R. de la Reza

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 ingredient that may affect the formation and evolution of planetary systems. Accurate stellar abundances require the determinations of reliable physical parameters, namely, the effective temperature, surface gravity, microturbulent velocity, and metallicity. This work presents the homogeneous derivation of such parameters for a large sample of stars hosting planets (N = 117), as well as a control sample of disk stars not known to harbor giant, closely orbiting planets (N = 145). Stellar parameters and iron abundances are derived from an automated analysis technique developed for this work. As previously found in the literature, the results in this study indicate that the metallicity distribution of planet-hosting stars is more metal rich by ~0.15 dex when compared to the control sample stars. A segregation of the sample according to planet mass indicates that the metallicity distribution of stars hosting only Neptunian-mass planets (with no Jovian-mass planets) tends to be more metal poor in comparison with that obtained for stars hosting a closely orbiting Jovian planet. The significance of this difference in metallicity arises from a homogeneous analysis of samples of FGK dwarfs which do not include the cooler and more problematic M dwarfs. This result would indicate that there is a possible link between planet mass and metallicity such that metallicity plays a role in setting the mass of the most massive planet. Further confirmation, however, must await larger samples.

Search for associations containing young stars (SACY) - III. Ages and Li abundances

Context. Our study is a follow-up of the SACY project, an extended high spectral resolution survey of more than two thousand optical counterparts of X-ray sources in the Southern Hemisphere targeted to search for young nearby associations. Nine associations have either been newly identified, or had their member list better defined. Groups belonging to the Sco-Cen-Oph complex are not considered in the present study. Aims. These nine associations, with ages between about 6Myr and 70Myr, form an excellent sample to study the Li depletion in the pre-main sequence(PMS) evolution. In the present paper we investigate the use of Li abundances as an independent clock to constrain the PMS evolution. Methods. Using our measurements of the equivalent widths of the Li resonance line and assuming fixed metallicities and microturbulence, we have calculated the LTE Li abundances for 376 members of different young associations. In addition we considered the effects of their projected stellar rotation. Results. We present the Li depletion as function of age in the first hundred million years for the first time for the most extended sample of Li abundances in young stellar associations. Conclusions. A clear Li depletion can be measured in the temperature range from 5000 K to 3500 K for the age span covered by the nine associations studied in this paper. The age sequence based on the Li-clock agrees well with the isochronal ages, ǫCha association being the only possible exception. The lithium depletion patterns for the associations presented here resemble those of the young open clusters with similar ages, strengthening the notion that the members proposed for these loose young associations have indeed a common physical origin. The observed scatter in the Li abundances hampers the use of Li to determine reliable ages for individual stars. For velocities above 20kms 1 rotation seems to play an important role inhibiting the Li depletion.

Discovery of new isolated T Tauri stars

Two new T Tauri stars (TTSs) have been discovered in the neighborhood of the isolated, strong H-alpha emitting, TTS (CTTS) TW Hya (b = 23 deg). The first object is a visual binary star formed by two CTTSs separated by 1.5 arcsec (b = 21 deg). The second object may be a weakly H-alpha emitting TTS (b = 28 deg). The high latitudes of the stars indicate that their distances cannot be very large. All these TTSs have similar radial velocities, suggesting the possibility that they are kinematically related. As the nearest cloud is quite far from this group of stars, star formation in situ must be considered. 20 refs.

The Origin of the β Pictoris Moving Group

The β Pictoris moving group (BPMG) as proposed by Zuckerman et al. is a group of 19 low-mass star systems containing β Pictoris and its protoplanetary disk. This nearest moving group is at a mean distance of 36 pc from the Earth, and these authors have proposed, by means of an H-R diagram, an evolutionary age of 12 Myr for this group. Here, by retracing the three-dimensional orbits of all the members of the BPMG and using a realistic Galactic potential, we find that a first maximum concentration of orbits occurs at 11.5 Myr and in a space region having a maximum size of 24 pc, 3 times smaller than its present size. We consider this region to be the birthplace of the BPMG. This interesting similarity of independently obtained evolutionary and kinematical ages indicates that this group could have already been formed as an unbound system, as observed today. The birthplace of the BPMG is located in a three-dimensional space, at ~45 pc from the region where the Lower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL) subgroups of the Scorpius-Centaurus OB association were when they were between 4 and 6 Myr old. At that age, both subgroups were able to produce supernovae (SNe) capable of triggering the formation of the BPMG. This interaction distance could even be smaller, up to ~26 pc, if SNe exploded in the outer regions of the LCC or UCL near the proposed birthplace of the BPMG. This scenario confirms, at least for the BPMG, the suggestion made by Mamajek & Feigelson and other authors that young unbound nearby stellar associations with ages around ~10 Myr originated in this OB complex. In contrast to the BPMG, for which we propose a coeval formation, the LCC and UCL subgroups appear not to be truly coeval.

Transiting exoplanets from the CoRoT space mission IV. CoRoT-Exo-4b : a transiting planet in a 9.2 day synchronous orbit

CoRoT, the first space-based transit search, provides ultra-high precision light curves with continuous time-sampling over periods, of up to 5 months. This allows the detection of transiting planets with relatively long periods, and the simultaneous study of the host stars photometric variability. In this letter, we report on the discovery of the transiting giant planet CoRoT-Exo-4b and use the CoRoT light curve to perform a detailed analysis of the transit and to determine the stellar rotation period. The CoRoT light curve was pre-processed to remove outliers and correct for orbital residuals and artefacts due to hot pixels on the detector. After removing stellar variability around each transit, the transit light curve was analysed to determine the transit parameters. A discrete auto-correlation function method was used to derive the rotation period of the star from the out-of-transit light curve. We derive periods for the planets orbit and stars rotation of 9.20205 +/- 0.00037 and 8.87 +/- 1.12 days respectively, consistent with a synchronised system. We also derive the inclination, i = 90.00 -0.085 +0.000 in degrees, the ratio of the orbital distance to the stellar radius, a/R_s = 17.36 -0.25 +0.05, and the planet to star radius ratio R_p/R_s = 0.1047 -0.0022 +0.0041. We discuss briefly the coincidence between the orbital period of the planet and the stellar rotation period and its possible implications for the systems migration and star-planet interaction history.

On the common origin of the AB Doradus moving group and the Pleiades cluster

AB Dor is the nearest identified moving group. As with other such groups, the age is important for understanding of several key questions. It is important, for example, in establishing the origin of the group and also in comparative studies of the properties of planetary systems, eventually surrounding some of the AB Dor group members, with those existing in other groups. For AB Dor two rather different estimates for its age have been proposed: a first one, of the order of 50 Myr, by Zuckerman and coworkers from a comparison with Tucana / Horologium moving group and a second one of about 100 125 Myr by Luhman and coworkers from color-magnitude diagrams (CMD). Using this last value and the closeness in velocity space of AB Dor and the Pleiades galactic cluster, Luhman and coworkers suggested coevality for these systems. Because strictly speaking such a closeness does not still guarantee coevality, here we address this problem by computing and comparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC 2602. The latter two open clusters have estimated ages of about 85 90 Myr and 50 Myr. The resulting age 119 ± 20 Myr is consistent with AB Dor and Pleiades being coeval. Our solution and the scenario of open cluster formation proposed by Kroupa and collaborators suggest that the AB Dor moving group may be identified with the expanding subpopulation (Group I) present in this scenario. We also discuss other related aspects as iron and lithium abundances, eventual stellar mass segregation during the formation of the systems and possible fraction of debris discs in AB Dor group.

MEASUREMENTS OF THE ISOTOPIC RATIO 6Li/7Li IN STARS WITH PLANETS

High-resolution (R = 143,000), high signal-to-noise (S/N = 700-1100) Gemini-S bHROS spectra have been analyzed in a search for 6Li in 5 stars which host extrasolar planets. The presence of detectable amounts of 6Li in these mature, solar-type stars is a good monitor of accretion of planetary disk material, or solid bodies themselves, into the outer layers of the parent stars. Detailed profile-fitting of the Li I resonance doublet at lambda 6707.8 A reveals no detectable amounts of 6Li in any star in our sample. The list of stars analyzed includes HD 82943 for which 6Li has been previouly detected at the level of 6Li/7Li = 0.05 +/- 0.02. The typical limits in the derived isotopic fraction are 6Li/7Li <= 0.00-0.02. These upper limits constrain the amount of accreted material to less than ~ 0.02 to 0.5 Jovian masses. The presence of detectable amounts of 6Li would manifest itself as a red asymmetry in the Li I line-profile and the derived upper limits on such asymmetries are discussed in light of three-dimensional hydrodynamic model atmospheres, where convective motions also give rise to slight red asymmetries in line profiles.

Transiting exoplanets from the CoRoT space mission VII. The "hot-Jupiter"-type planet CoRoT-5b

Aims. The CoRoT space mission continues to photometrically monitor about 12 000 stars in its field-of-view for a series of target fields to search for transiting extrasolar planets ever since 2007. Deep transit signals can be detected quickly in the “alarm-mode” in parallel to the ongoing target field monitoring. CoRoT’s first planets have been detected in this mode. Methods. The CoRoT raw lightcurves are filtered for orbital residuals, outliers, and low-frequency stellar signals. The phase folded lightcurve is used to fit the transit signal and derive the main planetary parameters. Radial velocity follow-up observations were initiated to secure the detection and to derive the planet mass. Results. We report the detection of CoRoT-5b, detected during observations of the LRa01 field, the first long-duration field in the galactic anticenter direction. CoRoT-5b is a “hot Jupiter-type” planet with a radius of 1.388+0.046 −0.047 RJup, a mass of 0.467 +0.047 −0.024 MJup, and therefore, a mean density of 0.217+0.031 −0.025 g cm −3. The planet orbits an F9V star of 14.0 mag in 4.0378962 ± 0.0000019 days at an orbital distance of 0.04947+0.00026 −0.00029 AU.Aims. The CoRoT space mission continues to photometrically monitor about 12 000 stars in its field-of-view for a series of target fields to search for transiting extrasolar planets ever since 2007. Deep transit signals can be detected quickly in the “alarm-mode” in parallel to the ongoing target field monitoring. CoRoT’s first planets have been detected in this mode. Methods. The CoRoT raw lightcurves are filtered for orbital residuals, outliers, and low-frequency stellar signals. The phase folded lightcurve is used to fit the transit signal and derive the main planetary parameters. Radial velocity follow-up observations were initiated to secure the detection and to derive the planet mass. Results. We report the detection of CoRoT-5b, detected during observations of the LRa01 field, the first long-duration field in the galactic anticenter direction. CoRoT-5b is a “hot Jupiter-type” planet with a radius of 1.388 +0.046 −0.047 RJup ,am ass of 0.467

Star formation triggered by SN explosions: an application to the stellar association of β Pictoris

In the present study, considering the physical conditions that are relevant in interactions between supernova remnants (SNRs) and dense molecular clouds for triggering star formation we have built a diagram of SNR radius versus cloud density in which the constraints above delineate a shaded zone where star formation is allowed. We have also performed fully 3D radiatively cooling numerical simulations of the impact between SNRs and clouds under different initial conditions in order to follow the initial steps of these interactions. We determine the conditions that may lead either to cloud collapse and star formation or to complete cloud destruction and find that the numerical results are consistent with those of the SNR-cloud density diagram. Finally, we have applied the results above to the Pictoris stellar association which is composed of low-mass post-T Tauri stars with an age of 11 Myr. It has been recently suggested that its formation could have been triggered by the shock wave produced by an SN explosion localized at a distance of about 62 pc that may have occurred either in the Lower Centaurus Crux or in the Upper Centaurus Lupus which are both nearby older subgroups of that association (Ortega and co-workers). Using the results of the analysis above we have shown that the suggested origin for the young association at the proposed distance is plausible only for a very restricted range of initial conditions for the parent molecular cloud, that is, a cloud with a radius of the order of 10 pc and density of the order of 20 cm -3 and a temperature of the order of 50-100 K.

On the Origin of the Very Young Groups η and Chamaeleontis

The three-dimensional orbits of the kinematical centers of the η Cha and Cha compact groups are retraced under the action of a general Galactic potential. The results show that both groups were formed in the same spatial region about 7 Myr ago. Their birthplace appears to be quite near the Lower Centaurus Crux subgroup of the Sco-Cen OB association. The epoch of minimum separation in the past between η Cha and Cha turns out to be consistent with the estimated evolutionary age.