320 research outputs found
DREAM II. The spin-orbit angle distribution of close-in exoplanets under the lens of tides
The spin-orbit angle, or obliquity, is a powerful observational marker that
allows us to access the dynamical history of exoplanetary systems. Here, we
have examined the distribution of spin-orbit angles for close-in exoplanets and
put it in a statistical context of tidal interactions between planets and their
stars. We confirm the observed trends between the obliquity and physical
quantities directly connected to tides, namely the stellar effective
temperature, the planet-to-star mass ratio, and the scaled orbital distance. We
further devised a tidal efficiency factor combining critical parameters that
control the strength of tidal effects and used it to corroborate the strong
link between the spin-orbit angle distribution and tidal interactions. In
particular, we developed a readily usable formula to estimate the probability
that a system is misaligned, which will prove useful in global population
studies. By building a robust statistical framework, we reconstructed the
distribution of the three-dimensional spin-orbit angles, allowing for a sample
of nearly 200 true obliquities to be analyzed for the first time. This
realistic distribution maintains the sky-projected trends, and additionally
hints toward a striking pileup of truly aligned systems. The comparison between
the full population and a pristine subsample unaffected by tidal interactions
suggests that perpendicular architectures are resilient toward tidal
realignment, providing evidence that orbital misalignments are sculpted by
disruptive dynamical processes that preferentially lead to polar orbits. On the
other hand, star-planet interactions seem to efficiently realign or quench the
formation of any tilted configuration other than for polar orbits, and in
particular for antialigned orbits.Comment: Accepted in A&
The JADE code: Coupling secular exoplanetary dynamics and photo-evaporation
Close-in planets evolve under extreme conditions, raising questions about
their origins and current nature. Two predominant mechanisms are orbital
migration, which brings them close to their star, and atmospheric escape under
the resulting increased irradiation. Yet, their relative roles remain unclear
because we lack models that couple the two mechanisms with high precision on
secular timescales. To address this need, we developed the JADE code, which
simulates the secular atmospheric and dynamical evolution of a planet around
its star, and can include the perturbation induced by a distant third body. On
the dynamical side, the 3D evolution of the orbit is modeled under stellar and
planetary tidal forces, a relativistic correction, and the action of the
distant perturber. On the atmospheric side, the vertical structure of the
atmosphere is integrated over time based on its thermodynamical properties,
inner heating, and the evolving stellar irradiation, which results, in
particular, in photo-evaporation. The JADE code is benchmarked on GJ436 b,
prototype of evaporating giants on eccentric, misaligned orbits at the edge of
the hot Neptunes desert. We confirm that its orbital architecture is well
explained by Kozai migration and unveil a strong interplay between its
atmospheric and orbital evolution. During the resonance phase, the atmosphere
pulsates in tune with the Kozai cycles, which leads to stronger tides and an
earlier migration. This triggers a strong evaporation several Gyr after the
planet formed, refining the paradigm that mass loss is dominant in the early
age of close-in planets. This suggests that the edge of the desert could be
formed of warm Neptunes whose evaporation was delayed by migration. It
strengthens the importance of coupling atmospheric and dynamical evolution over
secular timescales, which the JADE code will allow simulating for a wide range
of systems.Comment: 20 pages, 2 figures, accepted in A&
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&
LISA ON TABLE: AN OPTICAL SIMULATOR FOR LISA
LISA, the first space project for detecting gravitational waves, relies on two main technical challenges: the free falling masses and an outstanding precision on phase shift measurements (a few pm on 5 Mkm in the LISA band). The technology of the free falling masses, i.e. their isolation to forces other than gravity and the capability for the spacecraft to precisely follow the test masses, will soon be tested with the technological LISA Pathfinder mission. The performance of the phase measurement will be achieved by at least two stabilization stages: a pre-stabilisation of the laser frequency at a level of 10-13 (relative frequency stability) will be further improved by using numerical algorithms, such as Time Delay Interferometry, which have been theoretically and numerically demonstrated to reach the required performance level (10-21). Nevertheless, these algorithms, though already tested with numerical model of LISA, require experimental validation, including 'realistic' hardware elements. Such an experiment would allow to evaluate the expected noise level and the possible interactions between subsystems. To this end, the APC is currently developing an optical benchtop experiment, called LISA On Table (LOT), which is representative of the three LISA spacecraft. A first module of the LOT experiment has been mounted and is being characterized. After completion this facility may be used by the LISA community to test hardware (photodiodes, phasemeters) or software (reconstruction algorithms) components
Revisiting Kepler-444. II. Rotational, orbital and high-energy fluxes evolution of the system
Context. Kepler-444 is one of the oldest planetary systems known thus far.
Its peculiar configuration consisting of five sub-Earth-sized planets orbiting
the companion to a binary stellar system makes its early history puzzling.
Moreover, observations of HI-Ly- variations raise many questions
about the potential presence of escaping atmospheres today. Aims. We aim to
study the orbital evolution of Kepler-444-d and Kepler-444-e and the impact of
atmospheric evaporation on Kepler-444-e. Methods. Rotating stellar models of
Kepler-444-A were computed with the Geneva stellar evolution code and coupled
to an orbital evolution code, accounting for the effects of dynamical,
equilibrium tides and atmospheric evaporation. The impacts of multiple stellar
rotational histories and extreme ultraviolet (XUV) luminosity evolutionary
tracks are explored. Results. Using detailed rotating stellar models able to
reproduce the rotation rate of Kepler-444-A, we find that its observed rotation
rate is perfectly in line with what is expected for this old K0-type star,
indicating that there is no reason for it to be exceptionally active as would
be required to explain the observed HI-Ly- variations from a
stellar origin. We show that given the low planetary mass ( 0.03 M) and relatively large orbital distance ( 0.06 AU) of
Kepler-444-d and e, dynamical tides negligibly affect their orbits, regardless
of the stellar rotational history considered. We point out instead how
remarkable the impact is of the stellar rotational history on the estimation of
the lifetime mass loss for Kepler-444-e. We show that, even in the case of an
extremely slow rotating star, it seems unlikely that such a planet could retain
a fraction of the initial water-ice content if we assume that it formed with a
Ganymede-like composition
Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman- line
The TRAPPIST-1 system offers the opportunity to characterize terrestrial,
potentially habitable planets orbiting a nearby ultracool dwarf star. We
performed a four-orbit reconnaissance with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope to study the stellar emission
at Lyman-, to assess the presence of hydrogen exospheres around the two
inner planets, and to determine their UV irradiation. We detect the
Lyman- line of TRAPPIST-1, making it the coldest exoplanet host star
for which this line has been measured. We reconstruct the intrinsic line
profile, showing that it lacks broad wings and is much fainter than expected
from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as
Proxima Cen but a much lower Ly- emission. This suggests that
TRAPPIST-1 chromosphere is only moderately active compared to its transition
region and corona. We estimated the atmospheric mass loss rates for all
planets, and found that despite a moderate extreme UV emission the total XUV
irradiation could be strong enough to strip the atmospheres of the inner
planets in a few billions years. We detect marginal flux decreases at the times
of TRAPPIST-1b and c transits, which might originate from stellar activity, but
could also hint at the presence of extended hydrogen exospheres. Understanding
the origin of these Lyman- variations will be crucial in assessing the
atmospheric stability and potential habitability of the TRAPPIST-1 planets.Comment: Published in A&A as a Letter to the Edito
The long egress of GJ~436b's giant exosphere
The M dwarf GJ 436 hosts a transiting warm Neptune known to experience
atmospheric escape. Previous observations revealed the presence of a giant
hydrogen exosphere transiting the star for more than 5 h, and absorbing up to
56% of the flux in the blue wing of the stellar Lyman-{\alpha} line of neutral
hydrogen (H i Ly{\alpha}). The unexpected size of this comet-like exosphere
prevented observing the full transit of its tail. In this Letter, we present
new Ly{\alpha} observations of GJ 436 obtained with the Space Telescope Imaging
Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The
stability of the Ly{\alpha} line over six years allowed us to combine these new
observations with archival data sets, substantially expanding the coverage of
the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep
occulting the star for 10-25 h after the transit of the planet, remarkably
confirming a previous prediction based on 3D numerical simulations with the
EVaporating Exoplanet code (EVE). This result strengthens the interpretation
that the exosphere of GJ 436b is shaped by both radiative braking and charge
exchanges with the stellar wind. We further report flux decreases of 15 +/- 2%
and 47 +/- 10% in the red wing of the Ly{\alpha} line and in the line of
ionised silicon (Si iii). Despite some temporal variability possibly linked
with stellar activity, these two signals occur during the exospheric transit
and could be of planetary origin. Follow-up observations will be required to
assess the possibility that the redshifted Ly{\alpha} and Si iii absorption
signatures arise from interactions between the exospheric flow and the magnetic
field of the star.Comment: 10 pages, 7 figures, published in A&
Single-block rockfall dynamics inferred from seismic signal analysis
International audienceSeismic monitoring of mass movements can significantly help to mitigate the associated hazards; however, the link between event dynamics and the seismic signals generated is not completely understood. To better understand these relationships, we conducted controlled releases of single blocks within a soft-rock (black marls) gully of the Rioux-Bourdoux torrent (French Alps). A total of 28 blocks, with masses ranging from 76 to 472 kg, were used for the experiment. An instrumentation combining video cameras and seismometers was deployed along the travelled path. The video cameras allow reconstructing the trajectories of the blocks and estimating their velocities at the time of the different impacts with the slope. These data are compared to the recorded seismic signals. As the distance between the falling block and the seismic sensors at the time of each impact is known, we were able to determine the associated seismic signal amplitude corrected for propagation and attenuation effects. We compared the velocity, the potential energy lost, the kinetic energy and the momentum of the block at each impact to the true amplitude and the radiated seismic energy. Our results suggest that the amplitude of the seismic signal is correlated to the momentum of the block at the impact. We also found relationships between the potential energy lost, the kinetic energy and the seismic energy radiated by the impacts. Thanks to these relationships, we were able to retrieve the mass and the velocity before impact of each block directly from the seismic signal. Despite high uncertainties, the values found are close to the true values of the masses and the velocities of the blocks. These relationships allow for gaining a better understanding of the physical processes that control the source of high-frequency seismic signals generated by rockfalls
The space weather around the exoplanet GJ 436 b. II. Stellar wind-exoplanet interactions
The M dwarf star GJ 436 hosts a warm-Neptune that is losing substantial
amount of atmosphere, which is then shaped by the interactions with the wind of
the host star. The stellar wind is formed by particles and magnetic fields that
shape the exo-space weather around the exoplanet GJ 436 b. Here, we use the
recently published magnetic map of GJ 436 to model its 3D Alfv\'en-wave driven
wind. By comparing our results with previous transmission spectroscopic models
and measurements of non-thermal velocities at the transition region of GJ 436,
our models indicate that the wind of GJ 436 is powered by a smaller flux of
Alfv\'en waves than that powering the wind of the Sun. This suggests that the
canonical flux of Alfv\'en waves assumed in solar wind models might not be
applicable to the winds of old M dwarf stars. Compared to the solar wind, GJ
436's wind has a weaker acceleration and an extended sub-Alfv\'enic region.
This is important because it places the orbit of GJ 436 b inside the region
dominated by the stellar magnetic field (i.e., inside the Alfv\'en surface).
Due to the sub-Alfv\'enic motion of the planet through the stellar wind,
magnetohydrodynamic waves and particles released in reconnection events can
travel along the magnetic field lines towards the star, which could power the
anomalous ultraviolet flare distribution recently observed in the system. For
an assumed planetary magnetic field of G, we derive the power
released by stellar wind-planet interactions as --
erg s, which is consistent with the upper limit of
erg s derived from ultraviolet lines. We further highlight that, because
star-planet interactions depend on stellar wind properties, observations that
probe these interactions and the magnetic map used in 3D stellar wind
simulations should be contemporaneous for deriving realistic results.Comment: 12 pages, 7 figures, 1 table; accepted for publication in A&
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