2,834 research outputs found
Planet formation models: the interplay with the planetesimal disc
According to the sequential accretion model, giant planet formation is based
first on the formation of a solid core which, when massive enough, can
gravitationally bind gas from the nebula to form the envelope. In order to
trigger the accretion of gas, the core has to grow up to several Earth masses
before the gas component of the protoplanetary disc dissipates. We compute the
formation of planets, considering the oligarchic regime for the growth of the
solid core. Embryos growing in the disc stir their neighbour planetesimals,
exciting their relative velocities, which makes accretion more difficult. We
compute the excitation state of planetesimals, as a result of stirring by
forming planets, and gas-solid interactions. We find that the formation of
giant planets is favoured by the accretion of small planetesimals, as their
random velocities are more easily damped by the gas drag of the nebula.
Moreover, the capture radius of a protoplanet with a (tiny) envelope is also
larger for small planetesimals. However, planets migrate as a result of
disc-planet angular momentum exchange, with important consequences for their
survival: due to the slow growth of a protoplanet in the oligarchic regime,
rapid inward type I migration has important implications on intermediate mass
planets that have not started yet their runaway accretion phase of gas. Most of
these planets are lost in the central star. Surviving planets have either
masses below 10 ME or above several Jupiter masses. To form giant planets
before the dissipation of the disc, small planetesimals (~ 0.1 km) have to be
the major contributors of the solid accretion process. However, the combination
of oligarchic growth and fast inward migration leads to the absence of
intermediate mass planets. Other processes must therefore be at work in order
to explain the population of extrasolar planets presently known.Comment: Accepted for publication in Astronomy and Astrophysic
CHEOPS performance for exomoons: The detectability of exomoons by using optimal decision algorithm
Many attempts have already been made for detecting exomoons around transiting
exoplanets but the first confirmed discovery is still pending. The experience
that have been gathered so far allow us to better optimize future space
telescopes for this challenge, already during the development phase. In this
paper we focus on the forthcoming CHaraterising ExOPlanet Satellite
(CHEOPS),describing an optimized decision algorithm with step-by-step
evaluation, and calculating the number of required transits for an exomoon
detection for various planet-moon configurations that can be observable by
CHEOPS. We explore the most efficient way for such an observation which
minimizes the cost in observing time. Our study is based on PTV observations
(photocentric transit timing variation, Szab\'o et al. 2006) in simulated
CHEOPS data, but the recipe does not depend on the actual detection method, and
it can be substituted with e.g. the photodynamical method for later
applications. Using the current state-of-the-art level simulation of CHEOPS
data we analyzed transit observation sets for different star-planet-moon
configurations and performed a bootstrap analysis to determine their detection
statistics. We have found that the detection limit is around an Earth-sized
moon. In the case of favorable spatial configurations, systems with at least
such a large moon and with at least Neptune-sized planet, 80\% detection chance
requires at least 5-6 transit observations on average. There is also non-zero
chance in the case of smaller moons, but the detection statistics deteriorates
rapidly, while the necessary transit measurements increase fast. (abridged)Comment: 32 pages, 14 figures, accepted for publication in PAS
Astronomical spectrograph calibration with broad-spectrum frequency combs
Broadband femtosecond-laser frequency combs are filtered to
spectrographically resolvable frequency-mode spacing, and the limitations of
using cavities for spectral filtering are considered. Data and theory are used
to show implications to spectrographic calibration of high-resolution,
astronomical spectrometers
The match/mismatch hypothesis and the feeding success of fish larvae in ice-covered southeastern Hudson Bay
We studied the synchronism between the seasonal occurrence of fish larvae and their prey in ice-covered southeastern Hudson Bay, Canada, in spring 1988, 1989 and 1990. Arctic cod #Boreogadus saida and sand lance #Ammodytes sp. larvae hatched several weeks before ice break-up and fed primarily on copepod nauplii. The timing of 50% yolk resorption was the same every year (11 to 18 May for Arctic cod and 5 to 11 June for sand lance) but the availability of copepod nauplii varied substantially between years, both in magnitude (7-fold) and timing (4 to 6 wk). Interannual differences in the under-ice abundance of nauplii were linked to variations in the abundance of female cyclopoid copepods, and appeared unrelated to the timing of the ice-algal or phytoplankton blooms. Interannual differences (2- to 4-fold) in the feeding success of fish larvae (percent feeding incidence at length and mean feeding ratio at length) were related to the availability of copepod nauplii. Consistent with the match/mismatch hypothesis, the fixity of the spawning season in relation to a variable cycle of prey abundance accounted for the observed variations in feeding success and apparent growth (length at date) of fish larvae. Yet, in this particular ecosystem, a match or mismatch between Arctic cod or sand lance larvae and their prey may depend more on the dynamics of cyclopoid copepods during the previous winter than on the timing of the spring algal blooms. (Résumé d'auteur
Cognitive impairment in individuals with insomnia : clinical significance and correlates
Study Objectives:
The aims of this study were to (1) investigate the nature of cognitive impairment in individuals with insomnia, (2) document their clinical significance, (3) examine their correlates, and (4) explore differences among individuals with insomnia with and without cognitive complaints.
Design:
Participants underwent 3 consecutive nights of polysomnography. On the morning following the third night, they completed a battery of questionnaires and neuropsychological tests.
Participants:
The sample included 25 adults with primary insomnia (mean age: 44.4 ± 11.5 y, 56% women) and 16 controls (mean age: 42.8 ± 12.9 y, 50% women) matched for sex, age, and education.
Intervention:
N/A.
Measurement and Results:
Participants completed neuropsychological tests covering attention, memory, working memory, and executive functions, as well as questionnaires assessing the subjective perception of performance, depression, anxiety, fatigue, sleepiness, and hyperarousal. There were significant group differences for the attention and episodic memory domains. Clinically significant deficits were more frequent in the insomnia group. Within the insomnia group, individuals with cognitive complaints exhibited significantly poorer performance on a larger number of neuropsychological variables. All impaired aspects of performance were significantly associated with either subjective or objective sleep continuity, and some were also independently related to sleep microstructure (i.e., relative power for alpha frequencies) or selected psychological variables (i.e., beliefs or arousal).
Conclusions:
These findings suggest clinically significant alterations in attention and episodic memory in individuals with insomnia. Objective deficits were more pronounced and involved more aspects of performance in a subgroup of individuals with cognitive complaints. These deficits appear associated with sleep continuity, and may also be related to sleep microstructure and dysfunctional beliefs
Femtosecond frequency comb measurement of absolute frequencies and hyperfine coupling constants in cesium vapor
We report measurements of absolute transition frequencies and hyperfine
coupling constants for the 8S_{1/2}, 9S_{1/2}, 7D_{3/2}, and 7D_{5/2} states in
^{133}Cs vapor. The stepwise excitation through either the 6P_{1/2} or 6P_{3/2}
intermediate state is performed directly with broadband laser light from a
stabilized femtosecond laser optical-frequency comb. The laser beam is split,
counter-propagated and focused into a room-temperature Cs vapor cell. The
repetition rate of the frequency comb is scanned and we detect the fluorescence
on the 7P_{1/2,3/2} -> 6S_{1/2} branches of the decay of the excited states.
The excitations to the different states are isolated by the introduction of
narrow-bandwidth interference filters in the laser beam paths. Using a
nonlinear least-squares method we find measurements of transition frequencies
and hyperfine coupling constants that are in agreement with other recent
measurements for the 8S state and provide improvement by two orders of
magnitude over previously published results for the 9S and 7D states.Comment: 14 pages, 14 figure
Impact of Groundwater Flow on Permafrost Degradation and Transportation Infrastructure Stability
INE/AUTC 13.0
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