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Measuring the ages of low-mass stars and brown dwarfs
Age is among the most elusive, yet important, fundamental properties oflow-mass stars and brown dwarfs. M dwarfs have main-sequence lifetimesthat are estimated to be trillions of years, with little change inluminosity. In contrast, brown dwarfs cool and dim with time, resultingin a significant degeneracy between mass, age, and luminosity. Despitethese inherent challenges, there have been recent efforts on bothobservational and theoretical fronts that may yield precise ages forlow-mass stars and brown dwarfs. We feature some current observationalefforts focused on estimating ages of these objects as presented in ourCool Stars 17 splinter session.
| The Interstellar Magnetic Field Close to the Sun. II.
The magnetic field in the local interstellar medium (ISM) provides a keyindicator of the galactic environment of the Sun and influences theshape of the heliosphere. We have studied the interstellar magneticfield (ISMF) in the solar vicinity using polarized starlight for starswithin 40 pc of the Sun and 90° of the heliosphere nose. In Frischet al. (Paper I), we developed a method for determining the local ISMFdirection by finding the best match to a group of interstellarpolarization position angles obtained toward nearby stars, based on theassumption that the polarization is parallel to the ISMF. In this paper,we extend the analysis by utilizing weighted fits to the position anglesand by including new observations acquired for this study. We find thatthe local ISMF is pointed toward the galactic coordinates l, b =47°± 20°, 25° ± 20°. This direction is close tothe direction of the ISMF that shapes the heliosphere, l, b =33°± 4°, 55° ± 4°, as traced by the center of the"Ribbon" of energetic neutral atoms discovered by the InterstellarBoundary Explorer (IBEX) mission. Both the magnetic field direction andthe kinematics of the local ISM are consistent with a scenario where thelocal ISM is a fragment of the Loop I superbubble. A nearby orderedcomponent of the local ISMF has been identified in the region l≈0° → 80° and b ≈0° → 30°, wherePlanetPol data show a distance-dependent increase of polarizationstrength. The ordered component extends to within 8 pc of the Sun andimplies a weak curvature in the nearby ISMF of ~0fdg25pc-1. This conclusion is conditioned on the smallsample of stars available for defining this rotation. Variations fromthe ordered component suggest a turbulent component of ~23°. Theordered component and standard relations between polarization, colorexcess, and Ho column density predict a reasonable increaseof N(H) with distance in the local ISM. The similarity of the ISMFdirections traced by the polarizations, the IBEX Ribbon, and pulsarsinside the Local Bubble in the third galactic quadrant suggest that theISMF is relatively uniform over spatial scales of 8-200 pc and is moresimilar to interarm than spiral-arm magnetic fields. The ISMF directionfrom the polarization data is also consistent with small-scale spatialasymmetries detected in GeV-TeV cosmic rays with a galactic origin. Thepeculiar geometrical relation found earlier between the cosmic microwavebackground dipole moment, the heliosphere nose, and the ISMF directionis supported by this study. The interstellar radiation field at ~975Å does not appear to play a role in grain alignment for thelow-density ISM studied here.
| 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
| New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars?
Context. The Sun displays differential rotation that is intimatelyconnected to the solar dynamo and hence related to solar activity, thesolar cycle, and the solar wind. Considering the detectability andhabitability of planets around other stars it is important to understandthe role of differential rotation in other stars. Aims: Wepresent projected rotational velocities and new measurements of therotational profile of some 180 nearby stars with spectral types A-F. Theresults are consolidated by a homogeneous compilation of basic stellardata from photometry and the identification of multiple stellar systems.New and previous measurements of rotation by line profile analysis arecompiled and made available. Methods: The overall broadeningprofile is derived analysing spectral line shape from hundreds ofspectral lines by the method of least-squares deconvolution, reducingspectral noise to a minimum. The effect of differential rotation on thebroadening profile is best measured in inverse wavelength space by thefirst two zeros of its Fourier transform. Results: Projectedrotational velocity vsini is measured for more than 110 of the samplestars. Rigid and differential rotation can be distinguished in 56 caseswhere vsini > 12 km s-1. We detect differential rotationrates of δΩ/Ω = 5% and more. Ten stars withsignificant differential rotation rates are identified. The line shapesof 46 stars are consistent with rigid rotation, even though differentialrotation at very low rates might still be possible in these cases. Thestrongest amount of relative differential rotation (54%) detected byline profile analysis is found among F stars. Conclusions: As ofnow, 33 differential rotators detected by line profile analysis havebeen confirmed. The frequency of differential rotators decreases towardshigh effective temperature and rapid rotation. There is evidence for twopopulations of differential rotators, one of rapidly rotating A stars atthe granulation boundary with strong horizontal shear and one of mid- tolate-F type stars with moderate rates of rotation and less shear. Thegap in between can only partly be explained by an upper bound found forthe horizontal shear of F stars. Apparently, the physical conditionschange at early-F spectral types. The range of horizontal shear observedfor mid-type F stars is reproduced by theoretical calculations whilethere seems to be a discrepancy in period dependence for late-F stars.Based on observations made with ESO Telescopes at the La Silla ParanalObservatory under programme ID's 074.D-0008 and 075.D-0340.Tables 1 and2 are available in electronic form at http://www.aanda.orgTables 1 and 2 arealso 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/542/A116
| 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
| 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
| Effect of different stellar galactic environments on planetary discs - I. The solar neighbourhood and the birth cloud of the Sun
We have computed trajectories, distances and times of closest approachesto the Sun by stars in the solar neighbourhood with known position,radial velocity and proper motions. For this purpose, we have used afull potential model of the Galaxy that reproduces the local z-force,the Oort constants, the local escape velocity and the rotation curve ofthe Galaxy. From our sample, we constructed initial conditions, withinobservational uncertainties, with a Monte Carlo scheme for the 12 mostsuspicious candidates because of their small tangential motion. We findthat the star Gliese 710 will have the closest approach to the Sun, witha distance of approximately 0.34 pc in 1.36 Myr in the future. We showthat the effect of a flyby with the characteristics of Gliese 710 on a100 au test particle disc representing the Solar system is negligible.However, since there is a lack of 6D data for a large percentage ofstars in the solar neighbourhood, closer approaches may exist. Wecalculate parameters of passing stars that would cause notable effectson the solar disc. Regarding the birth cloud of the Sun, we performedexperiments to reproduce roughly the observed orbital parameters such aseccentricities and inclinations of the Kuiper belt. It is now known thatin Galactic environments, such as stellar formation regions, the stellardensities of new born stars are high enough to produce close encounterswithin 200 au. Moreover, in these Galactic environments, the velocitydispersion is relatively low, typically ?˜ 1-3 kms-1. We find that with a velocity dispersion of ˜1 kms-1 and an approach distance of about 150 au, typical ofthese regions, we obtain approximately the eccentricities andinclinations seen in the current Solar system. Simple analyticalcalculations of stellar encounters effects on the Oort Cloud arepresented.
| The Frequency of Low-mass Exoplanets. III. Toward ?? at Short Periods
Determining the occurrence rate of "super-Earth" planets (m sin i <10 M ?) is a critically important step on the pathtoward determining the frequency of Earth-like planets(??), and hence the uniqueness of our solar system.Current radial-velocity surveys, achieving precisions of 1 ms-1, are now able to detect super-Earths and providemeaningful estimates of their occurrence rate. We present an analysis of67 solar-type stars from the Anglo-Australian Planet Search specificallytargeted for very high precision observations. When corrected forincompleteness, we find that the planet occurrence rate increasessharply with decreasing planetary mass. Our results are consistent withthose from other surveys: in periods shorter than 50 days, we find that3.0% of stars host a giant (msin i > 100 M ?)planet, and that 17.4% of stars host a planet with msin i < 10 M?. The preponderance of low-mass planets inshort-period orbits is in conflict with formation simulations in whichthe majority of super-Earths reside at larger orbital distances. Thiswork gives a hint as to the size of ??, but to makemeaningful predictions on the frequency of terrestrial planets inlonger, potentially habitable orbits, low-mass terrestrial planetsearches at periods of 100-200 days must be made an urgent priority forground-based Doppler planet searches in the years ahead.
| Revised classification of the SBS carbon star candidates including the discovery of a new emission line dwarf carbon star
Context. Faint high-latitude carbon stars are rare objects commonlythought to be distant, luminous giants. For this reason, they are oftenused to probe the structure of the Galactic halo; however, more accurateinvestigation of photometric and spectroscopic surveys has revealed anincreasing percentage of nearby objects with luminosities of mainsequence stars. Aims: In the General Catalogue of the SecondByurakan Survey (SBS) only ten objects are indicated as carbon starcandidates. This work aims at clarifying the nature of these stars. Methods: We analyzed new optical spectra and photometry and usedastronomical databases available on the web. Results: We verifiedthat two stars are N-type giants already confirmed by other surveys. Wefound that four candidates are M type stars and confirmed the carbonnature of the remaining four stars; the characteristics of three of themare consistent with an early CH giant type. The fourth candidate,SBS 1310+561 identified with a high proper motionstar, is a rare type of dwarf carbon showing emission lines in itsoptical spectrum. We estimated absolute magnitudes and distances to thedwarf carbon and the three CH stars. Conclusions: Our limitedsample confirmed the increasing evidence that spectroscopy or colouralone are not conclusive luminosity discriminants for CH-type carbonstars.Based on observations made at the 1.52 m telescope of the BolognaObservatory and 1.83 m telescope of the Asiago Observatory.Figures 1, 3,and 5 are available in electronic form at http://www.aanda.orgThe spectra (asciifiles) are only available at 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/532/A69
| Carbon and Oxygen in Nearby Stars: Keys to Protoplanetary Disk Chemistry
We present carbon and oxygen abundances for 941 FGK stars—thelargest such catalog to date. We find that planet-bearing systems areenriched in these elements. We self-consistently measure NC/NO , which is thought to play a key role in planetformation. We identify 46 stars with NC /NO >=1.00 as potential hosts of carbon-dominated exoplanets. We measure adownward trend in [O/Fe] versus [Fe/H] and find distinct trends in thethin and thick disks, supporting the work of Bensby et al. Finally, wemeasure sub-solar NC /NO = 0.40+0.11- 0.07, for WASP-12, a surprising result as this star is hostto a transiting hot Jupiter whose dayside atmosphere was recentlyreported to have NC /NO >= 1 by Madhusudhan etal. Our measurements are based on 15,000 high signal-to-noise spectrataken with the Keck 1 telescope as part of the California Planet Search.We derive abundances from the [O I] and C I absorption lines at ?= 6300 and 6587 Å using the SME spectral synthesizer.Based in part on observations obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Time evolution of high-energy emissions of low-mass stars. I. Age determination using stellar chronology with white dwarfs in wide binaries
Context. Stellar ages are extremely difficult to determine and oftensubject to large uncertainties, especially for field low-mass stars. Weplan to carry out a calibration of the decrease in high-energy emissionsof low-mass GKM stars with time, and therefore precise age determinationis a key ingredient. The overall goal of our research is to study thetime evolution of these high-energy emissions as an essential input tostudying exoplanetary atmospheres. Aims: We propose to determinestellar ages with a methodology based on wide binaries. We areinterested in systems composed of a low-mass star and a white dwarf(WD), where the latter serves as a stellar chronometer for the system.We aim at obtaining reliable ages for a sample of late-type stars olderthan 1 Gyr. Methods: We selected a sample of wide binariescomposed by a DA type WD and a GKM companion. High signal-to-noise,low-resolution spectroscopic observations were obtained for most of theWD members of the sample. Atmospheric parameters were determined byfitting the spectroscopic data to appropiate WD models. The total agesof the systems were derived by using cooling sequences, an initial-finalmass relationship and evolutionary tracks, to account for the progenitorlife. Results: The spectroscopic observations have allowed us todetermine ages for the binary systems using WDs as cosmochronometers. Weobtained reliable ages for 27 stars between 1 and 5 Gyr, which is arange where age determination becomes difficult for field objects.Roughly half of these systems have cooling ages that contribute at least30% the total age. We select those for further study since their ageestimate should be less prone to systematic errors coming from theinitial-final mass relationship. Conclusions: We have determinedrobust ages for a sizeable sample of GKM stars that can be subsequentlyused to study the time evolution of their emissions associated tostellar magnetic activity.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).Based on observations made with the WHT (WilliamHerschel Telescope) operated on the island of La Palma by the IsaacNewton Group in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.
| 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
| On the Frequency of Jupiter Analogs
The Anglo-Australian Planet Search has now accumulated 12 years ofradial-velocity data with long-term instrumental precision better than 3m s-1. In this paper, we expand on earlier simulation work,to probe the frequency of near-circular, long-period gas-giant planetsresiding at orbital distances of 3-6 AU—the so-called Jupiteranalogs. We present the first comprehensive analysis of the frequency ofthese objects based on radial-velocity data. We find that 3.3% of starsin our sample host Jupiter analogs; detailed, star-by-star simulationsshow that no more than 37% of stars host a giant planet between 3 and 6AU.
| 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.
| Lithium Abundances in a Sample of Planet-hosting Dwarfs
This work presents a homogeneous determination of lithium abundances ina large sample of giant-planet-hosting stars (N = 117) and a controlsample of disk stars without detected planets (N = 145). The lithiumabundances were derived using a detailed profile fitting of the Li Idoublet at 6708 Å in LTE. The planet-hosting and comparison starswere chosen to have significant overlap in their respective physicalproperties, including effective temperatures, luminosities, masses,metallicities, and ages. The combination of uniform data and homogeneousanalysis with well-selected samples makes this study well suited toprobe for possible differences in the lithium abundances found inplanet-hosting stars. An overall comparison between the two samplesreveals no obvious differences between stars with and without planets. Acloser examination of the behavior of the Li abundances over a narrowrange of effective temperature (5700 K <= T eff <= 5850K) indicates subtle differences between the two stellar samples; thistemperature range is particularly sensitive to various physicalprocesses that can deplete lithium. In this T eff range,planet-hosting stars have lower Li abundances (by ~0.26 dex on average)than the comparison stars, although this segregation may be influencedby combining stars from a range of ages, metallicities, and masses. Whenstars with very restricted ranges in metallicity ([Fe/H] = 0.00 to +0.20dex) and mass (M ~ 1.05-1.15 M sun) are compared, however,both stars with and without planets exhibit similar behaviors in thelithium abundance with stellar age, suggesting that there are nodifferences in the lithium abundances between stars with planets andstars not known to have planets.Based on observations made with the 2.2 m telescope at the EuropeanSouthern Observatory (La Silla, Chile), under the agreementESO-Observatório Nacional/MCT.
| 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.
| X-ray, FUV, and UV Observations of α Centauri B: Determination of Long-term Magnetic Activity Cycle and Rotation Period
Over the last couple of decades we have been carrying out a study ofstellar magnetic activity, dynamos, atmospheric physics, and spectralirradiances from a sample of solar-type G0-5 V stars with differentages. One of the major goals of this program is to study the evolutionof the Sun's X-ray through NUV spectral irradiances with age. Ofparticular interest is the determination of the young Sun's elevatedlevels of high-energy fluxes because of the critical roles that X-ray(coronal) through FUV (transition region (TR), chromospheric) emissionsplay on the photochemical and photoionization evolution (and possibleerosion) of early, young planetary atmospheres and ionospheres.Motivated by the current exoplanetary search missions (such as Keplerand CoRoT, along with the planned Space Interferometry Mission andDarwin/Terrestrial Planet Finder missions) that are hunting forEarth-size planets in the habitable zones (liquid water) of nearbymain-sequence G-M stars, we are expanding our program to cooler, lessluminous, but very importantly, much more numerous main-sequence K-typestars, such as α Centauri B. The long life (2-3× longer thanthe Sun) and slow evolution of K stars provide nearly constant energysources for possible hosted planets. This program parallels our "Sun inTime" program, but extends the study to stars with deeper convectivezone depths. Presented here are X-ray (coronal; ROSAT, Chandra,XMM-Newton), UV (TR; International Ultraviolet Explorer (IUE)), NUV(chromospheric; IUE), and recently acquired FUV (TR/chromospheric; FUSECycles 7/8) observations of the K1 V star α Cen B (HD 128621; V =1.33; (B - V) = +0.88; τ = 5.6 ± 0.6 Gyr). These combinedhigh-energy measures provide a more complete look into the nature ofα Cen B's magnetic activity and X-ray-UV radiances. We find thatα Cen B has exhibited significant long-term variability in X-raythrough NUV emission fluxes, indicating a solar-like long-term activitycycle of P cycle = 8.84 ± 0.4 years. In addition,analysis of the short-term rotational modulation of mean light due tothe effects of magnetically active regions has yielded a well-determinedrotation period of P rotation = 36.2 ± 1.4 days.α Cen B is the only old main-sequence K star with a reliablydetermined age and rotation period, and for early K stars, as in thecase of the Sun for G2 V stars, is an important calibrator for stellarage/rotation/activity relations.
| 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
| Stellar Parameters and Metallicities of Stars Hosting Jovian and Neptunian Mass Planets: A Possible Dependence of Planetary Mass on Metallicity
The metal content of planet-hosting stars is an important ingredientthat may affect the formation and evolution of planetary systems.Accurate stellar abundances require the determinations of reliablephysical parameters, namely, the effective temperature, surface gravity,microturbulent velocity, and metallicity. This work presents thehomogeneous derivation of such parameters for a large sample of starshosting planets (N = 117), as well as a control sample of disk stars notknown to harbor giant, closely orbiting planets (N = 145). Stellarparameters and iron abundances are derived from an automated analysistechnique developed for this work. As previously found in theliterature, the results in this study indicate that the metallicitydistribution of planet-hosting stars is more metal rich by ~0.15 dexwhen compared to the control sample stars. A segregation of the sampleaccording to planet mass indicates that the metallicity distribution ofstars hosting only Neptunian-mass planets (with no Jovian-mass planets)tends to be more metal poor in comparison with that obtained for starshosting a closely orbiting Jovian planet. The significance of thisdifference in metallicity arises from a homogeneous analysis of samplesof FGK dwarfs which do not include the cooler and more problematic Mdwarfs. This result would indicate that there is a possible link betweenplanet mass and metallicity such that metallicity plays a role insetting the mass of the most massive planet. Further confirmation,however, must await larger samples.Based on observations made with the 2.2 m telescope at the EuropeanSouthern Observatory (La Silla, Chile), under the agreementESO-Observatório Nacional/MCT.
| 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
| 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
| 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.
| Searching for stars closely encountering with the solar system
Based on a new version of the Hipparcos catalog and currently availableradial velocity data, we have searched for stars that either haveencountered or will encounter the solar neighborhood within less than 3pc in the time interval from ?2 Myr to +2 Myr. Nine new candidateswithin 30 pc of the Sun have been found. To construct the stellar orbitsrelative to the solar orbit, we have used the epicyclic approximation.We show that, given the errors in the observational data, theprobability that the well-known star HIP 89 825 (GL 710) encounteringwith the Sun most closely falls into the Oort cloud is 0.86 in the timeinterval 1.45 ± 0.06 Myr. This star also has a nonzeroprobability, 1 × 10?4, of falling into the regiond < 1000 AU, where its influence on Kuiper Belt objects becomespossible.
| Speckle Interferometry at the Blanco and SOAR Telescopes in 2008 and 2009
The results of speckle interferometric measurements of binary andmultiple stars conducted in 2008 and 2009 at the Blanco and SOAR 4 mtelescopes in Chile are presented. A total of 1898 measurements of 1189resolved pairs or sub-systems and 394 observations of 285 un-resolvedtargets are listed. We resolved for the first time 48 new pairs, 21 ofwhich are new sub-systems in close visual multiple stars. Typicalinternal measurement precision is 0.3 mas in both coordinates, typicalcompanion detection capability is ?m ~ 4.2 at 0farcs15 separation.These data were obtained with a new electron-multiplication CCD camera;data processing is described in detail, including estimation ofmagnitude difference, observational errors, detection limits, andanalysis of artifacts. We comment on some newly discovered pairs andobjects of special interest.
| The Gemini Spectral Library of Near-IR Late-Type Stellar Templates and Its Application for Velocity Dispersion Measurements
We present a spectroscopic library of late spectral type stellartemplates in the near-IR range 2.15-2.42 μm, at R = 5300-5900resolution, oriented to support stellar kinematics studies in externalgalaxies, such as the direct determination of the masses of supermassiveblack holes in nearby active (or non-active) galaxies. The combinationof high spectral resolution and state-of-the-art instrumentationavailable in 8 m class telescopes has made the analysis of circumnuclearstellar kinematics using the near-IR CO band heads one of the most usedtechniques for such studies, and this library aims to provide thesupporting data sets required by the higher spectral resolution andlarger spectral coverage currently achieved with modern near-IRspectrographs. Examples of the application for kinematical analysis aregiven for data obtained with two Gemini instruments, but the templatescan be easily adjusted for use with other near-IR spectrographs atsimilar or lower resolution. The example data sets are also used torevisit the "template mismatch" effect and the dependence of thevelocity dispersion values obtained from the fitting process with thecharacteristics of the stellar templates. The library is available inelectronic form from the Gemini Web pages..
| Explorations Beyond the Snow Line: Spitzer/IRS Spectra of Debris Disks Around Solar-type Stars
We have observed 152 nearby solar-type stars with the InfraredSpectrometer (IRS) on the Spitzer Space Telescope. Including stars thatmet our criteria but were observed in other surveys, we get an overallsuccess rate for finding excesses in the long-wavelength IRS band (30-34μm) of 11.8% ± 2.4%. The success rate for excesses in theshort-wavelength band (8.5-12 μm) is ~1% including sources from othersurveys. For stars with no excess at 8.5-12 μm, the IRS data set3σ limits of around 1000 times the level of zodiacal emissionpresent in our solar system, while at 30-34 μm data set limits ofaround 100 times the level of our solar system. Two stars (HD 40136 andHD 10647) show weak evidence for spectral features; the excess emissionin the other systems is featureless. If the emitting material consistsof large (10 μm) grains as implied by the lack of spectral features,we find that these grains are typically located at or beyond the snowline, ~1-35 AU from the host stars, with an average distance of 14± 6 AU; however, smaller grains could be located at significantlygreater distances from the host stars. These distances correspond todust temperatures in the range ~50-450 K. Several of the disks are wellmodeled by a single dust temperature, possibly indicative of a ring-likestructure. However, a single dust temperature does not match the datafor other disks in the sample, implying a distribution of temperatureswithin these disks. For most stars with excesses, we detect an excess atboth IRS and Multiband Imaging Photometer for Spitzer (MIPS)wavelengths. Only three stars in this sample show a MIPS 70 μm excesswith no IRS excess, implying that very cold dust is rare aroundsolar-type stars.
| A New Color-Magnitude Diagram for 47 Tucanae: A Statistical Analysis
We present a statistical analysis of color-magnitude diagrams of 47 Tucderived from original and archival BVI photometry that produces the mostprobable locus for single stars. After adopting E(B-V) = 0.04, we derivean apparent distance modulus (m - M) V = 13.375 andachieve good matches to the most probable locus in the [B - V, V],[V - I, I], and [B - I, I] planes with 12 Gyr, [?/Fe]= +0.3, [Fe/H] = -0.83 isochrones from the Victoria-Regina models.This metallicity is generally lower than recent spectroscopicallyderived estimates for the cluster, but it is reinforced by themain-sequence match with a sample of subdwarfs.Based in part on observations made with the European SouthernObservatory (ESO) telescopes and obtained from the ESO/ST-ECF ScienceArchive facility.
| Survey of Nearby FGK Stars at 160 μm with Spitzer
The Spitzer Space Telescope has advanced debris disk sciencetremendously with a wealth of information on debris disks around nearbyA, F, G, K, and M stars at 24 and 70 μm with the MIPS photometerand at 8-34 μm with IRS. Here we present 160 μmobservations of a small subset of these stars. At this wavelength, thestellar photospheric emission is negligible and any detected emissioncorresponds to cold dust in extended Kuiper Belt analogs. However, theSpitzer 160 μm observations are limited in sensitivity by thelarge beam size which results in significant "noise" due to cirrus andextragalactic confusion. In addition, the 160 μm measurementssuffer from the added complication of a light leak next to the star'sposition whose flux is proportional to the near-infrared flux of thestar. We are able to remove the contamination from the leak and report160 μm measurements or upper limits for 24 stars. Three stars(HD 10647, HD 207129, and HD 115617) have excesses at 160 μmthat we use to constrain the properties of the debris disks around them.A more detailed model of the spectral energy distribution of HD 10647reveals that the 70 and 160 μm emission could be due to smallwater ice particles at a distance of 100 AU, consistent with HubbleSpace Telescope optical imaging of circumstellar material in the system.
| On the Relationship Between Debris Disks and Planets
Dust in debris disks is generated by collisions among planetesimals. Theexistence of these planetesimals is a consequence of the planetformation process, but the relationship between debris disks and planetshas not been clearly established. Here we analyze Spitzer/MIPS 24 and 70μm data for 150 planet-bearing stars, and compare the incidence ofdebris disks around these stars with a sample of 118 stars around whichplanets have been searched for, but not found. Together they comprisethe largest sample ever assembled to deal with this question. The use ofsurvival analysis techniques allows us to account for the large numberof nondetections at 70 μm. We discovered 10 new debris disks aroundstars with planets and one around a star without known planets. We foundthat the incidence of debris disks is marginally higher among stars withplanets, than among those without, and that the brightness of theaverage debris disk is not significantly different in the two samples.We conclude that the presence of a planet that has been detected viacurrent radial velocity techniques is not a good predictor of thepresence of a debris disk detected at infrared wavelengths.
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Dades d'Observació i Astrometria
| Constel·lació: | Sagittarius |
| Ascensió Recta: | 20h11m11.94s |
| Declinació: | -36°06'04.3" |
| Magnitud Aparent: | 5.324 |
| Moviment propi RA: | 458.4 |
| Moviment propi Dec: | -1569.3 |
| B-T magnitude: | 6.391 |
| V-T magnitude: | 5.413 |
Catàlegs i designacions:
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