT
诚请Continuing our census of late-type dwarfs in the Solar Neighbourhood,we prent BVRI photometry and optical spectroscopy of 800mid-type M dwarfs drawn from the NLTT proper motion catalogue.The targets are taken from both our own cross-referencing of the NLTT catalogue and the 2MASS Second Incremental relea,and from
the revid NLTT compiled by Salim&Gould(2003).All are identified as nearby-star
candidates bad on their location in the(m r,(m r-K S))diagram.Three hundred stars
discusd here have previous astrometric,photometric or spectroscopic obrvations.
We prent new BVRI photometry for101stars,together with low resolution spec-
troscopy of a further400dwarfs.In total,wefind that241stars are within20parcs
of the Sun,while a further70lie within1σof our distance limit.
Combining the prent results with previous analys,we have quantitative obrva-
tions for1910of the1913candidates in our NLTT nearby-star samples.Eight hundred
andfifteen of tho stars have distance estimates of20parcs or less,including312
全冠移植additions to the local census.
With our NLTT follow-up obrvations esntially complete,we have arched the liter-
ature for K and early-type M dwarfs within the sampling volume covered by the2MASS
Second Relea.Comparing the resultant20-parc census against predicted numbers,
derived from the8-parc luminosity function,shows an overall deficit of∼20%for
stellar systems and∼35%for individual stars.Almost all are likely to be fainter than
M J=7,and at least half are probably as-yet undiscovered companions of known nearby
stars.Our results suggest that there are relatively few missing systems at the lowest
luminosities,M J>8.5.We discuss possible means of identifying the missing stars.
Subject headings:stars:low-mass,brown dwarfs;stars:luminosity function,mass func-
马马虎虎的反义词tion;Galaxy:stellar content
1.Introduction
It is by now a clich´e(particularly after this t of papers)that M dwarfs are the most populous membe
rs of the Galactic Disk.M subdwarfs probably dominate the Galactic halo to a similar extent. It is also now well established that the most recently-published local census,the Third Catalogue of Nearby Stars(Glie&Jahreiss,1991,CNS3),becomes incomplete for early-type dwarfs at a distance of∼20−22parcs,and is barely complete within5parcs for spectral types later than M6(Reid,Gizis&Hawley,2002-PMSU4;Henry et al.,2003).Thus,while Hipparcos(ESA,1997) has given us a complete catalogue of over1000G dwarfs within30parcs of the Sun,the statistical properties of dwarfs like Proxima Cen rest on obrvations of a volume-complete sample of a mere half-dozen stars.
We are using data from the2-Micron All-Sky Survey(2MASS)(2MASS:Skrutskie et al,1997) to address this issue in a project carried out under the auspices of the NASA/NSF NStars Initiative. Our goal is to compile a statistically complete census of late-type dwarfs(spectral types late-K, M and L)within20parcs of the Sun.With the publication of the eighth paper in this ries,
we provide an asssment of progress toward that goal and map out likely developments in the immediate future.
So far,we have employed two distinct,complementary techniques to identify unrecognid constituen
奶奶我想对你说ts of the immediate Solar Neighbourhood:first,we have ud near-infrared photometry to identify ultracool dwarfs(spectral types M7and later)directly from the2MASS databa; cond,we have arched for nearby early-and mid-type M dwarfs by cross-referencing stars in the NLTT proper motion catalogue(Luyten,1980)against the2MASS data.We have pursued both avenues of exploration in parallel.In both cas,our analysis to date is restricted to the48%of the sky covered by the2MASS Second Incremental Relea(hereinafter,the2M2nd),with only limited coverage of regions within10degrees of the Galactic Plane.
Ultracool dwarfs can be identified directly from2MASS data alone since they have distinctively red near-infrared colours,with(J-K S)increasing from a colour of∼1.0mag at spectral type M7.5 to>1.8mag for late-type L dwarfs,until the ont of significant methane absorption revers this trend in L8dwarfs at temperatures of1300-1400K(Tsuji,1964;Kirkpatrick et al.,1999;Golimowski et al.,2004).We have ud the location of2MASS point sources in both the(J,(J-K S))colour-magnitude diagram and in the two-colour(J-H)/(H-K S)to lect candidates for dwarfs within20 parcs of the Sun.The results from this thread of our project are published as follows:•Paper IV(Reid et al.,2002)prents the initial result from the ultracool survey,the identi-fication of an M8dwarf within6parcs of the Sun.
•Paper V(Cruz et al.,2003)provides a thorough description of the lection criteria ud to compile the ultracool sample(the2MU2sample),and discuss optical spectroscopy of298 candidates.The2MU2sample is limited to|b|>10o,and specifically excludes areas near star-formation regions.
•Paper VI(Reid,2003)reprents afirst attempt to extend photometric ultracool surveys to the Galactic Plane,with only limited success.
•Paper IX(Cruz et al.,2004)prents obrvations of the remaining2MU2targets accessible to optical spectroscopy on4-metre class telescopes,discuss sample completeness and derives the luminosity function for spectral types M8to L7.
Building on the results,we have ud analysis of the2MASS All-Sky data Relea to compile an all-sky high-latitude(|b|>15o)sample of ultracool dwarfs.Follow-up obrvations of that sample are almost complete,and the initial results,bad on optical spectroscopy,will be prented in Paper X in this ries.In addition,we have obtained infrared spectroscopy of the faintest ultracool candidates from both samples,and tho results will be prented in Paper XI.
Early-and mid-type M dwarfs do not posss the easily recogniable near-infrared colours of ultrac
ool dwarfs,so additional information is required to gregate the nearby stars among the
108sources in the2M2nd point-source catalogue.Proper motion offers a well-tried method of identifying stars in the vicinity of the Sun.The most extensive proper motion catalogues currently available are tho compiled by W.Luyten using photographic material,primarily plates taken with the48-inch Palomar Oschin Schmidt.For a variety of reasons,principally the limited information offered by the catalogued photometry,the resources have been little exploited.However,Monte-Carlo simulations bad standard disk kinematics(PMSU4)show that between88and90%of stars within20parcs are expected to have motionsµ>0.18′′yr−1,the proper motion limit of Luyten’s NLTT survey.We have therefore cross-referenced the NLTT catalogue against the 2MASS databa,and arched for nearby-star candidates using the optical-infrared colours derived by combining Luyten’s red magnitudes,m r,with the2MASS JHK S photometry.The results from this cond thread have appeared as follows:
北京19号线
•Paper I(Reid&Cruz,2002)outlines the optical/near-infrared colour-magnitude criteria that have been ud to identify candidate nearby stars.Bad on tho criteria,we lected two parent samples of NLTT dwarfs,limiting the analysis to|b|>10o:NLTT1,consisting of 1237NLTT stars with2M2nd counterparts within10arcconds of the predicted NLTT location;and stars not included in the NLTT1
sample,but which have2M2nd sources within 60arcconds of the predicted position.Literature data were compiled for469stars,and distance estimates derived using(V-I),(I-J)and(V-K S)colour-magnitude relations.We place X of tho stars within20parcs of the Sun.
•Paper II(Reid,Kilkenny&Cruz,2002)prents BVRI photometry of180southern NLTT1 stars and the corresponding photometric parallax estimates.Y stars have formal distances estimates less than20parcs.
•Paper III(Cruz&Reid,2002)describes spectroscopic obrvations of venty NLTT dwarfs, and summaris the techniques adopted to derive both spectral types and spectrophotometric parallax.X stars fall within our20parc distance limit.
•Paper VII(Reid et al.,2003)includes photometric and spectroscopic obrvations of453 dwarfs from the NLTT1sample;wefind that111of tho stars are likely to lie within20 parcs of the Sun.
•The prent paper completes our NLTT survey,prenting photometric and spectroscopic obrvations and distance estimates for not only the remaining stars in the NLTT1sample, but also for candidate nearby stars from two other NLTT samples,NLTT2and NLTT3,where the latter is derived from Salim&Gould’s(2003)revid NLTT catalogue.
We do not propo to extend our NLTT-bad survey to cover the full celestial sphere by extending analysis to the2MASS All-Sky data relea-with obrvations of half the sky,we have sufficient numbers for statistically-significant analys of the intrinsic properties of K-and M-type dwarfs. However,as discusd in more detail in the concluding ctions of this paper,we are exploring the
potential for using other techniques to identify nearby early-and mid-type M dwarfs that lie within the area covered by the2M2nd data relea,but are not in the NLTT catalogue.
Previous papers in the NLTT ries have concentrated on follow-up obrvations of the NLTT1 sample.23,795of the58,818NLTT stars meet this criterion;distance estimates for1090of the1237 NLTT1nearby-star candidates are given in Papers I-VII.The current paper completes obrva-tions of that sample,and extends coverage to nearby-star candidates from two other NLTT-bad samples.First,as described in Paper I,the NLTT1stars exhibit distinct‘holes’in their(α,δ) -notably near the North Celestial Pole and the South Galactic Pole.Many of the unmatched NLTT stars in tho holes have2M2nd sources within60arcconds(e Figure3in Paper I), suggesting the prence of systematic offts in the NLTT astrometry.The latter stars,together with NLTT/2MASS pairs lying at low latitude(|b|<10o),constitute the NLTT2sample.
Second,we have taken advantage of the independent analysis of the NLTT catalogue by Salim &Gould(2003-SG03).As with our own study,the authors match NLTT stars against the 2M2nd databa,although their analysis is limited to regions which overlap the POSS I survey (effectively,δ>−33o).SG03have gone to considerable length to correct typographical errors in the original catalogue,and they recover97%of the NLTT dwarfs brighter than m r=17and85% to m r=18.Coupled with their analysis of Hipparcos and Tycho data for brighter stars(Gould &Salim,2003),the revid NLTT catalogue(rNLTT)includes data for28379stars with infrared photometry from the2M2nd databa.Covering almost the same ground as our own survey,the rNLTT provides both independent verification of our NLTT1and NLTT2samples,and a means of identifying new candidates.We have therefore compiled a third list of candidate nearby stars,the NLTT3sample,by applying our(m r,(m r-K S))lection criteria to the rNLTT.
The prent paper is organid in the following manner:Section2describes the NLTT2sample in more detail,summarising previously-published photometric and spectroscopic data available for tho stars.Section3provides a similar function for the NLTT3sample.New BVRI photometric obrvations of stars from both samples are prented in§4,while§5prents optical spectroscopy of stars from all three samples,NLTT1,NLTT2and NLTT3.Combining all of the results,we can derive r
eliable distance estimates for1910of the1913NLTT nearby-star candidates.Section 6expands the discussion to include early-type M and K dwarfs,drawn from the NLTT and other catalogues,and combines all of the datats to give a20-parc census for the part of the sky covered by the2M2nd.We discuss the statistical properties of the resulting catalogue,notably the luminosity function and the binary fraction,in§7and compare tho properties against similar data for the northern8-parc sample(Paper IV)and for the25-parc bright-star census compiled by PMSU4.Section8outlines possible methods for identifying additional stars within our20-parc limit,and thefinal ction,§9,prents our conclusions.
六种瘦腿睡姿2.The NLTT2Sample
2.1.Defining the sample
Approximately34,000NLTT stars either lack a2M2nd source within10arcconds of the predicted position,or lie within10degrees of the Galactic Plane.The stars are not included in the NLTT1sample,and the overwhelming majority lie beyond the bounds of the2M2nd.However, expanding the arch radius to60arcconds for proper motion stars with|b|>10o results in matches between5720NLTT stars and253052MASS sources.We have also cross-referenced the two catalogu
es at lower Galactic latitudes,but with the arch radius limited to10arcconds to minimi confusion.A total of886NLTT dwarfs are matched against19512MASS sources.The final NLTT2sample is derived by combining the two datats.
Most of the pairings generated by this exerci are obviously spurious-with a arch radius of 60arcconds,many NLTT stars are matched against six or more parate2MASS sources,even at moderate Galactic latitudes.We have trimmed the list by applying the(m r,(m r-K S))colour-magnitude lection criteria outlined in Paper I.This reduces the sample to1468NLTT/2MASS pairs with colours consistent with nearby M dwarfs,but many of tho pairings have incompatible near-infrared colours.Figure1plots the((m r-J)/(J-K S))and((J-H)/(H-K S))two-colour diagrams for all1468candidates.The latter diagram also shows the locii of main-quence dwarfs and red giants,and we have outlined schematically the regions occupied by dwarfs later than spectral type ≈M3((H-K S)≥0.19)).There are obviously many mismatched red giants among the current candidates,which is not surprising given that this datat includes many NLTT dwarfs clo to the Plane.Eliminating pairs which have JHK S colours inconsistent with M or L dwarfs reduces the NLTT2candidate list to514targets.
Thirty-ven of the remaining514candidates fall in the L dwarf gment of the JHK S diagram, althou
gh most lie suspiciously clo to the giant quence.Unfortunately,all of the candidates are found in implausible locations in the(m r-J)/(J-K S)diagram,with most having implausibly blue(m r-J)colours,so we have not succeeded infinding an L dwarf in the NLTT catalogue.The are likely to be mismatches to late-type red giants,lying on the redder edge of the giant quence in the JHK S diagram.We have eliminated all34accordingly.Visual inspection of2MASS and POSS/UKST sky survey photographic images shows that a further111pairings are mis-matches, either between components in a common proper-motion binary or with a randomfield star.Thus, thefinal NLTT2includes369candidate nearby M dwarfs.Of the,117,or approximately one-third,lie within10degrees of the Plane.
2.2.Known nearby stars
金牌销售员A substantial number of NLTT2stars are already known as denizens of the immediate Solar Neighbourhood.One hundred and nineteen stars are included in the CNS3,and most of the
