95 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&
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
Disentangling Stellar and Airglow Emission Lines from HST-COS Spectra
H I Ly (1215.67 \r{A}) and the O I triplet (1302.17, 1304.86, and
1306.03 \r{A}) are bright far-ultraviolet (FUV) emission lines that trace the
stellar chromosphere. Observations of stellar Ly and O I using the
Hubble Space Telescope's (HST) most sensitive FUV spectrograph, the Cosmic
Origins Spectrograph (COS), are contaminated with geocoronal emission, or
airglow. This study demonstrates that airglow emission profiles as observed by
COS are sufficiently stable to create airglow templates which can be reliably
subtracted from the data, recovering the underlying stellar flux. We developed
a graphical user interface to implement the airglow subtraction on a sample of
171 main sequence F, G, K, and M-type dwarfs from the COS data archive.
Correlations between recovered stellar emission and measures of stellar
activity were investigated. Several power law relationships are presented for
predicting the stellar Ly and O I emission. The apparent brightness of
the stellar emission relative to the airglow is a critical factor in the
success or failure of an airglow subtraction. We developed a predictor for the
success of an airglow subtraction using the signal-to-noise ratio (SNR) of the
nearby chromospheric emission line Si III (1206.51 \r{A}). The minimum
attenuated Ly flux which was successfully recovered is
1.3910 erg cm s, and we recommend this as a
minimum flux for COS Ly recoveries.Comment: 38 pages, 27 figures, to be published in Ap
Variational approach for nonsmooth elasto-plastic dynamics with contact and impacts
The objective of this article is the modelling and the numerical simulation of the response of elastoplastic structures to impacts. To this end, a numerical method is proposed that takes into account one-sided contact (Signorini condition) and impact phenomena together with plasticity in a monolithic solver, while accounting for the non-smooth character of the dynamics. The formulation of the plasticity and the contact laws are based on inclusions into normal cone of convex sets, or equivalently, variational inequalities following the pioneering work of Jean Jacques Moreau (1974) and Halphen and Son Nguyen (1975), who introduced the assumptions of normal dissipation and of generalised standard materials (GSM) in the framework of associated plasticity with strain hardening. The proposed time-stepping method is an extension of the Jean and J. J. Moreau (1987) scheme for nonsmooth dynamics. The discrete energy balance shows that spurious numerical damping can be removed and the scheme is practically unconditionally stable. Furthermore, the finite-dimensional variational inequality at each time-step is well-posed, can be solved by optimisation methods for convex quadratic programs, providing an interesting alternative to the return mapping algorithm. The paper is completed by numerical illustrative examples of impacts on metallic structures made of beams
CHEOPS's hunt for exocomets: photometric observations of 5 Vul
The presence of minor bodies in exoplanetary systems is in most cases
inferred through infra-red excesses, with the exception of exocomets. Even if
over 35 years have passed since the first detection of exocomets around beta
Pic, only ~ 25 systems are known to show evidence of evaporating bodies, and
most of them have only been observed in spectroscopy. With the appearance of
new high-precision photometric missions designed to search for exoplanets, such
as CHEOPS, a new opportunity to detect exocomets is available. Combining data
from CHEOPS and TESS we investigate the lightcurve of 5 Vul, an A-type star
with detected variability in spectroscopy, to search for non periodic transits
that could indicate the presence of dusty cometary tails in the system. While
we did not find any evidence of minor bodies, the high precision of the data,
along with the combination with previous spectroscopic results and models,
allows for an estimation of the sizes and spatial distribution of the
exocomets.Comment: Accepted for publication in MNRA
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