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A Catalog of Rotation and Activity in Early-M Stars
We present a catalog of rotation and chromospheric activity in a sampleof 334 M dwarfs of spectral types M0-M4.5 populating the parameter spacearound the boundary to full convection. We obtain high-resolutionoptical spectra for 206 targets and determine projected rotationalvelocity, vsin i, and H? emission. The data are combined withmeasurements of vsin i in field stars of the same spectral type from theliterature. Our sample adds 157 new rotation measurements to theexisting literature and almost doubles the sample of available vsin i.The final sample provides a statistically meaningful picture of rotationand activity at the transition to full convection in the solarneighborhood. We confirm a steep rise in the fraction of active stars atthe transition to full convection known from earlier work. In addition,we see a clear rise in rotational velocity in the same stars. In veryfew stars, no chromospheric activity but a detection of rotationalbroadening is reported. We argue that all of them are probably spuriousdetections; we conclude that in our sample all significantly rotatingstars are active, and all active stars are significantly rotating. Therotation-activity relation is valid in partially and in fully convectivestars. Thus, we do not observe any evidence for a transition from arotationally dominated dynamo in partially convective stars to arotation-independent turbulent dynamo in fully convective stars;turbulent dynamos in fully convective stars of spectral types around M4are still driven by rotation. Finally, we compare projected rotationalvelocities of 33 stars to rotational periods derived from photometry inthe literature and determine inclinations for a few of them.
| Near-infrared Spectroscopy of TW Hya: A Revised Spectral Type and Comparison with Magnetospheric Accretion Models
We present high signal-to-noise ratio (S/N), moderate spectralresolution (R ~ 2000-2500) near-infrared (0.8-5.0 ?m) spectroscopy ofthe nearby T Tauri star TW Hya. By comparing the spectrum and theequivalent widths of several atomic and molecular features with thosefor stars in the IRTF near-infrared library, we revise the spectral typeto M2.5V, which is later than what is usually adopted (K7V). Thisimplies a substantially cooler stellar temperature than previouslyassumed. Comparison with various pre-main-sequence models suggests thatTW Hya is only ~3 Myr old, much younger than the usually adopted 8-10Myr. Analysis of the relative strengths of the H lines seen in thespectrum yields estimates for the temperature and density of theemitting region of Te >= 7500 K and ne ~1012-1013 cm-3. The thickness of theemitting region is 102-104 km and the coveringfraction is f * ~ 0.04. Our derived physical parameter valuesagree with the predictions of the magnetospheric accretion scenario. Thehighest S/N H lines have profiles that indicate multiple emissioncomponents. We derive an excess spectrum (above that of the M2.5Vtemplate) that peaks in the H band. Although our derived veiling values(~0.1) agree with previous estimates, the excess spectrum does not matchthat of current models in which this flux is generated by an inneroptically thin disk. We suggest that the excess flux spectrum insteadreflects the differences in atmospheric opacity, gravity, and agebetween TW Hya and older, higher gravity, field M2.5 dwarfs.
| 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.
| The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars
We present a 0.8-5 μm spectral library of 210 cool stars observed ata resolving power of R ≡ λ/Δλ ~ 2000 with themedium-resolution infrared spectrograph, SpeX, at the 3.0 m NASAInfrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars havewell-established MK spectral classifications and are mostly restrictedto near-solar metallicities. The sample not only contains the F, G, K,and M spectral types with luminosity classes between I and V, but alsoincludes some AGB, carbon, and S stars. In contrast to some otherspectral libraries, the continuum shape of the spectra is measured andpreserved in the data reduction process. The spectra are absolutely fluxcalibrated using the Two Micron All Sky Survey photometry. Potentialuses of the library include studying the physics of cool stars,classifying and studying embedded young clusters and optically obscuredregions of the Galaxy, evolutionary population synthesis to studyunresolved stellar populations in optically obscured regions of galaxiesand synthetic photometry. The library is available in digital form fromthe IRTF Web site.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Further observations of Hipparcos red stars and standards for UBV(RI)C photometry
We present homogeneous and standardized UBV(RI)C JHKphotometry for over 100 M stars selected from an earlier paper on thebasis of apparent photometric constancy. L photometry has been obtainedfor stars brighter than about L = 6. Most of the stars have asubstantial number of UBV(RI)C observations and, it is hoped,will prove useful as red supplementary standards. Additionally, we listJHK photometry for nearly 300 Hipparcos red stars not selected asstandards, as well as L photometry for the brightest stars.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Speckle interferometry of nearby multiple stars. III.
Not Available
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| 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.
| The DA+dMe eclipsing binary EC13471-1258: its cup runneth over ... just
The optical spectrum and light curve of EC13471-1258 show that it is aneclipsing binary with an orbital period of 3h 37mcomprising a DA white dwarf and a dMe dwarf. Total eclipses of the whitedwarf are observed lasting 14 min, with the partial phases lasting 54 s.On one occasion, two pre-eclipse dips were seen. Timings of the eclipsesover 10 yr show jitter of up to 12 s. Flares from the M dwarf areregularly observed. The M dwarf also shows a large-amplitude ellipsoidalmodulation in the V-band light curve. The component stars emit almostequal amounts of light at 5500 Å.Hubble Space Telescope (HST) STIS spectra show strong Lyman-αabsorption with weak metal lines of C I,II and Si II superimposed. Modelatmosphere analysis yielded an effective temperature of 14 220 +/- 300 Kand logg of 8.34 +/- 0.20 for the white dwarf with these errors beingstrongly correlated. Its metal abundance is 1/30th solar with anuncertainty of 0.5 dex, and it is rapidly rotating with V1sin i= 400 +/- 100 km s-1. The white dwarf also shows radialvelocity variations with a semi-amplitude of 138 +/- 10 kms-1. The gravitational redshift of the white dwarf wasmeasured as 62 km s-1.From optical spectroscopy the spectral type of the M dwarf was found tobe M3.5-M4, its temperature 3100 +/- 75 K, its rotational velocity 140+/- 10 km s-1, its radial velocity semi-amplitude 266 +/- 5km s-1, its mean V-I colour 2.86 and its absolute V magnitude11.82. Intriguingly, its metal abundance is normal solar.The Hα emission line shows at least two distinct components, oneof which is uniformly distributed around the centre of mass of the Mdwarf and provided the estimate of the rotational velocity of the Mdwarf. The other arises from the other side of the binary centre ofmass, well within the white dwarf Roche lobe. This behaviour isconfirmed by Doppler tomography, which shows the presence of twodistinct velocity components within the primary Roche lobe. Theinterpretation of these features is uncertain. Variations in strength ofthe components with binary phase can be attributed to optical thicknessin the Balmer lines. Similar behaviour is seen in the observations ofthe other Balmer emission lines, although with a poorer signal-to-noiseratio. Flares in Hα were observed and are consistent with arisingfrom the vicinity of the M dwarf.Dynamical solutions for the binary are discussed and yield aninclination of 75.5°+/- 2.0o, a white dwarf mass andradius of 0.78 +/- 0.04 Msolar and 0.011 +/- 0.01Rsolar, and an M dwarf mass and radius of 0.43 +/- 0.04Msolar and 0.42 +/- 0.02 Rsolar. These parametersare consistent with the Wood mass-radius relation for white dwarfs andthe Clemens et al. mass-radius relation for M dwarfs; we argue that theM dwarf just fills its Roche lobe. The radius of the white dwarf and themodel fit imply a distance of 48 +/- 5 pc and an absolute V magnitude of11.74.The rapid rotation of the white dwarf strongly suggests that the systemhas undergone mass transfer in the past, and implies that it is ahibernating cataclysmic variable. The M dwarf shows the propertiesexpected of secondaries in cataclysmic variables: chromospheric activityand angular momentum loss.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| 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.
| Meeting the Cool Neighbors. II. Photometry of Southern NLTT Stars
We present BVRI photometry of 180 bright, southern nearby-starcandidates. The stars were selected from the New Luyten Two-Tenthsproper-motion catalog based on optical/infrared colors, constructed bycombining Luyten's mr estimates with near-infrared photometryfrom the Two Micron All Sky Survey. Photometric parallaxes derived fromV-Ks, V-I, and I-J colors, combined with the limitedavailable astrometry, show that as many as 108 stars may lie within 20pc of the Sun. Of these, 53 are new to nearby-star catalogs, includingthree within 10 pc of the Sun.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus
By combining deep optical imaging and infrared spectroscopy with datafrom the Two-Micron All-Sky Survey (2MASS) and from previous studies(e.g., Briceño et al.), I have measured the initial mass function(IMF) for a reddening-limited sample in four fields in the Taurusstar-forming region. This IMF is representative of the young populationswithin these fields for masses above 0.02 Msolar. Relative tothe similarly derived IMF for the Trapezium Cluster (Luhman et al.), theIMF for Taurus exhibits a modest deficit of stars above 1 solar mass(i.e., steeper slope), the same turnover mass (~0.8 Msolar),and a significant deficit of brown dwarfs. If the IMF in Taurus were thesame as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02Msolar) are expected in these Taurus fields where only onebrown dwarf candidate is found. These results are used to test theoriesof the IMF. Visiting Astronomer, Kitt Peak National Observatory,National Optical Astronomy Observatories, which is operated by theAssociation of Universities for Research in Astronomy, Inc. (AURA),under cooperative agreement with the National Science Foundation.
| New neighbours. I. 13 new companions to nearby M dwarfs
We present preliminary results of a long-term radial-velocity search forcompanions to nearby M dwarfs, started in September 95. The observedsample is volume-limited, and defined by the 127 northern(δ>-16(deg) ) M dwarfs listed in the Gliese and Jahreiss (CNS3)catalogue with d<=9 pc and V<=15. Observations are obtained withthe ELODIE spectrograph on the 1.93-m telescope of the Observatoire deHaute-Provence. The typical accuracy ranges between 10 {m s(-1}) (theinstrumental stability limit) for the brighter stars and 70 {m s(-1}) atour limiting magnitude. We complement the ELODIE velocities with oldermeasurements extracted from the CORAVEL database to extend our timebase, albeit obviously with lower precision. Simultaneously, we performadaptive optics imaging at CFHT and ESO to look for close(a>0.05-0.1'') visual companions in a larger volume-limited sample.For stellar companions the two techniques together cover the fullseparation range, to beyond the limiting distance of the sample. We willtherefore eventually obtain a statistically meaningful inventory of thestellar multiplicity of nearby M-dwarf systems. We also have usefulsensitivity to giant planets, as illustrated by our recent detection ofa planetary companion to Gl 876. After 2.5 years, we have discovered 12previously unknown components in this 127 stars sample, plus a companionto an additional star beyond its southern declination limit. 7 of theseare actually beyond the 9 pc limit, as they belong to systems includedin the sample on the basis of CNS3 photometric parallaxes which werebiased-down by the unrecognized companion. The remaining 5 companionsare true additions to the 9 pc inventory. More are certainlyforthcoming, given our present selection bias towards short periods andrelatively massive companions. We have derived orbital elements for 7 ofthe new systems, as well as for some known binaries with previouslyundetermined orbits. One system, G 203-47, associates an M3.5V star witha white dwarf in a rather tight orbit (a_1sin i = 15Rsol ) andrepresents a Post-Common-Envelope system. Some of the new M-dwarfbinaries will over the next few years provide very precise massdeterminations, and will thus better constrain the still poorlydetermined lower main-sequence mass-luminosity relation. The first suchresults are now being obtained, with some preliminary accuracies thatrange between 2% at 0.4-0.6 {M_sun} and 10% at 0.1 {M_sun}. We have alsodiscovered the third known detached M-dwarf eclipsing binary, anddetermined its masses with 0.4% accuracy. Partly based on observationsmade at Observatoire de Haute-Provence, operated by the Centre Nationalde la Recherche Scientifique de France and on observations made atCanada-France-Hawaii Telescope, operated by the National ResearchCouncil of Canada, the Centre National de la Recherche Scientifique deFrance and the University of Hawaii.
| Spectroscopy of Brown Dwarf Candidates in the rho Ophiuchi Molecular Core
We present an analysis of low-resolution infrared spectra for 20 browndwarf candidates in the core of the rho Ophiuchi molecular cloud.Fifteen of the sources display absorption-line spectra characteristic oflate-type stars. By comparing the depths of water vapor absorption bandsin our candidate objects with a grid of M dwarf standards, we derivespectral types that are independent of reddening. Optical spectroscopyof one brown dwarf candidate confirms the spectral type derived from thewater bands. Combining their spectral types with published near-infraredphotometry, effective temperatures and bolometric stellar luminositiesare derived, enabling us to place our sample on the Hertzsprung-Russelldiagram. We compare the positions of the brown dwarf candidates in thisdiagram with two sets of theoretical models in order to estimate theirmasses and ages. Considering uncertainties in placing the candidates inthe H-R diagram, six objects consistently lie in the brown dwarf regimeand another five objects lie in the transition region between stellarand substellar objects. The ages inferred for the sample are consistentwith those derived for higher mass association members. Three of thenewly identified brown dwarfs display infrared excesses at lambda=2.2mum, suggesting that young brown dwarfs can have active accretion disks.Comparing our mass estimates of the brown dwarf candidates with thosederived from photometric data alone suggests that spectroscopy is anessential component of investigations of the mass functions of youngclusters.
| Infrared Colors at the Stellar/Substellar Boundary
We present new infrared JHK photometry for 61 halo and disk stars aroundthe stellar/substellar boundary. We also present new L' photometry for21 of these stars and for 40 low-mass stars taken from the Leggettphotometry compilation. These data are combined with available opticalphotometry and astrometric data to produce color-color and absolutemagnitude-color diagrams; the current sample extends the similar workpresented in the 1992 paper into more metal-poor and lower mass regimes.The disk and halo sequences are compared to the predictions of thelatest model atmospheres and structural models. We find good agreementbetween observation and theory except for known problems in the V and Hpassbands, probably due to incomplete molecular data for TiO, metalhydrides, and H_2O. The metal-poor M subdwarfs are well matched by themodels since oxide opacity sources are less important in this case. Theknown extreme M subdwarfs have metallicities about one-hundredth solar,and the coolest subdwarfs have T_eff ~ 3000 K with masses~0.09M/M_ȯ. The grainless models are not able to reproduce the fluxdistributions of disk objects with T_eff < 2500 K; however, apreliminary version of the NextGen-Dusty models that includeshomogeneous formation and extinction by dust grains is able to match thecolors of these very cool objects. The least luminous objects in thissample are GD 165B, three DENIS objects-DBD 0205, DBD 1058, and DBD1228-and Kelu-1. These have T_eff ~ 2000 K and are at or below thestellar limit with masses <=0.075M/M_ȯ. Photometry alone cannotconstrain these parameters further since the age is unknown, butpublished lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass<= 0.06M/M_ȯ).
| Rotation and chromospheric activity in field M dwarfs
We have obtained high resolution spectra for a volume-limited sample of118 field M dwarfs. From these observations we derive projectedrotational velocities and fluxes in the H_alpha and H_beta lines. 8stars are double-lined spectroscopic binaries with measured or probableperiods short enough for rotation to be tidally synchronized with theorbit, and another 11 are visual binaries where we cannot yet separatethe lines of the two stars. Of the remaining 99 stars, 24 haverotational velocities above our detection limit of ~ 2 km.s(-1) , andsome are quite fast rotators, including two with v sin i\ =~ 30 km.s(-1)and one with v sin i\ =~ 50 km.s(-1) . Given the small radii of Mdwarfs, these moderate rotational velocities correspond to rather shortmaximum rotational periods, of only 7-8 hours. These three stars aregood candidates for Doppler imaging. We find that rotation is stronglycorrelated with both spectral type and kinematic population: all starswith measurable rotation are later than M3.5, and all but one havekinematic properties typical of the young disk, or intermediate betweenthe young disk and the the old disk. We interpret this correlation asevidence for a spin-down timescale that increases with decreasing mass.At the age of the old disk or halo, all stars earlier than M5-M6(0.1-0.15Msun) have spun-down to below our detection limit,while at the age of the young disk this has only happened for starsearlier than M3.5. The one star with measurable rotation and akinematics intermediate between old disk and population II has spectraltype M6. It is probably an old star whose mass is low enough that it hasretained significant rotation up to present, still consistently withlonger spin-down times for lower mass stars. We observe, on the otherhand, no conspicuous change in the v sin i\ distribution or activitypattern at the mass (M ~ 0.35 Msun) below which stars remainfully convective down to the main sequence. These new data areconsistent with a saturated correlation between rotation and activity,similar to the one observed for younger or more massive stars:L_X/Lbol and L_{H_alpha }/Lbol both correlate withv sin i\ for v sin i\ -5km.s^{-1} and then saturate at respectively10^{-2.5} and 10^{-3.5}$. Based on observations made at the Observatoirede Haute-Provence (CNRS), France Tables 2 and 4 are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| An H-alpha survey of neglected Vyssotsky Catalog stars
Consideration is given to a significant number of Vyssotsky K- andM-dwarfs that have either no published radial velocity (71 stars), or avelocity based on a single measure (22), or with an uncertaintly greaterthan 5 km/s (19). An 'in flare' observation of Vys 250A compared to itsquiescent state, and an example of the double-lined phase of Vys 250Bare illustrated. Mean values from the observations are given in tabularform.
| A Spectroscopic Study of the Dwarf Nova KT Persei
We present H$\alpha$ radial velocities of KT Per taken during quiescence(1989 November) and outburst (1990 February). Both data sets indicate anorbital period near 3.90 h, confirming the result of Ratering, Bruch,\& Diaz (1993). We could not refine the period unambiguously;combining our velocities with those of Ratering, Bruch \& Diaz(1993) shows that the period is most likely near 0.1627 d, but a periodnear 0.1618 d is just possible if one forces coherence between the theoutburst and quiescent velocities. Another possibility is that the phaseof the emission line velocities in outburst lags that of the quiescentvelocities by $\sim 1/4$ cycle. We detect the secondary star, estimateits spectral type as M3, and from this derive a distance of 245$\pm 100$pc. After subtracting the secondary contribution, we find that thenear-infrared continuum steepens considerably during outburst. KT Perappears to be a normal dwarf nova, although its period is among theshorter dwarf nova periods longward of the 2- to 3-hour gap. Anabsorption feature near $\lambda$6280, which had been noted by Rateringet al., is identified as a telluric O$_2$ ban. (SECTION: Special Issueon Cataclysmic Variable Stars)
| 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 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
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