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-$\alpha$ 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$^{-1}$ 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 $\sim$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 $v_{eq} \sin i_{\star}$ (1.59$\stackrel{+0.08}{_{-0.09}}$ km/s) and the sky-projected obliquity $\lambda$ (-123.0$\stackrel{+3.4}{_{-4.4}}^{\circ}$). Using our technique, we measured these values to be $v_{eq} \sin i_{\star}$ = 1.90$\pm$0.05 km/s and $\lambda$ = -143.0$\stackrel{+1.6}{_{-1.5}}^{\circ}$. 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-$\alpha$ 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-$\alpha$ 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-$\alpha$ 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-$\alpha$ of the exosphere of an Earth-like exoplanet transiting M dwarfs with radii between 0.1 and 0.6 R$_\odot$ produces an excess absorption between 50 and 600 ppm. The Lyman-$\alpha$ 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-$\alpha$ is observable using LUVOIR/LUMOS in M dwarfs up to a distance of $\sim$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 caractère déroutant de la tradition orale ancestrale, cette langue “mineure” archaïque qui traverse en profondeur les récits tribaux écrits en langue anglaise. Ce langage souterrain est la reconstruction imaginaire d’une “non réalité” - au sens où le souvenir n’est qu’une image de ce qui fut un temps réel - et en cela, il est clairement postmoderne. Apparemment absurde, intérieure et fuyante lorsqu’elle demeure non déchiffrée, la tradition orale offre une autre vision de ce monde là, la vision d’une autre monde. The Way to Rainy Mountain est l’ouvrage de Momaday le plus potsmoderne du fait même qu’il se construit sur la tradition orale, mais c’est également une magistrale démonstration de la manière dont les littératures amérindiennes parviennent en fait à dépasser le substrat postmoderne lorsque béances et fragments de la mémoire sont comblés par l’imaginatio