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High-metallicity, photoionized gas in intergalactic large-scale filaments
We present high-resolution ultraviolet spectra of absorption-linesystems towards the low-z quasi-stellar object (QSO) HS 0624+6907(zQSO= 0.3700). Coupled with ground-based imaging andspectroscopic galaxy redshifts, we find evidence that many of theseabsorbers do not arise in galaxy haloes but rather are trulyintergalactic gas clouds distributed within large-scale structures, andmoreover, the gas is cool (T < 105 K) and has relativelyhigh metallicity (Z > 0.9Zsolar). Hubble Space Telescope(HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramaticcluster of 13 HI Lyman α (Lyα) lines within a 1000 kms-1 interval at zabs= 0.0635. We find 10 galaxiesat this redshift with impact parameters ranging from ρ=135h-170 kpc to 1.37h-170Mpc. The velocities and velocity spread of the Lyα lines in thiscomplex are unlikely to arise in the individual haloes of the nearbygalaxies; instead, we attribute the absorption to intragroup medium gas,possibly from a large-scale filament viewed along its long axis.Contrary to theoretical expectations, this gas is not the shock-heatedwarm-hot intergalactic medium (WHIM); the width of the Lyα linesall indicate a gas temperature T<< 105 K, and metallines detected in the Lyα complex also favour photoionized, coolgas. No OVI absorption lines are evident, which is consistent withphotoionization models. Remarkably, the metallicity is near-solar,[M/H]=-0.05 +/- 0.4 (2σ uncertainty), yet the nearest galaxy whichmight pollute the intergalactic medium is at least135h-170 kpc away. Tidal stripping from nearbygalaxies appears to be the most likely origin of this highly enriched,cool gas. More than six Abell galaxy clusters are found within 4° ofthe sight line suggesting that the QSO line of sight passes near a nodein the cosmic web. At z~ 0.077, we find absorption systems as well asgalaxies at the redshift of the nearby clusters Abell 564 and Abell 559.We conclude that the sight line pierces a filament of gas and galaxiesfeeding into these clusters. The absorber at zabs= 0.07573associated with Abell 564/559 also has a high metallicity with [C/H]> -0.6, but again the closest galaxy is relatively far from the sightline (ρ= 293h-170 kpc). The Doppler parametersand HI column densities of the Lyα lines observed along the entiresight line are consistent with those measured towards other low-z QSOs,including a number of broad (b > 40kms-1) Lyα lines.
| 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.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| A Comparative Study on Lithium Abundances in Solar-Type Stars With and Without Planets
We have investigated the abundance anomalies of lithium for stars withplanets in the temperature range of 5600-5900 K reported by Israelianand coworkers, as compared to 20 normal stars in the same temperatureand metallicity ranges. Our result indicates a higher probability oflithium depletion for stars with planets in the main-sequence stage. Itseems that stellar photospheric abundances of lithium in stars withplanets may be somewhat affected by the presence of planets. Twopossible mechanisms are considered to account for the lower Liabundances of stars with planets. One is related to the rotation-inducedmixing due to the conservation of angular momentum by the protoplanetarydisk, and the other is a shear instability triggered by planetmigration. These results provide new information on stellar evolutionand the lithium evolution of the Galaxy.
| Lithium Abundances of F-, G-, and K-Type Stars: Profile-Fitting Analysis of the Li I 6708 Doublet
An extensive profile-fitting analysis was performed for the Li(+Fe)6707-6708Å feature of nearby 160 F-K dwarfs/subgiants (including27 planet-host stars) in the Galactic disk ( 7000 K ≳Teff ≳ 5000 K, -1 ≲ [Fe/H] ≲ +0.4), in orderto establish the photospheric lithium abundances of these stars. Thenon-LTE effect (though quantitatively insignificant) was taken intoaccount based on our statistical equilibrium calculations, which werecarried out on an adequate grid of models. Our results confirmed most ofthe interesting observational characteristics revealed by recentlypublished studies, such as the bimodal distribution of the Li abundancesfor stars at Teff ≳ 6000 K, the satisfactory agreementof the upper envelope of the A(Li) vs. [Fe/H] distribution with thetheoretical models, the existence of a positive correlation betweenA(Li) and the stellar mass, and the tendency of lower lithium abundancesof planet-host stars (as compared to stars without planets) at thenarrow ``transition'' region of 5900 K ≳ Teff ≳5800 K. The solar Li abundance derived from this analysis is 0.92 (H =12.00), which is by 0.24dex lower than the widely referenced standardvalue of 1.16.
| Spectroscopic Study on the Atmospheric Parameters of Nearby F--K Dwarfs and Subgiants
Based on a collection of high-dispersion spectra obtained at OkayamaAstrophysical Observatory, the atmospheric parameters (Teff,log g, vt, and [Fe/H]) of 160 mid-F through early-K starswere extensively determined by the spectroscopic method using theequivalent widths of Fe I and Fe II lines along with the numericaltechnique of Takeda et al. (2002, PASJ, 54, 451). The results arecomprehensively discussed and compared with the parameter values derivedby different approaches (e.g., photometric colors, theoreticalevolutionary tracks, Hipparcos parallaxes, etc.) as well as with thepublished values found in various literature. It has been confirmed thatour purely spectroscopic approach yields fairly reliable and consistentresults.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| Radial Velocities of Late-Type Field Subgiant Stars
High-dispersion coudé spectra were observed and measured for 43field subgiants of the G and K spectral types, with the intent ofsearching for hitherto undiscovered spectroscopic binaries. Statisticalanalysis of the measurements revealed no definite but two possible newbinaries. We present the data and discuss the statistical analysis usedto test for new binaries. Additionally, we discuss the techniques forhigh precision of radial-velocity measurement, and the systematic errorsthat interfere with that goal.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo not show dilution of metallicity, (3) no abundance variations for Na,Si, Ti, or Ni are found as a function of condensation temperature, and(4) no correlations between metallicity and orbital period oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| 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.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| The age of the Galactic thin disk from Th/Eu nucleocosmochronology. I. Determination of [Th/Eu] abundance ratios
The purpose of this work is to resume investigation of Galactic thindisk dating using nucleocosmochronology with Th/Eu stellar abundanceratios, a theme absent from the literature since 1990. A stellar sampleof 20 disk dwarfs/subgiants of F5 to G8 spectral types with-0.8≤[Fe/H]≤+0.3 was selected. In stars with such spectral typesand luminosity classes, spectral synthesis techniques must be employedif we wish to achieve acceptably accurate results. An homogeneous,self-consistent set of atmospheric parameters was determined. Effectivetemperatures were determined from photometric calibrations and Hαprofile fitting; surface gravities were obtained from Teff,stellar masses and luminosities; microturbulence velocities andmetallicities were obtained from detailed, differential spectroscopicanalysis, relative to the Sun, using equivalent widths of Fe I and Fe IIlines. Chemical abundances of the elements that contaminate the Th andEu spectral regions (Ti, V, Cr, Mn, Co, Ni, Ce, Nd, and Sm) weredetermined through spectroscopic analysis. Abundance uncertainties werethoroughly scrutinised, their average value (0.10±0.02) dex being found to be satisfactorily low. Eu and Th abundances weredetermined by spectral synthesis of one Eu II line (4129.72 Å) andone Th II line (4019.13 Å), taking into account the detailedhyperfine structures of contaminating Co lines, as well as the hyperfinestructure and isotope shift of the Eu line. Comparison of our abundanceswith literature data shows that our results exhibit a similar behaviour,but a considerably lower scatter (36% lower for Eu, and 61% lower forTh). The [Th/Eu] abundance ratios thus obtained were used, in the secondpaper of this series, to determine the age of the Galactic disk.
| α-, r-, and s-process element trends in the Galactic thin and thick disks
From a detailed elemental abundance analysis of 102 F and G dwarf starswe present abundance trends in the Galactic thin and thick disks for 14elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu).Stellar parameters and elemental abundances (except for Y, Ba and Eu)for 66 of the 102 stars were presented in our previous studies (Bensbyet al. [CITE], A&A, 410, 527, [CITE], A&A, 415, 155). The 36stars that are new in this study extend and confirm our previous resultsand allow us to draw further conclusions regarding abundance trends. Thes-process elements Y and Ba, and the r-element Eu have also beenconsidered here for the whole sample for the first time. With this newlarger sample we now have the following results: 1) smooth and distinctabundance trends that for the thin and thick disks are clearlyseparated; 2) the α-element trends for the thick disk show typicalsignatures from the enrichment of SN Ia; 3) the thick disk stellarsample is in the mean older than the thin disk stellar sample; 4) thethick disk abundance trends are invariant with galactocentric radii(R_m); 5) the thick disk abundance trends appear to be invariant withvertical distance (Z_max) from the Galactic plane. Adding furtherevidence from the literaure we argue that a merger/interacting scenariowith a companion galaxy to produce a kinematical heating of the stars(that make up today's thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk. The 102stars have -1 ≲ [Fe/H] ≲ +0.4 and are all in the solarneighbourhood. Based on their kinematics they have been divided into athin disk sample and a thick disk sample consisting of 60 and 38 stars,respectively. The remaining 4 stars have kinematics that make themkinematically intermediate to the two disks. Their chemical abundancesalso place them in between the two disks. Which of the two diskpopulations these 4 stars belong to, or if they form a distinctpopulation of their own, can at the moment not be settled. The 66 starsfrom our previous studies were observed with the FEROS spectrograph onthe ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-mtelescope. Of the 36 new stars presented here 30 were observed with theSOFIN spectrograph on the Nordic Optical Telescope on La Palma, 3 withthe UVES spectrograph on VLT/UT2, and 3 with the FEROS spectrograph onthe ESO 1.5-m telescope. All spectra have high signal-to-noise ratios(typically S/N≳ 250) and high resolution (R 80 000, 45 000,and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).Based on observations collected at the Nordic Optical Telescope on LaPalma, Spain, and at the European Southern Observatories on La Silla andParanal, Chile, Proposals # 65.L-0019(B), 67.B-0108(B), 69.B-0277. FullTables [see full text], [see full text] and [see full text] are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/185
| 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
| Fine structure of the chromospheric activity in Solar-type stars - The Hα line
A calibration of Hα as both a chromospheric diagnostic and an ageindicator is presented, complementing the works previously done on thissubject (\cite{herbig}; \cite{luca1}). The chromospheric diagnostic wasbuilt with a statistically significant sample, covering nine years ofobservations, and including 175 solar neighborhood stars. Regarding theage indicator, the presence of stars for which very accurate ages aredetermined, such as those belonging to clusters and kinematic groups,lends confidence to our analysis. We also investigate the possibilitythat stars of the same age might have gone through different tracks ofchromospheric decay, identifying - within the same age range - effectsof metallicity and mass. These parameters, however, as well as age, seemto be significant only for dwarf stars, losing their meaning when weanalyze stars in the subgiant branch. This result suggests that, inthese evolved stars, the emission mechanism cannot bemagnetohydrodynamical in nature, in agreement with recent models (Fawzyet al. 2002c, and references therein). The Sun is found to be a typicalstar in its Hα chromospheric flux, for its age, mass andmetallicity. As a byproduct of this work, we developed an automaticmethod to determine temperatures from the wings of Hα, which meansthe suppression of the error inherent to the visual procedure used inthe literature.Based on observations collected at Observatório do Pico dos Dias,operated by the Laboratório Nacional de Astrofísica, CNPq,Brazil.Table 5 is only available in electronic form at thehttp://www.edpsciences.org
| Orbital period variation in close binaries from radial velocity data and magnetic activity cycles. II. HR 1099
We studied orbital period changes in the non-eclipsing close binary HR1099, one of the most bright members of the RS CVn class of magneticallyactive binary systems. Intermediate-resolution optical spectroscopy andIUE archive spectra were used to build radial-velocity curves yieldingepochs of superior conjunction with an accuracy of 0.01 days. The finaldatabase ranged from 1976 to 2002 and allowed us a better assessment ofthe amplitude of the orbital period variation and its timescale. On thebasis of such results, the mechanisms proposed to explain the observedperiod change were briefly discussed giving further support to thepossible connection between the orbital period modulation and the changeof the gravitational quadrupole moment of the K1 subgiant component, inthe framework of the model elaborated by Lanza et al. (\cite{Lanza98}).Based on IUE archive data and observations collected at CataniaAstrophysical Observatory, Italy.Tables 1 and 2 are only 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/429/309
| 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 Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| First Planet Confirmation with a Dispersed Fixed-Delay Interferometer
The Exoplanet Tracker is a prototype of a new type of fiber-fedinstrument for performing high-precision relative Doppler measurementsto detect extrasolar planets. A combination of Michelson interferometerand medium-resolution spectrograph, this low-cost instrument facilitatesradial velocity measurements with high throughput over a small bandwidth(~300 Å) and has the potential to be designed for multiobjectoperation with moderate bandwidths (~1000 Å). We present the firstplanet detection with this new type of instrument, a successfulconfirmation of the well-established planetary companion to 51 Peg,showing an rms precision of 11.5 m s-1 over 5 days. We alsoshow comparison measurements of the radial velocity stable star, ηCas, showing an rms precision of 7.9 m s-1 over 7 days. Thesenew results are starting to approach the precision levels obtained withtraditional radial velocity techniques based on cross-dispersedechelles. We anticipate that this new technique could have an importantimpact in the search for extrasolar planets.
| Sodium abundances in nearby disk stars
We present sodium abundances for a sample of nearby stars. All resultshave been derived from NLTE statistical equilibrium calculations. Theinfluence of collisional interactions with electrons and hydrogen atomsis evaluated by comparison of the solar spectrum with very precise fitsto the Na I line cores. The NLTE effects are more pronounced inmetal-poor stars since the statistical equilibrium is dominated bycollisions of which at least the electronic component is substantiallyreduced. The resulting influence on the determination of sodiumabundances is in a direction opposite to that found previously for Mgand Al. The NLTE corrections are about -0.1 in thick-disk stars with[Fe/H] -0.6. Our [Na/Fe] abundance ratios are about solar forthick- and thin-disk stars. The increase in [Na/Fe] as a function of[Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93}) isconfirmed. Our results suggest that sodium yields increase with themetallicity, and quite large amounts of sodium may be produced by AGBstars. We find that [Na/Fe] ratios, together with either [Mg/Fe] ratio,kinematic data or stellar evolutionary ages, make possible theindividual discrimination between thin- and thick-disk membership.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.Tables \ref{table2} and \ref{table3} are only available in electronicform at http://www.edpsciences.org
| On the determination of oxygen abundances in chromospherically active stars
We discuss oxygen abundances derived from [O I] λ6300s and the OI triplet in stars spanning a wide range in chromospheric activitylevel, and show that these two indicators yield increasingly discrepantresults with higher chromospheric/coronal activity measures. While theforbidden and permitted lines give fairly consistent results forsolar-type disk dwarfs, spuriously high O I triplet abundances areobserved in young Hyades and Pleiades stars, as well as in individualcomponents of RS CVn binaries (up to 1.8 dex). The distinct behaviour ofthe [O I]-based abundances which consistently remain near-solar suggeststhat this phenomenon mostly results from large departures from LTEaffecting the O I triplet at high activity level that are currentlyunaccounted for, but also possibly from a failure to adequately modelthe atmospheres of K-type stars. These results suggest that some cautionshould be exercised when interpreting oxygen abundances in activebinaries or young open cluster stars.Based on observations collected at the European Southern Observatory,Chile (Proposals 64.L-0249 and 071.D-0260).Table \ref{tab_data} is only available in electronic form athttp://www.edpsciences.org
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only 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/423/517
| Oxygen trends in the Galactic thin and thick disks
We present oxygen abundances for 72 F and G dwarf stars in the solarneighbourhood. Using the kinematics of the stars we divide them into twosub-samples with space velocities that are typical for the thick andthin disks, respectively. The metallicities of the stars range from[Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of thestars to: (1) perform a differential study of the oxygen trends in thethin and the thick disk; (2) to follow the trend of oxygen in the thindisk to the highest metallicities. We analyze the forbidden oxygen linesat 6300 Å and 6363 Å as well as the (NLTE afflicted) tripletlines around 7774 Å. For the forbidden line at 6300 Å wehave spectra of very high S/N (>400) and resolution (R ≳ 215000). This has enabled a very accurate modeling of the oxygen line andthe blending Ni lines. The high internal accuracy in our determinationof the oxygen abundances from this line is reflected in the very tighttrends we find for oxygen relative to iron. From these abundances we areable to draw the following major conclusions: (i) That the [O/Fe] trendat super-solar [Fe/H] continues downward which is in concordance withmodels of Galactic chemical evolution. This is not seen in previousstudies as it has not been possible to take the blending Ni lines in theforbidden oxygen line at 6300 Å properly into account; (ii) Thatthe oxygen trends in the thin and the thick disks are distinctlydifferent. This confirms and extends previous studies of the otherα-elements; (iii) That oxygen does not follow Mg at super-solarmetallicities; (iv) We also provide an empirical NLTE correction for theinfrared O I triplet that could be used for dwarf star spectra with aS/N such that only the triplet lines can be analyzed well, e.g. stars atlarge distances; (v) Finally, we find that Gratton et al. (1999)overestimate the NLTE corrections for the permitted oxygen triplet linesat 7774 Å for the parameter space that our stars span.Based on observations collected at the European Southern Observatory, LaSilla and Paranal, Chile, Proposals #65.L-0019, 67.B-0108, and69.B-0277.The full Table 4 is only 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?/A+A/415/155
| The Age of the Oldest Stars in the Local Galactic Disk from Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars, the rolethat they played in the development of the early understanding ofstellar evolution, and their importance in the age dating of theGalactic disk. We use the cataloged data from the Hipparcos satellite inthis latter capacity.Based on Hipparcos parallaxes with relative accuracies ofσπ/π<=0.10, the absolute magnitude of thelower envelope of the nearly horizontal subgiant sequence for fieldstars in the H-R diagram for B-V colors between 0.85 and 1.05 ismeasured to be MV=4.03+/-0.06. New stellar evolutionarytracks calculated for metal abundances in the range-0.29<=[Fe/H]<=+0.37 are fitted to the main-sequence, subgiant,and giant-star distributions in the Hipparcos H-R diagram. Isochronesfor [Fe/H]=+0.37 provide the best fit to the reddest giants between+3>~MV>~0, as well as to the envelope of the reddestmain-sequence stars at MV>~5. The red edge of the densestpart of the distribution of field giants is, however, most readilymatched by isochrones having [Fe/H]~+0.23. Such high metal abundancesare evidently confirmed by the spectroscopically observed highmetallicity (between [Fe/H]=+0.2 and +0.4) of the old thick-diskGalactic cluster NGC 6791, whose color-magnitude diagram can be made tofit either of these red boundaries by adopting suitable values for thereddening and distance modulus (from within the observed ranges ofuncertainties of these quantities).The age of the field stars in the solar neighborhood is found to be7.9+/-0.7 Gyr (or 7.4+/-0.7 Gyr if the stellar models allow for theeffects of diffusive processes) by fitting the theoretical isochronesfor [Fe/H]=+0.37 to the lower envelope of the Hipparcos subgiants.However, this age is a function of metallicity. The models show adependence of δt=-3.99([Fe/H]-0.37) at MV~+4 formetallicities between 0.00 and +0.37. The same grid of isochrones yieldsages, in turn, of 4.0+/-0.2, 6.2+/-0.5, and 7.5 to 10 Gyr (depending onthe assumed reddening) for the old Galactic clusters M67, NGC 188, andNGC 6791 using metal abundances, distance moduli, and reddenings adoptedin the text. Although the distance (and hence age) of NGC 6791 issomewhat uncertain, the ages of both the Galactic disk in the solarneighborhood and of NGC 6791 are, nevertheless, likely between 3 and 5Gyr younger than the oldest halo globular clusters, which have ages of~13.5 Gyr. The conclusion is the same as that reached earlier by Liu& Chaboyer, who used Hipparcos parallax stars that were mainly nearthe main-sequence turnoff rather than at the lower bound of the subgiantluminosity that we determine here. The most significant results are (1)the supermetallicity of the oldest local disk stars and (2) the largeage difference between the most metal poor component of the halo and thethick and thin disk in the solar neighborhood, confirming Liu &Chaboyer. These facts are undoubtedly related and pose again the problemof the proper scenario for the timing of events in the formation of thehalo and the Galactic disk in the solar neighborhood.Part review of the history of the discovery of subgiants and part newresults from Hipparcos.
| The Metal Abundances of NGC 188 and NGC 6791 from Low-Resolution Spectra
Analysis of low-resolution spectra of K giants in the old open clustersNGC 188 and NGC 6791 yields [Fe/H]=0.075+/-0.045+/-σsysfor NGC 188 and [Fe/H]=0.320+/-0.023+/-σsys for NGC6791. The term σsys represents the drift between ourunderlying star catalog's abundance scale and the true abundance scale.Star R23 in NGC 6791 has [Fe/H]>0.6 according to our analysis anddeserves further study.
| All Sky Doppler Extrasolar Planet Surveys with a Multi-object Dispersed Fixed-delay Interferometer
Characterization of extrasolar planetary systems requires a radialvelocity (RV) survey for planets around hundreds of thousands of nearbystars of all spectral types over the next ten years. This survey will beextremely difficult to conduct using a current high resolution echellespectrometer due to its single object observing mode and low instrumentthroughput. Here we propose to use a high throughput multi-objectdispersed fixed-delay interferometer for the survey. This instrument, acombination of a fixed-delay interferometer with a moderate resolutionspectrometer, is completely different from current echellespectrometers. Doppler RV is measured through monitoring interferencefringe shifts of stellar absorption lines over a broad band. Couplingthis multi-object instrument with a wide field telescope (a few degree,such as Sloan and WIYN) and UV, visible and near-IR detectors will allowto simultaneously obtain hundreds of stellar fringing spectra forsearching for planets. The RV survey speed can be increased by more than2 orders of magnitude over that for the echelles.
| 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.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Vízöntő |
| Rektaszcenzió: | 19h24m58.20s |
| Deklináció: | +11°56'40.0" |
| Vizuális fényesség: | 5.16 |
| Távolság: | 15.149 parszek |
| RA sajátmozgás: | 721 |
| Dec sajátmozgás: | 642.8 |
| B-T magnitude: | 6.1 |
| V-T magnitude: | 5.243 |
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