124 research outputs found
Dynamical evolution of the Gliese 436 planetary system - Kozai migration as a potential source for Gliese 436b's eccentricity
The close-in planet orbiting GJ 436 presents a puzzling orbital eccentricity
considering its very short orbital period. Given the age of the system, this
planet should have been tidally circularized a long time ago. Many attempts to
explain this were proposed in recent years, either involving abnormally weak
tides, or the perturbing action of a distant companion. We address here the
latter issue based on Kozai migration. We propose that GJ 436b was formerly
located further away from the star and that it underwent a migration induced by
a massive, inclined perturber via Kozai mechanism. In this context, the
perturbations by the companion trigger high amplitude variations to GJ 436b
that cause tides to act at periastron. Then the orbit tidally shrinks to reach
its present day location. We numerically integrate the 3-body system including
tides and General Relativity correction. We first show that starting from the
present-day location of GJ 436b inevitably leads to damping the Kozai
oscillations and to rapidly circularizing the planet. Conversely, starting from
5-10 times further away allows the onset of Kozai cycles. The tides act in peak
eccentricity phases and reduce the semi-major axis of the planet. The net
result is an evolution characterized by two phases: a first one with Kozai
cycles and a slowly shrinking semi-major axis, and a second one once the planet
gets out of the Kozai resonance characterized by a more rapid decrease. The
timescale of this process appears in most cases much longer than the standard
circularization time of the planet by a factor larger than 50. This model can
provide a solution to the eccentricity paradox of GJ 436b. Depending on the
various orbital configurations, it can take several Gyrs to GJ 436b to achieve
a full orbital decrease and circularization. According to this scenario, we
could be witnessing today the second phase of the scenario where the semi-major
axis is already reduced while the eccentricity is still significant. We then
explore the parameter space and derive in which conditions this model can be
realistic given the age of the system. This yields constraints on the
characteristics of the putative companion.Comment: 13 pages To appear in Astronomy \& Astrophysic
Recherche et étude de planètes autour des naines M
In a decade of hunt for extrasolar planetary systems about 170 exoplanets have been found to orbit almost 150 stars. My thesis takes place in this new field of astrophysics by looking for planets around M dwarfs. These stars are very-low mass stars, and hence this work focuses on the caracterisation of the formation of planets depending on the host stars mass.First, I describe the paradigm of the planetary formation, on one hand by giving the statistical properties of the already detected systems and, on the other hand with a description of the standard model. Next I review different methods for the detection of exoplanets, with a particular stress on the radial-velocities method, its advantages and drawbacks. It is this last method we use for our different research programmes. Afetr what, I describe our stars samples and the obtained precisions according to the used instruments. From these programmes I present a detection of a planet with the mass of Neptune orbiting a M dwarf as well as the identification of a star which displays a spot on its surface. These two results, presented in a concurrent way, allow me to explain how it is possible to descriminate the signal induced by an exoplanet from false detections.Because in the case of sun-like stars a strong correlation is observed between the occurence of planets and the metal content of host stars, a part of my thesis is devoted to the study of the metallicity for M dwarfs.Finally, the last part of my work aims to estimate the frequency of giant planets orbiting M dwarfs. This is done through a comparison of our gathered data with output from numerical simulations. The computed frequency is analysed to the light of our determination of M-dwarf metallicity.En une décade la recherche de systèmes planétaires a fournit environ 170 exoplanètes en orbites autour de presque 150 étoiles. Mon travail de thèse s'inscrit dans ce nouveaux champ de l'astrophysique en cherchant les planètes autour des naines M. Ces étoiles sont des étoiles de très faibles masses, ce travail tente donc de caractériser la formation planétaire pour le domaine des petites masses stellaires. Dans un premier temps j'appréhende le paradigme de la formation planétaire en exposant les propriétés statistiques des systèmes détectés d'une part et le modèle standard d'autre part. Je m'intéresse aux méthodes de détection, avec une attention toute particulière pour la méthode des vitesses radiales, ses atouts et ses limitations. C'est cette dernière méthode que nous employons pour nos différents programmes de recherche. Après une description de nos échantillons et des précisions obtenues avec chacun des instruments utilisés, je présente la détection d'une planète de la masse de Neptune en orbite autour d'une naine M. De manière concurrente j'analyse aussi une étoile dont le signal mesuré est vraisemblablement causé par l'activité stellaire. Ce parallèle me permet d'expliquer comment nous différencions un signal induit par une planète des fausses détections.Parce que pour les étoiles de type solaire le taux d'étoiles-à-planètes est une fonction fortement croissante avec la teneur en métaux des étoiles, une partie de ma thèse est ensuite consacrée à l'analyse de la métallicité des naines M.Enfin, la dernière partie du manuscrit est consacrée à une étude statistique qui borne précisément la fréquence de planètes géantes en orbite autour de naines M. Le faible taux mesuré est regardé à la lumière de notre étude sur la métallicité des naines M
Stellar noise and planet detection. I. Oscillations, granulation and sun-like spots
Spectrographs like HARPS can now reach a sub-ms−1 precision in radial-velocity (RV) (Pepe & Lovis 2008). At this level of accuracy, we start to be confronted with stellar noise produced by 3 different physical phenomena: oscillations, granulation phenomena (granulation, meso- and super-granulation) and activity. On solar type stars, these 3 types of perturbation can induce ms−1 RV variation, but on different time scales: 3 to 15 minutes for oscillations, 15 minutes to 1.5 days for granulation phenomena and 10 to 50 days for activity. The high precision observational strategy used on HARPS, 1 measure per night of 15 minutes, on 10 consecutive days each month, is optimized, due to a long exposure time, to average out the noise coming from oscillations (Dumusque et al. 2011a) but not to reduce the noise coming from granulation and activity (Dumusque et al. 2011a and Dumusque et al. 2011b). The smallest planets found with this strategy (Mayor et al. 2009) seems to be at the limit of the actual observational strategy and not at the limit of the instrumental precision. To be able to find Earth mass planets in the habitable zone of solar-type stars (200 days for a K0 dwarf), new observational strategies, averaging out simultaneously all type of stellar noise, are require
Near-infrared transmission spectrum of the warm-uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope
The atmospheric composition of low-mass exoplanets is the object of intense
observational and theoretical investigations. GJ3470b is a warm uranus recently
detected in transit across a bright late-type star. The transit of this planet
has already been observed in several band passes from the ground and space,
allowing observers to draw an intriguing yet incomplete transmission spectrum
of the planet atmospheric limb. In particular, published data in the visible
suggest the existence of a Rayleigh scattering slope, making GJ3470b a unique
case among the known neptunes, while data obtained beyond 2 um are consistent
with a flat infrared spectrum. The unexplored near-infrared spectral region
between 1 and 2 um, is thus key to undertanding the atmospheric nature of
GJ3470b. Here, we report on the first space-borne spectrum of GJ3470, obtained
during one transit of the planet with WFC3 on board HST, operated in stare
mode. The spectrum covers the 1.1--1.7-um region with a resolution of about
300. We retrieve the transmission spectrum of GJ3470b with a chromatic
planet-to-star radius ratio precision of 0.15% (about one scale height) per
40-nm bins. At this precision, the spectrum appears featureless, in good
agreement with ground-based and Spitzer infrared data at longer wavelengths,
pointing to a flat transmission spectrum from 1 to 5 um. We present new
simulations of transmission spectra for GJ3470b, which allow us to show that
the HST/WFC3 observations rule out cloudless hydrogen-rich atmospheres (>10
sigma) as well as hydrogen-rich atmospheres with tholin haze (>5 sigma). Adding
our near-infrared measurements to the full set of previously published data
from 0.3 to 5 um, we find that a cloudy, hydrogen-rich atmosphere can explain
the full transmission spectrum if, at the terminator, the clouds are located at
low pressures (<1 mbar) or the water mixing ratio is extremely low (<1 ppm).Comment: Astronomy & Astrophysics, in press. 19 figures. 2 table
Telluric-line subtraction in high-accuracy velocimetry: a PCA-based approach
Optical velocimetry has led to the detection of more than 500 planets to date
and there is a strong effort to push m/s velocimetry to the near-infrared to
access cooler and lighter stars. The presence of numerous telluric absorption
lines in the nIR brings an important challenge. As the star's barycentric
velocity varies through the year, the telluric absorption lines effectively
varies in velocity relative to the star's spectrum by the same amount leading
to important systematic RV offsets. We present a novel principal component
analysis-based approach for telluric line subtraction and demonstrated its
effectiveness with archival HARPS data for GJ436 and {\tau} Ceti, over parts of
the R-band that contain strong telluric absorption lines. The main results are:
1) a better RV accuracy with excluding only a few percentage of the domain, 2)
better use of the entire spectrum to measure RV and 3) a higher telescope time
efficency by using A0V telluric standard from telescope archive.Comment: Presented at SPIE Astronomical Telescopes + Instrumentation 201
Disentangling stellar activity and planetary signals
High-precision radial-velocimetry (RV) is until now the more efficient way to discover planetary systems. Moreover, photometric transit search missions like CoRoT and Kepler, need spectroscopic RV measurements to establish the planetary nature of a transit candidate and to measure the true mass. An active star has on its photosphere dark spots and bright plages rotating with the star. These inhomogeneities of the stellar surface can induce a variation of the measurement of the RV, due to changes in lines shapes and not to a Doppler motion of the star (e.g. Queloz et al. 2001; Desort et al. 2007; Boisse et al. 2009). We study how the Keplerian fit used to search for planets in RV data is confused by spots and we test an approach to subtract RV jitter based on harmonic decomposition of the star rotation. We use simulations of spectroscopic measurements of rotating spotted stars and validate our approach on active stars monitored by high-precision spectrograph HARPS: CoRoT-7 and ι Ho
The HARPS search for southern extra-solar planets: XXVIII. Two giant planets around M0 dwarfs
Fewer giants planets are found around M dwarfs than around more massive
stars, and this dependence of planetary characteristics on the mass of the
central star is an important observational diagnostic of planetary formation
theories. In part to improve on those statistics, we are monitoring the radial
velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m
telescope. We present here the detection of giant planets around two nearby M0
dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1
Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a
factor of two, the most massive planet found by radial velocity monitoring of
an M dwarf, but its being found around an early M-dwarf is in approximate line
with the upper envelope of the planetary vs stellar mass diagram. HIP 12961
([Fe/H]=-0.07) is slightly more metal-rich than the average solar neighborhood
([Fe/H]=-0.17), and Gl 676A ([Fe/H=0.18) significantly so. The two stars
together therefore reinforce the growing trend for giant planets being more
frequent around more metal-rich M dwarfs, and the 5~Jupiter mass Gl 676Ab being
found around a metal-rich star is consistent with the expectation that the most
massive planets preferentially form in disks with large condensate masses.Comment: Corrected an error in the labelling of one line in Table
The Earth as an extrasolar transiting planet: Earth's atmospheric composition and thickness revealed by Lunar eclipse observations
An important goal within the quest for detecting an Earth-like extrasolar
planet, will be to identify atmospheric gaseous bio-signatures. Observations of
the light transmitted through the Earth's atmosphere, as for an extrasolar
planet, will be the first step for future comparisons. We have completed
observations of the Earth during a Lunar eclipse, a unique situation similar to
that of a transiting planet. We aim at showing what species could be detected
in its atmosphere at optical wavelengths, where a lot of photons are available
in the masked stellar light. We present observations of the 2008 August 16 Moon
eclipse performed with the SOPHIE spectrograph at the Observatoire de
Haute-Provence. Locating the spectrograph fibers in the penumbra of the
eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and
solar light that has passed through the Earth's limb. This mixture essentially
reproduces what is recorded during the transit of an extrasolar planet. We
report here the clear detection of several Earth atmospheric compounds in the
transmission spectra, such as ozone, molecular oxygen, and neutral sodium as
well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover,
we present a method that allows us to derive the thickness of the atmosphere
versus the wavelength for penumbra eclipse observations. We quantitatively
evaluate the altitude at which the atmosphere becomes transparent for important
species like molecular oxygen and ozone, two species thought to be tightly
linked to the presence of life. The molecular detections presented here are an
encouraging first attempt, necessary to better prepare for the future of
extremely-large telescopes and transiting Earth-like planets. Instruments like
SOPHIE will be mandatory when characterizing the atmospheres of transiting
Earth-like planets from the ground and searching for bio-marker signatures.Comment: 15 pages, 14 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
Exoplanets orbiting close to their parent stars could lose some fraction of
their atmospheres because of the extreme irradiation. Atmospheric mass loss
primarily affects low-mass exoplanets, leading to suggest that hot rocky
planets might have begun as Neptune-like, but subsequently lost all of their
atmospheres; however, no confident measurements have hitherto been available.
The signature of this loss could be observed in the ultraviolet spectrum, when
the planet and its escaping atmosphere transit the star, giving rise to deeper
and longer transit signatures than in the optical spectrum. Here we report that
in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese
436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69%
optical transit depth. The ultraviolet transits repeatedly start ~2 h before,
and end >3 h after the ~1 h optical transit, which is substantially different
from one previous claim (based on an inaccurate ephemeris). We infer from this
that the planet is surrounded and trailed by a large exospheric cloud composed
mainly of hydrogen atoms. We estimate a mass-loss rate in the range of
~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a
Neptune-like planet in the lifetime of the parent star, but would have been
much greater in the past.Comment: Published in Nature on 25 June 2015. Preprint is 28 pages, 12
figures, 2 table
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