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Ýlgili Makaleler
Discovery of 11 New T Dwarfs in the Two Micron All Sky Survey, Including a Possible L/T Transition Binary
We present the discovery of 11 new T dwarfs, found during the course ofa photometric survey for mid-to-late T dwarfs in the 2MASS Point SourceCatalog and from a proper-motion-selected sample of ultracool dwarfs inthe 2MASS Working Database. Using the NASA Infrared Telescope FacilitySpeX spectrograph, we obtained low-resolution (R~150) spectroscopy,allowing us to derive near-infrared spectral types of T2-T8. We alsopresent improved signal-to-noise ratio SpeX low-resolution spectroscopyof the near-infrared T0 standard SDSS J120747.17+024424.8 and T1standard SDSSp J083717.22-000018.3. One of these new T dwarfs, 2MASSJ13243559+6358284, was also discovered independently by Metchev et al.This object is spectroscopically peculiar and possibly a binary and/orvery young (<300 Myr). We specifically attempted to model thespectrum of this source as a composite binary to reproduce its peculiarspectral characteristics. The latest type object in our sample is a T8dwarf, 2MASS J07290002-3954043, now one of the four latest type T dwarfsknown. All 11 T dwarfs are nearby given their spectrophotometricdistance estimates, with one T dwarf within 10 pc and eight additional Tdwarfs within 25 pc, if single. These new additions increase the 25 pccensus of T dwarfs by ~14%. Their proximity offers an excellentopportunity to probe for companions at separations closer than arepossible for more distant T dwarfs.
| U Geminorum: A Test Case for Orbital Parameter Determination
High-resolution spectroscopy of U Gem was obtained during quiescence. Wedid not find a hot spot or gas stream around the outer boundaries of theaccretion disk. Instead, we detected a strong narrow emission regionnear the location of the secondary star. We measured the radial velocitycurve from the wings of the double-peaked Hα emission line andobtained a semiamplitude value that is in excellent agreement with thatobtained from observations in the ultraviolet spectral region by Sion etal. We also present a new method to obtain K2, which enhancesthe detection of absorption or emission features arising in thelate-type companion. Our results are compared with published valuesderived from the near-infrared Na I line doublet. From a comparison ofthe TiO band with those of late-type M stars, we find that a best fit isobtained for an M6 V star, contributing 5% of the total light at thatspectral region. Assuming that the radial velocity semiamplitudesaccurately reflect the motion of the binary components, then from ourresults Kem=107+/-2 km s-1 andKabs=310+/-5 km s-1 using the inclination anglegiven by Zhang & Robinson, i=69.7deg+/-0.7deg,the system parameters become MWD=1.20+/-0.05Msolar, MRD=0.42+/-0.04 Msolar, anda=1.55+/-0.02 Rsolar. Based on the separation of the doubleemission peaks, we calculate an outer disk radius ofRout/a~0.61, close to the distance of the inner Lagrangianpoint L1/a~0.63. Therefore, we suggest that, at the time ofobservations, the accretion disk was filling the Roche lobe of theprimary and the matter leaving the L1 point was colliding with the diskdirectly, producing the hot spot at this location.
| Hubble Space Telescope NICMOS Observations of NGC 1333: The Ratio of Stars to Substellar Objects
We present an analysis of NICMOS photometry and low-resolution grismspectroscopy of low-mass stars and substellar objects in the youngstar-forming region NGC 1333. Our goal is to constrain the ratio oflow-mass stars to substellar objects down to 20MJup in thecluster, as well as constrain the cluster initial mass function (IMF)down to 30MJup, in combination with a previous survey of NGC1333 by Wilking et al. Our survey covers four fields of 51.2" ×51.2", centered on brown dwarf candidates previously identified inWilking et al. We extend previous work based on the use of a water vaporindex for spectral typing to wavelengths accessible with NICMOS on theHubble Space Telescope. Spectral types were derived for the 14 brightestobjects in our fields, ranging from <=M0 to M8, which, at the age ofthe cluster (0.3 Myr), correspond to a range in mass of >=0.25-0.02Msolar. In addition to the spectra, we present an analysis ofthe color-magnitude diagram using pre-main-sequence evolutionary modelsof D'Antona & Mazzitelli. Using an extinction-limited sample, wederive the ratio of low-mass stars to brown dwarfs. Comparisons of theobserved ratio to that expected from the field IMF of Chabrier indicatethat the two results are consistent. We combine our data with those ofWilking et al. to compute the ratio of intermediate-mass stars (0.1-1.0Msolar) to low-mass objects (0.03-0.1 Msolar) inthe cluster. We also report the discovery of a faint companion to thepreviously confirmed brown dwarf ASR 28, as well as a possible outflowsurrounding ASR 16. If the faint companion is confirmed as a clustermember, it would have a mass of ~5MJup (mass ratio 0.15) at aprojected distance of 350 AU, similar to that of 2MASS 1207-3923B.
| An in-depth study of the pre-polar candidate WX Leonis Minoris
Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-rayobservations with full phase coverage are used for an in-depth study ofWX LMi, a system formerly termed a low-accretion rate polar. We find aconstant low-mass accretion rate, dot{M} ˜ 1.5 ×10-13 M_ȯ yr-1, a peculiar accretion geometrywith one spot not accessible via Roche-lobe overflow, a low temperatureof the white dwarf, T_eff< 8000 K, and the secondary very likelyRoche-lobe underfilling. All this lends further support to the changedview on WX LMi and related systems as detached binaries, i.e. magneticpost-common envelope binaries without significant Roche-lobe overflow inthe past. The transfer rate determined here is compatible with accretionfrom a stellar wind. We use cyclotron spectroscopy to determine theaccretion geometry and to constrain the plasma temperatures. Bothcyclotron spectroscopy and X-ray plasma diagnostics reveal low plasmatemperatures below 3 keV on both accretion spots. For the low-dot{m},high-B plasma at the accretion spots in WX LMi, cyclotron coolingdominates thermal plasma radiation in the optical. Optical spectroscopyand X-ray timing reveal atmospheric, chromospheric, and coronal activityat the saturation level on the dM4.5 secondary star.
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Testing Theoretical Evolutionary Models with AB Doradus C and the Initial Mass Function
We assess the constraints on the evolutionary models of young low-massobjects that are provided by the measurements of the companion AB Dor Cby Close and coworkers and by a new comparison of model-derived IMFs ofstar-forming regions to the well-calibrated IMF of the solarneighborhood. After performing an independent analysis of all of theimaging and spectroscopic data for AB Dor C that were obtained by Close,we find that AB Dor C (which has no methane) is not detected at asignificant level (S/N~1.2) in the SDI data when one narrowband image issubtracted from another but that it does appear in the individual SDIframes, as well as the images at J, H, and Ks. Although ourbroadband photometry for AB Dor C is consistent with that of Close, theuncertainties that we measure are larger. Using the age of τ=75-150Myr recently estimated for AB Dor by Luhman and coworkers, theluminosity predicted by the models of Chabrier and Baraffe is consistentwith the value that we estimate from the photometry for AB Dor C. Wemeasure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C,which is earlier than the value of M8+/-1 reported by Close and isconsistent with the model predictions when a dwarf temperature scale isadopted. In a test of these evolutionary models at much younger ages, weshow that the low-mass IMFs that they produce for star-forming regionsare similar to the IMF of the solar neighborhood. If the masses of thelow-mass stars and brown dwarfs in these IMFs of star-forming regionswere underestimated by a factor of 2 as suggested by Close, then the IMFcharacterizing the current generation of Galactic star formation wouldhave to be radically different from the IMF of the solar neighborhood.Based on observations made with ESO telescopes at the ParanalObservatories under program ID 60.A-9026. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a jointproject of the University of Massachusetts and the Infrared Processingand Analysis Center/California Institute of Technology, funded by theNational Aeronautics and Space Administration and the National ScienceFoundation.
| Three active M dwarfs within 8 pc: L 449-1, L 43-72, and LP 949-15
Three nearby star candidates were selected as bright 2MASS point sourceswithout associated optical sources, i.e. with potentially large propermotions, subsequently confirmed by multi-epoch optical data from theSuperCOSMOS Sky Surveys. All three objects are listed in the NLTTcatalogue of high proper motion stars. Follow-up spectroscopicobservations allowed us to classify L 449-1 as M 4.0e dwarf at 5.7 pc, L43-72 as M 4.5e dwarf at 7.2 pc, and LP 949-15 as M 5.0e dwarf at 6.1pc, using known absolute JHKs magnitudes of dwarfs withcorresponding spectral types, respectively. All three stars exhibitHα emission lines, and all three can be identified with brightX-ray sources. The available ROSAT light curves of two of the objectsshow typical flare-like variability. Thus, all three stars are active,very close potential neighbours of the Sun, previously not mentioned assuch, certainly deserving further attention. In particular, these arevery promising candidates for trigonometric parallax programs.
| The masses, radii and luminosities of the components of U Geminorum
We present a phase-resolved spectroscopic study of the secondary star inthe cataclysmic variable (CV) U Gem. We use our data to measure theradial velocity semi-amplitude, systemic velocity and rotationalvelocity of the secondary star. Combining this with literature dataallows us to determine masses and radii for both the secondary star andwhite dwarf, which are independent of any assumptions about theirstructure. We use these to compare their properties to those of fieldstars and find that both components follow field mass-radiusrelationships. The secondary star has the mass, radius, luminosity andphotometric temperature of an M2 star, but a spectroscopic temperatureof M4. The latter may well be due to a high metallicity. There is atroubling inconsistency between the radius of the white dwarf inferredfrom its gravitational redshift and inclination and that inferred fromits temperature, flux and astrometric distance.We find that there are two fundamental limits to the accuracy of theparameters we can derive. First, the radial velocity curve of thesecondary star deviates from a sinusoid, in part because of itsasphericity (which can be modelled) and in part because the line flux isnot evenly distributed over its surface. Secondly, we cannot be certainwhich spectral type is the best match for the lines of the secondarystar, and the derived rotational velocity is a function of the spectraltype of the template star used.
| An Infrared Spectroscopic Sequence of M, L, and T Dwarfs
We present a 0.6-4.1 μm spectroscopic sequence of M, L, and T dwarfs.The spectra have R≡λ/Δλ~2000 from 0.9 to 2.4μm and R=2500-200 from 2.9 to 4.1 μm. These new data nearly doublethe number of L and T dwarfs that have reported L-band spectra. Thenear-infrared spectra are combined with previously published red-opticalspectra to extend the wavelength coverage to ~0.6 μm. Prominentatomic and molecular absorption features are identified includingneutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weakCH4 absorption features in the H-band spectra of mid- tolate-type T dwarfs. In addition, we detect for the first time the 0-0band of the A 4Π-X 4Σ-transition of VO at ~1.06 μm in the spectra of L dwarfs and the P-and R-branches of the ν3 band of CH4 in thespectrum of a T dwarf. The equivalent widths of the refractory atomicfeatures all decrease with increasing spectral type and are absent by aspectral type of ~L0, except for the 1.189 μm Fe I line, whichpersists to at least ~L3. We compute the bolometric luminosities of thedwarfs in our sample with measured parallaxes and find good agreementwith previously published results that use L'-band photometry to accountfor the flux emitted from 2.5 to 3.6 μm. Finally, 2MASSJ2224381-0158521 (L4.5) has an anomalously red spectrum and thestrongest Δν=+2 CO bands in our sample. This may be indicativeof unusually thick condensate clouds and/or low surface gravity.Based in part on data collected at Subaru telescope, which is operatedby the National Astronomical Observatory of Japan.
| The χ Factor: Determining the Strength of Activity in Low-Mass Dwarfs
We describe a new, distance-independent method for calculating themagnetic activity strength in low-mass dwarfs,LHα/Lbol. Using a well-observed sample ofnearby stars and cool standards spanning spectral type M0.5 to L0, wecompute χ, the ratio between the continuum flux near Hα andthe bolometric flux, fλ6560/fbol. Thisratio can be multiplied by the measured equivalent width of the Hαemission line to yield LHα/Lbol. We provideχ values for all objects in our sample, and also fits to χ as afunction of color and average values by spectral type. This method wasused by West et al. to examine trends in magnetic activity strength inlow-mass stars.
| NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
We present a final summary of all ROSAT X-ray observations of nearbystars. All available ROSAT observations with the ROSAT PSPC, HRI and WFChave been matched with the CNS4 catalog of nearby stars and the resultsgathered in the Nearby X-ray and XUV-emitting Stars data base, availablevia www from the Home Page of the Hamburger Sternwarte at the URLhttp://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. Newvolume-limited samples of F/G-stars (dlim = 14 pc), K-stars(dlim = 12 pc), and M-stars (dlim = 6 pc) areconstructed within which detection rates of more than 90% are obtained;only one star (GJ 1002) remains undetected in a pointed follow-upobservation. F/G-stars, K-stars and M-stars have indistinguishablesurface X-ray flux distributions, and the lower envelope of the observeddistribution at FX ≈ 104 erg/cm2/sis the X-ray flux level observed in solar coronal holes. Large amplitudevariations in X-ray flux are uncommon for solar-like stars, but maybemore common for stars near the bottom of the main sequence; a largeamplitude flare is reported for the M star LHS 288. Long term X-raylight curves are presented for α Cen A/B and Gl 86, showingvariations on time scales of weeks and demonstrating that α Cen Bis a flare star.Tables 1-3 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/651
| Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars
We present the full target list and prioritization algorithm developedfor use by the microwave search for technological signals at the SETIInstitute. We have included the Catalog of Nearby Habitable StellarSystems (HabCat, described in Paper I), all of the nearest 100 stars and14 old open clusters. This is further augmented by a subset of theTycho-2 catalog based on reduced proper motions, and this larger catalogshould routinely provide at least three target stars within the largeprimary field of view of the Allen Telescope Array. The algorithm forprioritizing objects in the full target list includes scoring based onthe subset category of each target (i.e., HabCat, cluster, Tycho-2, ornearest 100), its distance (if known), and its proximity to the Sun onthe color-magnitude diagram.
| Rotation and Activity in Mid-M to L Field Dwarfs
We analyze rotation velocities and chromospheric (Hα) activity,derived from high-resolution spectra, in a large sample of mid-M to Lfield dwarfs. The projected rotation velocity is found to increase frommid-M to L. This is consistent with a lengthening of spin-down timescalewith later type, although in the L types the trend may also be afunction of the observational bias toward younger objects. From M4 toM8.5 a saturation-type rotation-activity relation is seen, similar tothat in earlier types, when activity is measured through eitherFHα or LHα/Lbol. However,we find that activity saturates at a significantly higher velocity (~10km s-1) in the M5.5-M8.5 dwarfs than in the M4-M5 ones(<~4 km s-1). This may result from a change in the dynamobehavior with later type (see also below). We note that the saturationlevel in Hα emission appears to vary somewhat less with spectraltype (from M4 to M8.5) when activity is measured throughLHα/Lbol instead of FHα. InM9 and later dwarfs, we observe a drastic drop in activity and a sharpbreak in the rotation-activity connection: Hα emission levels inthese dwarfs are much lower than in earlier types, and oftenundetectable, in spite of very rapid rotation. This may be caused by thevery high resistivities in the predominantly neutral atmospheres ofthese dwarfs, which would damp the magnetic energy available forsupporting a chromosphere. It is also possible that the rapid formationof dust in these cool atmospheres exacerbates this effect, as chargedparticles are soaked up by (more massive) dust grains. Finally, we notethat spectral type determination from low-resolution spectra may beaffected by gravity effects: cooler, lower gravity objects may mimichotter, higher gravity ones. Therefore, it is possible that the fewunsaturated fast rotators from M5.5 to M8.5 (whose presence leads us toascribe a higher saturation velocity to these spectral types, as notedabove) may actually be very low mass objects, with lower Teff(and gravity) than their spectral types suggest. If so, their behavior(low activity, fast rotation) would be compatible with that of the coolM9 and later dwarfs (and no change in dynamo behavior would have to bepostulated in the M5.5-M8.5 dwarfs). This interpretation is supported bya preliminary analysis of the high-resolution spectra of these anomolousobjects. It is also bolstered by the fact that a saturation-type Rossbynumber-activity relation is seen in the M5.5-M8.5 dwarfs when theseanomalous objects are removed from the sample, while the relationship ismuch weaker when they are included.
| 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.
| 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.
| Component Masses of the Young Spectroscopic Binary UZ Tau E
We report estimates of the masses of the component stars in thepre-main-sequence spectroscopic binary UZ Tau E. These results come fromthe combination of our measurements of the mass ratio,M2/M1=0.28+/-0.01, obtained using high-resolutionH-band spectroscopy, with the total mass of the system,(1.31+/-0.08)(D/140 pc) Msolar, derived from millimeterobservations of the circumbinary disk (Simon, Dutrey, & Guilloteau).The masses of the primary and secondary are (1.016+/-0.065)(D/140 pc)Msolar and (0.294+/-0.027)(D/140 pc) Msolar,respectively. Using the orbital parameters determined from our sixepochs of observation, we find that the inclination of the binary orbit,59.8d+/-4.4d, is consistent with that determined for the circumbinarydisk from the millimeter observations, indicating that the disk andbinary orbits are probably coplanar.
| 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.
| Characterization of M, L, and T Dwarfs in the Sloan Digital Sky Survey
An extensive sample of M, L, and T dwarfs identified in the SloanDigital Sky Survey (SDSS) has been compiled. The sample of 718 dwarfsincludes 677 new objects (629 M dwarfs and 48 L dwarfs), together with41 that have been previously published. All new objects and some of thepreviously published ones have new optical spectra obtained either withthe SDSS spectrographs or with the Apache Point Observatory 3.5 m ARCtelescope. Spectral types and SDSS colors are available for all objects;approximately 35% also have near-infrared magnitudes measured by 2MASS(Two Micron All Sky Survey) or on the Mauna Kea system. We use thissample to characterize the color-spectral type and color-color relationsof late-type dwarfs in the SDSS filters and to derive spectroscopic andphotometric parallax relations for use in future studies of theluminosity and mass functions based on SDSS data. We find that thei*-z* and i*-J colors provide goodspectral type and absolute magnitude (Mi*) estimates for Mand L dwarfs. Our distance estimates for the current sample indicatethat SDSS is finding early M dwarfs out to ~1.5 kpc, L dwarfs to ~100pc, and T dwarfs to ~20 pc. The T dwarf photometric data show largescatter and are therefore less reliable for spectral type and distanceestimation. Based on observations obtained with the Sloan Digital SkySurvey and the Apache Point Observatory 3.5 m telescope, which are ownedand operated by the Astrophysical Research Consortium.
| 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.
| The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high-resolution near-infrared spectroscopy with the KeckTelescope, we have detected the radial velocity signatures of the coolsecondary components in four optically identified pre-main-sequence,single-lined spectroscopic binaries. All are weak-lined T Tauri starswith well-defined center-of-mass velocities. The mass ratio for oneyoung binary, NTTS 160905-1859, isM2/M1=0.18+/-0.01, the smallest yet measureddynamically for a pre-main-sequence spectroscopic binary. These newresults demonstrate the power of infrared spectroscopy for the dynamicalidentification of cool secondaries. Visible-light spectroscopy, to date,has not revealed any pre-main-sequence secondary stars with masses<0.5 Msolar, while two of the young systems reported hereare in that range. We compare our targets with a compilation of thepublished young, double-lined spectroscopic binaries and discuss ourunique contribution to this sample.
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| 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.
| A Coronagraphic Survey for Companions of Stars within 8 Parsecs
We present the technique and results of a survey of stars within 8 pc ofthe Sun with declinations δ>-35° (J2000.00). The survey,designed to find without color bias faint companions, consists ofoptical coronagraphic images of the 1' field of view centered on eachstar and infrared direct images with a 32" field of view. The imageswere obtained through the optical Gunn r and z filters and the infraredJ and K filters. The survey achieves sensitivities up to 4 absolutemagnitudes fainter than the prototype brown dwarf, Gliese 229B. However,this sensitivity varies with the seeing conditions, the intrinsicbrightness of the star observed, and the angular distance from the star.As a result, we tabulate sensitivity limits for each star in the survey.We used the criterion of common proper motion to distinguish companionsand to determine their luminosities. In addition to the brown dwarf Gl229B, we have identified six new stellar companions of the sample stars.Since the survey began, accurate trigonometric parallax measurements formost of the stars have become available. As a result, some of the starswe originally included should no longer be included in the 8 pc sample.In addition, the 8 pc sample is incomplete at the faint end of the mainsequence, complicating our calculation of the binary fraction of browndwarfs. We assess the sensitivity of the survey to stellar companionsand to brown dwarf companions of different masses and ages.
| High proper motion stars in Kapteyn Selected Area 94
We have applied the astrometric techniques devised by Murray foranalysis of 48-inch Schmidt photographic data to SuperCOSMOS scans of UKSchmidt plates centred on Kapteyn's Selected Area 94(α=2h 53m, δ=0°). In thispreliminary study, we combine astrometric data from four short-exposureV-band plates, taken in 1987 August (2 plates) and 1993 August (2plates), with BVRI photometry from sky-limited plate material, toidentify stars with proper motions exceeding 0.1arcsecyr-1.This paper discusses the completeness of the resulting sample andpresents spectroscopy of 30 stars with μ>0.2arcsecyr-1.Based on the latter observations and the distribution in the[HV, (V-I)] reduced proper-motion diagram, we have classifiedstars in the complete sample as candidate white dwarfs, main-sequencedwarfs and halo subdwarfs, and derived estimates of the disc and haloluminosity functions.
| From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348
We investigate the low-mass population of the young cluster IC 348 downto the deuterium-burning limit, a fiducial boundary between brown dwarfand planetary mass objects, using a new and innovative method for thespectral classification of late-type objects. Using photometric indices,constructed from HST/NICMOS narrowband imaging, that measure thestrength of the 1.9 μm water band, we determine the spectral type andreddening for every M-type star in the field, thereby separating clustermembers from the interloper population. Due to the efficiency of ourspectral classification technique, our study is complete from ~0.7 to0.015 Msolar. The mass function derived for the cluster inthis interval, dN/dlogM~M0.5, is similar to that obtained forthe Pleiades, but appears significantly more abundant in brown dwarfsthan the mass function for companions to nearby Sunlike stars. Thisprovides compelling observational evidence for different formation andevolutionary histories for substellar objects formed in isolation versusas companions. Because our determination of the IMF is complete to verylow masses, we can place interesting constraints on the role of physicalprocesses such as fragmentation in the star and planet formation processand the fraction of dark matter in the Galactic halo that resides insubstellar objects. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| A systematic analysis of X-ray variability of dM stars
We have systematically analyzed X-ray variability of dM stars. Our database is the sample of all dM stars listed in the CNS3 (Gliese &Jahreiss 1991) catalog which have been observed with the ROSAT PSPC. Ourdata sample includes 86 pointed observations of 55 distinct stars ormultiple systems. A large fraction of stars shows significantvariations, regardless of their quiescent flux. Variability is detectedon all observable time scales. The amplitudes of these variations areindependent of both stellar X-ray and visual luminosity. Compared tosolar X-ray variability properties our results suggest that theamplitude distribution of X-ray variability in dM stars is consistentwith the analogous distribution for solar flares. We discuss the effectof variability on the spread observed in the X-ray luminosity functionof M stars. The comparison of our data with those obtained with EinsteinIPC shows that variations on time scales shorter than a few month aremore common than long term variations comparable to, e.g., the 11 yearssolar cycle.
| Photometric modelling of starspots - I. A Barnes-Evans-like surface brightness-colour relation using (Ic-K)
In the first part of this work, the empirical correlation of stellarsurface brightness FV with (Ic-K) broad-bandcolour is investigated by using a sample of stars cooler than the Sun. Abilinear correlation is found to represent well the brightness of G, Kand M giant stars. The change in slope occurs at (Ic-K)~2.1or at about the transition from K to M spectral types. The samerelationship is also investigated for dwarf stars and found to bedistinctly different from that of the giants. The dwarf star correlationdiffers by an average of -0.4 in (Ic-K) or by a maximum inFV of ~-0.1, positioning it below that of the giants, withboth trends tending towards convergence for the hotter stars in oursample. The flux distribution derived from theFV-(Ic-K) relationship for the giant stars,together with that derived from an FV-(V-K) relationship andthe blackbody flux distribution, is then utilized to compute syntheticlight V and colour (V-R)c, (V-I)c and (V-K) curvesof cool spotted stars. We investigate the effects on the amplitudes ofthe curves by using these FV-colour relations and by assumingthe effective gravity of the spots to be lower than the gravity of theunspotted photosphere. We find that the amplitudes produced by using theFV-(Ic-K) relationship are larger than thoseproduced by the other two brightness correlations, meaning smallerand/or warmer spots.
| Young Low-Mass Stars and Brown Dwarfs in IC 348
I present new results from a continuing program to identify andcharacterize the low-mass stellar and substellar populations in theyoung cluster IC 348 (0.5-10 Myr). Optical spectroscopy has revealedyoung objects with spectral types as late as M8.25. The intrinsic J-Hand H-K colors of these sources are dwarflike, whereas the R-I and I-Jcolors appear intermediate between the colors of dwarfs and giants.Furthermore, the spectra from 6500 to 9500 Å are reproduced wellwith averages of standard dwarf and giant spectra, suggesting that suchaverages should be used in the classification of young late-typesources. An H-R diagram is constructed for the low-mass population in IC348 (K6-M8). The presumably coeval components of the young quadruplesystem GG Tau (White et al.) and the locus of stars in IC 348 are usedas empirical isochrones to test the theoretical evolutionary models. Thecalculations of Burrows et al. do not appear to be consistent with thedata at these earliest stages of stellar evolution. There is fairagreement between the data and the model isochrones of D'Antona &Mazzitelli, except near the hydrogen-burning limit. The agreement cannotbe improved by changing the conversion between spectral types andeffective temperatures. On the other hand, for the models of Baraffe etal., an adjustment of the temperature scale to progressively warmertemperatures at later M types, intermediate between dwarfs and giants,brings all components of GG Tau onto the same model isochrone and givesthe population of IC 348 a constant age and age spread as a function ofmass. When other observational constraints are considered, such as thedynamical masses of GM Aur, DM Tau, and GG Tau A, the models of Baraffeet al. are the most consistent with observations of young systems. Withcompatible temperature scales, the models of both D'Antona &Mazzitelli and Baraffe et al. suggest that the hydrogen-burning masslimit occurs near M6 at ages of <~10 Myr. Thus, several likely browndwarfs are discovered in this study of IC 348, with masses down to~20-30 MJ.
| Barnes-Evans relations for late-type giants and dwarfs
The visual surface brightness of K/M giants and dwarfs with near-solarmetallicity differ slightly in agreement with the gravity effectspredicted by recent theoretical models. We show that M-dwarfs displayalso a metallicity dependence of the surface brightness in the infraredK-band in agreement with theory. Based on these results, we presentimproved Barnes-Evans type relations and estimate the radii of 60 singleor presumed M and K-dwarfs.
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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| Takýmyýldýz: | Avci |
| Sað Açýklýk: | 05h42m09.28s |
| Yükselim: | +12°29'21.6" |
| Görünürdeki Parlaklýk: | 11.308 |
| Uzaklýk: | 5.788 parsek |
| özdevim Sað Açýklýk: | 2001.4 |
| özdevim Yükselim: | -1572.9 |
| B-T magnitude: | 13.084 |
| V-T magnitude: | 11.455 |
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