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 {approx}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.
The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, ...their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III–V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II–VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures.
We report results from a high-resolution optical spectroscopic survey aimed to search for nearby young associations and young stars among optical counterparts of ROSAT All-Sky Survey X-ray sources in ...the Southern Hemisphere. We selected 1953 late-type ( B-V similar to \geq similar to 0.6), potentially young, optical counterparts out of a total of 9574 1RXS sources for follow-up observations. At least one high-resolution spectrum was obtained for each of 1511 targets. This paper is the first in a series presenting the results of the SACY survey. Here we describe our sample and our observations. We describe a convergence method in the ( UVW) velocity space to find associations. As an example, we discuss the validity of this method in the framework of the \beta Pic Association.
ABSTRACT
Dwarf stars with debris discs and planets appear to be excellent laboratories to study the core accretion theory of planets formation. These systems are however, insufficiently studied. In ...this paper we present the main metallicity and lithium abundance properties of these stars together with stars with only debris discs and stars with only planets. Stars without detected planets or discs are also considered. The analysed sample is formed by main-sequence FGK field single stars. Apart from the basic stellar parameters, we include the use of dusty discs masses. The main results show for the first time that the dust mass of debris disc stars with planets correlate with metallicity. We confirm that these disc dust masses are related to their central stellar masses.
Separately, the masses of stars and those of planets also correlate with metallicity. We conclude that two conditions are necessary to form giant planets: to have a sufficient metallicity and also a sufficient protoplanetary mass of gas and dust. The debris discs masses of stars without giant planets do not correlate with metallicity, because they do not fulfil these two conditions. Concerning lithium, by adopting a stellar model for lithium depletion based on a strong interaction between the star and a protoplanetary disc, we found that in agreement with the model predictions, observations indicate that the main lithium depletion occurs during this initial protoplanetary evolution stage. We show that the ultimately lithium depletion is independent of the presence or absence of planets and appears to be only age dependent.
This work presents a homogeneous determination of lithium abundances in a large sample of giant-planet-hosting stars (N = 117) and a control sample of disk stars without detected planets (N = 145). ...The lithium abundances were derived using a detailed profile fitting of the Li I doublet at 6708 A in LTE. The planet-hosting and comparison stars were chosen to have significant overlap in their respective physical properties, including effective temperatures, luminosities, masses, metallicities, and ages. The combination of uniform data and homogeneous analysis with well-selected samples makes this study well suited to probe for possible differences in the lithium abundances found in planet-hosting stars. An overall comparison between the two samples reveals no obvious differences between stars with and without planets. A closer examination of the behavior of the Li abundances over a narrow range of effective temperature (5700 K <= T eff <= 5850 K) indicates subtle differences between the two stellar samples; this temperature range is particularly sensitive to various physical processes 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 influenced by combining stars from a range of ages, metallicities, and masses. When stars with very restricted ranges in metallicity (Fe/H = 0.00 to +0.20 dex) and mass (M ~ 1.05-1.15 M ) are compared, however, both stars with and without planets exhibit similar behaviors in the lithium abundance with stellar age, suggesting that there are no differences in the lithium abundances between stars with planets and stars not known to have planets.
Context. The CoRoT mission, a pioneer in exoplanet searches from space, has completed its first 150 days of continuous observations of similar to 12 000 stars in the galactic plane. An analysis of ...the raw data identifies the most promising candidates and triggers the ground-based follow-up. Aims. We report on the discovery of the transiting planet CoRoT-Exo-2b, with a period of 1.743 days, and characterize its main parameters. Methods. We filter the CoRoT raw light curve of cosmic impacts, orbital residuals, and low frequency signals from the star. The folded light curve of 78 transits is fitted to a model to obtain the main parameters. Radial velocity data obtained with the SOPHIE, CORALIE and HARPS spectrographs are combined to characterize the system. The 2.5 min binned phase-folded light curve is affected by the effect of sucessive occultations of stellar active regions by the planet, and the dispersion in the out of transit part reaches a level of 1.09\times10 super(-4) in flux units. Results. We derive a radius for the planet of 1.465 \pm 0.029 R_{\rm Jup} and a mass of 3.31 \pm 0.16 M_{\rm Jup}, corresponding to a density of 1.31 \pm 0.04 g/cm super(3). The large radius of CoRoT-Exo-2b cannot be explained by current models of evolution of irradiated planets.
The integration of III–V semiconductors with Si in device fabrication is facilitated by the use of nanoscale structures such as nanowires. Nanowires are predominantly grown using Au seed particles; ...however, the seed material is known to affect the nanowire growth and properties. Here we present growth of GaAs nanowires using three different seed particle materials: Au, Ag, and a AgAu alloy. By comparing the results from the different seeds, we found that the growths of Au- and AgAu-seeded nanowires were in general very similar, with homogeneous and vertical nanowires observed in both cases. The Ag-seeded growths instead revealed a lower yield of vertical nanowires with large variations in lengths. Different Ga-concentrations were measured in the different seed particles, which suggested that the Au and the AgAu seed particles were liquid during growth, whereas Ag particles were solid. The chemical potential of Ga was however found to be similar for all three seed materials. We propose that the Ga concentrations are determined by the chemical potential of Ga, which in turn explains why Ag-seeded nanowire growth proceeds with a solid particle. Overall, this study shows that varying the seed material can be a powerful tool to gain a deeper understanding of particle assisted nanowire growth.
AB Doradus (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: the first one, of the order of 50 Myr, by Zuckerman and coworkers from a comparison with the Tucana/Horologium moving group and a second one of about 100–125 Myr by Luhman and coworkers from colour–magnitude diagrams. 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 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 the AB Dor group.
We report on the spectroscopic transit of the massive hot-Jupiter CoRoT- Exo-2b observed with the high-precision spectrographs SOPHIE and HARPS. By modeling the radial velocity anomaly occurring ...during the transit due to the Rossiter-McLaughlin (RM) effect, we determine the sky-projected angle between the stellar spin and the planetary orbital axis to be close to zero \lambda = 7.2 \pm 4.5 deg, and we secure the planetary nature of CoRoT-Exo-2b. We discuss the influence of the stellar activity on the RM modeling. Spectral analysis of the parent star from HARPS spectra are presented.
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