The orbit of Beta Pic b as a transiting planet

In 1981, Beta Pictoris showed strong and rapid photometric variations possibly due to a transiting giant planet. Later, a planetary mass companion to the star, Beta Pic b, was identified using imagery. Observations at different epochs (2003 and 2009-2015) detected the planet at a projected distance of 6 to 9 AU from the star and showed that the planet is on an edge-on orbit. The observed motion is consistent with an inferior conjunction in 1981, and Beta Pic b can be the transiting planet proposed to explain the photometric event observed at that time. Assuming that the 1981 event is related to the transit or the inferior conjunction of Beta Pic b on an edge-on orbit, we search for the planetary orbit in agreement with all the measurements of the planet position published so far. We find two different orbits that are compatible with all these constraints: (i) an orbit with a period of 17.97$\pm$0.08 years along with an eccentricity of around 0.12 and (ii) an orbit with a period of 36.38$\pm$0.13 years and a larger eccentricity of about 0.32. In the near future, new imaging observations should allow us to discriminate between these two different orbits. We also estimate the possible dates for the next transits, which could take place as early as 2017 or 2018, even for a long-period orbit.Comment: Accepted for publication in A&

Hydrogen column density evaluations toward Capella: consequences on the interstellar deuterium abundance

The deuterium abundance evaluation in the direction of Capella has for a long time been used as a reference for the local interstellar medium (ISM) within our Galaxy. We show here that broad and weak HI components could be present on the Capella line of sight, leading to a large new additional systematic uncertainty on the N(HI) evaluation. The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with almost identical chi^2 for all the fits (this range includes only the systematic error; the 2 sigma statistical one is almost negligible in comparison). It is concluded that D/H evaluations over HI column densities below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional observations) may present larger uncertainties than previously anticipated. It is mentionned that the D/O ratio might be a better tracer for DI variations in the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer (FUSE).Comment: Accepted for publication in the Astrophysical Journal Letter

The role of goal structures and peer climate in trajectories of social achievement goals during high school

Students’ social goals—reasons for engaging in interpersonal relationships with peers—are consequential for students’ interactions with their peers at school and for their well-being. Despite the salience of peer relationships during adolescence, research on social goals is generally lacking compared with academic goals, and it is unknown how these social goals develop over time, especially among high school students. The aim of the study was to assess trajectories of students’ social goals and to determine how relevant individual and contextual variables predicted initial levels and trajectories of students’ social goals. Participants were 9th through 12th grade students (N = 526) attending a U.S. high school. Students filled out surveys of their social goals (social development, social demonstration-approach, and social demonstration-avoidance) 6 times across 2 school years. Nonlinear growth curve analyses and piecewise growth curve analyses were used to assess trajectories of social goals across time. Students’ initial levels of social goals differed based on their gender, grade level, prior achievement, and perceptions of classroom goals structures and peer climate. Furthermore, despite substantial stability over time, the shapes of these goal trajectories were predicted by students’ gender, grade level, and perceptions of classroom goal structures and peer climate. In particular, students who perceived an increase in performance-avoidance classroom goals maintained higher demonstration social goals and decreased in developmental social goals over time, and students who perceived an increase in positive peer climate decreased in demonstration-avoidance social goals. Implications and directions for future research on social goals are discussed

Modeling magnesium escape from HD209458b atmosphere

Transit observations in the MgI line of HD209458b revealed signatures of neutral magnesium escaping the upper atmosphere of the planet, while no atmospheric absorption was found in the MgII doublet. Here we present a 3D particle model of the dynamics of neutral and ionized magnesium populations, coupled with an analytical modeling of the atmosphere below the exobase. Theoretical MgI absorption line profiles are directly compared with the absorption observed in the blue wing of the line during the planet transit. Observations are well-fitted with an escape rate of neutral magnesium in the range 2x10^7-3.4x10^7 g/s, an exobase close to the Roche lobe (Rexo in the range 2.1-4.3 Rp, where Rp is the planet radius) and a planetary wind velocity at the exobase vpl=25km/s. The observed velocities of the planet-escaping magnesium up to -60km/s are well explained by radiation pressure acceleration, provided that UV-photoionization is compensated for by electron recombination up to about 13Rp. If the exobase properties are constrained to values given by theoretical models of the deeper atmosphere (Rexo=2Rp and vpl=10km/s), the best fit to the observations is found at a similar electron density and escape rate within 2 sigma. In all cases, the mean temperature of the atmosphere below the exobase must be higher than about 6100 K. Simulations predict a redward expansion of the absorption profile from the beginning to the end of the transit. The spatial and spectral structure of the extended atmosphere is the result of complex interactions between radiation pressure, planetary gravity, and self-shielding, and can be probed through the analysis of transit absorption profiles in the MgI line.Comment: 16 pages, 24 figure

The MgI line: a new probe of the atmospheres of evaporating exoplanets

Transit observations of HD209458b in the UV revealed signatures of neutral magnesium escaping the planet's upper atmosphere. The absorption detected in the MgI line provides unprecedented information on the physical conditions at the altitude where the atmospheric blow-off takes place. Here we use a 3D model of atmospheric escape to estimate the transit absorption signatures in the MgI line of their host stars. The detectability of these signatures depends on the brightness of the star and the escape rate of neutral magnesium. We identify a sample of potentially evaporating exoplanets that covers a wide range of stellar and planetary properties, and whose extended exospheres might be detected through MgI line observations with current UV facilities, allowing further steps in comparative exoplanetology.Comment: 4 pages, 2 figure

D/H Measurements

Primordial evaluations of the deuterium abundance should provide one of the best tests of Big Bang nucleosynthesis models. Space as well as ground based observations seem however to result in different values. This asks for more observations in different astrophysical sites in order to link present day interstellar medium D/H evaluations to primordial ones. New investigations, made with FUSE (the Far Ultraviolet Spectroscopic Explorer launched in June 1999), are presented and in the case of the white dwarf G191-B2B line of sight a low D/H evaluation of 1.16 (+/-0.24) x 10^-5 (2 sigma) is confirmed. This seems to indicate that D/H variations are probably present in the nearby interstellar medium. The FUSE observations should help us reach in a near future a better global view of the evolution of that key element.Comment: 7 pages, 2 figures, Proceedings of "Cosmic Evolution" Conference, Paris November 200