1,314 research outputs found
Detecting the spin-orbit misalignment of the super-Earth 55 Cnc e
We present time-resolved spectroscopy of transits of the super-Earth 55 Cnc e
using HARPS-N observations. We devised an empirical correction for the "color
effect" affecting the radial velocity residuals from the Keplerian fit, which
significantly improves their dispersion with respect to the HARPS-N pipeline
standard data-reduction. Using our correction, we were able to detect the
smallest Rossiter-McLaughlin anomaly amplitude of an exoplanet so far (~60
cm/s). The super-Earth 55 Cnc e is also the smallest exoplanet with a
Rossiter-McLaughlin anomaly detection. We measured the sky-projected obliquity
lambda = 72.4 (+12.7 -11.5 deg), indicating that the planet orbit is prograde,
highly misaligned and nearly polar compared to the stellar equator. The entire
55 Cancri system may have been highly tilted by the presence of a stellar
companion.Comment: 11 pages, 7 figure
3D model of hydrogen atmospheric escape from HD209458b and HD189733b: radiative blow-out and stellar wind interactions
Transit observations in Ly-alpha of HD209458b and HD189733b revealed
signatures of neutral hydrogen escaping the planets. We present a 3D particle
model of the dynamics of the escaping atoms, and calculate theoretical Ly-alpha
absorption line profiles, which can be directly compared with the absorption
observed in the blue wing of the line. For HD209458b the observed velocities of
the escaping atoms up to -130km/s are naturally explained by radiation-pressure
acceleration. The observations are well-fitted with an ionizing flux of about
3-4 times solar and a hydrogen escape rate in the range 10^9-10^11g/s, in
agreement with theoretical predictions. For HD189733b absorption by neutral
hydrogen was observed in 2011 in the velocity range -230 to -140km/s. These
velocities are higher than for HD209458b and require an additional acceleration
mechanism for the escaping hydrogen atoms, which could be interactions with
stellar wind protons. We constrain the stellar wind (temperature ~3x10^4K,
velocity 200+-20km/s and density in the range 10^3-10^7/cm3) as well as the
escape rate (4x10^8-10^11g/s) and ionizing flux (6-23 times solar). We also
reveal the existence of an 'escape-limited' saturation regime in which most of
the escaping gas interacts with the stellar protons. In this regime, which
occurs at proton densities above ~3x10^5/cm3, the amplitude of the absorption
signature is limited by the escape rate and does not depend on the wind
density. The non-detection of escaping hydrogen in earlier observations in 2010
can be explained by the suppression of the stellar wind at that time, or an
escape rate of about an order of magnitude lower than in 2011. For both
planets, best-fit simulations show that the escaping atmosphere has the shape
of a cometary tail.Comment: 21 pages, 26 figures, accepted for publication in A&
Sketching for Large-Scale Learning of Mixture Models
Learning parameters from voluminous data can be prohibitive in terms of
memory and computational requirements. We propose a "compressive learning"
framework where we estimate model parameters from a sketch of the training
data. This sketch is a collection of generalized moments of the underlying
probability distribution of the data. It can be computed in a single pass on
the training set, and is easily computable on streams or distributed datasets.
The proposed framework shares similarities with compressive sensing, which aims
at drastically reducing the dimension of high-dimensional signals while
preserving the ability to reconstruct them. To perform the estimation task, we
derive an iterative algorithm analogous to sparse reconstruction algorithms in
the context of linear inverse problems. We exemplify our framework with the
compressive estimation of a Gaussian Mixture Model (GMM), providing heuristics
on the choice of the sketching procedure and theoretical guarantees of
reconstruction. We experimentally show on synthetic data that the proposed
algorithm yields results comparable to the classical Expectation-Maximization
(EM) technique while requiring significantly less memory and fewer computations
when the number of database elements is large. We further demonstrate the
potential of the approach on real large-scale data (over 10 8 training samples)
for the task of model-based speaker verification. Finally, we draw some
connections between the proposed framework and approximate Hilbert space
embedding of probability distributions using random features. We show that the
proposed sketching operator can be seen as an innovative method to design
translation-invariant kernels adapted to the analysis of GMMs. We also use this
theoretical framework to derive information preservation guarantees, in the
spirit of infinite-dimensional compressive sensing
Radiative braking in the extended exosphere of GJ436b
The recent detection of a giant exosphere surrounding the warm Neptune GJ436
b has shed new light on the evaporation of close-in planets, revealing that
moderately irradiated, low-mass exoplanets could make exceptional targets for
studying this mechanism and its impact on the exoplanet population. Three
HST/STIS observations were performed in the Lyman- line of GJ436 at
different epochs, showing repeatable transits with large depths and extended
durations. Here, we study the role played by stellar radiation pressure on the
structure of the exosphere and its transmission spectrum. We found that the
neutral hydrogen atoms in the exosphere of GJ436 b are not swept away by
radiation pressure as shown to be the case for evaporating hot Jupiters.
Instead, the low radiation pressure from the M-dwarf host star only brakes the
gravitational fall of the escaping hydrogen toward the star and allows its
dispersion within a large volume around the planet, yielding radial velocities
up to about -120 km s that match the observations. We performed
numerical simulations with the EVaporating Exoplanets code (EVE) to study the
influence of the escape rate, the planetary wind velocity, and the stellar
photoionization. While these parameters are instrumental in shaping the
exosphere and yield simulation results in general agreement with the
observations, the spectra observed at the different epochs show specific,
time-variable features that require additional physics.Comment: 10 pages, 5 figure
Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis
Probing the trajectory of a transiting planet across the disk of its star
through the analysis of its Rossiter-McLaughlin effect can be used to measure
the differential rotation of the host star and the true obliquity of the
system. Highly misaligned systems could be particularly conducive to these
mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b
using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This
approach allows us to isolate the local stellar CCF emitted by the
planet-occulted regions. As a result we identified a 35% variation in the
local CCF contrast along the transit chord, which might trace a deepening of
the stellar lines from the equator to the poles. Whatever its origin, such an
effect cannot be detected when analyzing the RV centroids of the
disk-integrated CCFs through a traditional velocimetric analysis of the RM
effect. Consequently it injected a significant bias into the results obtained
by Queloz et al. (2010) for the projected rotational velocity (1.59 km/s) and the sky-projected
obliquity (-123.0). Using our
technique, we measured these values to be =
1.900.05 km/s and = -143.0. We
found no compelling evidence for differential rotation of the star, although
there are hints that WASP-8 is pointing away from us with the stellar poles
rotating about 25% slower than the equator. Measurements at higher accuracy
during ingress/egress will be required to confirm this result. In contrast to
the traditional analysis of the RM effect, the reloaded RM technique directly
extracts the local stellar CCFs, allowing us to analyze their shape and to
measure their RV centroids, unbiased by variations in their contrast or FWHM.Comment: Accepted for publication in A&A. 12 page
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
Observability of hydrogen-rich exospheres in Earth-like exoplanets
(Abridged) The existence of an extended neutral hydrogen exosphere around
small planets can be used as an evidence for the presence of water in their
lower atmosphere but, to date, such feature has not been securely detected in
rocky exoplanets. Planetary exospheres can be observed using transit
spectroscopy of the Lyman- line, which is limited mainly by
interstellar medium absorption in the core of the line, and airglow
contamination from the geocorona when using low-orbit space telescopes. Our
objective is to assess the detectability of the neutral hydrogen exosphere of
an Earth-like planet transiting a nearby M dwarf using Lyman-
spectroscopy and provide the necessary strategies to inform future
observations. The spatial distribution in the upper atmosphere is provided by
an empirical model of the geocorona, and we assume a velocity distribution
based on radiative pressure as the main driver in shaping the exosphere. We
compute the excess absorption in the stellar Lyman- line while in
transit, and use realistic estimates of the uncertainties involved in
observations to determine the observability of the signal. We found that the
signal in Lyman- of the exosphere of an Earth-like exoplanet transiting
M dwarfs with radii between 0.1 and 0.6 R produces an excess absorption
between 50 and 600 ppm. The Lyman- flux of stars decays exponentially
with distance because of interstellar medium absorption, which is the main
observability limitation. Other limits are related to the stellar radial
velocity and instrumental setup. The excess absorption in Lyman- is
observable using LUVOIR/LUMOS in M dwarfs up to a distance of 15 pc. The
analysis of noise-injected data suggests that it would be possible to detect
the exosphere of an Earth-like planet transiting TRAPPIST-1 within 20 transits.Comment: 12 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an
ultracool dwarf star recently discovered to host three transiting and temperate
Earth-sized planets. We find the star is a relatively strong and variable
coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun,
despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with
the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation
of the planets that is many times stronger than experienced by the present-day
Earth. Using a simple energy-limited model we show that the relatively close-in
Earth-sized planets, which span the classical habitable zone of the star, are
subject to sufficient X-ray and EUV irradiation to significantly alter their
primary and any secondary atmospheres. Understanding whether this high-energy
irradiation makes the planets more or less habitable is a complex question, but
our measured fluxes will be an important input to the necessary models of
atmospheric evolution.Comment: 5 pages, published as a letter in MNRAS (accepted 16 September 2016
N. Scott Momaday: a postmodern rebel with a cause?
Tous les textes de N. Scott Momaday insistent sur le caractĂšre dĂ©routant de la tradition orale ancestrale, cette langue âmineureâ archaĂŻque qui traverse en profondeur les rĂ©cits tribaux Ă©crits en langue anglaise. Ce langage souterrain est la reconstruction imaginaire dâune ânon rĂ©alitĂ©â - au sens oĂč le souvenir nâest quâune image de ce qui fut un temps rĂ©el - et en cela, il est clairement postmoderne. Apparemment absurde, intĂ©rieure et fuyante lorsquâelle demeure non dĂ©chiffrĂ©e, la tradition orale offre une autre vision de ce monde lĂ , la vision dâune autre monde. The Way to Rainy Mountain est lâouvrage de Momaday le plus potsmoderne du fait mĂȘme quâil se construit sur la tradition orale, mais câest Ă©galement une magistrale dĂ©monstration de la maniĂšre dont les littĂ©ratures amĂ©rindiennes parviennent en fait Ă dĂ©passer le substrat postmoderne lorsque bĂ©ances et fragments de la mĂ©moire sont comblĂ©s par lâimaginatio
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