596 research outputs found
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 17.970.08 years
along with an eccentricity of around 0.12 and (ii) an orbit with a period of
36.380.13 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&
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
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