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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
| Chromospheric activities and kinematics for solar type dwarfs and subgiants: analysis of the activity distribution and the AVR
Aims: In this work we present chromospheric activity indices,kinematics, radial-velocities, and rotational velocities for more than850 FGK-type dwarfs and subgiant stars in the southern hemisphere andtest how best to calibrate and measure S-indices from echelle spectra. Methods: We measured our parameters using the high-resolution andhigh-S/N FEROS echelle spectra acquired for this purpose. Results: We confirm the bimodal distribution of chromospheric activitiesfor such stars and highlight the role that the more active K-dwarfs playin biasing the number of active stars. We show that the age-activityrelationship does appear to continue to ages older than the Sun if wesimply compare main sequence stars and subgiant stars with an offset ofaround 2.5 Gyr between the peaks of both distributions. Also we showevidence of an increased spin-down timescale for cool K dwarfs comparedwith earlier F and G type stars. We highlight that activities drawn fromlow-resolution spectra (R < 2500) significantly increase the rmsscatter when calibrating onto common systems of measurements like theMt. Wilson system. Also we show that older and widely used catalogues ofactivities in the south appear to be offset compared to more recentworks at the 0.1 dex level in log R'HK through calibratordrift. In addition, we show how kinematics can be used to preselectinactive stars for future planet search projects. We see the well knowntrend between projected rotational velocity and activity, however wealso find a correlation between kinematic space velocity andchromospheric activity. It appears that after the Vaughan-Preston gapthere is a quick step function in the kinematic space motion towards asignificantly broader spread in velocities. We speculate on reasons forthis correlation and provide some model scenarios to describe thebimodal activity distribution through magnetic saturation, residual lowlevel gas accretion, or accretion by the star of planets orplanetesimals. Finally, we provide a new empirical measurement for thedisk-heating law, using the latest age-activity relationships toreconstruct the age-velocity distribution for local disk stars. We finda value of 0.337 ± 0.045 for the exponent of this power law (i.e.σtot ∝ t0.337), in excellent agreementwith those found using isochrone fitting methods and with theoreticaldisk-heating models.Based on observations made with the ESO telescopes at the La SillaParanal observatory under programme ID's 076.C-0578(B), 077.C-0192(A),082.C-0446(A) and 082.C-0446(B).Full Table 4 is only available 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/A8
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
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