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LP 261-75/2MASSW J09510549+3558021: A Young, Wide M4.5/L6 Binary
We present new observations of the LP 261-75/2MASSW J09510549+3558021M4.5/L6 wide, common-proper-motion binary system. Optical spectroscopyof LP 261-75 shows strong Hα emission, and the star may also beassociated with the ROSAT FSC source 1RXS J095102.7+355824. The derivedchromospheric and coronal activity levels are consistent with those ofPleiades stars of the same spectral type, and we infer an age of 100-200Myr for the system. In that case, theoretical models suggest that the L6dwarf 2MASSW J09510549+3558021 is a low-mass brown dwarf, withM~0.02+0.01-0.005 Msolar.Based partly on observations obtained with the Apache Point Observatory3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| 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.
| Spot patterns and differential rotation in the eclipsing pre-cataclysmic variable binary, V471 Tau
We present surface spot maps of the K2V primary star in thepre-cataclysmic variable binary system, V471 Tau. The spot maps show thepresence of large high-latitude spots located at the sub-white dwarflongitude region. By tracking the relative movement of spot groups overthe course of four nights (eight rotation cycles), we measure thesurface differential rotation rate of the system. Our results revealthat the star is rotating rigidly with a surface shear rate, dΩ=1.6 +/- 6mradd-1. The single active star AB Dor has a similarspectral type, rotation period and activity level as the K star in V471Tau, but displays much stronger surface shear (46 < dΩ <58mradd-1). Our results suggest that tidal locking mayinhibit differential rotation; this reduced shear, however, does notaffect the overall magnetic activity levels in active K dwarfs.
| The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours
The United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Surveyis a set of five surveys of complementary combinations of area, depthand Galactic latitude, which began in 2005 May. The surveys use theUKIRT Wide Field Camera (WFCAM), which has a solid angle of0.21deg2. Here, we introduce and characterize the ZY JHKphotometric system of the camera, which covers the wavelength range0.83-2.37μm. We synthesize response functions for the five passbands,and compute colours in the WFCAM, Sloan Digital Sky Survey (SDSS) andtwo-Micron All Sky Survey (2MASS) bands, for brown dwarfs, stars,galaxies and quasars of different types. We provide a recipe for othersto compute colours from their own spectra. Calculations are presented inthe Vega system, and the computed offsets to the AB system are provided,as well as colour equations between WFCAM filters and the SDSS and 2MASSpassbands. We highlight the opportunities presented by the new Y filterat 0.97-1.07μm for surveys for hypothetical Y dwarfs (brown dwarfscooler than T), and for quasars of very high redshift, z > 6.4.
| Exploring the Frequency of Close-in Jovian Planets around M Dwarfs
We discuss our high-precision radial velocity results of a sample of 90M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J.Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLTand the Keck I telescopes, within the context of the overall frequencyof Jupiter-mass planetary companions to main-sequence stars. None of thestars in our sample show variability indicative of a giant planet in ashort-period orbit, with a<=1 AU. We estimate an upper limit of thefrequency f of close-in Jovian planets around M dwarfs as <1.27% (atthe 1 σ confidence level). Furthermore, we determine that theefficiency of our survey in noticing planets in circular orbits is 98%for companions with msini>3.8MJ and a<=0.7 AU. Foreccentric orbits (e=0.6) the survey completeness is 95% for all planetswith msini>3.5MJ and a<=0.7 AU. Our results pointtoward a generally lower frequency of close-in Jovian planets for Mdwarfs as compared to FGK-type stars. This is an important piece ofinformation for our understanding of the process of planet formation asa function of stellar mass.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen. Also based on observationscollected at the European Southern Observatory, Chile (ESO programs65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044,71.C-0498, 072.C-0495, 173.C-0606). Additional data were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration(NASA), and with the McDonald Observatory Harlan J. Smith 2.7 mtelescope.
| A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs
We present a low-resolution (R≡λ/Δλ~90),5.5-38 μm spectral sequence of a sample of M, L, and T dwarfsobtained with the Infrared Spectrograph (IRS) on board the Spitzer SpaceTelescope. The spectra exhibit prominent absorption bands ofH2O at 6.27 μm, CH4 at 7.65 μm, andNH3 at 10.5 μm and are relatively featureless atλ>~15 μm. Three spectral indices that measure the strengthsof these bands are presented; H2O absorption features arepresent throughout the MLT sequence, while the CH4 andNH3 bands first appear at roughly the L/T transition.Although the spectra are, in general, qualitatively well matched bysynthetic spectra that include the formation of spatially homogeneoussilicate and iron condensate clouds, the spectra of the mid-type Ldwarfs show an unexpected flattening from roughly 9 to 11 μm. Wehypothesize that this may be a result of a population of small silicategrains that are not predicted in the cloud models. The spectrum of thepeculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5 to 7.5 μmrelative to typical T6 dwarfs and may be a consequence of its mildlymetal-poor/high surface gravity atmosphere. Finally, we computebolometric luminosities of a subsample of the M, L, and T dwarfs bycombining the IRS spectra with previously published 0.6-4.1 μmspectra and find good agreement with the values of Golimowski et al.,who use L'- and M'-band photometry to account forthe flux emitted at λ>2.5 μm.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| Low-mass companions to solar-type stars
We present preliminary results from a coronagraphic survey of youngnearby Sun-like stars using the Palomar and Keck adaptive opticssystems. We have targeted 251 solar analogs (F5-K5) at 20-160 pc fromthe Sun, spanning the 3-3000 Myr age range. The youngest (<500 Myr)≈100 of these have been imaged with deeper exposures to search forsub-stellar companions. The deep survey is sensitive to brown-dwarfcompanions at separations >0.5arcsec from their host stars, withsensitivity extending to planetary-mass (5-15M_Jup) objects at wider(>3arcsec ) separations. Based on the discovery of a number of newlow-mass (<0.2M_ȯ) stellar companions, we infer that theirfrequency at >20 AU separations (probed via direct imaging) may begreater (≈12%) than that found from radial velocity surveys probing<4 AU separations \citep[≈6%;][]{mazeh_etal03}. We also report theastrometric confirmation of the first sub-stellar companion from thesurvey -- an L4 brown dwarf at a projected distance of 44 AU from the≈500 Myr-old star HD 49197. Based on this detection, we estimate thatfrequency of sub-stellar companions to solar-type stars is at least 1%,and possibly of order a few per cent.
| New Low Accretion Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
Discoveries of two new white dwarf plus M star binaries with strikingoptical cyclotron emission features from the Sloan Digital Sky Survey(SDSS) brings to six the total number of X-ray-faint, magnetic accretionbinaries that accrete at rates M˙<~10-13Msolar yr-1, or <1% of the values normallyencountered in cataclysmic variables. This fact, coupled with donorstars that underfill their Roche lobes and very cool white dwarfs, brandthe binaries as post-common-envelope systems whose orbits have not yetdecayed to the point of Roche lobe contact. They are premagneticcataclysmic variables, or pre-Polars. The systems exhibit spin-orbitsynchronism and apparently accrete by efficient capture of the stellarwind from the secondary star, a process that has been dubbed a``magnetic siphon.'' Because of this, period evolution of the binarieswill occur solely by gravitational radiation, which is very slow forperiods >3 hr. Optical surveys for the cyclotron harmonics appear tobe the only means of discovery, so the space density of pre-Polars couldrival that of Polars, and the binaries provide an important channel ofprogenitors (in addition to the asynchronous intermediate Polars). Bothphysical and SDSS observational selection effects are identified thatmay help to explain the clumping of all six systems in a narrow range ofmagnetic field strength around 60 MG.A portion of the results presented here was obtained with the MMTObservatory, a facility operated jointly by the University of Arizonaand the Smithsonian Institution.Based in part on observations with the Apache Point Observatory 3.5 mtelescope and the Sloan Digital Sky Survey, which are owned and operatedby the Astrophysical Research Consortium (ARC).
| 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.
| Low mass companions to white dwarfs
This paper summarizes the results of over 17 years of work searching forlow mass stellar and substellar companions to more than 370 nearby whitedwarfs. Roughly 60 low mass, unevolved companions were found and studiedall together, with over 20 discovered in the last few years, includingthe first unambiguous brown dwarf companion to a white dwarf, GD 1400B.The resulting spectral type distributions for companions to white dwarfsand nearby cool field dwarfs are compared, and the implications forbinary star formation are discussed. A brief analysis of GD 1400B,including new data, is also presented.
| A coronagraphic search for brown dwarfs and planets around nearby stars
We have started a corongraphic search for brown dwarfs and planetsaround young nearby stars within 20 pc of the Sun, using the adaptiveoptics coronagraph, CIAO, on Subaru. The dynamic range we have achievedis \Delta K = 13 at 2.5'' from the central star. For atypical target with K=7 at 10 pc, the limiting absolute magnitude isM_K=20. We apply two kinematical age criteria to select M and K dwarfsstatistically younger than 350Myr. The first criterion is a smallvelocity deviation from the velocity of LSR. The second is a (U,V,W)velocity vector similar to a particular young moving group. Thecombination of the age and magnitude limits implies that the mass limitfor giant planet detection is about 2M_J. We show a sample image of atarget field at 3 pc of the Sun with faint companion candidates, to befollowed up for the common proper motion test. We give a briefdescription of our procedures for data acquisition, reduction, andanalysis.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| 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).
| PHOENIX model chromospheres of mid- to late-type M dwarfs
We present semi-empirical model chromospheres computed with theatmosphere code PHOENIX. The models are designed to fit the observedspectra of five mid- to late-type M dwarfs. Next to hydrogen lines fromthe Balmer series we used various metal lines, e.g. from Fe i, for thecomparison between data and models. Our computations show that an NLTEtreatment of C, N, O impacts on the hydrogen line formation, while NLTEtreatment of less abundant metals such as nickel influences the lines ofthe considered species itself. For our coolest models we investigatedalso the influence of dust on the chromospheres and found that dustincreases the emission line flux. Moreover we present an (electronicallypublished) emission line list for the spectral range of 3100 to 3900 and4700 to 6800 Å for a set of 21 M dwarfs and brown dwarfs. The linelist includes the detection of the Na i D lines in emission for a L3dwarf.
| Imaging exoplanets.
Not Available
| 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.
| 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.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| L' and M' Photometry of Ultracool Dwarfs
We have compiled L' (3.4-4.1 μm) and M' (4.6-4.8 μm) photometry of63 single and binary M, L, and T dwarfs obtained at the United KingdomInfrared Telescope using the Mauna Kea Observatory filter set. Thiscompilation includes new L' measurements of eight L dwarfs and 13 Tdwarfs and new M' measurements of seven L dwarfs, five T dwarfs, and theM1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6,respectively, the numbers of ultracool dwarfs (Teff<~2400K) for which L' and M' measurements have been reported. We computeLbol, BCK, and Teff for 42 dwarfs whoseflux-calibrated JHK spectra, L' photometry, and trigonometric parallaxesare available, and we estimate these quantities for nine other dwarfswhose parallaxes and flux-calibrated spectra have been obtained.BCK is a well-behaved function of near-infrared spectral typewith a dispersion of ~0.1 mag for types M6-T5 it is significantly morescattered for types T5-T9. Teff declines steeply andmonotonically for types M6-L7 and T4-T9, but it is nearly constant at~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constantTeff is evidenced by nearly unchanging values of L'-M'between types L6 and T3. It also supports recent models that attributethe changing near-infrared luminosities and spectral features across theL-T transition to the rapid migration, disruption, and/or thinning ofcondensate clouds over a narrow range of Teff. The L' and M'luminosities of early-T dwarfs do not exhibit the pronounced humps orinflections previously noted in the I through K bands, but insufficientdata exist for types L6-T5 to assert that ML' andMM' are strictly monotonic within this range of types. Wecompare the observed K, L', and M' luminosities of L and T dwarfs in oursample with those predicted by precipitating-cloud and cloud-free modelsfor varying surface gravities and sedimentation efficiencies. The modelsindicate that the L3-T4.5 dwarfs generally have higher gravities(logg=5.0-5.5) than the T6-T9 dwarfs (logg=4.5-5.0). The predicted M'luminosities of late-T dwarfs are 1.5-2.5 times larger than thosederived empirically for the late-T dwarfs in our sample. Thisdiscrepancy is attributed to absorption at 4.5-4.9 μm by CO, which isnot expected under the condition of thermochemical equilibrium assumedin the models. Our photometry and bolometric calculations indicate thatthe L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57-041403.5 areprobable binary systems. We computelog(Lbol/Lsolar)=-5.73+/-0.05 andTeff=600-750 K for the T9 dwarf 2MASSI J0415195-093506, whichsupplants Gl 570D as the least luminous and coolest brown dwarfpresently known.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Fe XIII coronal line emission in cool M dwarfs
We report on a search for the Fe XIII forbidden coronal line at 3388.1Å in a sample of 15 M-type dwarf stars covering the whole spectralclass as well as different levels of activity. A clear detection wasachieved for LHS 2076 during a major flare and for CN Leo, where theline had been discovered before. For some other stars the situation isnot quite clear. For CN Leo we investigated the timing behaviour of theFe XIII line and report a high level of variability on a timescale ofhours which we ascribe to microflare heating.Based on observations collected at the European Southern Observatory,Paranal, Chile, 68.D-0166A.
| 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
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| Searching for Faint Companions with the TRIDENT Differential Simultaneous Imaging Camera
We present the first results obtained at CFHT with the TRIDENT infraredcamera, dedicated to the detection of faint companions close to brightnearby stars. Its main feature is the acquisition of three simultaneousimages in three wavelengths (simultaneous differential imaging) acrossthe methane absorption bandhead at 1.6 microns, that enables a precisesubtraction of the primary star's PSF while keeping the companionsignal. Gl229 and 55 Cnc observations are presented to demonstrateTRIDENT subtraction performances. It is shown that a faint companionwith an H magnitude difference of 10 magnitudes would be detected at 0.5arcsec from the primary.
| MEDI: An Instrument for Direct Detection of Massive Extrasolar Planets
We have developed an instrument, MEDI (Massive Exoplanet DifferentialImager), that takes advantage of a novel method of starlight rejection,simultaneous differential imaging, in order to image massive planetsaround nearby stars. Using this technique we expect to achievesuppression of starlight to the photon-noise limit, which means thatincreased exposure time will translate into higher sensitivities. Thisis in contrast to past sequential and two-color simultaneous studiesthat reach a sensitivity floor due to speckle-noise limitations. Basedon lab results, we expect to be able to detect objects 106times fainter than their primaries in the H band at 0.5 arc-secseparations in 2 hours. This suggests that we will be sensitive toobjects with masses as small as 5 Jupiters at separations of greaterthan about 5 AU for G2 V stars that are about 300 Myr old and withinabout 10 pc.
| 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.
| 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.
| Kinematics and Luminosity Function of Dwarf Populations in Three Areas of the Calán-ESO Proper-Motion Catalog
We have completed the analysis of a sample of 112 stars in the solarneighborhood taken from the statistically complete Calán-ESOcatalog. From medium-resolution spectroscopy we classified every star,both by direct comparison with spectroscopic standards and by usingspectral indices. The latter also allowed discrimination betweenmain-sequence (MS) dwarfs and subdwarfs. Several useful spectral typeversus color relations were obtained from CCD photometry of the sample(observed magnitudes were dereddened). Distances and absolute magnitudeswere determined. From measured radial velocities and proper motions, wedetermined the kinematics [Galactocentric velocity components (U,V,W)],which allowed the classification of each star as belonging to the diskor halo population. Luminosity functions (LFs) were then obtained usingthe 1/Vmax method for the different populations. The maximumin the LF for MS dwarfs was found to be near MV=12.5+/-0.5,in accord with previous determinations. On the other hand, we found anincrease in the LF of the subdwarf at its faint end, which is in strongdisagreement with determinations by other authors. A mass density of MSdwarfs of ~0.047+/-0.021 Msolar pc-3 was derived,while the contribution of subdwarfs was found to be negligible.Based on observations obtained with the VLT (ESO), project 67.D-0224A.
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
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