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Asteroseismology of solar-type stars with CORALIE and HARPS . I: Observations and modelling of single stars
We present here the detection and characterization of solar-likeoscillations in several targets such as beta Vir, eta Boo, delta Eri,chi Eri and the Am star HD 209625 obtained with the CORALIE and HARPSspectrographs based at the ESO La Silla Observatory. The measurement ofthe frequencies of p-mode oscillations provides an insight into theinternal structure and is nowadays the most powerful constraint on thetheory of stellar evolution.
| Bisectors of the cross-correlation function applied to stellar spectra. Discriminating stellar activity, oscillations and planets
Context: .Bisectors of strong, single spectral lines, usually the Fe I6252 line, have traditionally been used to examine the velocity fieldsin stellar atmospheres. This requires high S/N often achieved by summingmany individual spectra. Aims: .We investigate whether bisectorsderived from cross-correlation functions (CCF) of single-exposurespectra can be used to provide information on stellar atmospheres, andwhether they can be used to discriminate between radial velocity changescaused by planets, magnetic activity and oscillations. Methods:.Using a sample of bright stars observed with the HARPS spectrograph, weexamine the shapes of the bisectors of individual strong spectral linesin summed spectra, comparing with similar studies in the literature.Moreover, we examine four different quantitative CCF bisector measuresfor correlations with radial velocity and stellar parameters.Results: .We show that CCF bisector measures can be used forquantitative analysis, employing both the absolute values and thevariations. From absolute values, log g and absolute magnitude can beapproximated, and from the correlations with radial velocity one candistinguish between magnetic activity, oscillations and orbiting planetsas the probable cause of radial velocity variations. We confirm thatdifferent isolated spectral lines show different bisector shapes, evenbetween lines of the same element, calling for caution in trying toderive global stellar properties from the bisector of a CCF. For theactive star HR 1362 we suggest from the bisector shape an extraphotospheric heating caused by the chromosphere of several hundreddegrees. We confirm the fill-in of spectral lines of the Sun taken onthe daylight sky caused by Rayleigh-Brillouin and aerosol scattering,and we show for the first time that the fill-in has an asymmetriccomponent.
| The dependence of the Rossby number and XUV-Lyα emission flux with age for solar-like G-type stars
Stellar parameters of 11 G-type stars with ages ranging from 0.1 to 8.5Gyr, from the Sun in Time programme, were used to compute the Rossbynumber, ℜ, for each star. The Rossby number for each star wascalculated from the rotation period and the convective overturn timederived from spectral type (B-V). It was found to vary essentially ast0.5, where t is the stellar age in Gyr. The Rossby number isused as an index of X-ray-ultraviolet (XUV) (1-1200 Å) andLyα activity, defined as the ratio of the total emission flux inthese spectral regions to the total bolometric emission. Expressions forthe ratio of the stellar surface XUV and Lyα emission fluxrelative to present mean solar surface flux values are given in terms ofℜ. It is shown that the observed activity in these stars varies asℜ-β, where β takes values of 2.5 and 1.5 forXUV and Lyα, respectively. Expressions for deriving the Rossbynumber from B-V and age are also given. Thus, one can use the stellarB-V and effective temperature variation with age to calculate the XUVand Lyα emission flux relative to present solar values. As anexample, the evolution of the solar XUV and Lyα with age from 0.1to 8.5 Gyr is given. The variation of the stellar ultraviolet flux withage can be used in photochemical models to study the evolution ofplanetary atmospheres orbiting such stars.
| The use of frequency-separation ratios for asteroseismology
The systematic patterns of separations between frequencies of modes ofdifferent degree and order are a characteristic of p-mode oscillationsof stars. The frequency separations depend on the internal structure ofthe star and so measuring them in the observed oscillation spectra ofvariable stars gives valuable diagnostics of the interior of a star.Roxburgh & Vorontsov proposed using the ratio of the so-called smallfrequency separation to the large frequency separation as a diagnosticof the stellar interior, and demonstrated that this ratio was lesssensitive than the individual frequency separations themselves touncertain details of the near-surface structure. Here we derive kernelsrelating the frequency separation ratio to structure, and show why theratio is relatively insensitive to the near-surface structure in termsof the very small amplitude of the kernels in the near-surface layers.We also investigate the behaviour of the separation ratio for stars ofdifferent masses and ages, and demonstrate the usefulness of the ratioin the so-called asteroseismic Hertzsprung-Russell diagram.
| Asterosismologie.
Not Available
| Excitation of Solar-like Oscillations: From PMS to MS Stellar Models
The amplitude of solar-like oscillations results from a balance betweenexcitation and damping. As in the sun, the excitation is attributed toturbulent motions that stochastically excite the p modes in theupper-most part of the convective zone. We present here a model for theexcitation mechanism. Comparisons between modeled amplitudes and helioand stellar seismic constraints are presented and the discrepanciesdiscussed. Finally the possibility and the interest of detecting suchstochastically excited modes in pre-main sequence stars are alsodiscussed.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| 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.
| Solar-like Oscillations in α Centauri B
We have made velocity observations of the star α Centauri B fromtwo sites, allowing us to identify 37 oscillation modes with l=0-3.Fitting to these modes gives the large and small frequency separationsas a function of frequency. The mode lifetime, as measured from thescatter of the oscillation frequencies about a smooth trend, is similarto that in the Sun. Limited observations of the star δ Pav showoscillations centered at 2.3 mHz, with peak amplitudes close to solar.We introduce a new method of measuring oscillation amplitudes fromheavily smoothed power density spectra, from which we estimatedamplitudes for α Cen α and B, β Hyi, δ Pav, andthe Sun. We point out that the oscillation amplitudes may depend onwhich spectral lines are used for the velocity measurements.Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Programme 71.D-0618).
| 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.
| Single-Visit Photometric and Obscurational Completeness
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
| Three Low-Mass Planets from the Anglo-Australian Planet Search
We report the detection of three new low-mass planets from theAnglo-Australian Planet Search. The three parent stars of these planetsare chromospherically quiet main-sequence G dwarfs with metallicitiesranging from roughly solar (HD 117618 and HD 208487) to metal enriched(HD 102117). The orbital periods range from 20.8 to 130 days, theminimum masses from roughly 0.5MSat to 0.5MJup,and the eccentricities from 0.08 to 0.37, with the planet in thesmallest orbit (HD 102117) having the smallest eccentricity. Withsemiamplitudes of 10.6-19 m s-1, these planets induce Doppleramplitudes similar to those of Jupiter analogs, albeit with shorterperiods. Many of the most interesting future Doppler planets will bedetected at these semiamplitude levels, placing a premium on measurementprecision. The detection of such amplitudes in data extending back 6 yrgives confidence in the Anglo-Australian Planet Search's ability todetect Jupiter analogs as our time baseline extends to 12 yr. We discussthe criticality of such detections for the design of the next generationof extremely large telescopes and also highlight prospects for suitableobserving strategies to push to below 1 m s-1 precisions forbright stars in a search for sub-Neptunian planets.Based on observations obtained at the Anglo-Australian Telescope, SidingSpring, Australia.
| 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.
| Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)
We report on the results of the Sun in Time multiwavelength program(X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chiefscience goals are to study the solar magnetic dynamo and to determinethe radiative and magnetic properties of the Sun during its evolutionacross the main sequence. The present paper focuses on the latter goal,which has the ultimate purpose of providing the spectral irradianceevolution of solar-type stars to be used in the study and modeling ofplanetary atmospheres. The results from the Sun in Time program suggestthat the coronal X-ray-EUV emissions of the young main-sequence Sun were~100-1000 times stronger than those of the present Sun. Similarly, thetransition region and chromospheric FUV-UV emissions of the young Sunare expected to be 20-60 and 10-20 times stronger, respectively, than atpresent. When we consider the integrated high-energy emission from 1 to1200 Å, the resulting relationship indicates that about 2.5 Gyrago the solar high-energy flux was about 2.5 times the present value andabout 3.5 Gyr ago was about 6 times the present value (when lifesupposedly arose on Earth). The strong radiation emissions inferredshould have had major influences on the thermal structure,photochemistry, and photoionization of planetary atmospheres and haveplayed an important role in the development of primitive life in thesolar system. Some examples of the application of the Sun in Timeresults on exoplanets and on early solar system planets are discussed.
| Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages
We investigate the long-term evolution of X-ray coronae of solar analogsbased on high-resolution X-ray spectroscopy and photometry withXMM-Newton. Six nearby main-sequence G stars with ages between ~0.1 and~1.6 Gyr and rotation periods between ~1 and 12.4 days have beenobserved. We use the X-ray spectra to derive coronal element abundancesof C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measuredistribution (EMD). We find that the abundances change from an inversefirst ionization potential (FIP) distribution in stars with ages around0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr andbeyond. This transformation is coincident with a steep decline ofnonthermal radio emission. The results are in qualitative agreement witha simple model in which the stream of electrons in magnetic fieldssuppresses diffusion of low-FIP ions from the chromosphere into thecorona. The coronal emission measure distributions show shapescharacterized by power laws on each side of the EMD peak. The lattershifts from temperatures of about 10 MK in the most rapidly rotating,young stars to temperatures around 4 MK in the oldest target consideredhere. The power-law index on the cooler side of the EMD exceeds expectedslopes for static loops, with typical values being 1.5-3. We interpretthis slope with a model in which the coronal emission is due to asuperposition of stochastically occurring flares, with an occurrencerate that is distributed in radiated energy E as a power law,dN/dE~E-α, as previously found for solar and stellarflares. We obtain the relevant power-law index α from the slope ofthe high-temperature tail of the EMD. Our EMDs indicate α~2.2-2.8,in excellent agreement with values previously derived from light curvesof magnetically active stars. Modulation with timescales reminiscent offlares is found in the light curves of all our targets. Several strongflares are also observed. We use our α-values to simulate lightcurves and compare them with the observed light curves. We thus derivethe range of flare energies required to explain the light-curvemodulation. More active stars require a larger range of flare energiesthan less active stars within the framework of this simplistic model. Inan overall scenario, we propose that flaring activity plays a largerrole in more active stars. In this model, the higher flare rate isresponsible both for the higher average coronal temperature and the highcoronal X-ray luminosity, two parameters that are indeed found to becorrelated.
| Abundances of Mn, Co and Eu in a sample of 20 F-G disk stars: the influence of hyperfine structure splitting
We present Mn, Co and Eu abundances for a sample of 20 disk F and Gdwarfs and subgiants with metallicities in the range-0.8≤[Fe/H]≤+0.3. We investigate the influence of hyperfinestructure (HFS) on the derived abundances of Mn and Co by using HFS datafrom different sources in the literature, as well as calculated HFS frominteraction factors A and B. Eu abundances were obtained from spectralsynthesis of one Eu II line that takes into account HFS from a series ofrecent laboratory measurements. For the lines analysed in this study, wefind that for manganese, the differences between abundances obtainedwith different HFSs are no greater than 0.10 dex. Our cobalt abundancesare even less sensitive to the choice of HFS than Mn, presenting a 0.07dex maximum difference between determinations with different HFSs.However, the cobalt HFS data from different sources are significantlydifferent. Our abundance results for Mn offer an independentconfirmation of literature results, favouring type Ia supernovae as themain nucleosynthesis site of Mn production, in contrast to trends of Mnversus metallicity previously reported in the literature. For Co, weobtain [Co/Fe]0.0 in the range -0.3<[Fe/H]<+0.3 and [Co/Fe]rising to a level of +0.2 when [Fe/H] decreases from -0.3 to -0.8, indisagreement with recent results in the literature. The observeddiscrepancies may be attributed to the lack of HFS in the works we usedfor comparison. Our results for Eu are in accordance with low-mass typeII supernovae being the main site of the r-process nucleosynthesis.
| Rotation profile inversion in solar-like stars. In the COROT framework
The observation of a few mixed modes on solar-like oscillating starswould enable their rotation profile to be inverted with success. Heresimulated data are used to show that it is possible to find models forsolar-like stars that present stochastically excited mixed modes withdetectable amplitudes. We take special care to build the mode set bycomputing the mode amplitudes and selecting those modes with amplitudescompatible with the performance of the forthcoming seismic spaceexperiment, COROT. The frequency set is inverted for various cases whereinput and trial stellar models differ and where random noise is added tothe splittings. We show it is possible to localize a rotation gradientand assess its magnitude. Moreover the use of inverse and forwardprocedures in parallel gives access to a large part of the profile. Weprovide several constraints to help the selection of such stars. Onelooks for a relatively evolved star (still on the main sequence) of≃ 1.5 ~ M_ȯ and the rotation rate on the surface should notbe too small.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| Spectroscopic metallicities for planet-host stars: Extending the samples
We present stellar parameters and metallicities for 29 planet-hoststars, as well as for a large volume-limited sample of 53 stars notknown to be orbited by any planetary-mass companion. These stars add tothe results presented in our previous series of papers, providing twolarge and uniform samples of 119 planet-hosts and 94“single” stars with accurate stellar parameters and [Fe/H]estimates. The analysis of the results further confirms that stars withplanets are metal-rich when compared with average field dwarfs.Important biases that may compromise future studies are also discussed.Finally, we compare the metallicity distributions for singleplanet-hosts and planet-hosts in multiple stellar systems. The resultsshow that a small difference cannot be excluded, in the sense that thelatter sample is slighly overmetallic. However, more data are needed toconfirm this correlation.
| 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.
| 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
| 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
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| 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
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Multiple Companions to HD 154857 and HD 160691
Precise Doppler measurements from the Anglo-Australian Telescope (AAT)UCLES spectrometer reveal two companions to both HD 154857 and HD160691. The inner companion to HD 154857 has a period of 398 days, aneccentricity of 0.51, and a minimum mass of 1.8MJ. An outercompanion has a period much longer than 2 yr and is currently detectedonly as a linear trend of 14 m s-1 yr-1. The innercompanion to HD 160691, previously announced from AAT data, has a periodof 645 days, an eccentricity of 0.20, and a minimum mass of1.7MJ. For the outer planet, whose orbit is less wellconstrained, a two-Keplerian fit yields a period of 8.2 yr, aneccentricity of 0.57, and a minimum mass of 3.1MJ. With theseorbital parameters, its maximum separation from the star of 0.4" makesit a viable target for direct imaging.Based on observations obtained at the Anglo-Australian Telescope, SidingSpring, Australia.
| Ultra-High-Precision Velocity Measurements of Oscillations in α Centauri A
We have made differential radial velocity measurements of the starα Centauri A using two spectrographs, the UV-Visual EchelleSpectrograph (UVES) and the University College London EchelleSpectrograph (UCLES), both with iodine absorption cells for wavelengthreferencing. Stellar oscillations are clearly visible in the timeseries. After removing jumps and slow trends in the data, we show thatthe precision of the velocity measurements per minute of observing timeis 0.42 m s-1 for UVES and 1.0 m s-1 for UCLES,while the noise level in the Fourier spectrum of the combined data is1.9 cm s-1. As such, these observations represent the mostprecise velocities ever measured on any star apart from the Sun.
| The science case of the PEPSI high-resolution echelle spectrograph and polarimeter for the LBT
We lay out the scientific rationale for and present the instrumentalrequirements of a high-resolution adaptive-optics Echelle spectrographwith two full-Stokes polarimeters for the Large Binocular Telescope(LBT) in Arizona. Magnetic processes just like those seen on the Sun andin the space environment of the Earth are now well recognized in manyastrophysical areas. The application to other stars opened up a newfield of research that became widely known as the solar-stellarconnection. Late-type stars with convective envelopes are all affectedby magnetic processes which give rise to a rich variety of phenomena ontheir surface and are largely responsible for the heating of their outeratmospheres. Magnetic fields are likely to play a crucial role in theaccretion process of T-Tauri stars as well as in the acceleration andcollimation of jet-like flows in young stellar objects (YSOs). Anotherarea is the physics of active galactic nucleii (AGNs) , where themagnetic activity of the accreting black hole is now believed to beresponsible for most of the behavior of these objects, including theirX-ray spectrum, their notoriously dramatic variability, and the powerfulrelativistic jets they produce. Another is the physics of the centralengines of cosmic gamma-ray bursts, the most powerful explosions in theuniverse, for which the extreme apparent energy release are explainedthrough the collimation of the released energy by magnetic fields.Virtually all the physics of magnetic fields exploited in astrophysicsis somehow linked to our understanding of the Sun's and the star'smagnetic fields.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Hydrus |
| Ascensió Recta: | 00h25m45.10s |
| Declinació: | -77°15'15.0" |
| Magnitud Aparent: | 2.8 |
| Distancia: | 7.475 parsecs |
| Moviment propi RA: | 2221.9 |
| Moviment propi Dec: | 322.6 |
| B-T magnitude: | 3.543 |
| V-T magnitude: | 2.855 |
Catàlegs i designacions:
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