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Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data*
The MOST (Microvariability and Oscillations of Stars) satellite obtainsultraprecise photometry from space with high sampling rates and dutycycles. Astronomical photometry or imaging missions in low Earth orbits,like MOST, are especially sensitive to scattered light from Earthshine,and all these missions have a common need to extract target informationfrom voluminous data cubes. They consist of upwards of hundreds ofthousands of two-dimensional CCD frames (or subrasters) containing fromhundreds to millions of pixels each, where the target information,superposed on background and instrumental effects, is contained only ina subset of pixels (Fabry Images, defocused images, mini-spectra). Wedescribe a novel reduction technique for such data cubes: resolvinglinear correlations of target and background pixel intensities. Thisstep-wise multiple linear regression removes only those targetvariations which are also detected in the background. The advantage ofregression analysis versus background subtraction is the appropriatescaling, taking into account that the amount of contamination may differfrom pixel to pixel. The multivariate solution for all pairs oftarget/background pixels is minimally invasive of the raw photometrywhile being very effective in reducing contamination due to, e.g. straylight. The technique is tested and demonstrated with both simulatedoscillation signals and real MOST photometry.
| Turbulence in models of a star other than Sun: matching η Bootis observed p-modes
Standard stellar models for η Boo fail to reproduce the newly observedlow frequency p-modes from space and the high frequency p-modes observedfrom the ground, simultaneously. This discrepancy can be removed byincluding turbulence in the modeling of the outer layers of η Boo. Weinclude turbulence by applying the effects of turbulent pressure andturbulent kinetic energy -- extracted from a hydrodynamical 3Dconvection simulation for the Sun -- at the correct depth in the 1Dmodels of η Boo.
| Observing solar-like oscillations: recent results.
We review recent progress in observations of ground-based oscillations.Excellent observations now exist for a few stars (alpha Cen A{} and B,mu Ara), while there is some controversy over others (Procyon, etaBoo). We have reached the stage where single-site observations are oflimited value and where careful planning is needed to ensure the futureof asteroseismology.
| Progress in understanding and exploiting stellar oscillation spectra .
Rich oscillation spectra of dwarf-like pulsators contain a wealth ofinformation about the object interiors and, in particular, aboutmacroscopic transport processes, which is the most difficult aspect ofstellar physics. Examples of extracting such information from data onsolar-like and opacity driven pulsators are given. Problems inunderstanding new oscillation spectra are discussed. Importance ofemploying various data on excited mode is emphasized.
| 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.
| Observations of solar-like oscillations and asteroseismic models including rotation .
Since the success of helioseismology, numerous efforts have been made todetect solar-like oscillations on other stars. Thanks to newspectrographs developed for extra-solar planet searches, the accuracyneeded to detect such oscillations has recently been achieved. In thispaper, we present new asteroseismic measurements obtained with theCORALIE and HARPS spectrographs as well as new theoretical analysesbased on these observations. In particular, we focus on the effects ofrotation on the modelling of solar-type stars and on its influence onthe determination of fundamental stellar parameters.
| Space- and Ground-based Pulsation Data of η Bootis Explained with Stellar Models Including Turbulence
The space telescope MOST is now providing us with extremely accuratelow-frequency p-mode oscillation data for the star η Boo. Wedemonstrate in this paper that these data, when combined withground-based measurements of the high-frequency p-mode spectrum, can bereproduced with stellar models that include the effects of turbulence intheir outer layers. Without turbulence, the l=0 modes of our modelsdeviate from either the ground-based or the space data by about 1.5-4μHz. This discrepancy can be completely removed by includingturbulence in the models, and we can exactly match 12 out of 13 MOSTfrequencies that we identified as l=0 modes, in addition to 13 out of 21ground-based frequencies within their observational 2 σtolerances. The better agreement between model frequencies and observedfrequencies depends for the most part on the turbulent kinetic energythat was taken from a three-dimensional convection simulation for theSun.
| 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.
| Modeling β Virginis using seismological data
This paper presents the modeling of the F9 V star βVirginis based on seismological measurements. Using the Genevaevolution code including rotation and atomic diffusion, we find that twodistinct solutions reproduce all existing asteroseismic andnon-asteroseismic observational constraints well: a main-sequence modelwith a mass of 1.28 ± 0.03~Mȯ and an age t=3.24± 0.20 Gyr, or a model in the post-main sequence phase ofevolution with a lower mass of 1.21 ± 0.02~Mȯ andan age t=4.01 ± 0.30 Gyr. The small spacings δν02 and the ratio r02 between small and largespacings are sensitive to the differences in the structure of thecentral layers between these two solutions and are also sensitive to thestructural changes due to the rotational mixing. They can therefore beused to unambiguously determine the evolutionary state ofβ Vir and to study the effects of rotation onthe inner structure of the star. Unfortunately, existing asteroseismicdata do not enable such precise determination. We also show that thescatter in frequencies introduced by the rotational splittings canaccount for the larger dispersion of the observed large spacings for thenon-radial modes than for the radial modes.
| Prospects for population synthesis in the H band: NeMo grids of stellar atmospheres compared to observations
Context: .For applications in population synthesis, libraries oftheoretical stellar spectra are often considered an alternative totemplate libraries of observed spectra, because they allow a completesampling of stellar parameters. Most of the attention in publishedtheoretical spectral libraries has been devoted to the visual wavelengthrange.Aims.The goal of the present work is to explore the near-infraredrange where few observed fully calibrated spectra and no theoreticallibraries are available.Methods.We make a detailed comparison oftheoretical spectra in the range 1.57-1.67 μm for spectral types fromA to early M and for giant and dwarf stars, with observed stellarspectra at resolutions around 3000, which would be sufficient todisentangle the different groups of late-type stars. We selected theNeMo grids of stellar atmospheres to perform this comparison.Results.Wefirst demonstrate that observed spectral flux distributions can bematched very well with theoretical ones for almost the entire parameterrange covered by the NeMo grids at moderate resolution in the visualrange. In the infrared range, although the overall shape of the observedflux distributions still matches reasonably well, the individualspectral features are reproduced by the theoretical spectra only forstars earlier than mid F type. For later spectral types the differencesincrease, and theoretical spectra of K type stars have systematicallyweaker line features than those found in observations. Thesediscrepancies are traced back to stem primarily from incomplete data onneutral atomic lines, although some of them are also related tomolecules.Conclusions.Libraries of theoretical spectra for A to early Mtype stars can be successfully used in the visual regions for populationsynthesis, but their application in the infrared is restricted to earlyand intermediate type stars. Improving atomic data in the near infraredis a key element in making the construction of reliable libraries ofstellar spectra feasible in the infrared.
| Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F andlater were obtained in order to search for signatures of differentialrotation in line profiles. In 147 stars the rotation law could bemeasured, with 28 of them found to be differentially rotating.Comparison to rotation laws in stars of spectral type A reveals thatdifferential rotation sets in at the convection boundary in theHR-diagram; no star that is significantly hotter than the convectionboundary exhibits the signatures of differential rotation. Four lateA-/early F-type stars close to the convection boundary and at v sin{i}≈ 100 km s-1 show extraordinarily strong absolute shear atshort rotation periods around one day. It is suggested that this is dueto their small convection zone depth and that it is connected to anarrow range in surface velocity; the four stars are very similar inTeff and v sin{i}. Detection frequencies of differentialrotation α = ΔΩ/Ω > 0 were analyzed in starswith varying temperature and rotation velocity. Measurable differentialrotation is more frequent in late-type stars and slow rotators. Thestrength of absolute shear, ΔΩ, and differential rotationα are examined as functions of the stellar effective temperatureand rotation period. The highest values of ΔΩ are found atrotation periods between two and three days. In slower rotators, thestrongest absolute shear at a given rotation rateΔΩmax is given approximately byΔΩmax ∝ P-1, i.e.,αmax ≈ const. In faster rotators, bothαmax and ΔΩmax diminish lessrapidly. A comparison with differential rotation measurements in starsof later spectral type shows that F-stars exhibit stronger shear thancooler stars do and the upper boundary in absolute shear ΔΩwith temperature is consistent with the temperature-scaling law found inDoppler Imaging measurements.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| 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.
| Why are G and K giants radial velocity variables?
Not Available
| Predicting accurate stellar angular diameters by the near-infrared surface brightness technique
I report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids.
| 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.
| Stellar Model Analysis of the Oscillation Spectrum of η Bootis Obtained from MOST
Eight consecutive low-frequency radial p-modes are identified in the G0IV star η Bootis based on 27 days of ultraprecise rapid photometryobtained by the MOST (Microvariability and Oscillations of Stars)satellite. The MOST data extend smoothly, to lower overtones, thesequence of radial p-modes reported in earlier ground-based spectroscopyby other groups. The sampling is nearly continuous; hence, theambiguities in p-mode identifications due to aliases, such as the cycleday-1 alias found in ground observations, are not an issue.The lower overtone modes from the MOST data constrain the interiorstructure of the model of η Boo, giving a best fit on a grid of~300,000 stellar models for a composition of (X,Z)=(0.71,0.04), a massof M=1.71+/-0.05 Msolar, and an age of t=2.40+/-0.03 Gyr. Thesurface temperature and luminosity of this model, which were constrainedonly by using the oscillation modes, are close (1 σ) to currentbest estimates of η Boo's surface temperature and luminosity. Withthe interior fit anchored by the lower overtone modes seen by MOST,standard models are not able to fit the higher overtone modes with thesame level of accuracy. The discrepancy, model minus observed frequency,increases from 0.5 μHz at 250 μHz to 5 μHz at 1000 μHz andis similar to the discrepancy that exists between the Sun's observedp-mode frequencies and the p-mode frequencies of the standard solarmodel. This discrepancy promises to be a powerful constraint on modelsof three-dimensional convection.
| 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.
| The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
| The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method
The infrared flux method (IRFM) has been applied to a sample of 135dwarf and 36 giant stars covering the following regions of theatmospheric parameter space: (1) the metal-rich ([Fe/H]>~0) end(consisting mostly of planet-hosting stars), (2) the cool(Teff<~5000 K) metal-poor (-1<~[Fe/H]<~-3) dwarfregion, and (3) the very metal-poor ([Fe/H]<~-2.5) end. These starswere especially selected to cover gaps in previous works onTeff versus color relations, particularly the IRFMTeff scale of A. Alonso and collaborators. Our IRFMimplementation was largely based on the Alonso et al. study (absoluteinfrared flux calibration, bolometric flux calibration, etc.) with theaim of extending the ranges of applicability of their Teffversus color calibrations. In addition, in order to improve the internalaccuracy of the IRFM Teff scale, we recomputed thetemperatures of almost all stars from the Alonso et al. work usingupdated input data. The updated temperatures do not significantly differfrom the original ones, with few exceptions, leaving the Teffscale of Alonso et al. mostly unchanged. Including the stars withupdated temperatures, a large sample of 580 dwarf and 470 giant stars(in the field and in clusters), which cover the ranges3600K<~Teff<~8000K and -4.0<~[Fe/H]<~+0.5, haveTeff homogeneously determined with the IRFM. The meanuncertainty of the temperatures derived is 75 K for dwarfs and 60 K forgiants, which is about 1.3% at solar temperature and 4500 K,respectively. It is shown that the IRFM temperatures are reliable in anabsolute scale given the consistency of the angular diameters resultingfrom the IRFM with those measured by long baseline interferometry, lunaroccultation, and transit observations. Using the measured angulardiameters and bolometric fluxes, a comparison is made between IRFM anddirect temperatures, which shows excellent agreement, with the meandifference being less than 10 K for giants and about 20 K for dwarfstars (the IRFM temperatures being larger in both cases). This resultwas obtained for giants in the ranges 3800K
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| Seismic Diagnostics of Mixing Beyond the Convective Core in Intermediate Mass Main-Sequence Stars
We study prospects for seismic sounding the layer of a partial mixingabove the convective core in main-sequence stars with masses in the1.2-1.9 M_odot range. There is an initial tendency to increase of theconvective core mass in such stars and this leads to ambiguities inmodeling. Solar-like oscillations are expected to be excited in suchobjects. Frequencies of such oscillations provide diagnostics, which aresensitive to the structure of the innermost part of the star and theyare known as the small separations. We construct evolutionary models ofstars in this mass range assuming various scenarios for element mixing,which includes formation of element abundance jumps, as well assemiconvective and overshooting layers. We find that the three pointsmall separations employing frequencies of radial and dipole modesprovide the best probe of the element distribution above the convectivecore. With expected accuracy of frequency measurement from the spaceexperiments, a discrimination between various scenarios should bepossible.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| 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.
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Посматрања и Астрометриски подаци
| Сазвежђа: | Волар |
| Ректацензија: | 13h54m41.10s |
| Deклинација: | +18°23'52.0" |
| Apparent магнитуда: | 2.68 |
| Даљина: | 11.342 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 3.38 |
| V-T magnitude: | 2.719 |
Каталог и designations:
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INHABITED SKY
