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The Infrared Telescope Facility (IRTF) spectral library:. Spectral diagnostics for cool stars
Context. The near-infrared (NIR) wavelength range offers some uniquespectral features, and it is less prone to the extinction than theoptical one. Recently, the first flux calibrated NIR library of coolstars from the NASA Infrared Telescope Facility (IRTF) have becomeavailable, and it has not been fully exploited yet. Aims: We wantto develop spectroscopic diagnostics for stellar physical parametersbased on features in the wavelength range 1-5 μm. In this work wetest the technique in the I and K bands. The study of the Y, J, H, and Lbands will be presented in the following paper. Methods: Anobjective method for semi-empirical definition of spectral featuressensitive to various physical parameters is applied to the spectra. Itis based on sensitivity map - i.e., derivative of the flux in thespectra with respect to the stellar parameters at a fixed wavelength.New optimized indices are defined and their equivalent widths (EWs) aremeasured. Results: The method is applied in the I- and K-bandwindows of the IRTF stellar spectra to verify the new technique bycomparing the results with the known behavior of well-studied spectralfeatures. A number of sensitive features to the effective temperatureand surface gravity are re-identified or newly identified clearlyshowing the reliability of the sensitivity map analysis. Conclusions: The sensitivity map allows to identify the best bandpasslimits for the line and nearby continuum. It reliably predicts thetrends of spectral features with respect to a given physical parameterbut not their absolute strengths. Line blends are easy to recognize whenblended features have different behavior with respect to some physicalstellar parameter. The use of sensitivity map is therefore complementaryto the use of indices. We give the EWs of the new indices measured forthe IRTF star sample. This new and homogeneous set of EWs will be usefulfor stellar population synthesis models and can be used to getelement-by-element abundances for unresolved stellar population studiesin galaxies.
| Stellar Diameters and Temperatures. II. Main-sequence K- and M-stars
We present interferometric angular diameter measurements of 21 low-mass,K- and M-dwarfs made with the CHARA Array. This sample is enhanced byadding a collection of radius measurements published in the literatureto form a total data set of 33 K-M-dwarfs with diameters measured tobetter than 5%. We use these data in combination with the Hipparcosparallax and new measurements of the star's bolometric flux to computeabsolute luminosities, linear radii, and effective temperatures for thestars. We develop empirical relations for ~K0 to M4 main-sequence starsthat link the stellar temperature, radius, and luminosity to theobserved (B - V), (V - R), (V - I), (V - J), (V - H), and (V - K)broadband color index and stellar metallicity [Fe/H]. These relationsare valid for metallicities ranging from [Fe/H] = -0.5 to +0.1 dex andare accurate to ~2%, ~5%, and ~4% for temperature, radius, andluminosity, respectively. Our results show that it is necessary to usemetallicity-dependent transformations in order to properly convertcolors into stellar temperatures, radii, and luminosities.Alternatively, we find no sensitivity to metallicity on relations weconstruct to the global properties of a star omitting color information,e.g., temperature-radius and temperature-luminosity. Thus, we are ableto empirically quantify to what order the star's observed color index isimpacted by the stellar iron abundance. In addition to the empiricalrelations, we also provide a representative look-up table via stellarspectral classifications using this collection of data. Robustexaminations of single star temperatures and radii compared toevolutionary model predictions on the luminosity-temperature andluminosity-radius planes reveal that models overestimate thetemperatures of stars with surface temperatures <5000 K by ~3%, andunderestimate the radii of stars with radii <0.7 R &sun;by ~5%. These conclusions additionally suggest that the models overaccount for the effects that the stellar metallicity may have on theastrophysical properties of an object. By comparing theinterferometrically measured radii for the single star population tothose of eclipsing binaries, we find that for a given mass, single andbinary star radii are indistinguishable. However, we also find that fora given radius, the literature temperatures for binary stars aresystematically lower compared to our interferometrically derivedtemperatures of single stars by ~200 to 300 K. The nature of this offsetis dependent on the validation of binary star temperatures, wherebringing all measurements to a uniform and correctly calibratedtemperature scale is needed to identify any influence stellar activitymay have on the physical properties of a star. Lastly, we present anempirically determined H-R diagram using fundamental propertiespresented here in combination with those in Boyajian et al. for a totalof 74 nearby, main-sequence, A- to M-type stars, and define regions ofhabitability for the potential existence of sub-stellar mass companionsin each system.
| The Solar Neighborhood. XXVIII. The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert around M Dwarfs
We report on our analysis of Hubble Space Telescope/NICMOS snapshothigh-resolution images of 255 stars in 201 systems within ~10 pc of theSun. Photometry was obtained through filters F110W, F180M, F207M, andF222M using NICMOS Camera 2. These filters were selected to permit clearidentification of cool brown dwarfs through methane contrast imaging.With a plate scale of 76 mas pixel-1, NICMOS can easilyresolve binaries with subarcsecond separations in the19farcs5×19farcs5 field of view. We previously reported fivecompanions to nearby M and L dwarfs from this search. No new companionswere discovered during the second phase of data analysis presented here,confirming that stellar/substellar binaries are rare. We establishmagnitude and separation limits for which companions can be ruled outfor each star in the sample, and then perform a comprehensivesensitivity and completeness analysis for the subsample of 138 M dwarfsin 126 systems. We calculate a multiplicity fraction of0.0+3.5 -0.0% for L companions to M dwarfsin the separation range of 5-70 AU, and 2.3+5.0-0.7% for L and T companions to M dwarfs in theseparation range of 10-70 AU. We also discuss trends in thecolor-magnitude diagrams using various color combinations and presentastrometry for 19 multiple systems in our sample. Considering theseresults and results from several other studies, we argue that theso-called brown dwarf desert extends to binary systems with low-massprimaries and is largely independent of primary mass, mass ratio, andseparations. While focusing on companion properties, we discuss how thequalitative agreement between observed companion mass functions andinitial mass functions suggests that the paucity of brown dwarfs ineither population may be due to a common cause and not due to binaryformation mechanisms.
| Further Defining Spectral Type "Y" and Exploring the Low-mass End of the Field Brown Dwarf Mass Function
We present the discovery of another seven Y dwarfs from the Wide-fieldInfrared Survey Explorer (WISE). Using these objects, as well as thefirst six WISE Y dwarf discoveries from Cushing et al., we furtherexplore the transition between spectral types T and Y. We find that theT/Y boundary roughly coincides with the spot where the J - Hcolors of brown dwarfs, as predicted by models, turn back to the red.Moreover, we use preliminary trigonometric parallax measurements to showthat the T/Y boundary may also correspond to the point at which theabsolute H (1.6 ?m) and W2 (4.6 ?m) magnitudes plummet. We usethese discoveries and their preliminary distances to place them in thelarger context of the solar neighborhood. We present a table thatupdates the entire stellar and substellar constituency within 8 pc ofthe Sun, and we show that the current census has hydrogen-burning starsoutnumbering brown dwarfs by roughly a factor of six. This factor willdecrease with time as more brown dwarfs are identified within thisvolume, but unless there is a vast reservoir of cold brown dwarfsinvisible to WISE, the final space density of brown dwarfs is stillexpected to fall well below that of stars. We also use these new Y dwarfdiscoveries, along with newly discovered T dwarfs from WISE, toinvestigate the field substellar mass function. We find that the overallspace density of late-T and early-Y dwarfs matches that from simulationsdescribing the mass function as a power law with slope -0.5
| Values of v sin i for late-type stars from spectral synthesis in the K-band region
We analyse medium-resolution spectra (R˜ 18 000) of 19 late typedwarfs in order to determine v sin i values using synthetic rather thanobservational template spectra. For this purpose, observational dataaround 2.2 μm for stars with spectral classes from G8V to M9.5V weremodelled. We find that the Na I (2.2062- and 2.2090-μm) and12CO 2-0 band features are modelled well enough to use for vsin i determination without the need for suitable observational templatespectra. Within the limit of the resolution of our spectra, we usesynthetic spectra templates to derive v sin i values consistent withthose derived in the optical regime using observed templates. Wequantify the errors in our v sin i determination due to incorrect choiceof model parameters Teff, log g, vmicro, [Fe/H]or full width at half-maximum and show that they are typically less than10 per cent. We note that the spectral resolution of our data (˜16km s-1) limited this study to relatively fast rotators andthat a resolution of 60 000 will be required to access most late-typedwarfs.
| Metallicity of solar-type stars with debris discs and planets?
Context. Around 16% of the solar-like stars in our neighbourhood showIR-excesses due to dusty debris discs and a fraction of them are knownto host planets. Determining whether these stars follow any specialtrend in their properties is important to understand debris disc andplanet formation. Aims: We aim to determine in a homogeneous waythe metallicity of a sample of stars with known debris discs andplanets. We attempt to identify trends related to debris discs andplanets around solar-type stars. Methods: Our analysis includesthe calculation of the fundamental stellar parameters Teff,log g, microturbulent velocity, and metallicity by applying the ironionisation equilibrium conditions to several isolated Fe i and Fe iilines. High-resolution échelle spectra (R ~ 57 000) from 2, 3 mclass telescopes are used. Our derived metallicities are compared withother results in the literature, which finally allows us to extend thestellar samples in a consistent way. Results: The metallicitydistributions of the different stellar samples suggest that there is atransition toward higher metallicities from stars with neither debrisdiscs nor planets to stars hosting giant planets. Stars with debrisdiscs and stars with neither debris nor planets follow a similarmetallicity distribution, although the distribution of the first onesmight be shifted towards higher metallicities. Stars with debris discsand planets have the same metallicity behaviour as stars hostingplanets, irrespective of whether the planets are low-mass or gas giants.In the case of debris discs and giant planets, the planets are usuallycool, - semimajor axis larger than 0.1 AU (20 out of 22 planets), even?65% have semimajor axis larger than 0.5 AU. The data also suggestthat stars with debris discs and cool giant planets tend to have a lowdust luminosity, and are among the less luminous debris discs known. Wealso find evidence of an anticorrelation between the luminosity of thedust and the planet eccentricity. Conclusions: Our data show thatthe presence of planets, not the debris disc, correlates with thestellar metallicity. The results confirm that core-accretion modelsrepresent suitable scenarios for debris disc and planet formation. Theseconclusions are based on a number of stars with discs and planetsconsiderably larger than in previous works, in particular stars hostinglow-mass planets and debris discs. Dynamical instabilities produced byeccentric giant planets could explain the suggested dust luminositytrends observed for stars with debris discs and planets.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC); observations made with the Italian TelescopioNazionale Galileo (TNG) operated on the island of La Palma by theFundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica); observations made with the Nordic Optical Telescope,operated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias; and data obtainedfrom the ESO Science Archive Facility.Full Tables 1 and 5 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A40
| The first high-precision radial velocity search for extra-solar planets
The reflex motion of a star induced by a planetary companion is toosmall to detect by photographic astrometry. The apparent discovery inthe 1960s of planetary systems around certain nearby stars, inparticular Barnard's star, turned out to be spurious. Conventionalstellar radial velocities determined from photographic spectra at thattime were also too inaccurate to detect the expected reflex velocitychanges. In the late 1970s and early 1980s, the introduction ofsolid-state, signal-generating detectors and absorption cells to imposewavelength fiducials directly on the starlight, reduced radial velocityerrors to the point where such a search became feasible. Beginning in1980, our team from UBC introduced an absorption cell of hydrogenfluoride gas in front of the CFHT coudé spectrograph and, for 12years, monitored the radial velocities of some 29 solar-type stars.Since it was assumed that extra-solar planets would most likely resembleJupiter in mass and orbit, we were awarded only three or four two-nightobserving runs each year. Our survey highlighted three potential planethosting stars, γ Cep (K1 IV), β Gem (K0 III), and ɛEri (K2 V). The putative planets all resembled Jovian systems withperiods and masses of: 2.5 years and 1.4 MJ, 1.6 years and2.6 MJ, and 6.9 years and 0.9 MJ, respectively.All three were subsequently confirmed from more extensive data by theTexas group led by Cochran and Hatzes who also derived the currentlyaccepted orbital elements.None of these three systems is simple. Allfive giant stars and the supergiant in our survey proved to be intrinsicvelocity variables. When we first drew attention to a possible planetarycompanion to γ Cep in 1988 it was classified as a giant, and therewas the possibility that its radial velocity variations and those ofβ Gem (K0 III) were intrinsic to the stars. A further complicationfor γ Cep was the presence of an unseen secondary star in an orbitwith a period initially estimated at some 30 years. The implication wasthat the planetary orbit might not be stable, and a Jovian planetsurviving so close to a giant then seemed improbable. Later observationsby others showed the stellar binary period was closer to 67 years, theprimary was only a sub-giant and a weak, apparently synchronouschromospheric variation disappeared. Chromospheric activity wasconsidered important because κ1 Cet, one of our programstars, showed a significant correlation of its radial velocity curvewith chromospheric activity.ɛ Eri is a young, magneticallyactive star with spots making it a noisy target for radial velocities.While the signature of a highly elliptical orbit (e = 0.6) has persistedfor more than three planetary orbits, some feel that even more extensivecoverage is needed to confirm the identification despite an apparentcomplementary astrometric acceleration detected with the Hubble SpaceTelescope.We confined our initial analyses of the program stars tolooking for circular orbits. In retrospect, it appears that some 10% ofour sample did in fact have Jovian planetary companions in orbits withperiods of years.
| Optimizing exoplanet transit searches around low-mass stars with inclination constraints
Aims: We investigate a method to increase the efficiency of atargeted exoplanet search with the transit technique by preselecting asubset of candidates from large catalogs of stars. Assuming spin-orbitalignment, this can be achieved by considering stars that have a higherprobability to be oriented nearly equator-on (inclination close to90°). Methods: We used activity-rotation velocity relationsfor low-mass stars with a convective envelope to study the dependence ofthe position in the activity-vsini diagram on the stellar axisinclination. We composed a catalog of G-, K-, M-type main-sequencesimulated stars using isochrones, an isotropic inclination distributionand empirical relations to obtain their rotation periods and activityindexes. Then the activity-vsini diagram was completed and statisticswere applied to trace the areas containing the higher ratio of starswith inclinations above 80°. A similar statistics was applied tostars from real catalogs with log(R'HK) and vsini data tofind their probability of being oriented equator-on. Results: Wepresent our method to generate the simulated star catalog and thesubsequent statistics to find the highly inclined stars from realcatalogs using the activity-vsini diagram. Several catalogs from theliterature are analyzed and a subsample of stars with the highestprobability of being equator-on is presented. Conclusions:Assuming spin-orbit alignment, the efficiency of an exoplanet transitsearch in the resulting subsample of probably highly inclined stars isestimated to be two to three times higher than with a general searchwithout preselection.Table 4 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/537/A147
| The non-local thermodynamic equilibrium barium abundance in dwarf stars in the metallicity range of -1 < [Fe/H] < +0.3
We present the results of determination of the barium abundanceconsidering the non-local thermodynamic equilibrium (NLTE) effects in172 dwarf stars in the metallicity range of -1 < [Fe/H] < +0.3,assigned to different Galactic substructures by kinematic criteria. Weused a model of the Ba atom with 31 levels of Ba I and 101 levels of BaII. The atmosphere models for the investigated stars were computed usingthe ATLAS9 code modified by new opacity distribution functions byCastelli & Kurucz. The NLTE profiles of the unblended Ba II (4554,5853, 6496 Å) were computed and then compared to those observed.The line 6141 Å was also used, but with an allowance for itscorrelation with the iron line. The average barium abundances in thethin and thick discs are 0.01 ± 0.08 and -0.03 ± 0.07,respectively. The comparison to the calculations of the Galacticchemical evolution by Serminato et al. was conducted. The trend obtainedfor the Ba abundance versus [Fe/H] suggests a complex barium productionprocess in the thin and thick discs.
| The copper and zinc abundances in stars of galactic sub-structures
We have determined abundances of copper, zinc, sodium, and aluminum inthe atmospheres of 172 F, G, and K dwarf stars (-1.0 < [Fe/H] <0.3) belonging to the Galaxy's thin and thick disks and to the Herculesmoving group. Our observations were performed with the ELODIEéchelle spectrometer on the 1.93-m telescope of the HauteProvence Observatory, with a resolving power of R = 42 000 andsignal-to-noise ratio S/N > 100. The Na, Al, Cu, and Zn abundanceswere derived in an LTE approximation; the synthetic spectrum for thecopper lines was calculated taking into account super-fine structure ofthe lines. We analyzed the abundances of these elements as a function ofmetallicity [Fe/H] for stars of the thin and thick disks of the Galaxyand the Hercules moving group. The Cu abundances and their trends withmetallicity are essentially the same in the three studiedsub-structures. The mean Al and Zn abundances for stars of the thin andthick disks differ significantly.
| The atmospheric parameters and spectral interpolator for the MILES stars
Context. Empirical libraries of stellar spectra are used to classifystars and synthetize stellar populations. MILES is a mediumspectral-resolution library in the optical domain covering a wide rangeof temperatures, surface gravities and metallicities. Aims: Weredetermine the atmospheric parameters of these stars in order toimprove the homogeneity and accuracy. We build an interpolating functionthat returns a spectrum as a function of the three atmosphericparameters, and finally we characterize the precision of the wavelengthcalibration and stability of the spectral resolution. Methods: Weused the ULySS program with the ELODIE library as a reference andcompared the results with those in the literature. Results: Weobtain precisions of 60 K, 0.13, and 0.05 dex, respectively, forTeff, log g, and [Fe/H] for the FGK stars. For the M stars,the mean errors are 38 K, 0.26, and 0.12 dex and 3.5%, 0.17, and 0.13dex for the OBA. We construct an interpolator that we test against theMILES stars themselves. We test it also by measuring the atmosphericparameters of the CFLIB stars with MILES as reference and find it to bemore reliable than the ELODIE interpolator for the evolved hot stars,like those of the blue horizontal branch in particular.FITS files are only and Table 1 also available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A165
| Element abundances in the stars of the MILES spectral library: the Mg/Fe ratio
We have obtained [Mg/Fe] measurements for 76.3 per cent of the stars inthe Mid-resolution Isaac Newton Telescope Library of Empirical Spectra(MILES) spectral library used for understanding stellar atmospheres andstellar populations in galaxies and star clusters. These abundanceratios were obtained through (1) a compilation of values from theliterature using abundances from high-resolution (HR) spectroscopicstudies and (2) a robust spectroscopic analysis using the MILESmid-resolution (MR) optical spectra. All the [Mg/Fe] values werecarefully calibrated to a single uniform scale, by using an extensivecontrol sample with results from HR spectra. The small averageuncertainties in the calibrated [Mg/Fe] values [respectively 0.09 and0.12 dex with methods (1) and (2)] and the good coverage of the starswith [Mg/Fe] over stellar atmospheric parameter space of the librarywill permit the building of new simple stellar populations (SSPs) withempirical ?-enhancements. These will be available for a range of[Mg/Fe], including both sub-solar and super-solar values, and forseveral metallicities and ages. These models will open up new prospectsfor testing and applications of evolutionary stellar populationsynthesis.
| New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
We present a re-analysis of the Geneva-Copenhagen survey, which benefitsfrom the infrared flux method to improve the accuracy of the derivedstellar effective temperatures and uses the latter to build a consistentand improved metallicity scale. Metallicities are calibrated onhigh-resolution spectroscopy and checked against four open clusters anda moving group, showing excellent consistency. The new temperature andmetallicity scales provide a better match to theoretical isochrones,which are used for a Bayesian analysis of stellar ages. With respect toprevious analyses, our stars are on average 100 K hotter and 0.1 dexmore metal rich, which shift the peak of the metallicity distributionfunction around the solar value. From Strömgren photometry we areable to derive for the first time a proxy for [?/Fe] abundances,which enables us to perform a tentative dissection of the chemical thinand thick disc. We find evidence for the latter being composed of anold, mildly but systematically alpha-enhanced population that extends tosuper solar metallicities, in agreement with spectroscopic studies. Ourrevision offers the largest existing kinematically unbiased sample ofthe solar neighbourhood that contains full information on kinematics,metallicities, and ages and thus provides better constraints on thephysical processes relevant in the build-up of the Milky Way disc,enabling a better understanding of the Sun in a Galactic context.Catalogue (Table 2) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A138
| Coudé-feed stellar spectral library - atmospheric parameters
Context. Empirical libraries of stellar spectra play an important rolein different fields. For example, they are used as reference for theautomatic determination of atmospheric parameters, or for buildingsynthetic stellar populations to study galaxies. The CFLIB(Coudé-feed library, Indo-US) database is at present one of themost complete libraries, in terms of its coverage of the atmosphericparameters space (T{eff}, log g and [Fe/H]) and wavelengthcoverage 3460-9464 Å at a resolution of 1 Å FWHM. Althoughthe atmospheric parameters of most of the stars were determined fromdetailed analyses of high-resolution spectra, for nearly 300 of the 1273stars of the library at least one of the three parameters is missing.For the others, the measurements, compiled from the literature, areinhomogeneous. Aims: In this paper, we re-determine theatmospheric parameters, directly using the CFLIB spectra, and comparethem to the previous studies. Methods: We use the ULySS programto derive the atmospheric parameters, using the ELODIE library as areference. Results: Based on comparisons with several previousstudies we conclude that our determinations are unbiased. For the 958 F,G, and K type stars the precision on T{eff}, log g, and[Fe/H] is respectively 43 K, 0.13 dex and 0.05 dex. For the 53 M starsthey are 82 K, 0.22 dex and 0.28 dex. And for the 260 OBA type stars therelative precision on T{eff} is 5.1%, and on log g, and[Fe/H] the precision is respectively 0.19 dex and 0.16 dex. Theseparameters will be used to re-calibrate the CFLIB fluxes and to producesynthetic spectra of stellar populations.Tables 2 and 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/525/A71
| Chromospheric Activity and Jitter Measurements for 2630 Stars on the California Planet Search
We present time series measurements of chromospheric activity for morethan 2600 main-sequence and subgiant stars on the California PlanetSearch (CPS) program with spectral types ranging from about F5V to M4Vfor main-sequence stars and from G0IV to about K5IV for subgiants. Thelarge data set of more than 44,000 spectra allows us to identify anempirical baseline floor for chromospheric activity as a function ofcolor and height above the main sequence. We define ?S as anexcess in emission in the Ca II H and K lines above the baselineactivity floor and define radial velocity jitter as a function of?S and B - V for main-sequence and subgiant stars. Although thejitter for any individual star can always exceed the baseline level, wefind that K dwarfs have the lowest level of jitter. The lack ofcorrelation between observed jitter and chromospheric activity in Kdwarfs suggests that the observed jitter is dominated by instrumental oranalysis errors and not astrophysical noise sources. Thus, given thelong-term precision for the CPS program, radial velocities are notcorrelated with astrophysical noise for chromospherically quiet K dwarfstars, making these stars particularly well suited for the highestprecision Doppler surveys. Chromospherically quiet F and G dwarfs andsubgiants exhibit higher baseline levels of astrophysical jitter than Kdwarfs. Despite the fact that the rms in Doppler velocities iscorrelated with the mean chromospheric activity, it is rare to seeone-to-one correlations between the individual time series activity andDoppler measurements, diminishing the prospects for correctingactivity-induced velocity variations in F and G dwarfs.Based on observations obtained at the Keck Observatory and LickObservatory, which are operated by the University of California.
| Flux Calibrations from Nearby Eclipsing Binaries and Single Stars
Eclipsing binaries (EBs) measure distance without need or use for nearbysimilar objects, with many applications over recent decades. EBs are nowconsidered the most reliable and accurate distance indicators for thevery important lower rungs in the cosmic distance ladder, within theLocal Group of Galaxies. Among several EB distance algorithms, directcomparison of observed and theoretical fluxes is particularlystraightforward, although it requires absolute flux calibrations forwhich only a modest number of publications exist. Here, we measure UBVRI and uvby flux calibrations and calibration ratios from astronomicalobjects in ways not previously tried, specifically for EBs, single starswithin 80 pc, and the Sun. All of the single stars are below about 6500K temperature. Interstellar extinction is avoided by a restriction tonearby targets. Some photometric band calibrations in the literature areaccurate enough for very good EB distance determinations if startemperatures are accurately known, especially considering that estimateddistance has only a square-root dependence on calibration constant, butaccurate band-to-band calibration ratios are keys to the combinedtemperature-distance problem. Band-independent canceling factors (starradii and distances) allow calibration ratio measurement with enhancedaccuracy, compared to individual band calibrations. A physical EB modelwith embedded stellar atmosphere emission optimally matches theory toobservations for the binaries. Single star candidates are identified asreliably single if their radial velocity variations are below 100 ms-1. For the most part, we find good agreement with some ofthe previous calibrations and the ratios are improved.
| The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters
The questions of how planets form and how common Earth-like planets arecan be addressed by measuring the distribution of exoplanet masses andorbital periods. We report the occurrence rate of close-in planets (withorbital periods less than 50 days), based on precise Dopplermeasurements of 166 Sun-like stars. We measured increasing planetoccurrence with decreasing planet mass (M). Extrapolation of a power-lawmass distribution fitted to our measurements, df/dlogM = 0.39M-0.48, predicts that 23% of stars harbor a close-inEarth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoreticalmodels of planet formation predict a deficit of planets in the domainfrom 5 to 30 Earth masses and with orbital periods less than 50 days.This region of parameter space is in fact well populated, implying thatsuch models need substantial revision.
| A High-Contrast Imaging Survey of SIM Lite Planet Search Targets
With the development of extreme high contrast ground-based adaptiveoptics instruments and space missions aimed at detecting andcharacterizing Jupiter- and terrestrial-mass planets, it is criticalthat each target star be thoroughly vetted to determine whether it is aviable target, given both the instrumental design and scientific goalsof the program. With this in mind, we have conducted a high-contrastimaging survey of mature AFGKM stars with the PALAO/PHARO instrument onthe Palomar 200 inch telescope. The survey reached sensitivitiessufficient to detect brown dwarf companions at separations of >50 AU.The results of this survey will be utilized both by future directimaging projects such as GPI, SPHERE, and P1640 and indirect detectionmissions such as SIM Lite. Out of 84 targets, all but one have noclose-in (0.45-1") companions and 64 (76%) have no stars at all withinthe 25" field of view. The sensitivity contrasts in the Kspassband ranged from 4.5 to 10 for this set of observations. These starswere selected as the best nearby targets for habitable planet searchesbecause of their long-lived habitable zones (>1 billion years). Wereport two stars, GJ 454 and GJ 1020, with previously unpublished propermotion companions. In both cases, the companions are stellar in natureand are most likely M dwarfs based on their absolute magnitudes andcolors. Based on our mass sensitivities and level of completeness, wecan place an upper limit of ˜17% on the presence of brown dwarfcompanions with masses >40 MJ at separations of >1". Wealso discuss the importance of including statistics on those stars withno detected companions in their field of view for the sake of futurecompanion searches and an overall understanding of the population oflow-mass objects around nearby stars.
| The ability of intermediate-band Strömgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
Context. Several large scale photometric and spectroscopic surveys arebeing undertaken to provide a more detailed picture of the Milky Way.Given the necessity of generalisation in the determination of, e.g.,stellar parameters when tens and hundred of thousands of stars areconsidered it remains important to provide independent, detailed studiesto verify the methods used in the surveys. Aims: Our first aim isto critically evaluate available calibrations for deriving [M/H] fromStrömgren photometry. Secondly, we develop the standard sequencesfor dwarf stars to reflect their inherent metallicity dependence.Finally, we test how well metallicities derived from ugriz photometryreproduce metallicities derived from the well-tested system ofStrömgren photometry. Methods: We evaluate availablemetallicity calibrations based on Strömgren uvby photometry fordwarf stars using a catalogue of stars with both uvby photometry andspectroscopically determined iron abundances ([Fe/H]). The catalogue wascreated for this project. Using this catalogue, we also evaluateavailable calibrations that determine log g. A larger catalogue, inwhich metallicity is determined directly from uvby photometry, is usedto trace metallicity-dependent standard sequences for dwarf stars. Wealso perform comparisons, for both dwarf and giant stars, ofmetallicities derived from ugriz photometry with metallicities derivedfrom Strömgren photometry. Results: We provide a homogenisedcatalogue of 451 dwarf stars with 0.3 < (b-y)0 < 1.0.All stars in the catalogue have uvby photometry and [Fe/H] determinedfrom spectra with high resolution and high signal-to-noise ratios (S/N).Using this catalogue, we test how well various photometric metallicitycalibrations reproduce the spectroscopically determined [Fe/H]. Usingthe preferred metallicity calibration for dwarf stars, we derive newstandard sequences in the c1,0 versus (b-y)0 planeand in the c1,0 versus (v-y)0 plane for dwarfstars with 0.40 < (b-y)0 < 0.95 and 1.10 <(v-y)0 < 2.38. Conclusions: We recommend thecalibrations of Ramírez & Meléndez (2005) in derivingmetallicities from Strömgren photometry and find that intermediateband photometry, such as Strömgren photometry, more accurately thanbroad band photometry reproduces spectroscopically determined [Fe/H].Strömgren photometry is also better at differentiating betweendwarf and giant stars. We conclude that additional investigations of thedifferences between metallicities derived from ugriz photometry andintermediate-band photometry, such as Strömgren photometry, arerequired.Appendix A is only available in electronic form at http://www.aanda.org, Appendix B isavailable in electronic form at http://www.aanda.org and at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A40
| A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups
Context. Nearby late-type stars are excellent targets for seeking youngobjects in stellar associations and moving groups. The origin of thesestructures is still misunderstood, and lists of moving group membersoften change with time and also from author to author. Most members ofthese groups have been identified by means of kinematic criteria,leading to an important contamination of previous lists by old fieldstars. Aims: We attempt to identify unambiguous moving groupmembers among a sample of nearby-late type stars by studying theirkinematics, lithium abundance, chromospheric activity, and otherage-related properties. Methods: High-resolution echelle spectra(R ~ 57 000) of a sample of nearby late-type stars are used to deriveaccurate radial velocities that are combined with the precise Hipparcosparallaxes and proper motions to compute galactic-spatial velocitycomponents. Stars are classified as possible members of the classicalmoving groups according to their kinematics. The spectra are also usedto study several age-related properties for young late-type stars, i.e.,the equivalent width of the lithium Li i 6707.8 Å line or theR'HK index. Additional information like X-ray fluxes from theROSAT All-Sky Survey or the presence of debris discs is also taken intoaccount. The different age estimators are compared and the moving groupmembership of the kinematically selected candidates are discussed. Results: From a total list of 405 nearby stars, 102 have beenclassified as moving group candidates according to their kinematics.i.e., only ~25.2% of the sample. The number reduces when age estimatesare considered, and only 26 moving group candidates (25.5% of the 102candidates) have ages in agreement with the star having the same age asan MG member.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC) and observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.Appendices and Tables 1,5-15 are available in electronic form at http://www.aanda.orgTable 1 is alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A12
| Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter
Context. Chromospheric activity produces both photometric andspectroscopic variations that can be mistaken as planets. Large spotscrossing the stellar disc can produce planet-like periodic variations inthe light curve of a star. These spots clearly affect the spectral lineprofiles, and their perturbations alter the line centroids creating aradial velocity jitter that might “contaminate” thevariations induced by a planet. Precise chromospheric activitymeasurements are needed to estimate the activity-induced noise thatshould be expected for a given star. Aims: We obtain precisechromospheric activity measurements and projected rotational velocitiesfor nearby (d ? 25 pc) cool (spectral types F to K) stars, toestimate their expected activity-related jitter. As a complementaryobjective, we attempt to obtain relationships between fluxes indifferent activity indicator lines, that permit a transformation oftraditional activity indicators, i.e., Ca ii H & K lines, to othersthat hold noteworthy advantages. Methods: We used high resolution(~50 000) echelle optical spectra. Standard data reduction was performedusing the IRAF echelle package. To determine the chromospheric emissionof the stars in the sample, we used the spectral subtraction technique.We measured the equivalent widths of the chromospheric emission lines inthe subtracted spectrum and transformed them into fluxes by applyingempirical equivalent width and flux relationships. Rotational velocitieswere determined using the cross-correlation technique. To inferactivity-related radial velocity (RV) jitter, we used empiricalrelationships between this jitter and the R'_HK index. Results:We measured chromospheric activity, as given by different indicatorsthroughout the optical spectra, and projected rotational velocities for371 nearby cool stars. We have built empirical relationships among themost important chromospheric emission lines. Finally, we used themeasured chromospheric activity to estimate the expected RV jitter forthe active stars in the sample.Based on observations made with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto (Spain)and the Telescopio Nazionale Galileo (TNG) operated on the island of LaPalma by the Istituto Nazionale de Astrofisica Italiano (INAF), in theSpanish Observatorio del Roque de los Muchachos. This research has beensupported by the Programa de Acceso a InfraestructurasCientíficas y Tecnológicas Singulares (ICTS).Tables A1 toA4 are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/520/A79
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| Unveiling the Structure of Pre-transitional Disks
In the past few years, several disks with inner holes that arerelatively empty of small dust grains have been detected and are knownas transitional disks. Recently, Spitzer has identified a new class of"pre-transitional disks" with gaps based on near-infrared photometry andmid-infrared spectra; these objects have an optically thick inner diskseparated from an optically thick outer disk by an optically thin diskgap. A near-infrared spectrum provided the first confirmation of a gapin the pre-transitional disk of LkCa 15 by verifying that thenear-infrared excess emission in this object was due to an opticallythick inner disk. Here, we investigate the difference between the natureof the inner regions of transitional and pre-transitional disks usingthe same veiling-based technique to extract the near-infrared excessemission above the stellar photosphere. However, in this work we usedetailed disk models to fit the excess continua as opposed to the simpleblackbody fits previously used. We show that the near-infrared excessemission of the previously identified pre-transitional disks of LkCa 15and UX Tau A in the Taurus cloud as well as the newly identifiedpre-transitional disk of ROX 44 in Ophiuchus can be fit with an innerdisk wall located at the dust destruction radius. We also presentdetailed modeling of the broadband spectral energy distributions ofthese objects, taking into account the effect of shadowing by the innerdisk on the outer disk, but considering the finite size of the star,unlike other recent treatments. The near-infrared excess continua ofthese three pre-transitional disks, which can be explained by opticallythick inner disks, are significantly different from that of thetransitional disks of GM Aur, whose near-infrared excess continuum canbe reproduced by emission from sub-micron-sized optically thin dust, andDM Tau, whose near-infrared spectrum is consistent with a disk hole thatis relatively free of small dust. The structure of pre-transitionaldisks may be a sign of young planets forming in these disks and futurestudies of pre-transitional disks will provide constraints to aid intheoretical modeling of planet formation.
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| New Completeness Methods for Estimating Exoplanet Discoveries by Direct Detection
We report on new methods for evaluating realistic observing programsthat search stars for planets by direct imaging, where observations areselected from an optimized star list and stars can be observed multipletimes. We show how these methods bring critical insight into the designof the mission and its instruments. These methods provide an estimate ofthe outcome of the observing program: the probability distribution ofdiscoveries (detection and/or characterization) and an estimate of theoccurrence rate of planets (η). We show that these parameters can beaccurately estimated from a single mission simulation, without the needfor a complete Monte Carlo mission simulation, and we prove the accuracyof this new approach. Our methods provide tools to define a mission fora particular science goal; for example, a mission can be defined by theexpected number of discoveries and its confidence level. We detail howan optimized star list can be built and how successive observations canbe selected. Our approach also provides other critical missionattributes, such as the number of stars expected to be searched and theprobability of zero discoveries. Because these attributes dependstrongly on the mission scale (telescope diameter, observingcapabilities and constraints, mission lifetime, etc.), our methods aredirectly applicable to the design of such future missions and provideguidance to the mission and instrument design based on scientificperformance. We illustrate our new methods with practical calculationsand exploratory design reference missions for the James Webb SpaceTelescope (JWST) operating with a distant starshade to reduce scatteredand diffracted starlight on the focal plane. We estimate that fivehabitable Earth-mass planets would be discovered and characterized withspectroscopy, with a probability of zero discoveries of 0.004, assuminga small fraction of JWST observing time (7%), η = 0.3, and 70observing visits, limited by starshade fuel.
| Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the asteroid and Kuiper-Edgeworth belts inthe Solar system - have so far mostly been identified and studied inthermal emission shortward of 100?m. The Herschel space observatoryand the Submillimetre Common-User Bolometer Array-2 (SCUBA-2) camera onthe James Clerk Maxwell Telescope will allow efficient photometricsurveying at 70 to 850?m, which allows for the detection of coolerdiscs not yet discovered, and the measurement of disc masses andtemperatures when combined with shorter wavelength photometry. TheSCUBA-2 Unbiased Nearby Stars survey (SUNS) and the Disc Emission via aBias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) HerschelOpen Time Key Project are complementary legacy surveys observing samplesof ~500 nearby stellar systems. To maximize the legacy value of thesesurveys, great care has gone into the target selection process. Thispaper describes the target selection process and presents the targetlists of these two surveys.
| The Multiple Pre-main-sequence System HBC 515 in L1622
The bright pre-main-sequence star HBC 515 (HD 288313) located in theL1622 cometary cloud in Orion has been studied extensively withoptical/infrared imaging and ultraviolet/optical/infrared spectroscopy.The spectra indicate that HBC 515 is a weakline T Tauri star (TTS) ofspectral type K2V. Adaptive optics imaging in the K band reveals thatHBC 515 is a binary with two equally bright components separated by0farcs5. A very faint third component is found 5'' to the northwest.Spitzer IRAC and MIPS observations show that at mid-infrared wavelengthsthis third source dominates the system, suggesting that it is aprotostar still embedded in the nascent cloud of HBC 515. The closeassociation of a weakline TTS with a newborn protostar in a multiplesystem is noteworthy. Two nearby TTSs are likely associated with the HBC515 multiple system, and the dynamical evolution of the complex thatwould lead to such a configuration is considered.
| F, G, K, M Spectral Standards in the Y Band (0.95-1.11 μm)
We take advantage of good atmospheric transparency and the availabilityof high-quality instrumentation in the 1 μm near-infrared atmosphericwindow to present a grid of F, G, K, and M spectral standards observedat high spectral resolution (R ≈ 25, 000). In addition to a spectralatlas, we present a catalog of atomic line absorption features in the0.95-1.11 μm range. The catalog includes a wide range of lineexcitation potentials, from 0-13 eV, arising from neutral and singlyionized species, most frequently those of Fe I and Ti I at lowexcitation, Cr I, Fe I, and Si I at moderate excitation, and C I, S I,and Si I having relatively high excitation. The spectra also includeseveral prominent molecular bands from CN and FeH. For the atomicspecies, we analyze trends in the excitation potential, line depth, andequivalent width across the grid of spectroscopic standards to identifytemperature and surface gravity diagnostics near 1 μm. We identifythe line ratios that appear especially useful for spectral typing asthose involving Ti I and C I or S I, which are temperature sensitive inopposite directions, and Sr II, which is gravity sensitive at allspectral types. ASCII versions of all spectra are available in theonline version of the journal.
| The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars
We present a 0.8-5 μm spectral library of 210 cool stars observed ata resolving power of R ≡ λ/Δλ ~ 2000 with themedium-resolution infrared spectrograph, SpeX, at the 3.0 m NASAInfrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars havewell-established MK spectral classifications and are mostly restrictedto near-solar metallicities. The sample not only contains the F, G, K,and M spectral types with luminosity classes between I and V, but alsoincludes some AGB, carbon, and S stars. In contrast to some otherspectral libraries, the continuum shape of the spectra is measured andpreserved in the data reduction process. The spectra are absolutely fluxcalibrated using the Two Micron All Sky Survey photometry. Potentialuses of the library include studying the physics of cool stars,classifying and studying embedded young clusters and optically obscuredregions of the Galaxy, evolutionary population synthesis to studyunresolved stellar populations in optically obscured regions of galaxiesand synthetic photometry. The library is available in digital form fromthe IRTF Web site.
| REM near-IR and optical photometric monitoring of pre-main sequence stars in Orion. Rotation periods and starspot parameters
Aims. We aim at determining the rotational periods and the starspotproperties in very young low-mass stars belonging to the Ori OB1c starforming region, contributing to the study of the angular momentum andmagnetic activity evolution in these objects. Methods: Weperformed an intensive photometric monitoring of the PMS stars fallingin a field of about 10 arcmin× 10 arcmin in the vicinity of theOrion nebula cluster (ONC), also containing the BD eclipsing system2MASS J05352184-0546085. Photometric data were collected betweenNovember 2006 and January 2007 with the REM telescope in the {VRIJHK}'bands. The largest number of observations is in the I band (about 2700images) and in J and H bands (about 500 images in each filter). From theobserved rotational modulation, induced by the presence of surfaceinhomogeneities, we derived the rotation periods. The long time-baseline(nearly three months) allowed us to detect rotation periods, also forthe slowest rotators, with sufficient accuracy (? P/P<2%). Theanalysis of the spectral energy distributions and, for some stars, ofhigh-resolution spectra provided us with the main stellar parameters(luminosity, effective temperature, mass, age, and v sin i) which areessential for the discussion of our results. Moreover, the simultaneousobservations in six bands, spanning from optical to near-infraredwavelengths, enabled us to derive the starspot properties for these veryyoung low-mass stars. Results: In total, we were able to determinethe rotation periods for 29 stars, spanning from about 0.6 to 20 days.Thanks to the relatively long time-baseline of our photometry, wederived periods for 16 stars and improved previous determinations forthe other 13. We also report the serendipitous detection of two strongflares in two of these objects. In most cases, the light-curveamplitudes decrease progressively from the R to H band as expected forcool starspots, while in a few cases, they can only be modelled by thepresence of hot spots, presumably ascribable to magnetosphericaccretion. The application of our own spot model to the simultaneouslight curves in different bands allowed us to deduce the spot parametersand particularly to disentangle the spot temperature and size effects onthe observed light curves. Based on observations collected at the ESO REM telescope (La Silla,Chile) and at the MMT Observatory, a joint facility of the SmithsonianInstitution and the University of Arizona. Tables 6, 7 and the lightcurves of the variable stars are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/508/1313 Figures 21-24 areonly available in electronic form at http://www.aanda.org
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