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Rotational Velocities for M Dwarfs
We present spectroscopic rotation velocities (v sin i) for 56 M dwarfstars using high-resolution Hobby-Eberly Telescope High ResolutionSpectrograph red spectroscopy. In addition, we have also determinedphotometric effective temperatures, masses, and metallicities ([Fe/H])for some stars observed here and in the literature where we couldacquire accurate parallax measurements and relevant photometry. We haveincreased the number of known v sin i values for mid M stars by around80% and can confirm a weakly increasing rotation velocity withdecreasing effective temperature. Our sample of v sin is peak at lowvelocities (~3 km s-1). We find a change in therotational velocity distribution between early M and late M stars, whichis likely due to the changing field topology between partially and fullyconvective stars. There is also a possible further change in therotational distribution toward the late M dwarfs where dust begins toplay a role in the stellar atmospheres. We also link v sin i to age andshow how it can be used to provide mid-M star age limits. When allliterature velocities for M dwarfs are added to our sample, there are198 with v sin i <= 10 km s-1 and 124 in themid-to-late M star regime (M3.0-M9.5) where measuring precision opticalradial velocities is difficult. In addition, we also search the spectrafor any significant Hα emission or absorption. Forty three percentwere found to exhibit such emission and could represent young, activeobjects with high levels of radial-velocity noise. We acquired twoepochs of spectra for the star GJ1253 spread by almost one month and theHα profile changed from showing no clear signs of emission, toexhibiting a clear emission peak. Four stars in our sample appear to below-mass binaries (GJ1080, GJ3129, Gl802, and LHS3080), with both GJ3129and Gl802 exhibiting double Hα emission features. The tablespresented here will aid any future M star planet search target selectionto extract stars with low v sin i.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| Radio Interferometric Planet Search. I. First Constraints On Planetary Companions For Nearby, Low-Mass Stars From Radio Astrometry
Radio astrometry of nearby, low-mass stars has the potential to be apowerful tool for the discovery and characterization of planetarycompanions. We present a Very Large Array survey of 172 active M dwarfsat distances of less than 10 pc. Twenty-nine stars were detectedwith flux densities greater than 100 μJy. We observed seven ofthese stars with the Very Long Baseline Array at milliarcsecondresolution in three separate epochs. With a detection threshold of500 μJy in images of sensitivity 1σ ~ 100 μJy, wedetected three stars three times (GJ 65B, GJ 896A, GJ 4247), one startwice (GJ 285), and one star once (GJ 803). Two stars were undetected(GJ 412B and GJ 1224). For the four stars detected in multiple epochs,residuals from the optically determined apparent motions have anroot-mean-square deviation of ~0.2 milliarcseconds, consistent withstatistical noise limits. Combined with previous optical astrometry,these residuals provide acceleration upper limits that allow us toexclude planetary companions more massive than 3-6 M Jup at adistance of ~1 AU with a 99% confidence level.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| Nondetection of the Neptune-Mass Planet Reported Around GJ 176
Endl et al. reported a Neptune-mass planet in a 10.24 day orbit aroundGJ 176. This planet has raised interest because of its low mass(Msin i = 24 M Earth), correspondingly smallvelocity amplitude(K = 11.7 m s-1), and becauseGJ 176 is an M star. We report 41 precise Doppler measurements ofGJ 176 obtained with the Keck-HIRES spectrometer over a 10 yeartime span. These measurements show no evidence of the 10.24 daycompanion, at a threshold of 4 m s-1, afactor of 3 less than the amplitude reported by Endl et al. The Keckvelocities are consistent with instrumental noise and stellar jitter.The existence of the planet is thus called into question.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.
| Magnetic Properties of Young Stars in the TW Hydrae Association
We present an analysis of infrared echelle spectra of five stars in theTW Hydrae Association (TWA). We model the Zeeman broadening in fourmagnetic-sensitive Ti I lines near 2.2 μm and measure the value ofthe photospheric magnetic field averaged over the surface of each star.To ensure that other broadening mechanisms are properly taken intoaccount, we also inspect several magnetically insensitive CO lines near2.3 μm and find no excess broadening above that produced by stellarrotation and instrumental broadening, providing confidence in themagnetic interpretation of the width of the Ti I lines. We then utilizeour results to test the relationship between stellar magnetic flux andX-ray properties and compare the measured fields with equipartitionfield values. Finally, we use our results and recent results on a largesample of stars in Taurus to discuss the potential evolution of magneticfield properties between the age of Taurus (~2 Myr) and the age of TWA(~10 Myr). We find that the average stellar field strength increaseswith age; however, the total unsigned magnetic flux decreases as thestars contract onto the main sequence.
| M dwarfs: effective temperatures, radii and metallicities
We empirically determine effective temperatures and bolometricluminosities for a large sample of nearby M dwarfs, for which highaccuracy optical and infrared photometry is available. We introduce anew technique which exploits the flux ratio in different bands as aproxy of both effective temperature and metallicity. Our temperaturescale for late-type dwarfs extends well below 3000K (almost to the browndwarf limit) and is supported by interferometric angular diametermeasurements above 3000K. Our metallicities are in excellent agreement(usually within 0.2dex) with recent determinations via independenttechniques. A subsample of cool M dwarfs with metallicity estimatesbased on hotter Hipparcos common proper motion companions indicates ourmetallicities are also reliable below 3000K, a temperature rangeunexplored until now. The high quality of our data allows us to identifya striking feature in the bolometric luminosity versus temperatureplane, around the transition from K to M dwarfs. We have compared oursample of stars with theoretical models and conclude that thistransition is due to an increase in the radii of the M dwarfs, a featurewhich is not reproduced by theoretical models.
| Dynamical Mass of GJ 802B: A Brown Dwarf in a Triple System
We report a dynamical measurement of the mass of the brown dwarf GJ 802Busing aperture-masking interferometry and astrometry. In addition, wereport the discovery that GJ 802A is itself a close spectroscopicnoneclipsing binary with a 19 hr period. We find the mass of GJ 802B tobe 0.063+/-0.005 Msolar. GJ 802 has kinematics inconsistentwith a young star and more consistent with the thick-disk population,implying a system age of ~10 Gyr. However, model evolutionary tracks forGJ 802B predict system ages of ~2 Gyr, suggesting that brown dwarfevolutionary models may be underestimating luminosity for old browndwarfs.
| Characterizing the Near-UV Environment of M Dwarfs
We report the results of our Hubble Space Telescope (HST) snapshotsurvey with the ACS HRC PR200L prism, designed to measure the near-UVemission in a sample of nearby M dwarfs. Thirty-three stars wereobserved, spanning the mass range from 0.1 to 0.6 solar masses(Teff~2200-4000 K) where the UV energy distributions varywidely between active and inactive stars. These observations providemuch needed constraints on models of the habitability zone and theatmospheres of possible terrestrial planets orbiting M dwarf hosts andwill be useful in refining the target selection for future spacemissions such as Terrestrial Planet Finder (TPF). We compare our datawith a new generation of M dwarf atmospheric models and discuss theirimplications for the chromospheric energy budget. These NUV data willalso be valuable in conjunction with existing optical, FUV, and X-raydata to explore unanswered questions regarding the dynamo generation andmagnetic heating in low-mass stars.
| Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
We examine the implications for the distribution of extrasolar planetsbased on the null results from two of the largest direct imaging surveyspublished to date. Combining the measured contrast curves from 22 of thestars observed with the VLT NACO adaptive optics system by Masciadri andcoworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDIdevices by Biller and coworkers (for a total of 60 unique stars), weconsider what distributions of planet masses and semimajor axes can beruled out by these data, based on Monte Carlo simulations of planetpopulations. We can set the following upper limit with 95% confidence:the fraction of stars with planets with semimajor axis between 20 and100 AU, and mass above 4 MJup, is 20% or less. Also, with adistribution of planet mass of dN/dM~M-1.16 in the range of0.5-13 MJup, we can rule out a power-law distribution forsemimajor axis (dN/da~aα) with index 0 and upper cutoffof 18 AU, and index -0.5 with an upper cutoff of 48 AU. For thedistribution suggested by Cumming et al., a power-law of index -0.61, wecan place an upper limit of 75 AU on the semimajor axis distribution. Ingeneral, we find that even null results from direct imaging surveys arevery powerful in constraining the distributions of giant planets (0.5-13MJup) at large separations, but more work needs to be done toclose the gap between planets that can be detected by direct imaging,and those to which the radial velocity method is sensitive.
| 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
| The Gemini Deep Planet Survey
We present the results of the Gemini Deep Planet Survey, a near-infraredadaptive optics search for giant planets and brown dwarfs around 85nearby young stars. The observations were obtained with the Altairadaptive optics system at the Gemini North telescope, and angulardifferential imaging was used to suppress the speckle noise of thecentral star. Typically, the observations are sensitive to angularseparations beyond 0.5" with 5 σ contrast sensitivities inmagnitude difference at 1.6 μm of 9.5 at 0.5", 12.9 at 1", 15.0 at2", and 16.5 at 5". These sensitivities are sufficient to detect planetsmore massive than 2 MJ with a projected separation in therange 40-200 AU around a typical target. Second-epoch observations of 48stars with candidates (out of 54) have confirmed that all candidates areunrelated background stars. A detailed statistical analysis of thesurvey results is presented. Assuming a planet mass distributiondn/dm~m-1.2 and a semimajor-axis distributiondn/da~a-1, the 95% credible upper limits on the fraction ofstars with at least one planet of mass 0.5-13 MJ are 0.28 forthe range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; thisresult is weakly dependent on the semimajor-axis distribution power-lawindex. The 95% credible interval for the fraction of stars with at leastone brown dwarf companion having a semimajor axis in the range 25-250 AUis 0.019+0.083-0.015, irrespective of anyassumption on the mass and semimajor-axis distributions. Theobservations made as part of this survey have resolved the stars HD14802, HD 166181, and HD 213845 into binaries for the first time.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
| 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.
| The narrowest M-dwarf line profiles and the rotation-activity connection at very slow rotation
Context: The rotation-activity connection explains stellar activity interms of rotation and convective overturn time. It is well establishedin stars of spectral types F-K as well as in M-type stars of youngclusters, in which rotation is still very rapid even among M-dwarfs. Therotation-activity connection is not established in field M-dwarfs,because they rotate very slowly, and detecting rotation periods orrotational line broadening is a challenge. In field M-dwarfs, saturationsets in below v_rot = 5 km s-1, hence they are expected topopulate the non-saturated part of the rotation-activity connection. Aims: This work for the first time shows intrinsically resolved spectrallines of slowly rotating M-dwarfs and makes a first comparison toestimates of convective velocities. By measuring rotation velocities ina sample of mostly inactive M-dwarfs, the unsaturated part of therotation-activity connection is followed into the regime of very lowactivity. Methods: Spectra of ten M-dwarfs are taken at a resolvingpower of R = 200000 at the CES in the near infrared region wheremolecular FeH has strong absorption bands. The intrinsically very narrowlines are compared to model calculations of convective flows, androtational broadening is measured. Results: In one star, an upper limitof v sin i = 1 km s-1 was found, significant rotation wasdetected in the other nine objects. All inactive stars show rotationbelow or equal to 2 km s-1. In the two active stars AD Leoand YZ CMi, rotation velocities are found to be 40-50% below the resultsfrom earlier studies. Conclusions: The rotation activity connectionholds in field early-M stars, too. Activity and rotation velocities ofthe sample stars are well in agreement with the relation found inearlier and younger stars. The intrinsic absorption profiles ofmolecular FeH lines are consistent with calculations from atomic Felines. Investigation of FeH line profiles is a very promising tool tomeasure convection patterns at the surfaces of M-stars.Based on observationscarried out at the European Southern Observatory, La Silla,PID 076.D-0092.
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar-mass companions have focusedprimarily on stars older than 1 Gyr. Increased levels of stellaractivity in young stars hinders the detection of solar system analogs,and therefore until recently there has been a prejudice againstinclusion of young stars in radial velocity surveys. Adaptive opticssurveys of young stars have given us insight into the multiplicity ofyoung stars, but only for massive, distant companions. Understanding thelimit of the radial velocity technique, restricted to high-mass,close-orbiting planets and brown dwarfs, we began a survey of youngstars of various ages. While the number of stars needed to carry outfull analysis of the problems of planetary and brown dwarf populationand evolution is large, the beginning of such a sample is included here.We report on 61 young stars ranging in age from the β Pictorisassociation (~12 Myr) to the Ursa Major association (~300 Myr). Thisinitial search resulted in no stars showing evidence of companionslarger than ~1MJup-2MJup in short-period orbits atthe 3 σ level. We also present derived stellar parameters, as mosthave unpublished values. The chemical homogeneity of a cluster, andpresumably of an association, may help to constrain true membership, sowe present [Fe/H] abundances for the stars in our sample.
| 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.
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| An Atlas of Spectrophotometric Landolt Standard Stars
We present CCD observations of 102 Landolt standard stars obtained withthe Ritchey-Chrétien spectrograph on the Cerro TololoInter-American Observatory 1.5 m telescope. Using stellar atmospheremodels, we have extended the flux points to our six spectrophotometricsecondary standards, in both the blue and the red, allowing us toproduce flux-calibrated spectra that span a wavelength range from 3050Å to 1.1 μm. Mean differences between UBVRI spectrophotometrycomputed using Bessell's standard passbands and Landolt's publishedphotometry were determined to be 1% or less. Observers in bothhemispheres will find these spectra useful for flux-calibrating spectra,and through the use of accurately constructed instrumental passbands,will be able to compute accurate corrections to bring instrumentalmagnitudes to any desired standard photometric system (S-corrections).In addition, by combining empirical and modeled spectra of the Sun,Sirius, and Vega, we calculate and compare synthetic photometry toobserved photometry taken from the literature for these three stars.
| A non-main-sequence secondary in SY Cancri
Simultaneous spectroscopic and photometric observations of the Z Camtype dwarf nova SY Cancri were used to obtain absolute fluxcalibrations. A comparison of the photometric calibration with awide-slit spectrophotometric calibration showed that either method isequally satisfactory. A radial velocity study of the secondary star,made using the far-red NaI doublet, yielded a semi-amplitude ofK2= 127 +/- 23 km s-1. Taking the published valueof 86 +/- 9 km s-1 for K1 gives a mass ratio ofq=M2/M1= 0.68 +/- 0.14; this is very differentfrom the value of 1.13 +/- 0.35 quoted in the literature. Using the newlower mass ratio, and constraining the mass of the white dwarf to bewithin reasonable limits, then leads to a mass for the secondary starthat is substantially less than would be expected for its orbital periodif it satisfied a main-sequence mass-radius relationship. We find aspectral type of M0 that is consistent with that expected for amain-sequence star of the low mass we have found. However, in order tofill its Roche lobe, the secondary must be significantly larger than amain-sequence star of that mass and spectral type. The secondary isdefinitely not a normal main-sequence star.
| A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO
We report on a survey devoted to the search of exoplanets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets versus the angular separationfrom the star is presented. The nondetection of any planetary masscompanion in our survey is used to derive, for the first time, thefrequency of the upper limit of the projected planet-star separation. Inparticular, we find that in 50% of the cases, no 5MJ (or moremassive) planet has been detected at projected separations larger than14 AU, and no 10MJ (or more massive) planet has been detectedat projected separations larger than 8.5 AU. In 100% of the cases, thesevalues increase to 36 and 65 AU, respectively. The excellent sensitivityreached by our study leads to a much lower upper limit of the projectedplanet-star separation compared with previous studies. For example, forthe β Pictoris group (~12 Myr), we did not detect any10MJ planet at distances larger than 15 AU. A previous studycarried out with 4 m class telescopes put an upper limit for10MJ planets at ~60 AU. For our closest target (V2306 Oph;d=4.3 pc), it is shown that it would be possible to detect a10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to mechanisms explainingplanet formation and migration and forthcoming observational strategiesand future planet-finder observations from the ground.Based on observations collected at the European Southern Observatory,Chile. Program 70.C-0777D, 70.C-0777E, and 71.C-0029A.
| Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component
We present results of an investigation of the X-ray properties, agedistribution, and kinematical characteristics of a high-galacticlatitude sample of late-type field stars selected from the ROSAT All-SkySurvey (RASS). The sample comprises 254 RASS sources with opticalcounterparts of spectral types F to M distributed over six study areaslocated at |b| 20 °, and Dec ≥ -9 °. A detailed studywas carried out for the subsample of ~200 G, K, and M stars. Lithiumabundances were determined for 179 G-M stars. Radial velocities weremeasured for most of the 141 G and K type stars of the sample. Combinedwith proper motions these data were used to study the age distributionand the kinematical properties of the sample. Based on the lithiumabundances half of the G-K stars were found to be younger than theHyades (660 Myr). About 25% are comparable in age to the Pleiades (100Myr). A small subsample of 10 stars is younger than the Pleiades. Theyare therefore most likely pre-main sequence stars. Kinematically the PMSand Pleiades-type stars appear to form a group with space velocitiesclose to the Castor moving group but clearly distinct from the LocalAssociation.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, operated by the Max-Planck-Institut fürAstronomie, Heidelberg, jointly with the Spanish National Commission forAstronomy, and at the European Southern Observatory, La Silla, Chile.Tables A2-A4 are only available in electronic form athttp://www.edpsciences.org
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Searching for massive extrasolar planets around young and nearby stars: from NACO to CHEOPS
We report on a survey devoted to the search of exo-planets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets vs. the angular separation fromthe star is presented. The non-detection of any planetary mass companionin our survey is used to derive, for the first time, the frequency ofthe upper limit of the projected separation planet/stars. In particular,we find that in 50% of cases, no 5MJ planet (or more massive)has been detected at projected separations larger than 14 AU and no10MJ planet (or more massive) has been detected at projectedseparations larger than 8.5 AU. The excellent sensitivity reached by ourstudy leads to a much lower upper limit of the projected separationplanet-star compared with previous studies. For our closest target(V2306 Oph - d = 4.3 pc) it is shown that it would be possible to detecta 10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to forthcomingobservational strategies (Simultaneous Differential Imaging technique)and future planet finder observations from the ground.
| 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.
| Multi-order TODCOR: Application to observations taken with the CORALIE echelle spectrograph. II. A planet in the system HD 41004
In previous works we reported the detection of a brown-dwarf companionorbiting the faint stellar component of the visual binary HD41004. This had been achieved by applying the multi-orderTwO-Dimensional CORrelation (TODCOR) algorithm to CORALIE spectra. Along-term trend of the primary component had also been detected. In thispaper we report follow-up observations that seem to unravel the natureof this long-term trend. The new measurements are consistent with thepresence of a 2.5 MJ planet in an eccentric orbit around HD 41004 A. Thus, HD 41004 is aunique hierarchical system with a brown dwarf that accompanies the faintM-component, and a planet that orbits the brighter K-component.Based on observations collected at the La Silla Observatory, ESO(Chile), with the CORALIE spectrograph at the 1.2-m Euler Swisstelescope.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbrg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbrg.fr/cgi-bin/qcat?J/A+A/426/695
| Further Results of TiO-Band Observations of Starspots
We present measurements of starspot parameters (temperature and fillingfactor) on five highly active stars, using absorption bands of TiO, fromobservations made between 1998 March and 2001 December. We determinedstarspot parameters by fitting TiO bands using spectra of inactive G andK stars as proxies for the unspotted photospheres of the active starsand spectra of M stars as proxies for the spots. For three evolved RSCVn systems, we find spot filling factors between 0.28 and 0.42 for DMUMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; thesevalues are similar to those found by other investigators usingphotometry and Doppler imaging. Among active dwarfs, we measured a lowerspot temperature (3350 K) for EQ Vir than found in a previous study ofTiO bands, and for EK Dra a lower spot temperature (~3800 K) than foundthrough photometry. For all active stars but XX Tri, we achieved goodphase coverage through a stellar rotational period. We also present ourfinal, extensive grid of spot and nonspot proxy stars.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Detection of 13CO and Other Apparent Abundance Anomalies in the Secondary Stars of Long-Period Cataclysmic Variables
We present moderate-resolution (R>1800) infrared K-band spectra of 12long-period (Porb>6 hr) cataclysmic variables (CVs). Wedetect absorption lines from the photospheres of the secondary stars inevery system, even though two of them were undergoing outbursts. We haveattempted to assign a spectral type to each of the secondary stars, andthese classifications are generally consistent with previousdeterminations. We find evidence for abundance anomalies that includeenhancements and/or deficits for all of the species commonly found inK-band spectra of G- and K-type dwarfs. There is, however, only onecommon abundance anomaly: extremely weak CO features. Only two of the 12objects appeared to have normal levels of CO absorption. We interpretthis as evidence of low carbon abundances. In addition, we detect13CO absorption in four of the 12 objects. Depleted levels of12C and enhanced levels of 13C indicate thatmaterial that has been processed in the CNO cycle is finding its wayinto the photospheres of CV secondary stars. In systems with luminousaccretion disks, we find that the spectrum of the secondary star iscontaminated by a source that flattens (reddens) the continuum. Whilefree-free or classical accretion disk spectra are flatter than theblackbody-like spectra of G and K dwarfs, removal of such contaminationfrom the K-band data results in spectra in which the absorption featuresbecome too strong to be consistent with those of G and K dwarfs.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Szextáns |
| Rektaszcenzió: | 10h28m55.55s |
| Deklináció: | +00°50'27.6" |
| Vizuális fényesség: | 9.6 |
| Távolság: | 7.231 parszek |
| RA sajátmozgás: | -601.9 |
| Dec sajátmozgás: | -734.5 |
| B-T magnitude: | 11.516 |
| V-T magnitude: | 9.759 |
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