898 research outputs found
Time-scales of close-in exoplanet radio emission variability
We investigate the variability of exoplanetary radio emission using stellar
magnetic maps and 3D field extrapolation techniques. We use a sample of hot
Jupiter hosting stars, focusing on the HD 179949, HD 189733 and tau Boo
systems. Our results indicate two time-scales over which radio emission
variability may occur at magnetised hot Jupiters. The first is the synodic
period of the star-planet system. The origin of variability on this time-scale
is the relative motion between the planet and the interplanetary plasma that is
co-rotating with the host star. The second time-scale is the length of the
magnetic cycle. Variability on this time-scale is caused by evolution of the
stellar field. At these systems, the magnitude of planetary radio emission is
anticorrelated with the angular separation between the subplanetary point and
the nearest magnetic pole. For the special case of tau Boo b, whose orbital
period is tidally locked to the rotation period of its host star, variability
only occurs on the time-scale of the magnetic cycle. The lack of radio
variability on the synodic period at tau Boo b is not predicted by previous
radio emission models, which do not account for the co-rotation of the
interplanetary plasma at small distances from the star.Comment: 10 pages, 7 figures, 2 tables, accepted in MNRA
On the environment surrounding close-in exoplanets
Exoplanets in extremely close-in orbits are immersed in a local
interplanetary medium (i.e., the stellar wind) much denser than the local
conditions encountered around the solar system planets. The environment
surrounding these exoplanets also differs in terms of dynamics (slower stellar
winds, but higher Keplerian velocities) and ambient magnetic fields (likely
higher for host stars more active than the Sun). Here, we quantitatively
investigate the nature of the interplanetary media surrounding the hot Jupiters
HD46375b, HD73256b, HD102195b, HD130322b, HD179949b. We simulate the
three-dimensional winds of their host stars, in which we directly incorporate
their observed surface magnetic fields. With that, we derive mass-loss rates
(1.9 to 8.0 /yr) and the wind properties at the
position of the hot-Jupiters' orbits (temperature, velocity, magnetic field
intensity and pressure). We show that these exoplanets' orbits are
super-magnetosonic, indicating that bow shocks are formed surrounding these
planets. Assuming planetary magnetic fields similar to Jupiter's, we estimate
planetary magnetospheric sizes of 4.1 to 5.6 planetary radii. We also derive
the exoplanetary radio emission released in the dissipation of the stellar wind
energy. We find radio fluxes ranging from 0.02 to 0.13 mJy, which are
challenging to be observed with present-day technology, but could be detectable
with future higher sensitivity arrays (e.g., SKA). Radio emission from systems
having closer hot-Jupiters, such as from tau Boo b or HD189733b, or from nearby
planetary systems orbiting young stars, are likely to have higher radio fluxes,
presenting better prospects for detecting exoplanetary radio emission.Comment: 15 pages, 5 figures, accepted to MNRA
PASTIS: Bayesian extrasolar planet validation. I. General framework, models, and performance
A large fraction of the smallest transiting planet candidates discovered by
the Kepler and CoRoT space missions cannot be confirmed by a dynamical
measurement of the mass using currently available observing facilities. To
establish their planetary nature, the concept of planet validation has been
advanced. This technique compares the probability of the planetary hypothesis
against that of all reasonably conceivable alternative false-positive (FP)
hypotheses. The candidate is considered as validated if the posterior
probability of the planetary hypothesis is sufficiently larger than the sum of
the probabilities of all FP scenarios. In this paper, we present PASTIS, the
Planet Analysis and Small Transit Investigation Software, a tool designed to
perform a rigorous model comparison of the hypotheses involved in the problem
of planet validation, and to fully exploit the information available in the
candidate light curves. PASTIS self-consistently models the transit light
curves and follow-up observations. Its object-oriented structure offers a large
flexibility for defining the scenarios to be compared. The performance is
explored using artificial transit light curves of planets and FPs with a
realistic error distribution obtained from a Kepler light curve. We find that
data support for the correct hypothesis is strong only when the signal is high
enough (transit signal-to-noise ratio above 50 for the planet case) and remains
inconclusive otherwise. PLATO shall provide transits with high enough
signal-to-noise ratio, but to establish the true nature of the vast majority of
Kepler and CoRoT transit candidates additional data or strong reliance on
hypotheses priors is needed.Comment: Accepted for publication in MNRAS; 23 pages, 11 figure
High-contrast imaging of Sirius~A with VLT/SPHERE: Looking for giant planets down to one astronomical unit
Sirius has always attracted a lot of scientific interest, especially after
the discovery of a companion white dwarf at the end of the 19th century. Very
early on, the existence of a potential third body was put forward to explain
some of the observed properties of the system. We present new coronagraphic
observations obtained with VLT/SPHERE that explore, for the very first time,
the innermost regions of the system down to 0.2" (0.5 AU) from Sirius A. Our
observations cover the near-infrared from 0.95 to 2.3 m and they offer the
best on-sky contrast ever reached at these angular separations. After detailing
the steps of our SPHERE/IRDIFS data analysis, we present a robust method to
derive detection limits for multi-spectral data from high-contrast imagers and
spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at
0.2", ~16.3 mag in the 0.4-1.0" range and down to 19 mag at 3.7". In physical
units, our observations are sensitive to giant planets down to 11 at
0.5 AU, 6-7 in the 1-2 AU range and ~4 at 10 AU. Despite
the exceptional sensitivity of our observations, we do not report the detection
of additional companions around Sirius A. Using a Monte Carlo orbital analysis,
we show that we can reject, with about 50% probability, the existence of an 8
planet orbiting at 1 AU. In addition to the results presented in the
paper, we provide our SPHERE/IFS data reduction pipeline at
http://people.lam.fr/vigan.arthur/ under the MIT license.Comment: 16 pages, 10 figures, accepted for publication in MNRA
Comparing the performance of stellar variability filters for the detection of planetary transits
We have developed a new method to improve the transit detection of
Earth-sized planets in front of solar-like stars by fitting stellar
microvariability by means of a spot model. A large Monte Carlo numerical
experiment has been designed to test the performance of our approach in
comparison with other variability filters and fitting techniques for stars of
different magnitudes and planets of different radius and orbital period, as
observed by the space missions CoRoT and Kepler. Here we report on the results
of this experiment.Comment: 4 pages, 3 postscript figures, Transiting Planets Proceeding IAU
Symposium No.253, 200
Modeling the RV jitter of early M dwarfs using tomographic imaging
In this paper we show how tomographic imaging (Zeeman Doppler Imaging, ZDI)
can be used to characterize stellar activity and magnetic field topologies,
ultimately allowing to filter out the radial velocity (RV) activity jitter of
M-dwarf moderate rotators. This work is based on spectropolarimetric
observations of a sample of five weakly-active early M-dwarfs (GJ 205, GJ 358,
GJ 410, GJ479, GJ 846) with HARPS-Pol and NARVAL. These stars have v sin i and
RV jitters in the range 1-2 km/s and 2.7-10.0 m/s rms respectively. Using a
modified version of ZDI applied to sets of phase-resolved Least-Squares- Decon-
volved (LSD) profiles of unpolarized spectral lines, we are able to
characterize the distribution of active regions at the stellar surfaces. We
find that darks spots cover less than 2% of the total surface of the stars of
our sample. Our technique is e cient at modeling the rotationally mod- ulated
component of the activity jitter, and succeeds at decreasing the amplitude of
this com- ponent by typical factors of 2-3 and up to 6 in optimal cases. From
the rotationally modulated time-series of circularly polarized spectra and with
ZDI, we also reconstruct the large-scale magnetic field topology. These fields
suggest that bi-stability of dynamo processes observed in active M dwarfs may
also be at work for moderately active M dwarfs. Comparing spot distributions
with field topologies suggest that dark spots causing activity jitter
concentrate at the magnetic pole and/or equator, to be confirmed with future
data on a larger sample.Comment: 34 pages, accepted for publication in MNRA
Adaptive optics imaging of P Cygni in Halpha
We obtained Halpha diffraction limited data of the LBV star P Cyg using the
ONERA Adaptive Optics (AO) facility BOA at the OHP 1.52m telescope on October
1997. Taking P Cyg and the reference star 59 Cyg AO long exposures we find that
P Cyg clearly exhibits a large and diffuse intensity distribution compared to
the 59 Cyg's point-like source. A deconvolution of P Cyg using 59 Cyg as the
Point Spread Function was performed by means of the Richardson-Lucy algorithm.
P Cyg clearly appears as an unresolved star surrounded by a clumped envelope.
The reconstructed image of P Cyg is compared to similar spatial resolution maps
obtained from radio aperture synthesis imaging. We put independent constraints
on the physics of P Cyg which agree well with radio results. We discuss future
possibilities to constrain the wind structure of P Cyg by using
multi-resolution imaging, coronagraphy and long baseline interferometry to
trace back its evolutionary status.Comment: 10 pages, 19 Encapsulated Postscript figure
Numerical analysis of timber fracture due to mechanical and thermal loads: an approach based on invariant integral
International audienc
Spectropolarimetric observations of the transiting planetary system of the K dwarf HD 189733
With a Jupiter-mass planet orbiting at a distance of only 0.031 AU, the
active K2 dwarf HD 189733 is a potential candidate in which to study the
magnetospheric interactions of a cool star with its recently-discovered
close-orbiting giant planet. We decided to explore the strength and topology of
the large-scale magnetosphere of HD 189733, as a future benchmark for
quantitative studies for models of the star/planet magnetic interactions. To
this end, we used ESPaDOnS, the new generation spectropolarimeter at the
Canada-France-Hawaii 3.6m telescope, to look for Zeeman circular polarisation
signatures in the line profiles of HD 189733 in 2006 June and August. Zeeman
signatures in the line profiles of HD 189733 are clearly detected in all
spectra, demonstrating that a field is indeed present at the surface of the
star. The Zeeman signatures are not modulated with the planet's orbital period
but apparently vary with the stellar rotation cycle. The reconstructed
large-scale magnetic field, whose strength reaches a few tens of G, is
significantly more complex than that of the Sun; it involves in particular a
significant toroidal component and contributions from magnetic multipoles of
order up to 5. The CaII H & K lines clearly feature core emission, whose
intensity is apparently varying mostly with rotation phase. Our data suggest
that the photosphere and magnetic field of HD 189733 are sheared by a
significant amount of differential rotation. Our initial study confirms that HD
189733 is an optimal target for investigating activity enhancements induced by
closely orbiting planets. More data are needed, densely covering both the
orbital and rotation cycles, to investigate whether and how much the planet
contributes to the overall activity level of HD 189733.Comment: Accepted in Astronomy and Astrophysics, 12 page
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