346 research outputs found
DEM MODELING OF ROCKFALL REBOUND ON PROTECTIVE EMBANKMENTS
Design of Rockfall Protection Embankments and estimation of their capacity to control the trajectory of rock boulders are complex issues, which give considerable room for research and improvement. A lack of detailed models for the simulation of block rebound in the embankment vicinity is mainly due to the large number of parameters that influences the phenomenon. Therefore, the evaluation of the embankment efficiency in satisfactorily acting on the block trajectory, as a function of the site characteristics and boulder kinematics, is still precluded to design engineers.In the present paper, the open-source code YADE, based on a discrete element method (DEM), is used to model the bouncing of a rock block on the embankment face, while taking into account a certain number of parameters with influence on the impact.By contrast with previously developed models (DEM, FEM or coupled approaches), the aim is here to propose a model with limited computation cost. In this purpose, the embankment is modelled as a membrane interacting with the rock block. The embankment body is not represented because it would require a large number of particles, and, consequently, a high computational time. Various elements implemented in YADE are used to model the embankment surface, with the aim of mimicking the mechanisms involved during the rock boulder rebound. The validity of the approach is addressed comparing simulation results with the few experimental data available from the literature. The influence of characteristics of the impacting block (radius and weight) and kinematic parameters (impact angle and velocity) on the restitution coefficients is explored. In particular, the normal (R-n), tangential (R-t) and energetic (R-TE) coefficients of restitution are monitored. The goal of defining an efficient model in a realistic range of these parameters is pursued
DEM modeling of rockfall rebound on protective embankments
Design of Rockfall Protection Embankments and estimation of their capacity to control the trajectory of rock boulders are complex issues, which give considerable room for research and improvement. A lack of detailed models for the simulation of block rebound in the embankment vicinity is mainly due to the large number of parameters that influences the phenomenon. Therefore, the evaluation of the embankment efficiency in satisfactorily acting on the block trajectory, as a function of the site characteristics and boulder kinematics, is still precluded to design engineers. In the present paper, the open-source code YADE, based on a discrete element method (DEM), is used to model the bouncing of a rock block on the embankment face, while taking into account a certain number of parameters with influence on the impact. By contrast with previously developed models (DEM, FEM or coupled approaches), the aim is here to propose a model with limited computation cost. In this purpose, the embankment is modelled as a membrane interacting with the rock block. The embankment body is not represented because it would require a large number of particles, and, consequently, a high computational time. Various elements implemented in YADE are used to model the embankment surface, with the aim of mimicking the mechanisms involved during the rock boulder rebound. The validity of the approach is addressed comparing simulation results with the few experimental data available from the literature. The influence of characteristics of the impacting block (radius and weight) and kinematic parameters (impact angle and velocity) on the restitution coefficients is explored. In particular, the normal (Rn), tangential (Rt) and energetic (RTE) coefficients of restitution are monitored. The goal of defining an efficient model in a realistic range of these parameters is pursued
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
in most recent versio
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&
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
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
Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) I. Detection of hot neutral sodium at high altitudes on WASP-49b
High-resolution optical spectroscopy during the transit of HD 189733b, a
prototypical hot Jupiter, allowed the resolution of the Na I D sodium lines in
the planet, giving access to the extreme conditions of the planet upper
atmosphere. We have undertaken HEARTS, a spectroscopic survey of exoplanet
upper atmospheres, to perform a comparative study of hot gas giants and
determine how stellar irradiation affect them. Here, we report on the first
HEARTS observations of the hot Saturn-mass planet WASP-49b. We observed the
planet with the HARPS high-resolution spectrograph at ESO 3.6m telescope. We
collected 126 spectra of WASP-49, covering three transits of WASP-49b. We
analyzed and modeled the planet transit spectrum, while paying particular
attention to the treatment of potentially spurious signals of stellar origin.
We spectrally resolve the Na I D lines in the planet atmosphere and show that
these signatures are unlikely to arise from stellar contamination. The large
contrasts of (D) and (D) require the
presence of hot neutral sodium ( K) at high altitudes
(1.5 planet radius or 45,000 km). From estimating the cloudiness
index of WASP-49b, we determine its atmosphere to be cloud free at the
altitudes probed by the sodium lines. WASP-49b is close to the border of the
evaporation desert and exhibits an enhanced thermospheric signature with
respect to a farther-away planet such as HD 189733b.Comment: Accepted for publication in A&A. 14 page
SOPHIE velocimetry of Kepler transit candidates XVI. Tomographic measurement of the low obliquity of KOI-12b, a warm Jupiter transiting a fast rotator
We present the detection and characterization of the transiting warm Jupiter
KOI-12b, first identified with Kepler with an orbital period of 17.86 days. We
combine the analysis of Kepler photometry with Doppler spectroscopy and
line-profile tomography of time-series spectra obtained with the SOPHIE
spectrograph to establish its planetary nature and derive its properties. To
derive reliable estimates for the uncertainties on the tomographic model
parameters, we devised an empirical method to calculate statistically
independent error bars on the time-series spectra. KOI-12b has a radius of
1.430.13 and a 3 upper mass limit of
10. It orbits a fast-rotating star (sin =
60.00.9 km s) with mass and radius of 1.450.09
and 1.630.15 , located at 42640 pc
from the Earth. Doppler tomography allowed a higher precision on the obliquity
to be reached by comparison with the analysis of the Rossiter-McLaughlin radial
velocity anomaly, and we found that KOI-12b lies on a prograde, slightly
misaligned orbit with a low sky-projected obliquity =
12.6. The properties of this planetary system,
with a 11.4 magnitude host-star, make of KOI-12b a precious target for future
atmospheric characterization.Comment: 19 pages, 10 figure
NIGHT: a compact, near-infrared, high-resolution spectrograph to survey helium in exoplanet systems
Among highly irradiated exoplanets, some have been found to undergo
significant hydrodynamic expansion traced by atmospheric escape. To better
understand these processes in the context of planetary evolution, we propose
NIGHT (the Near-Infrared Gatherer of Helium Transits). NIGHT is a
high-resolution spectrograph dedicated to surveying and temporally monitoring
He I triplet absorption at 1083nm in stellar and planetary atmospheres. In this
paper, we outline our scientific objectives, requirements, and cost-efficient
design. Our simulations, based on previous detections and modelling using the
current exoplanet population, determine our requirements and survey targets.
With a spectral resolution of 70,000 on a 2-meter telescope, NIGHT can
accurately resolve the helium triplet and detect 1% peak absorption in 118
known exoplanets in a single transit. Additionally, it can search for
three-sigma temporal variations of 0.4% in 66 exoplanets in-between two
transits. These are conservative estimates considering the ongoing detections
of transiting planets amenable to atmospheric characterisation. We find that
instrumental stability at 40m/s, less stringent than for radial velocity
monitoring, is sufficient for transmission spectroscopy in He I. As such, NIGHT
can utilize mostly off-the-shelf components, ensuring cost-efficiency. A
fibre-fed system allows for flexibility as a visitor instrument on a variety of
telescopes, making it ideal for follow-up observations after JWST or
ground-based detections. Over a few years of surveying, NIGHT could offer
detailed insights into the mechanisms shaping the hot Neptune desert and
close-in planet population by significantly expanding the statistical sample of
planets with known evaporating atmospheres. First light is expected in 2024.Comment: 15 pages, 20 figures, this manuscript has been accepted for
publication in MNRAS. This is a pre-copyedited, author-produced PD
No hydrogen exosphere detected around the super-Earth HD 97658 b
The exoplanet HDâ97658âb provides a rare opportunity to probe the atmospheric composition and evolution of moderately irradiated super-Earths. It transits a bright K star at a moderate orbital distance of 0.08âau. Its low density is compatible with a massive steam envelope that could photodissociate at high altitudes and become observable as escaping neutral hydrogen. Our analysis of three transits with HST/STIS at Lyman-α reveals no such signature, suggesting that the thermosphere of HDâ97658âb is not hydrodynamically expanding and is subjected to a low escape of neutral hydrogen (<108âgâs-1 at 3Ï). Using HST/STIS Lyman-α observations and Chandra/ACIS-S and XMM-Newton/EPIC X-ray observations at different epochs, we find that HDâ97658 is in fact a weak and soft X-ray source with signs of chromospheric variability in the Lyman-α line core. We determine an average reference for the intrinsic Lyman-α line and X-EUV (XUV) spectrum of the star, and show that HDâ97658 b is in mild conditions of irradiation compared to other known evaporating exoplanets with an XUV irradiation about three times lower than the evaporating warm Neptune GJ436 b. This could be the reason why the thermosphere of HDâ97658âb is not expanding: the low XUV irradiation prevents an efficient photodissociation of any putative steam envelope. Alternatively, it could be linked to a low hydrogen content or inefficient conversion of the stellar energy input. The HDâ97658 system provides clues for understanding the stability of low-mass planet atmospheres in terms of composition, planetary density, and irradiation. Our study of HDâ97658 b can be seen as a control experiment of our methodology, confirming that it does not bias detections of atmospheric escape and underlining its strength and reliability. Our results show that stellar activity can be efficiently discriminated from absorption signatures by a transiting exospheric cloud. They also highlight the potential of observing the upper atmosphere of small transiting planets to probe their physical and chemical properties
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