Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Accurate M Dwarf Metallicities from Spectral Synthesis: A Critical Test of Model Atmospheres
We describe a method for accurately determining M dwarf metallicitieswith spectral synthesis based on abundance analyses of visual binarystars. We obtained high-resolution, high-signal-to-noise ratio spectraof each component of five visual binary pairs at McDonald Observatory.The spectral types of the components range from F7 to K3 V for theprimaries and from M0.5 to M3.5 V for the secondaries. We havedetermined the metallicities of the primaries differentially withrespect to the Sun by fitting synthetic spectra to Fe I line profiles inthe observed spectra. In the course of our analysis of the M dwarfsecondaries, we have made significant improvements to the PHOENIXcool-star model atmospheres and the spectrum analysis code MOOG. Ouranalysis yields an rms deviation of 0.11 dex in metallicity valuesbetween the binary pairs. We estimate the uncertainties in the derivedstellar parameters for the M dwarfs to be 48 K, 0.10 dex, 0.12 dex, 0.15km s-1, and 0.20 km s-1 for Teff, logg,[M/H], ξ, and η, respectively. Accurate stellar evolutionarymodels are needed to progress further in the analysis of cool-starspectra; the new model atmospheres warrant recalculation of theevolutionary models.
| 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.
| The Variability and Rotation of Pre-Main-Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?
A variability study of the young cluster IC 348 at Van Vleck Observatoryhas been extended to a total of 7 yr. Twelve new periodic stars havebeen found in the last 2 yr, bringing the total discovered by thisprogram to 40. In addition, we confirm 16 of the periods reported byothers and resolve some discrepancies. The total number of knownrotation periods in the cluster from all studies has now reached 70.This is sufficient to demonstrate that the parent population of K5-M2stars is rotationally indistinguishable from that in the Orion NebulaCluster, even though their radii are 20% smaller and they would beexpected to spin about twice as fast if angular momentum were conserved.The median radius and therefore the inferred age of the IC 348 starsactually closely matches that of NGC 2264, but the stars spinsignificantly more slowly. This suggests that another factor besidesmass and age plays a role in establishing the rotation properties withina cluster, and we suggest that it is environment. If disk locking wereto persist for longer times in less harsh environments because the disksthemselves persist for longer times, it could explain the generallyslower rotation rates observed for stars in this cluster, whose earliesttype star is of class B5. We have also obtained radial velocities, thefirst for pre-main-sequence stars in IC 348, and vsini measurements for30 cluster stars to assist in the study of rotation and as anindependent check on stellar radii. Several unusual variable stars arediscussed; in some or all cases their behavior may be linked tooccultations by circumstellar material. A strong correlation existsbetween the range of photometric variability and the slope of thespectral energy distribution in the infrared. Nineteen of the 21 starswith I ranges exceeding 0.4 mag show infrared evidence for circumstellardisks.
| 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.
| First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars
We have measured the angular diameters of six M dwarfs with the CHARAArray, a long-baseline optical interferometer located at Mount WilsonObservatory. Spectral types range from M1.0 V to M3.0 V and linear radiifrom 0.38 to 0.69 Rsolar. These results are consistent withthe seven other M dwarf radii measurements from optical interferometryand with those for 14 stars in eclipsing binary systems. We compare alldirectly measured M dwarf radii to model predictions and find thatcurrent models underestimate the true stellar radii by up to 15%-20%.The differences are small among the metal-poor stars but becomesignificantly larger with increasing metallicity. This suggests thattheoretical models for low-mass stars may be missing some opacity sourcethat alters the computed stellar radii.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| 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.
| Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
| Calibrating M Dwarf Metallicities Using Molecular Indices
We report progress in the calibration of a method to determine cooldwarf star metallicities using molecular band strength indices. Themolecular band index to metallicity relation can be calibrated usingchemical abundances calculated from atomic-line equivalent widthmeasurements in high-resolution spectra. Building on previous work, wehave measured Fe and Ti abundances in 32 additional M and K dwarf starsto extend the range of temperature and metallicity covered. A test ofour analysis method using warm star-cool star binaries shows we cancalculate reliable abundances for stars warmer than 3500 K. We have usedabundance measurements for warmer binary or cluster companions toestimate abundances in six additional cool dwarfs. Adding stars measuredin our previous work and others from the literature provides 76 starswith Fe abundance and CaH2 and TiO5 index measurements. The CaH2molecular index is directly correlated with temperature. TiO5 depends ontemperature and metallicity. Metallicity can be estimated to within+/-0.3 dex within the bounds of our calibration, which extends fromroughly [Fe/H]=+0.05 to -1.0, with a limited extension to -1.5.
| Anharmonic and standing dynamo waves: theory and observation of stellar magnetic activity
The familiar decadal cycle of solar activity is one expression ofinterannual variability of surface magnetism observed in stars on ornear the lower main sequence. From studies of time-series of CaII H andK emission fluxes that go back more than 35 yr and have been accumulatedfor such stars at the Mount Wilson Observatory by the HK Project, wedefine a quantitative measure, called anharmonicity, of the cycliccomponent of interannual magnetic variability. Anharmonicity provides aconnection between observed variations in magnetic activity and thetwo-dimensional description of a Parker dynamo model. We explore theparameter space of the Parker dynamo model and find an excellentcounterpart in the records of several of the lowest-mass (late K-type toearly M-type) active stars in the HK Project sample to the solutionscontaining highly anharmonic, standing dynamo waves. We interpretanharmonicity apparent in the records as resulting from non-propagatingor standing dynamo waves, which operate in a regime that issubstantially supercriticial. There, for the majority of a cycle, orpulse of decadal-to-interdecadal variability, the large-scale magneticfields are generated and maintained by winding of field by differentialrotation rather than by the joint action of differential rotation andhelical convection. Among the less active stars (the Sun is consideredsuch a star in the HK Project sample) we find a correspondence betweenanharmonicity and Parker dynamo model solutions that include simpleharmonic, migratory and/or intermediate-type dynamo wave patterns over abroad range of dynamo parameters.
| Calibrating models of ultralow-mass stars
Evolutionary and atmospheric models have become available for youngultralow-mass objects. These models are being used to determinefundamental parameters from observational properties. TiO bands are usedto determine effective temperatures in ultralow-mass objects, andtogether with Na- and K-lines to derive gravities at the substellarboundary. Unfortunately, model calibrations in (young) ultralow-massobjects are rare. As a first step towards a calibration of syntheticspectral features, I show molecular bands of TiO, which is a mainopacity source in late M-dwarfs. The TiO \epsilon-band at 8450Å issystematically too weak. This implies that temperatures determined fromthat band are underestimated, and I discuss implications for determiningfundamental parameters from high resolution spectra.
| 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.
| Astrometric Methods and Instrumentation to Identify and Characterize Extrasolar Planets: A Review
I present a review of astrometric techniques and instrumentation used tosearch for, detect, and characterize extrasolar planets. First, Ibriefly summarize the properties of the current sample of extrasolarplanets, in connection with predictions from theoretical models ofplanet formation and evolution. Next, the generic approach to planetdetection with astrometry is described, with significant discussion of avariety of technical, statistical, and astrophysical issues to be facedby future ground-based and space-borne efforts in order to achieve therequired degree of measurement precision. After a brief summary of pastand present efforts to detect planets via milliarcsecond astrometry, Ithen discuss the planet-finding capabilities of future astrometricobservatories aiming at microarcsecond precision. Finally, I outline anumber of experiments that can be conducted by means of high-precisionastrometry during the next decade, to illustrate its potential forimportant contributions to planetary science, compared to other indirectand direct methods for the detection and characterization of planetarysystems.
| 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.
| A Map of the Universe
We have produced a new conformal map of the universe illustrating recentdiscoveries, ranging from Kuiper Belt objects in the solar system to thegalaxies and quasars from the Sloan Digital Sky Survey. This mapprojection, based on the logarithm map of the complex plane, preservesshapes locally and yet is able to display the entire range ofastronomical scales from the Earth's neighborhood to the cosmicmicrowave background. The conformal nature of the projection, preservingshapes locally, may be of particular use for analyzing large-scalestructure. Prominent in the map is a Sloan Great Wall of galaxies 1.37billion light-years long, 80% longer than the Great Wall discovered byGeller and Huchra and therefore the largest observed structure in theuniverse.
| 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).
| 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.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Magnetospheric radio emission from extrasolar giant planets: the role of the host stars
We present a new analysis of the expected magnetospheric radio emissionfrom extrasolar giant planets (EGPs) for a distance limited sample ofthe nearest known extrasolar planets. Using recent results on thecorrelation between stellar X-ray flux and mass-loss rates from nearbystars, we estimate the expected mass-loss rates of the host stars ofextrasolar planets that lie within 20 pc of the Earth. We find that someof the host stars have mass-loss rates that are more than 100 times thatof the Sun and, given the expected dependence of the planetarymagnetospheric radio flux on stellar wind properties, this has a verysubstantial effect. Using these results and extrapolations of the likelymagnetic properties of the extrasolar planets, we infer their likelyradio properties.We compile a list of the most promising radio targets and conclude thatthe planets orbiting Tau Bootes, Gliese 86, Upsilon Andromeda and HD1237(as well as HD179949) are the most promising candidates, with expectedflux levels that should be detectable in the near future with upcomingtelescope arrays. The expected emission peak from these candidate radioemitting planets is typically ~40-50 MHz. We also discuss a range ofobservational considerations for detecting EGPs.
| Metallicity measurements using atomic lines in M and K dwarf stars
We report the first survey of chemical abundances in M and K dwarf starsusing atomic absorption lines in high-resolution spectra. We havemeasured Fe and Ti abundances in 35 M and K dwarf stars using equivalentwidths measured from λ/Δλ~ 33000 spectra. Ouranalysis takes advantage of recent improvements in model atmospheres oflow-temperature dwarf stars. The stars have temperatures between 3300and 4700 K, with most cooler than 4100 K. They cover an iron abundancerange of -2.44 < [Fe/H] < +0.16. Our measurements show [Ti/Fe]decreasing with increasing [Fe/H], a trend similar to that measured forwarmer stars, where abundance analysis techniques have been tested morethoroughly. This study is a step towards the observational calibrationof procedures to estimate the metallicity of low-mass dwarf stars usingphotometric and low-resolution spectral indices.
| A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436
We report precise Doppler measurements of GJ 436 (M2.5 V) obtained atKeck Observatory. The velocities reveal a planetary companion withorbital period of 2.644 days, eccentricity of 0.12 (consistent withzero), and velocity semiamplitude of K=18.1 m s-1. Theminimum mass (Msini) for the planet is0.067MJup=1.2MNep=21MEarth, making itthe lowest mass exoplanet yet found around a main-sequence star and thefirst candidate in the Neptune-mass domain. GJ 436 (mass = 0.41Msolar) is only the second M dwarf found to harbor a planet,joining the two-planet system around GJ 876. The low mass of the planetraises questions about its constitution, with possible compositions ofprimarily H and He gas, ice/rock, or rock-dominated. The impliedsemimajor axis is a=0.028AU=14 stellar radii, raising issues of planetformation, migration, and tidal coupling with the star. GJ 436 is morethan 3 Gyr old, based on both kinematic and chromospheric diagnostics.The star exhibits no photometric variability on the 2.644 day Dopplerperiod to a limiting amplitude of 0.0004 mag, supporting the planetaryinterpretation of the Doppler periodicity. Photometric transits of theplanet across the star are ruled out for gas giant compositions and arealso unlikely for solid compositions. As the third closest knownplanetary system, GJ 436 warrants follow-up observations byhigh-resolution optical and infrared imaging and by the SpaceInterferometry Mission.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.
| 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.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Extra-solar system planets: searches, discoveries and characteristics
The study of planets outside our solar system constitutes a new branchof astronomy that literally did not exist a decade ago. This discussionbegins with how people have thought about other worlds in the past andsome of the reported detections that turned out not to be true. Itcontinues with a brief description of several successful ways of findingexoplanets and the properties of the planets found and their host stars,and concludes with an attempt to look ahead. Most of the planets nowknown revealed themselves because their mutual orbits with their parentstars impose small, periodic radial velocity shifts in the stellarspectra, and most of the host stars are rich in heavy elements by thestandards of the solar neighbourhood. The inventory of actual andpotential detection methods has reached about two dozen.
| 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
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Большая Медведица |
| Прямое восхождение: | 11h03m20.20s |
| Склонение: | +35°58'11.6" |
| Видимая звёздная величина: | 7.506 |
| Расстояние: | 2.548 парсек |
| Собственное движение RA: | -577 |
| Собственное движение Dec: | -4761.8 |
| B-T magnitude: | 9.357 |
| V-T magnitude: | 7.659 |
Каталоги и обозначения:
|
