Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Stellar population synthesis in the presence of diluting components. Application to the infrared range for MCG-6-30-15
Aims.Numerous studies of the host galaxy of Seyfert nuclei are beingconducted in the optical/visible range. However, in the case of Seyfert1, the spectra of the inner galactic core can be dominated by broademission lines coming from the nucleus that totally flood the underlyinggalactic spectrum, preventing any study of stellar populations. Methods: Because part of the IR H-band is free of the strongest AGNemission lines, we developed a method that allows the stellar populationof the very internal regions of the Seyfert 1 galaxies to be studied inthe presence of a diluting continuum. Results: A new inverse methodusing the flux as observables was developed and deeply tested. Thespecificity of the method is to take the non stellar parameters(reddening, dust emission, and non-stellar continuum) into accountdirectly in the synthetic distance to be minimised. Conclusions: Themethod is powerful for deriving the stellar content of the very centralpart of AGN. However, the results for the stellar population are stilltentative, as the incompleteness of the stellar base (lack ofsupermetallic giant stars) could lead to large uncertainties. Newobservations of stars in the infrared at high spectral resolution, inparticular metal-rich stars, are mandatory to build a complete stellarlibrary that can be used to synthesise the host galaxies of AGN with asmuch accuracy as possible.
| High Spectral Resolution Near-IR Observations of ESO-Hα 279A and 279B
We present high spectral resolution near-IR observations of the starsESO-Hα 279A and 279B located in the Serpens star formationcomplex. ESO-Hα 279A is a known young T Tauri star driving aHerbig-Haro flow, while ESO-Hα 279B is a nearby (8", 2500 AU)extremely red companion. Previous work has suggested that this is not aphysically bound system but a chance alignment of a young star and abackground giant. Below, we further investigate the nature ofESO-Hα 279B and probe the emission characteristics of ESO-Hα279A. We find that ESO-Hα 279B shows many narrow absorptionfeatures typical of late-type giant stars and unlike those seen in FUOrionis objects and Herbig-Haro energy sources, the only young starsthat exhibit such deep CO absorption. This result confirms thatESO-Hα 279B is a background M-type giant viewed through themolecular cloud hosting ESO-Hα 279A, and therefore, theassociation of ESO-Hα 279A and 279B is fortuitous. ForESO-Hα 279A we find that the Na doublet lines are broadened andself-absorbed and that the v=2-0 CO overtone band head is similar inshape to that observed in the young pre-main-sequence object SVS 13 inNGC 1333, and not characteristic of a rotating-disk velocity dispersion.We consider the origin of the CO emission in relation to current modelsand suggest that it perhaps is more indicative of creation in a wind orfunnel flow rather than in the inner hot regions of a circumstellardisk.
| New Debris Disks around Nearby Main-Sequence Stars: Impact on the Direct Detection of Planets
Using the MIPS instrument on Spitzer, we have searched for infraredexcesses around a sample of 82 stars, mostly F, G, and K main-sequencefield stars, along with a small number of nearby M stars. These starswere selected for their suitability for future observations by a varietyof planet-finding techniques. These observations provide information onthe asteroidal and cometary material orbiting these stars, data that canbe correlated with any planets that may eventually be found. We havefound significant excess 70 μm emission toward 12 stars. Combinedwith an earlier study, we find an overall 70 μm excess detection rateof 13%+/-3% for mature cool stars. Unlike the trend for planets to befound preferentially toward stars with high metallicity, the incidenceof debris disks is uncorrelated with metallicity. By newly identifyingfour of these stars as having weak 24 μm excesses (fluxes ~10% abovethe stellar photosphere), we confirm a trend found in earlier studieswherein a weak 24 μm excess is associated with a strong 70 μmexcess. Interestingly, we find no evidence for debris disks around 23stars cooler than K1, a result that is bolstered by a lack of excessaround any of the 38 K1-M6 stars in two companion surveys. Onemotivation for this study is the fact that strong zodiacal emission canmake it hard or impossible to detect planets directly with futureobservatories such as the Terrestrial Planet Finder (TPF). Theobservations reported here exclude a few stars with very high levels ofemission, >1000 times the emission of our zodiacal cloud, from directplanet searches. For the remainder of the sample, we set relatively highlimits on dust emission from asteroid belt counterparts.
| On the Use of Line Depth Ratios to Measure Starspot Properties on Magnetically Active Stars
Photometric and spectroscopic techniques have proven to be effectiveways to measure the properties of dark, cool starspots on magneticallyactive stars. Recently, a technique was introduced using atomic linedepth ratios (LDRs) to measure starspot properties. Carefullyreproducing this technique using a new set of spectroscopic observationsof active stars, we find that the LDR technique encounters difficulties,specifically by overestimating spot temperatures (because the atomiclines blend with titanium oxide absorption in cooler spots) and by nottightly constraining the filling factor of spots. While the use of LDRsfor active star studies has great promise, we believe that theseconcerns need to be addressed before the technique is more widelyapplied.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| Observations of the binary star 61 Cyg on the 26 inch refractor at the Pulkovo observatory
Results from an analysis of a forty year series of photographicobservations of the binary star 61 Cyg on the 26 inch refractor at thePulkovo Observatory are presented. The orbit is constructed and the sumof the masses of the components is determined from the relativepositions of the components. A study of the individual motions of thecomponents of 61 Cyg relative to the surrounding stars yields their massratio and the masses of the main and secondary components, 0.74 and 0.46solar masses, respectively. The relative motion of the components isfound to have a fluctuating component with a period of 6.5 years whichmay be caused, in particular, by the presence in the system of a dark,low-mass companion.
| Data Mining for Double Stars in Astrometric Catalogs
The US Naval Observatory has mined over 140 astrometric catalogs,including the Astrographic Catalogue and the Two Micron All Sky Survey,for measures of double stars. This resulted in 114,218 new measures of47,007 different systems spanning 110 years; these are now included inthe Washington Double Star catalog (WDS). This is the single largestdata set ever added to the WDS. The measures are typically of widerpairs, most between 4" and 30" thus, their value in aiding orbitdetermination is limited. However, they have proven invaluable in theverification of systems and the determination of rectilinear motions ofsystems.
| 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.
| Prospects for population synthesis in the H band: NeMo grids of stellar atmospheres compared to observations
Context: .For applications in population synthesis, libraries oftheoretical stellar spectra are often considered an alternative totemplate libraries of observed spectra, because they allow a completesampling of stellar parameters. Most of the attention in publishedtheoretical spectral libraries has been devoted to the visual wavelengthrange.Aims.The goal of the present work is to explore the near-infraredrange where few observed fully calibrated spectra and no theoreticallibraries are available.Methods.We make a detailed comparison oftheoretical spectra in the range 1.57-1.67 μm for spectral types fromA to early M and for giant and dwarf stars, with observed stellarspectra at resolutions around 3000, which would be sufficient todisentangle the different groups of late-type stars. We selected theNeMo grids of stellar atmospheres to perform this comparison.Results.Wefirst demonstrate that observed spectral flux distributions can bematched very well with theoretical ones for almost the entire parameterrange covered by the NeMo grids at moderate resolution in the visualrange. In the infrared range, although the overall shape of the observedflux distributions still matches reasonably well, the individualspectral features are reproduced by the theoretical spectra only forstars earlier than mid F type. For later spectral types the differencesincrease, and theoretical spectra of K type stars have systematicallyweaker line features than those found in observations. Thesediscrepancies are traced back to stem primarily from incomplete data onneutral atomic lines, although some of them are also related tomolecules.Conclusions.Libraries of theoretical spectra for A to early Mtype stars can be successfully used in the visual regions for populationsynthesis, but their application in the infrared is restricted to earlyand intermediate type stars. Improving atomic data in the near infraredis a key element in making the construction of reliable libraries ofstellar spectra feasible in the infrared.
| 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 Physical Natures of Class I and Flat-Spectrum Protostellar Photospheres: A Near-Infrared Spectroscopic Study
We present high-resolution (R~=18,000), high signal-to-noise ratio, 2μm spectra of 52 IR-selected Class I and flat-spectrum young stellarobjects in the Taurus-Auriga, ρ Ophiuchi, Serpens, Perseus, andCorona Australis dark clouds. We detect key absorption lines in 41objects and fit synthetic spectra generated from pre-main-sequencemodels to deduce the effective temperatures, surface gravities, near-IRveilings, rotation velocities, and radial velocities of each of these 41sources. We find these objects to span ranges in effective temperature,surface gravity, and stellar luminosity that appear similar to those oflate spectral type Class II sources and classical T Tauri stars.However, because of significant but uncertain corrections for scatteringand extinction, the derived luminosities for the embedded protostellarobjects must be regarded as being highly uncertain. We determine thatthe mean 2 μm veiling of Class I and flat-spectrum objects issignificantly higher than that of Class II objects in the same regionwhere both types of objects are extensively observed (ρ Oph). Wefind that a significant fraction of our protostellar sample alsoexhibits emission lines. Twenty-three objects show H2emission, which is usually indicative of the presence of energeticoutflows. Thirty-four sources show H I Brγ emission, and a numberof these exhibit profile asymmetries consistent with infall. Eightsources show significant Δv=2 CO emission suggestive of emissionfrom a circumstellar disk. Overall, these observations indicate thatClass I and flat-spectrum objects are self-embedded protostarsundergoing significant mass accretion, although the objects appear tospan a broad range of mass accretion activity.Data presented herein were obtained at the W. M. Keck Observatory fromtelescope time allocated to the National Aeronautics and SpaceAdministration through the agency's scientific partnership with theCalifornia Institute of Technology and the University of California. TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| 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.
| 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 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
| HD 77407 and GJ 577: Two new young stellar binaries. Detected with the Calar Alto Adaptive Optics system ALFA
We present the first results from our search for close stellar andsub-stellar companions to young nearby stars on the northern sky. Ourinfrared imaging observations are obtained with the 3.5 m Calar Altotelescope and the AO system ALFA. With two epoch observations which wereseparated by about one year, we found two co-moving companioncandidates, one close to HD 77407 and one close to GJ 577. For thecompanion candidate near GJ 577, we obtained an optical spectrum showingspectral type M 4.5; this candidate is a bound low-mass stellarcompanion confirmed by both proper motion and spectroscopy. We estimatethe masses for HD 77407 B and GJ 577 B to be 0.3 to 0.5Mȯ and 0.16 to 0.2 Mȯ,respectively. Compared to Siess et al. (\cite{Siess2000}) models, eachof the two pairs appears co-eval with HD 77407 A, B being 10 to 40 Myrsand GJ 577 A, B being ≥100 Myrs old. We also took multi-epochhigh-resolution spectra of HD 77407 to search for sub-stellarcompanions, but did not find any with 3 MJup as upper mass (msin i) limit (for up to 4 year orbits); however, we detected a long-termradial velocity trend in HD 77407 A, consistent with a 0.3Mȯ companion at 50 AU separation, i.e. the onedetected by the imaging. Hence, HD 77407 B is confirmed to be a boundcompanion to HD 77407 A. We also present limits for undetected, butdetectable companions using a deep image of HD 77407 A and B, alsoobserved with the Keck NIRC2 AO system; any brown dwarfs were detectableoutside of 0.5 arcsec (17 AU at HD 77407), giant planets with massesfrom 6.5 to 12 MJup were detectable at ≥1.5 arcsec.
| New Hipparcos-based Parallaxes for 424 Faint Stars
We present a catalog of 424 common proper-motion companions to Hipparcosstars with good (>3 σ) parallaxes, thereby effectively providingnew parallaxes for these companions. Compared with typical stars in theHipparcos catalog, these stars are substantially dimmer. The catalogincludes 20 white dwarfs and an additional 29 stars withMV>14, the great majority of the latter being M dwarfs.
| 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.
| A Spectroscopic Technique for Measuring Stellar Properties of Pre-Main-Sequence Stars
We describe a technique for deriving effective temperatures, surfacegravities, rotation velocities, and radial velocities fromhigh-resolution near-IR spectra. The technique matches the observednear-IR spectra to spectra synthesized from model atmospheres. Ouranalysis is geared toward characterizing heavily reddenedpre-main-sequence stars, but the technique also has potentialapplications in characterizing main-sequence and post-main-sequencestars when these lie behind thick clouds of interstellar dust. For thepre-main-sequence stars, we use the same matching process to measure theamount of excess near-IR emission (which may arise in the protostellardisks) in addition to the other stellar parameters. The informationderived from high-resolution spectra comes from line shapes and therelative line strengths of closely spaced lines. The values for thestellar parameters we derive are therefore independent of those derivedfrom low-resolution spectroscopy and photometry. The new method offersthe promise of improved accuracy in placing young stellar objects onevolutionary model tracks. Tests with an artificial noisy spectrum withtypical stellar parameters and a signal-to-noise ratio of 50 indicate a1 σ error of 100 K in Teff, 2 km s-1 invsini, and 0.13 in continuum veiling for an input veiling of 1. If theflux ratio between the sum of the Na, Sc, and Si lines at 2.2 μm andthe (2-0) 12CO band head at 2.3 μm is known to an accuracyof 10%, the errors in our best-fit value for logg will beΔlogg=0.1-0.2. We discuss the possible systematic effects on ourdetermination of the stellar parameters and evaluate the accuracy of theresults derivable from high-resolution spectra. In the context of thisevaluation, we quantitatively explore the degeneracy between temperatureand gravity that has bedeviled efforts to type young stellar objectsusing low-resolution spectra. The analysis of high-resolution near-IRspectra of MK standards shows that the technique yields very accuratevalues for the effective temperature. The greatest uncertainty incomparing our results with optical spectral typing of MK standards is inthe spectral type-to-effective temperature conversion for the standardsthemselves. Even including this uncertainty, the 1 σ differencebetween the optical and infrared temperatures for dwarfs at 3000-5800 Kis only 140 K. In a companion paper, we present an analysis of heavilyextincted young stellar objects in the ρ Ophiuchi molecular cloud.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| 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.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| 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.
| Lithium and Hα in stars and brown dwarfs of sigma Orionis.
We present intermediate- and low-resolution optical spectra aroundHα and Li I lambda 6708 Åfor a sample of 25 low mass starsand 2 brown dwarfs with confirmed membership in the pre-main sequencestellar sigma Orionis cluster. Our observations are intended toinvestigate the age of the cluster. The spectral types derived for ourtarget sample are found to be in the range K6-M8.5, which corresponds toa mass interval of roughly 1.2-0.02 Msun on the basis ofstate-of-the-art evolutionary models. Radial velocities (except for oneobject) are found to be consistent with membership in the Orion complex.All cluster members show considerable Hα emission and the Li Iresonance doublet in absorption, which is typical of very young ages. Wefind that our pseudo-equivalent widths of Hα and Li I (measuredrelative to the observed local pseudo-continuum formed by molecularabsorptions) appear rather dispersed (and intense in the case ofHα ) for objects cooler than M3.5 spectral class, occurring at theapproximate mass where low mass stars are expected to become fullyconvective. The least massive brown dwarf in our sample, S Ori 45 (M8.5,~ 0.02 Msun), displays variable Hα emission and aradial velocity that differs from the cluster mean velocity. Tentativedetection of forbidden lines in emission indicates that this brown dwarfmay be accreting mass from a surrounding disk. We also present recentcomputations of Li I lambda 6708 Åcurves of growth for lowgravities and for the temperature interval (about 4000-2600 K) of oursample. The comparison of our observations to these computations allowsus to infer that no lithium depletion has yet taken place in sigmaOrionis, and that the observed pseudo-equivalent widths are consistentwith a cluster initial lithium abundance close to the cosmic value.Hence, the upper limit to the sigma Orionis cluster age can be set at 8Myr, with a most likely value around 2-4 Myr. Based on observations madewith the following telescopes: 3.5-m telescope at the Spanish-GermanCalar Alto Observatory (Spain) operated by the Max-Planck-Institutfür Astronomie in Heidelberg (Germany); 2.5-m Isaac Newtontelescope operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de Los Muchachos of the Institutode Astrofísica de Canarias; 2.1-m Otto Struve telescope atMcDonald Observatory (USA); and the 10-m Keck II telescope of the W. M.Keck Observatory, which is operated as a scientific partnership amongthe California Institute of Technology, the University of California andthe National Aeronautics and Space Administration (the Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation).
| Hydroxyl 1.563 Micron Absorption from Starspots on Active Stars
We present results from a study of starspots on active stars using apair of vibrational-rotational absorption lines of the OH molecule near1.563 μm. We detect excess OH absorption due to dark, cool starspotson several active stars of the RS CVn and BY Dra classes. Our resultsfor the single-lined spectroscopic binaries II Pegasi, V1762 Cygni, andλ Andromedae augment those from a previous study that used a lesssensitive detector. In this study, we were able for the first time touse molecular absorption features to measure starspot properties ondouble-lined spectroscopic binaries. Measuring the equivalent widths ofthese OH lines in inactive giant and dwarf stars of spectral types G, K,and M, we find that the total equivalent width of the line pairincreases approximately linearly as effective temperature decreases from5000 to 3000 K. We measure starspot filling factors by fitting thespectra of active stars with linear combinations of comparison starspectra representing the spot and nonspot regions of the star.
| A Coronagraphic Survey for Companions of Stars within 8 Parsecs
We present the technique and results of a survey of stars within 8 pc ofthe Sun with declinations δ>-35° (J2000.00). The survey,designed to find without color bias faint companions, consists ofoptical coronagraphic images of the 1' field of view centered on eachstar and infrared direct images with a 32" field of view. The imageswere obtained through the optical Gunn r and z filters and the infraredJ and K filters. The survey achieves sensitivities up to 4 absolutemagnitudes fainter than the prototype brown dwarf, Gliese 229B. However,this sensitivity varies with the seeing conditions, the intrinsicbrightness of the star observed, and the angular distance from the star.As a result, we tabulate sensitivity limits for each star in the survey.We used the criterion of common proper motion to distinguish companionsand to determine their luminosities. In addition to the brown dwarf Gl229B, we have identified six new stellar companions of the sample stars.Since the survey began, accurate trigonometric parallax measurements formost of the stars have become available. As a result, some of the starswe originally included should no longer be included in the 8 pc sample.In addition, the 8 pc sample is incomplete at the faint end of the mainsequence, complicating our calculation of the binary fraction of browndwarfs. We assess the sensitivity of the survey to stellar companionsand to brown dwarf companions of different masses and ages.
| A database of high and medium-resolution stellar spectra
We present a database of 908 spectra of 709 stars obtained with theELODIE spectrograph at the Observatoire de Haute-Provence. 52 orders ofthe echelle spectra have been carefully fitted together to providecontinuous, high-resolution spectra in the wavelength range lambdalambda = 410-680 nm. The archive provides a large coverage of the spaceof atmospheric parameters: T_eff from 3700 K to 13 600 K, log g from0.03 to 5.86 and [Fe/H] from -2.8 to +0.7. At the nominal resolution,R=42 000, the mean signal-to-noise ratio is 150 per pixel. The spectragiven at this resolution are normalized to their pseudo-continuum andare intended to serve for abundance studies, spectral classification andtests of stellar atmosphere models. A lower resolution version of thearchive, at R=10 000, is calibrated in physical flux with a broad-bandphotometric precision of 2.5% and narrow-band precision of 0.5%. It iswell suited to stellar population synthesis of galaxies and clusters,and to kinematical investigations of stellar systems. The archive isdistributed in FITS format through the HYPERCAT and CDS databases. Basedon observations made on the 193 cm telescope at the Haute-ProvenceObservatory, France. Table 1 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/1048
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orsa Maggiore |
| Ascensione retta: | 09h14m22.77s |
| Declinazione: | +52°41'11.8" |
| Magnitudine apparente: | 7.642 |
| Distanza: | 6.189 parsec |
| Moto proprio RA: | -1555.6 |
| Moto proprio Dec: | -570 |
| B-T magnitude: | 9.412 |
| V-T magnitude: | 7.789 |
Cataloghi e designazioni:
|
