457 research outputs found
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
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
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
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
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
High-energy environment of super-Earth 55 Cnc e I: Far-UV chromospheric variability as a possible tracer of planet-induced coronal rain
The irradiation of close-in planets by their star influences their evolution
and might be responsible for a population of ultra-short period planets eroded
to their bare core. In orbit around a bright, nearby G-type star, the
super-Earth 55 Cnc e offers the possibility to address these issues through UV
transit observations. We used the Hubble Space Telescope to observe the transit
in the FUV over 3 epochs in Apr. 2016, Jan. 2017, and Feb. 2017. These
observations reveal significant short- and long-term variability in 55 Cnc
chromospheric emission lines. In the last 2 epochs, we detected a larger flux
in the C III, Si III, and Si IV lines after the planet passed the approaching
quadrature, followed by a flux decrease in the Si IV doublet. In the second
epoch these variations are contemporaneous with flux decreases in the Si II and
C II doublet. All epochs show flux decreases in the N V doublet as well, albeit
at different orbital phases. These flux decreases are consistent with
absorption from optically thin clouds of gas, are mostly localized at low and
redshifted radial velocities in the star rest frame, and occur preferentially
before and during the transit. These 3 points make it unlikely that the
variations are purely stellar, yet we show that the occulting material is also
unlikely to originate from the planet. We tentatively propose that the motion
of 55 Cnc e at the fringes of the stellar corona leads to the formation of a
cool coronal rain. The inhomogeneity and temporal evolution of the stellar
corona would be responsible for the differences between the visits. Additional
variations are detected in the C II doublet in the first epoch and in the O I
triplet in all epochs with a different behavior that points toward intrinsic
stellar variability. Further observations at FUV wavelengths are required to
disentangle between star-planet interactions and the activity of the starComment: 22 pages, 20 figures, accepted for publication in A&
A cautionary tale: limitations of a brightness-based spectroscopic approach to chromatic exoplanet radii
Determining wavelength-dependent exoplanet radii measurements is an excellent
way to probe the composition of exoplanet atmospheres. In light of this, Borsa
et al. (2016) sought to develop a technique to obtain such measurements by
comparing ground-based transmission spectra to the expected brightness
variations during an exoplanet transit. However, we demonstrate herein that
this is not possible due to the transit light curve normalisation necessary to
remove the effects of the Earth's atmosphere on the ground-based observations.
This is because the recoverable exoplanet radius is set by the planet-to-star
radius ratio within the transit light curve; we demonstrate this both
analytically and with simulated planet transits, as well as through a
reanalysis of the HD 189733b data.Comment: 5 pages, 2 figures, 1 table, accepted to A&
Strong HI Lyman- variations from the 11 Gyr-old host star Kepler-444: a planetary origin ?
Kepler-444 provides a unique opportunity to probe the atmospheric composition
and evolution of a compact system of exoplanets smaller than the Earth. Five
planets transit this bright K star at close orbital distances, but they are too
small for their putative lower atmosphere to be probed at optical/infrared
wavelengths. We used the Space Telescope Imaging Spectrograph instrument
onboard the Hubble Space Telescope to search for the signature of the planet's
upper atmospheres at six independent epochs in the Ly- line. We detect
significant flux variations during the transits of both Kepler-444e and f
(~20%), and also at a time when none of the known planets was transiting
(~40%). Variability in the transition region and corona of the host star might
be the source of these variations. Yet, their amplitude over short time scales
(~2-3 hours) is surprisingly strong for this old (11.2+-1.0Gyr) and apparently
quiet main-sequence star. Alternatively, we show that the in-transits
variations could be explained by absorption from neutral hydrogen exospheres
trailing the two outer planets (Kepler-444e and f). They would have to contain
substantial amounts of water to replenish such hydrogen exospheres, which would
reveal them as the first confirmed ocean-planets. The out-of-transit
variations, however, would require the presence of a yet-undetected Kepler-444g
at larger orbital distance, casting doubt on the planetary origin scenario.
Using HARPS-N observations in the sodium doublet, we derived the properties of
two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This
allowed us to reconstruct the stellar Ly- line profile and to estimate
the XUV irradiation from the star, which would still allow for a moderate mass
loss from the outer planets after 11.2Gyr. Follow-up of the system at XUV
wavelengths will be required to assess this tantalizing possibility.Comment: Accepted for publication in A&A Name of the system added to the title
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