תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| 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.
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| 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.
| Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. III. Spectroscopic distances of 322 NLTT stars
Distance estimates based on low-resolution spectroscopy and Two MicronAll Sky Survey (2MASS) J magnitudes are presented for a large sample of322 nearby candidates from Luyten's NLTT catalogue. Mainly relativelybright (typically 7 < Ks < 11) and red high propermotion stars have been selected according to their 2MASS magnitudes andoptical-to-infrared colours (+1 < R-Ks < +7). Some LHSstars previously lacking spectroscopy have also been included. We haveclassified the majority of the objects as early-M dwarfs (M 2-M 5). Morethan 70% of our targets turned out to lie within the 25 pc horizon ofthe catalogue of nearby stars, with 50 objects placed within 15 pc and 8objects being closer than 10 pc. Three objects in the 10 pc sample haveno previously published spectral type: LP 876-10 (M 4), LP 870-65 (M4.5), and LP 869-26 (M 5). A large fraction of the objects in our sample(57%) have independent distance estimates, mainly by the recent effortsof Reid and collaborators. Our distance determinations are generally ingood agreement with theirs. 11 rather distant (d > 100 pc) objectshave also been identified, including a probable halo, but relatively hot(T_eff13 000 K) white dwarf (LHS 1200) and 10 red dwarfs withextremely large tangential velocities (250 < vt < 1150km s-1). Altogether, there are 11 red dwarfs (including onewithin 70 pc) with tangential velocities larger than about 250 kms-1. All these objects are suspected to be in fact subdwarfs,if so, their distances would be only about half of our originalestimates. The three most extreme objects in that respect are the K andearly M dwarfs LP 323-168, LHS 5343 and LP 552-21 with correcteddistances between 180 pc and 400 pc and resulting tangential velocitiesstill larger than about 400 km s-1.
| 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).
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| 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.
| Membership, binarity and accretion among very low-mass stars and brown dwarfs of the σ Orionis cluster
Intermediate-resolution (R~ 7000) spectroscopy is presented for 76photometrically selected very low-mass (0.04 < M < 0.3Msolar) candidate members of the young cluster around σOrionis (σ Ori). More than two-thirds appear to be genuine clustermembers on the basis that they exhibit LiI 6708-Åabsorption, weakNaI 8183/8195 Åfeatures and a radial velocity consistent with thecluster mean. Photometric selection alone therefore appears to be veryeffective in identifying cluster members in this mass range. Only sixobjects appear to be certain non-members; however, a substantial subsetof 13 candidates have ambiguous or contradictory indications ofmembership and lack Li absorption. Together with an observed spread inthe equivalent width of the Li absorption feature in the cooler stars ofour sample, this indicates that there may be deficiencies in ourunderstanding of the formation of this line in cool, low-gravityobjects.Four candidate binary cluster members are identified. Consideration ofsampling and radial velocity measurement precision leads us to concludethat either the fraction of very low-mass stars and brown dwarfs insmall separation (a < 1 au) binary systems is larger than in fieldM-dwarfs, or the distribution of separations is much less skewed towardslarge separations. This conclusion hinges critically on the correctidentification of the small number of binary candidates, although itremains significant even when only the candidate members displaying Liabsorption are considered.Broadened Hα emission, indicative of circum(sub)stellar accretiondiscs is found in five or six of the candidate cluster members, three ofwhich probably have substellar masses. The fraction of accretors (10 +/-5 per cent) is similar to that found in stars of higher mass in theσ Ori cluster using Hα emission as a diagnostic, but muchlower than found for very low-mass stars and brown dwarfs of youngerclusters. The time-scale for accretion rates to drop to<~10-11 Msolar yr-1 is hence lessthan the age of the σ Ori cluster (3-7 Myr) for most low-massobjects.
| 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.
| 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.
| 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.
| The DA+dMe eclipsing binary EC13471-1258: its cup runneth over ... just
The optical spectrum and light curve of EC13471-1258 show that it is aneclipsing binary with an orbital period of 3h 37mcomprising a DA white dwarf and a dMe dwarf. Total eclipses of the whitedwarf are observed lasting 14 min, with the partial phases lasting 54 s.On one occasion, two pre-eclipse dips were seen. Timings of the eclipsesover 10 yr show jitter of up to 12 s. Flares from the M dwarf areregularly observed. The M dwarf also shows a large-amplitude ellipsoidalmodulation in the V-band light curve. The component stars emit almostequal amounts of light at 5500 Å.Hubble Space Telescope (HST) STIS spectra show strong Lyman-αabsorption with weak metal lines of C I,II and Si II superimposed. Modelatmosphere analysis yielded an effective temperature of 14 220 +/- 300 Kand logg of 8.34 +/- 0.20 for the white dwarf with these errors beingstrongly correlated. Its metal abundance is 1/30th solar with anuncertainty of 0.5 dex, and it is rapidly rotating with V1sin i= 400 +/- 100 km s-1. The white dwarf also shows radialvelocity variations with a semi-amplitude of 138 +/- 10 kms-1. The gravitational redshift of the white dwarf wasmeasured as 62 km s-1.From optical spectroscopy the spectral type of the M dwarf was found tobe M3.5-M4, its temperature 3100 +/- 75 K, its rotational velocity 140+/- 10 km s-1, its radial velocity semi-amplitude 266 +/- 5km s-1, its mean V-I colour 2.86 and its absolute V magnitude11.82. Intriguingly, its metal abundance is normal solar.The Hα emission line shows at least two distinct components, oneof which is uniformly distributed around the centre of mass of the Mdwarf and provided the estimate of the rotational velocity of the Mdwarf. The other arises from the other side of the binary centre ofmass, well within the white dwarf Roche lobe. This behaviour isconfirmed by Doppler tomography, which shows the presence of twodistinct velocity components within the primary Roche lobe. Theinterpretation of these features is uncertain. Variations in strength ofthe components with binary phase can be attributed to optical thicknessin the Balmer lines. Similar behaviour is seen in the observations ofthe other Balmer emission lines, although with a poorer signal-to-noiseratio. Flares in Hα were observed and are consistent with arisingfrom the vicinity of the M dwarf.Dynamical solutions for the binary are discussed and yield aninclination of 75.5°+/- 2.0o, a white dwarf mass andradius of 0.78 +/- 0.04 Msolar and 0.011 +/- 0.01Rsolar, and an M dwarf mass and radius of 0.43 +/- 0.04Msolar and 0.42 +/- 0.02 Rsolar. These parametersare consistent with the Wood mass-radius relation for white dwarfs andthe Clemens et al. mass-radius relation for M dwarfs; we argue that theM dwarf just fills its Roche lobe. The radius of the white dwarf and themodel fit imply a distance of 48 +/- 5 pc and an absolute V magnitude of11.74.The rapid rotation of the white dwarf strongly suggests that the systemhas undergone mass transfer in the past, and implies that it is ahibernating cataclysmic variable. The M dwarf shows the propertiesexpected of secondaries in cataclysmic variables: chromospheric activityand angular momentum loss.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Narrow-Angle Astrometry with the Space Interferometry Mission: The Search for Extrasolar Planets. I. Detection and Characterization of Single Planets
A decade after the publication of the Hipparcos Catalogue, the SpaceInterferometry Mission (SIM) will be capable of making selectedhigh-precision astrometric measurements about 3 orders of magnitude moreaccurate than the Hipparcos survey. We present results from a detailedset of end-to-end numerical simulations of SIM narrow-angle astrometricmeasurements and data analysis to illustrate the enormous potential thatSIM has for the discovery and characterization of planets outside thesolar system. Utilizing a template observing scenario, we quantify SIMsensitivity to single planets orbiting single normal nearby stars as afunction of measurement errors and properties of the planet: SIM willdetect over 95% of the planets with periods between a few days and the 5yr nominal mission lifetime that produces astrometric signatures ~2.2times larger than the single-measurement accuracy. We provide accuracyestimates of full-orbit reconstruction and planet mass determination: attwice the discovery limit, orbital elements will be determined with atypical accuracy of 20%-30% the astrometric signature must be ~10 and~15 times the minimum signal required for detection to derive mass andinclination-angle estimates accurate to 10%. We quantify the impact ofdifferent observing strategies on the boundaries for secure detectionand accurate orbit estimation: the results scale with the square root ofboth the number of observations and the number of reference stars. Weinvestigate SIM discovery space, to gauge the instrument ability indetecting very low mass planets: around the nearest stars, SIM will findplanets as small as Earth, if they are present. Some of these might beorbiting inside the parent star's habitable zone. Extrasolar planetsfigure prominently among SIM scientific goals: our results reaffirm theimportance of high-precision astrometric measurements as a uniquecomplement to spectroscopic surveys based on radial velocity. Forexample, establishing the existence of rocky, perhaps habitable planetswould constitute both a fundamental test of theoretical models andprogress toward the understanding of formation and evolution processesof planetary systems. Such discoveries would also provide theTerrestrial Planet Finder with prime targets to investigate with directspectroscopy in terms of the potential for life.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| Spectroscopy and polarimetry of the AM Herculis system RX J1313.32-3259
We present polarimetric and spectroscopic observations of the ROSATsource RX J1313.32-3259, recently identified as a polar. Circularpolarization is modulated over the orbital period with an amplitude of~10 per cent. A brief reversal in circular polarization is detected atphase ~0.0, when the main accretion region is seen face on to theobserver. We deduce that the main cyclotron emission is absorbed by theaccretion shock and/or stream at this phase, allowing a second accretionshock, located at the opposite ends of the field lines that feed themain pole, to be seen. Linear polarization is also detected withpossible linear pulses at orbital phases where the accretion region isexpected to be on the limb of the white dwarf. The general morphology ofthe photopolarimetry arises as a result of a combination of theabsorption of cyclotron radiation by the accretion shock/stream andprojection/cyclotron beaming effects of a main cyclotron emission regionas the white dwarf rotates. The main cyclotron emission region remainson the visible hemisphere of the white dwarf throughout the orbit.Doppler maps of the emission lines show emission at or near the expectedlocation of the secondary star and an extended arc-like featurecoincident with the ballistic and possibly the magnetically confinedaccretion stream.
| 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 u'g'r'i'z' Standard-Star System
We present the 158 standard stars that define the u'g'r'i'z' photometricsystem. These stars form the basis for the photometric calibration ofthe Sloan Digital Sky Survey. The defining instrument system andfilters, the observing process, the reduction techniques, and thesoftware used to create the stellar network are all described. Webriefly discuss the history of the star selection process, thederivation of a set of transformation equations for theUBVRCIC system, and plans for future work.
| 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.
| BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters
We have initiated a long-term project, the BOAO photometric survey ofopen clusters, to enlarge our understanding of Galactic structure usingUBVI CCD photometry of open clusters, which have been little studiedbefore. This is the second paper of the project, in which we present thephotometry of 12 open clusters. We have determined the clusterparameters by fitting the Padua isochrones to the color-magnitudediagrams of the clusters. All the clusters except for Be 60 and NGC 1348are found to be intermediate-age to old (0.2-4.0 Gyrs) open clusterswith a mean metallicity of [Fe/H]~0.0.
| The 2dF Galaxy Redshift Survey: spectra and redshifts
The 2dF Galaxy Redshift Survey (2dFGRS) is designed to measure redshiftsfor approximately 250000 galaxies. This paper describes the surveydesign, the spectroscopic observations, the redshift measurements andthe survey data base. The 2dFGRS uses the 2dF multifibre spectrograph onthe Anglo-Australian Telescope, which is capable of observing 400objects simultaneously over a 2° diameter field. The sourcecatalogue for the survey is a revised and extended version of the APMgalaxy catalogue, and the targets are galaxies with extinction-correctedmagnitudes brighter than bJ=19.45. The main survey regionsare two declination strips, one in the southern Galactic hemispherespanning 80°×15° around the SGP, and the other in thenorthern Galactic hemisphere spanning 75°×10° along thecelestial equator; in addition, there are 99 fields spread over thesouthern Galactic cap. The survey covers 2000deg2 and has amedian depth of z=0.11. Adaptive tiling is used to give a highly uniformsampling rate of 93 per cent over the whole survey region. Redshifts aremeasured from spectra covering 3600-8000Å at a two-pixelresolution of 9.0Å and a median S/N of 13pixel-1. Allredshift identifications are visually checked and assigned a qualityparameter Q in the range 1-5 Q>=3 redshifts are 98.4 per centreliable and have an rms uncertainty of 85kms-1. The overallredshift completeness for Q>=3 redshifts is 91.8 per cent, but thisvaries with magnitude from 99 per cent for the brightest galaxies to 90per cent for objects at the survey limit. The 2dFGRS data base isavailable on the World Wide Web at http://www.mso.anu.edu.au/2dFGRS.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | כלב קטן |
| התרוממות ימנית: | 07h27m24.50s |
| סירוב: | +05°13'32.9" |
| גודל גלוי: | 9.87 |
| מרחק: | 3.799 פארסק |
| תנועה נכונה: | 573.6 |
| תנועה נכונה: | -3691 |
| B-T magnitude: | 11.771 |
| V-T magnitude: | 10.027 |
קטלוגים וכינוים:
|
