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The Palomar/Keck Adaptive Optics Survey of Young Solar Analogs: Evidence for a Universal Companion Mass Function
We present results from an adaptive optics survey for substellar andstellar companions to Sun-like stars. The survey targeted 266 F5-K5stars in the 3 Myr-3 Gyr age range with distances of 10-190 pc.Results from the survey include the discovery of two brown dwarfcompanions (HD 49197B and HD 203030B), 24 new stellar binaries, and atriple system. We infer that the frequency of 0.012-0.072Msun brown dwarfs in 28-1590 AU orbits around young solaranalogs is 3.2+3.1 -2.7% (2σ limits).The result demonstrates that the deficiency of substellar companions atwide orbital separations from Sun-like stars is less pronounced than inthe radial velocity "brown dwarf desert." We infer that the massdistribution of companions in 28-1590 AU orbits around solar-mass starsfollows a continuous dN/dM 2 vprop M -0.42 relation over the 0.01-1.0 M sun secondary massrange. While this functional form is similar to that for isolatedobjects less than 0.1 M sun, over the entire 0.01-1.0 Msun range, the mass functions of companions and of isolatedobjects differ significantly. Based on this conclusion and on similarresults from other direct imaging and radial velocity companion surveysin the literature, we argue that the companion mass function follows thesame universal form over the entire range between 0 and 1590 AU inorbital semimajor axis and ≈ 0.01-20 M sun in companionmass. In this context, the relative dearth of substellar versus stellarsecondaries at all orbital separations arises naturally from theinferred form of the companion mass function.
| Seismic analysis of 70 Ophiuchi A: A new quantity proposed
The basic intent of this paper is to model 70 Ophiuchi A using thelatest asteroseismic observations as complementary constraints and todetermine the fundamental parameters of the star. Additionally, wepropose a new quantity to lift the degeneracy between the initialchemical composition and stellar age. Using the Yale stellar evolutioncode (YREC7), we construct a series of stellar evolutionary tracks forthe mass range M = 0.85 0.93 Mȯ with differentcomposition Yi (0.26 0.30) and Zi (0.017 0.023).Along these tracks, we select a grid of stellar model candidates thatfall within the error box in the HR diagram to calculate the theoreticalfrequencies, the large- and small-frequency separations usingGuenther’s stellar pulsation code. Following the asymptoticformula of stellar p-modes, we define a quantity r01 which iscorrelated with stellar age. Also, we test it by theoretical adiabaticfrequencies of many models. Many detailed models of 70 Ophiuchi A arelisted in Table 3. By combining all non-asteroseismic observationsavailable for 70 Ophiuchi A with these seismological data, we think thatModel 60, Model 125 and Model 126, listed in Table 3, are the optimummodels presently. Meanwhile, we predict that the radius of this star isabout 0.860 0.865 Rȯ and the age is about 6.8 7.0 Gyrwith mass 0.89 0.90 Mȯ. Additionally, we prove that thenew quantity r01 can be a useful indicator of stellar age.
| Analysis of 70 Ophiuchi AB including seismic constraints
Context: The analysis of solar-like oscillations for stars belonging toa binary system provides a unique opportunity to probe the internalstellar structure and to test our knowledge of stellar physics. Suchoscillations have been recently observed and characterized for the Acomponent of the 70 Ophiuchi system. Aims: We determined theglobal parameters of 70 Ophiuchi AB using the new asteroseismicmeasurements now available for 70 Oph A and testedthe input physics introduced in stellar evolution codes. Methods:Three different stellar evolution codes and two different calibrationmethods were used to perform a comprehensive analysis of the 70 Ophiuchisystem. Results: A model of 70 Ophiuchi AB that correctlyreproduces all observational constraints available for both stars isdetermined. An age of 6.2 ± 1.0 Gyr is found with an initialhelium mass fraction Yi=0.266 ± 0.015 and an initialmetallicity (Z/X)i=0.0300 ± 0.0025 when atomicdiffusion is included and a solar value of the mixing-length parameterassumed. A precise and independent determination of the value of themixing-length parameter needed to model 70 Oph Arequires accurate measurement of the mean small separation, which is notavailable yet. Current asteroseismic observations, however, suggest thatthe value of the mixing-length parameter of 70 Oph Ais lower or equal to the solar calibrated value. The effects of atomicdiffusion and of the choice of the adopted solar mixture were alsostudied. We finally found that the different evolution codes andcalibration methods we used led to perfectly coherent results.
| Nearby stars of the Galactic disc and halo - IV
The Milky Way Galaxy has an age of about 13 billion years. Solar-typestars evolve all the long way to the realm of degenerate objects onessentially this time-scale. This, as well as the particular advantagethat the Sun offers through reliable differential spectroscopicanalyses, render these stars the ideal tracers for the fossil record ofour parent spiral. Astrophysics is a science that is known to benotoriously plagued by selection effects. The present work - with amajor focus in this fourth contribution on model atmosphere analyses ofspectroscopic binaries and multiple star systems - aims at avolume-complete sample of about 300 nearby F-, G-, and K-type stars thatparticularly avoids any kinematical or chemical pre-selection from theoutset. It thereby provides an unbiased record of the local stellarpopulations - the ancient thick disc and the much younger thin disc. Onthis base, the detailed individual scrutiny of the long-lived stars ofboth populations unveils the thick disc as a single-burst component witha local normalization of no less than 20 per cent. This enormousfraction, combined with its much larger scaleheight, implies a mass forthe thick disc that is comparable to that of the thin disc. On accountof its completely different mass-to-light ratio the thick disc therebybecomes the dark side of the Milky Way, an ideal major source forbaryonic dark matter. This massive, ancient population consequentlychallenges any gradual build-up scenario for our parent spiral. Evenmore, on the supposition that the Galaxy is not unusual, the thick disc- as it emerges from this unbiased spectroscopic work - particularlychallenges the hierarchical cold-dark-matter-dominated formation picturefor spiral galaxies in general.
| The helium abundance and ΔY/ΔZ in lower main-sequence stars
We use nearby K dwarf stars to measure the helium-to-metal enrichmentratio ΔY/ΔZ, a diagnostic of the chemical history of thesolar neighbourhood. Our sample of K dwarfs has homogeneously determinedeffective temperatures, bolometric luminosities and metallicities,allowing us to fit each star to the appropriate stellar isochrone anddetermine its helium content indirectly. We use a newly computed set ofPadova isochrones which cover a wide range of helium and metal content.Our theoretical isochrones have been checked against a congruous set ofmain-sequence binaries with accurately measured masses, to discuss andvalidate their range of applicability. We find that the stellar massesdeduced from the isochrones are usually in excellent agreement withempirical measurements. Good agreement is also found with empiricalmass-luminosity relations.Despite fitting the masses of the stars very well, we find thatanomalously low helium content (lower than primordial helium) isrequired to fit the luminosities and temperatures of the metal-poor Kdwarfs, while more conventional values of the helium content are derivedfor the stars around solar metallicity.We have investigated the effect of diffusion in stellar models and theassumption of local thermodynamic equilibrium (LTE) in derivingmetallicities. Neither of these is able to resolve the low-heliumproblem alone and only marginally if the cumulated effects are included,unless we assume a mixing-length which is strongly decreasing withmetallicity. Further work in stellar models is urgently needed.The helium-to-metal enrichment ratio is found to be ΔY/ΔZ =2.1 +/- 0.9 around and above solar metallicity, consistent with previousstudies, whereas open problems still remain at the lowest metallicities.Finally, we determine the helium content for a set of planetary hoststars.
| Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors
We here develop an improved way of using a rotating star as a clock, setit using the Sun, and demonstrate that it keeps time well. Thistechnique, called gyrochronology, derives ages for low-massmain-sequence stars using only their rotation periods and colors. Thetechnique is developed here and used to derive ages for illustrativegroups of nearby field stars with measured rotation periods. We firstdemonstrate the reality of the interface sequence, the unifying featureof the rotational observations of cluster and field stars that makes thetechnique possible, and extend it beyond the proposal of Skumanich byspecifying the mass dependence of rotation for these stars. We delineatewhich stars it cannot currently be used on. We then calibrate the agedependence using the Sun. The errors are propagated to understand theirdependence on color and period. Representative age errors associatedwith the technique are estimated at ~15% (plus possible systematicerrors) for late F, G, K, and early M stars. Gyro ages for the MountWilson stars are shown to be in good agreement with chromospheric agesfor all but the bluest stars, and probably superior. Gyro ages are thencalculated for each of the active main-sequence field stars studied byStrassmeier and collaborators. These are shown to have a median age of365 Myr. The sample of single field stars assembled by Pizzolato andcollaborators is then assessed and shown to have gyro ages ranging fromunder 100 Myr to several Gyr, with a median age of 1.2 Gyr. Finally, wedemonstrate that the individual components of the three wide binariesξ Boo AB, 61 Cyg AB, and α Cen AB yield substantially the samegyro ages.
| X-ray emission from T Tauri stars and the role of accretion: inferences from the XMM-Newton extended survey of the Taurus molecular cloud
Context: T Tau stars display different X-ray properties depending onwhether they are accreting (classical T Tau stars; CTTS) or not(weak-line T Tau stars; WTTS). X-ray properties may provide insight intothe accretion process between disk and stellar surface. Aims: We usedata from the XMM-Newton Extended Survey of the Taurus molecular cloud(XEST) to study differences in X-ray properties between CTTS and WTTS. Methods: XEST data are used to perform correlation and regressionanalysis between X-ray parameters and stellar properties. Results: Weconfirm the existence of a X-ray luminosity (L_X) vs. mass (M) relation,L_X∝ M1.69~± 0.11, but this relation is aconsequence of X-ray saturation and a mass vs. bolometric luminosity(L_*) relation for the TTS with an average age of 2.4 Myr. X-raysaturation indicates LX = const.L_*, although the constant isdifferent for the two subsamples: const. = 10-3.73~±0.05 for CTTS and const. = 10-3.39~± 0.06 forWTTS. Given a similar L* distribution of both samples, theX-ray luminosity function also reflects a real X-ray deficiency in CTTS,by a factor of ≈2 compared to WTTS. The average electron temperaturesT_av are correlated with LX in WTTS but not in CTTS; CTTSsources are on average hotter than WTTS sources. At best marginaldependencies are found between X-ray properties and mass accretion ratesor age. Conclusions: The most fundamental properties are the twosaturation laws, indicating suppressed LX for CTTS. Wespeculate that some of the accreting material in CTTS is cooling activeregions to temperatures that may not significantly emit in the X-rayband, and if they do, high-resolution spectroscopy may be required toidentify lines formed in such plasma, while CCD cameras do not detectthese components. The similarity of the LX vs. T_avdependencies in WTTS and main-sequence stars as well as their similarX-ray saturation laws suggests similar physical processes for the hotplasma, i.e., heating and radiation of a magnetic corona.
| First Doppler images of the very young K2-dwarf PW Andromedae = HD 1405
Aims.We present the first Doppler images of the single pre-main-sequencestar PW And. Its position in the HR-diagram suggests it to be in therapid-braking phase just prior to arrival on the ZAMS. Methods:.Images are obtained from seven individual spectral lines as well asfrom 5-nm wide chunks of spectrum that invert a total of 58 lineprofiles simultaneously. Results: .Cool spots with temperaturedifferences with respect to the stellar photosphere of up to 1200 K aredetected. Spot occurrence is mostly within an equatorial band up to±40° of the stellar equator and thus contradictsmagnetic-flux tube emergence models based on young K-star propertiesthat predict an emerging latitude in two distinct bands of +45-55°.This confirms previous suggestions that current magnetic-flux tubemodels predict emergence latitudes too low for G-dwarfs and too high forK-dwarfs, which may be caused by the fact that the G-dwarf modelsheavily rely on comparisons with the Sun. We also redetermine theabsolute parameters of PW And in the light of a missing Hipparcosparallax and revise its age to be more near 20 Myr, in agreement withits logarithmic lithium abundance of 2.87±0.05. A preciserotational period of 1.76159d±0.00006 was found from ourphotometric data in 2004.
| HD 203030B: An Unusually Cool Young Substellar Companion near the L/T Transition
We present the discovery of a brown dwarf companion to the star HD203030: a G8 V Solar analog with an estimated age between 130 and 400Myr old G8 V. Separated by 11.9" (487 AU in projection) from its hoststar, HD 203030B has an estimated mass of0.023+0.008-0.011 Msolar. The K-bandspectral type of L7.5+/-0.5 places HD 203030B near the critical L/Ttransition in brown dwarfs, which is characterized by the rapiddisappearance of dust in substellar photospheres. From a comparativeanalysis with well-characterized field L/T transition dwarfs, we findthat, despite its young age, HD 203030B has a bolometric luminositysimilar to the >1 Gyr old field dwarfs. Adopting a radius fromcurrent models of substellar evolution, we hence obtain that theeffective temperature of HD 203030B is only1206+74-116 K, markedly lower than the ~1440 Keffective temperatures of field L/T transition dwarfs. The temperaturediscrepancy can be resolved if either (1) the ages of field brown dwarfshave been overestimated by a factor of ~1.5, leading to underestimatedradii, or (2) the lower effective temperature of HD 203030B is relatedto its young age, implying that the effective temperature at the L/Ttransition is gravity dependent.
| Detection Limits from the McDonald Observatory Planet Search Program
Based on the long-term radial velocity surveys carried out with theMcDonald Observatory 2.7 m Harlan J. Smith Telescope from 1988 to thepresent, we derive upper limits to long-period giant planet companionsfor 31 nearby stars. Data from three phases of the McDonald Observatory2.7 m planet-search program have been merged together, and for 17objects data from the pioneering Canada-France-Hawaii Telescope radialvelocity program have also been included in the companion-limitsdetermination. For those 17 objects, the baseline of observations is inexcess of 23 yr, enabling the detection or exclusion of giant planets inorbits beyond 8 AU. We also consider the possibility of eccentric orbitsin our computations. At an orbital separation of 5.2 AU, we can excludeon average planets of Msini>~(2.0+/-1.1)MJ (e=0) andMsini>~(4.0+/-2.8)MJ (e=0.6) for 25 of the 31 stars inthis survey. However, we are not yet able to rule out ``true Jupiters,''i.e., planets of Msini~1MJ in 5.2 AU orbits. These limits areof interest for the Space Interferometry Mission, Terrestrial PlanetFinder, and Darwin missions, which will search for terrestrial planetsorbiting nearby stars, many of which are included in this work.
| Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed by Chandra
We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph(K1 V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-Sspectra of all four of these stars are presented and compared with anarchival LETG spectrum of another moderately active K dwarf, ɛEri. Coronal densities are estimated from O VII line ratios and emissionmeasure distributions are computed for all five of these stars. We seeno substantial differences in coronal density or temperature among thesestars, which is not surprising considering that they are all early Kdwarfs with similar activity levels. However, we do see significantdifferences in coronal abundance patterns. Coronal abundance anomaliesare generally associated with the first ionization potential (FIP) ofthe elements. On the Sun, low-FIP elements are enhanced in the coronarelative to high-FIP elements, the so-called FIP effect. Differentlevels of FIP effect are seen for our stellar sample, ranging from 70Oph A, which shows a prominent solar-like FIP effect, to 70 Oph B, whichhas no FIP bias at all or possibly even a weak inverse FIP effect. Thestrong abundance difference exhibited by the two 70 Oph stars isunexpected considering how similar these stars are in all other respects(spectral type, age, rotation period, X-ray flux). It will be difficultfor any theoretical explanation for the FIP effect to explain how twostars so similar in all other respects can have coronae with differentdegrees of FIP bias. Finally, for the stars in our sample exhibiting aFIP effect, a curious difference from the solar version of thephenomenon is that the data seem to be more consistent with the high-FIPelements being depleted in the corona rather than with a low-FIPenhancement.
| Asteroseismology of the visual binary 70 Ophiuchi
Context: .Convection in stars excites resonant acoustic waves. Thefrequencies of these oscillations depend on the sound speed inside thestar, which in turn depends on density, temperature, gas motion, andother properties of the stellar interior. Therefore, analysis of theoscillations provides an unrivaled method to probe the internalstructure of a star. Aims: .Solar-like oscillations in the primaryof the visual binary 70 Ophiuchi are investigated. Methods: .70Ophiuchi A was observed with the Harps spectrograph mounted on the 3.6-mtelescope at the ESO La Silla Observatory (Chile) during 6 nights inJuly 2004 allowing us to collect 1758 radial velocity measurements witha standard deviation of about 1.39 m s-1. Results: .Thepower spectrum of the high precision velocity time series clearlypresents several identifiable peaks between 3 and 6 mHz showingregularity with a large spacing of Δν = 161.7 ± 0.3μHz. Fourteen individual modes were identified with amplitudes in therange 11 to 14 cm s-1.
| Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution
We present an H-band image of the light scattered from circumstellardust around the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ803, HD 197481), obtained with the Keck adaptive optics system. Weresolve the disk both vertically and radially, tracing it over 17-60 AUfrom the star. Our high angular resolution (0.04 or 0.4 AU perresolution element) observations thus offer the opportunity to probe themorphology of the AU Mic debris disk on solar system scales. We identifysubstructures (dust clumps and gaps) in the disk that may point to theexistence of orbiting planets. No planets are seen in our H-band imagedown to a limiting mass of 1MJ at >20 AU, although theexistence of smaller planets cannot be excluded from the current data.Modeling of the disk surface brightness distribution at H band and Rband, and of the optical to submillimeter spectral energy distribution,allows us to constrain the disk geometry and the dust grain properties.We confirm the nearly edge-on orientation of the disk inferred fromprevious observations and deduce that the disk may be clear inward of1-10 AU. We find evidence for a lack of small grains at <50 AU,either as a result of grain growth or because of destruction byPoynting-Robertson and/or corpuscular drag. A change in the power-lawindex of the surface brightness profile is observed near 33 AU, similarto a feature known in the profile of the β Pic circumstellar debrisdisk. By comparing the timescales for interparticle collisions andPoynting-Robertson drag between the two systems, we argue that thebreaks are linked to one or both of these processes.
| 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.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| A High-Resolution Spectrum of the TrES-1 Parent Star
We present a high-resolution (R=120,000) spectrum of the lithium regionof the parent star of the newly-discovered transiting planet TrES-1(GSC02652-01324, K0V, V=11.8mag). No lithium seems to be present at thesurface of TrES-1.
| 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.
| Intermediate-metallicity, high-velocity stars and Galactic chemical evolution
High signal-to-noise ratio spectra were obtained of 10high-proper-motion stars having -1 <~[Fe/H] < 0, and a comparablenumber of disc stars. All but two of the high-proper-motion stars wereconfirmed to have [Fe/H] > -1.0, some approaching solar metallicity,but, even so, earlier measurements overestimated the metallicities andvelocities of some of these stars. Models of stellar populations wereused to assign membership probabilities to the Galactic components towhich the high-velocity stars might belong. Many were found to be moreprobably thick-disc than halo objects, despite their large spacemotions, and two might be associated with the inner Galaxy. It may benecessary to reassess contamination of previous halo samples, such asthose used to define the metallicity distribution, to account forcontamination by high-velocity thick-disc stars, and to considerpossible subcomponents of the halo.The change in [α/Fe] ratios at [Fe/H]~=-1.0 is often used toconstrain the degree and timing of Type Ia supernova nucleosynthesis inGalactic chemical-evolution models. [Ti/Fe] values were measured foreight of the high-velocity stars. Both high- and low-[Ti/Fe] halo starsexist; likewise high- and low-[Ti/Fe] thick-disc stars exist. Weconclude that the [Ti/Fe]`break' is not well defined for a givenpopulation; nor is there a simple, continuous evolutionary sequencethrough the break. Implications for the interpretation of the[α/Fe] break in terms of SN Ia time-scales and progenitors arediscussed. The range of [Ti/Fe] found for high-velocity (low rotation)thick-disc stars contrasts with that for the low-velocity (highrotation) thick-disc sample studied by Prochaska et al.
| Selection criteria for targets of asteroseismic campaigns
Various dedicated satellite projects are underway or in advanced stagesof planning to perform high-precision, long duration time seriesphotometry of stars, with the purpose of using the frequencies ofstellar oscillations to put new constraints on the internal structure ofstars. It is known (cf. \cite{Bro+94}) that the effectiveness ofoscillation frequencies in constraining stellar model parameters issignificantly higher if classical parameters such as effectivetemperature and luminosity are known with high precision. In order tooptimize asteroseismic campaigns it is therefore useful to selecttargets from among candidates for which good spectroscopic andastrometric data already exists. This paper presents selection criteria,as well as redeterminations of stellar luminosity and reddening forstars satisfying these criteria.
| 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.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| 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.
| 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 electron pressure in the outer atmosphere of ɛ Eri (K2 V)
Observations of ɛ Eri (K2 V) have been made with the SpaceTelescope Imaging Spectrograph on the Hubble Space Telescope. Thespectra obtained show a number of emission lines which can be used todetermine, or place limits on, the electron density and pressure. Valuesof the electron pressure are required in order to make quantitativemodels of the transition region and inner corona from absolute linefluxes, and to constrain semi-empirical models of the chromosphere.Using line flux ratios in Siiii and Oiv a mean electron pressure ofPe=NeTe=4.8×1015cm-3Kis derived. This value is compatible with the lower and upper limits toPe found from flux ratios in Ciii, Ov and Fexii. Someinconsistencies which may be because of small uncertainties in theatomic data used are discussed.
| Detecting magnetic fields of upper-main-sequence stars with FORS1 at ANTU.
Not Available
| 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.
| Detection of magnetic dipole lines of Fexii in the ultraviolet spectrum of the dwarf star ɛ Eri
We report observations of the dwarf star ɛ Eri (K2V) made withthe Space Telescope Imaging Spectrograph (STIS) on the Hubble SpaceTelescope. The high sensitivity of the STIS instrument has allowed us todetect the magnetic dipole transitions of Fexii at 1242.00 and1349.38Å for the first time in a star other than the Sun. Thewidth of the stronger line at 1242.00Å has also been measured;such measurements are not possible for the permitted lines of Fexii inthe extreme-ultraviolet. To within the accuracy of the measurements, theNv and the Fexii lines occur at their rest wavelengths. Electrondensities and linewidths have been measured from other transition regionlines. Together, these can be used to investigate the non-thermal energyflux in the lower and upper transition regions, which is useful inconstraining possible heating processes. The Fexii lines are alsopresent in archival STIS spectra of other G/K-type dwarfs.
| 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
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