547 research outputs found
Comparison of flow resistance relations for debris flows using a one-dimensional finite element simulation model
International audienceThis paper describes a one-dimensional finite element code for debris flows developed to model the flow within a steep channel and the stopping conditions on the fan. The code allows the systematic comparison of a wide variety of previously proposed one-phase flow resistance laws using the same finite element solution method. The one-dimensional depth-averaged equations of motion and the numerical model are explained. The model and implementation of the flow resistance relations was validated using published analytical results for the dam break case. Reasonable agreement for the front velocities and stopping location for a debris-flow event in the Kamikamihori torrent in Japan can be achieved with turbulent flow resistance relations including "stop" terms which allow the flow to come to rest on a gently sloping surface. While it is possible to match the overall bulk flow behavior using relatively simple flow resistance relations, they must be calibrated. A sensitivity analysis showed that the shape of the upstream input hydrograph does not much affect the flow conditions in the lower part of the flow path, whereas the event volume is much more important
Formation of the resonant system HD 60532
Among multi-planet planetary systems there are a large fraction of resonant
systems. Studying the dynamics and formation of these systems can provide
valuable informations on processes taking place in protoplanetary disks where
the planets are thought have been formed. The recently discovered resonant
system HD 60532 is the only confirmed case, in which the central star hosts a
pair of giant planets in 3:1 mean motion resonance. We intend to provide a
physical scenario for the formation of HD 60532, which is consistent with the
orbital solutions derived from the radial velocity measurements. Observations
indicate that the system is in an antisymmetric configuration, while previous
theoretical investigations indicate an asymmetric equilibrium state. The paper
aims at answering this discrepancy as well. We performed two-dimensional
hydrodynamical simulations of thin disks with an embedded pair of massive
planets. Additionally, migration and resonant capture are studied by
gravitational N-body simulations that apply properly parametrized
non-conservative forces. Our simulations suggest that the capture into the 3:1
mean motion resonance takes place only for higher planetary masses, thus
favouring orbital solutions having relatively smaller inclination i=20 degrees.
The system formed by numerical simulations qualitatively show the same
behaviour as HD 60532. We also find that the presence of an inner disk (between
the inner planet and the star) plays a very important role in determining the
final configurations of resonant planetary systems. Its damping effect on the
inner planet's eccentricity is responsible for the observed antisymmetric state
of HD 60532.Comment: 7 pages, 7 figures, Accepted for publication in Astronomy &
Astrophysic
Nuclear spin relaxation rates in two-leg spin ladders
Using the transfer-matrix DMRG method, we study the nuclear spin relaxation
rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain
(J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the
q_y=0 and \pi contributions and show that the later contribute significantly to
the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling
and temperature range. We compare our results to both theoretical predictions
and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures,
accepted for publication in PR
No planet for HD 166435
The G0V star HD166435 has been observed by the fiber-fed spectrograph ELODIE
as one of the targets in the large extra-solar planet survey that we are
conducting at the Observatory of Haute-Provence. We detected coherent,
low-amplitude, radial-velocity variations with a period of 3.7987days,
suggesting a possible close-in planetary companion. Subsequently, we initiated
a series of high-precision photometric observations to search for possible
planetary transits and an additional series of CaII H and K observations to
measure the level of surface magnetic activity and to look for possible
rotational modulation. Surprisingly, we found the star to be photometrically
variable and magnetically active. A detailed study of the phase stability of
the radial-velocity signal revealed that the radial-velocity variability
remains coherent only for durations of about 30days. Analysis of the time
variation of the spectroscopic line profiles using line bisectors revealed a
correlation between radial velocity and line-bisector orientation. All of these
observations, along with a one-quarter cycle phase shift between the
photometric and the radial-velocity variationss, are well explained by the
presence of dark photospheric spots on HD166435. We conclude that the
radial-velocity variations are not due to gravitational interaction with an
orbiting planet but, instead, originate from line-profile changes stemming from
star spots on the surface of the star. The quasi-coherence of the
radial-velocity signal over more than two years, which allowed a fair fit with
a binary model, makes the stability of this star unusual among other active
stars. It suggests a stable magnetic field orientation where spots are always
generated at about the same location on the surface of the star.Comment: 9 pages, 8 figures, Accepted for publication in A&
The HARPS search for southern extrasolar planets: XXXIII. New multi-planet systems in the HARPS volume limited sample: a super-Earth and a Neptune in the habitable zone
The vast diversity of planetary systems detected to date is defying our
capability of understanding their formation and evolution. Well-defined
volume-limited surveys are the best tool at our disposal to tackle the problem,
via the acquisition of robust statistics of the orbital elements. We are using
the HARPS spectrograph to conduct our survey of ~850 nearby solar-type stars,
and in the course of the past nine years we have monitored the radial velocity
of HD103774, HD109271, and BD-061339. In this work we present the detection of
five planets orbiting these stars, with m*sin(i) between 0.6 and 7 Neptune
masses, four of which are in two multiple systems, comprising one super-Earth
and one planet within the habitable zone of a late-type dwarf. Although for
strategic reasons we chose efficiency over precision in this survey, we have
the capability to detect planets down to the Neptune and super-Earth mass
range, as well as multiple systems, provided that enough data points are made
available.Comment: 7 pages, 14 figures, accepted for publication by A&A, 04-01-201
The HARPS search for southern extrasolar planets. XXIII. 8 planetary companions to low-activity solar-type stars
In this paper, we present our HARPS radial-velocity data for eight
low-activity solar-type stars belonging to the HARPS volume-limited sample:
HD6718, HD8535, HD28254, HD290327, HD43197, HD44219, HD148156, and HD156411.
Keplerian fits to these data reveal the presence of low-mass companions around
these targets. With minimum masses ranging from 0.58 to 2.54 MJup, these
companions are in the planetary mass domain. The orbital periods of these
planets range from slightly less than one to almost seven years. The eight
orbits presented in this paper exhibit a wide variety of eccentricities: from
0.08 to above 0.8.Comment: 8 pages, 2 figures, accepted for publication in A&
Integration of Light and Temperature in the Regulation of Circadian Gene Expression in Drosophila
Circadian clocks are aligned to the environment via synchronizing signals, or Zeitgebers, such as daily light and temperature cycles, food availability, and social behavior. In this study, we found that genome-wide expression profiles from temperature-entrained flies show a dramatic difference in the presence or absence of a thermocycle. Whereas transcript levels appear to be modified broadly by changes in temperature, there is a specific set of temperature-entrained circadian mRNA profiles that continue to oscillate in constant conditions. There are marked differences in the biological functions represented by temperature-driven or circadian regulation. The set of temperature-entrained circadian transcripts overlaps significantly with a previously defined set of transcripts oscillating in response to a photocycle. In follow-up studies, all thermocycle-entrained circadian transcript rhythms also responded to light/dark entrainment, whereas some photocycle-entrained rhythms did not respond to temperature entrainment. Transcripts encoding the clock components Period, Timeless, Clock, Vrille, PAR-domain protein 1, and Cryptochrome were all confirmed to be rhythmic after entrainment to a daily thermocycle, although the presence of a thermocycle resulted in an unexpected phase difference between period and timeless expression rhythms at the transcript but not the protein level. Generally, transcripts that exhibit circadian rhythms both in response to thermocycles and photocycles maintained the same mutual phase relationships after entrainment by temperature or light. Comparison of the collective temperature- and light-entrained circadian phases of these transcripts indicates that natural environmental light and temperature cycles cooperatively entrain the circadian clock. This interpretation is further supported by comparative analysis of the circadian phases observed for temperature-entrained and light-entrained circadian locomotor behavior. Taken together, these findings suggest that information from both light and temperature is integrated by the transcriptional clock mechanism in the adult fly head
The HARPS search for southern extra-solar planets. XXIV. Companions to HD 85390, HD 90156 and HD 103197: A Neptune analogue and two intermediate mass planets
We report the detection of three new extrasolar planets orbiting the solar
type stars HD 85390, HD 90156 and HD 103197 with the HARPS spectrograph mounted
on the ESO 3.6-m telescope at La Silla observatory. HD 85390 has a planetary
companion with a projected intermediate mass (42.0 Earth masses) on a 788-day
orbit (a=1.52 AU) with an eccentricity of 0.41, for which there is no analogue
in the solar system. A drift in the data indicates the presence of another
companion on a long period orbit, which is however not covered by our
measurements. HD 90156 is orbited by a warm Neptune analogue with a minimum
mass of 17.98 Earth masses (1.05 Neptune masses), a period of 49.8 days (a=0.25
AU) and an eccentricity of 0.31. HD 103197 has an intermediate mass planet on a
circular orbit (P=47.8 d, Msini=31.2 Earth masses). We discuss the formation of
planets of intermediate mass (about 30-100 Earth masses) which should be rare
inside a few AU according to core accretion formation models.Comment: 9 pages, 5 figures. Accepted to A&
Control of Daily Transcript Oscillations in Drosophila by Light and the Circadian Clock
The transcriptional circuits of circadian clocks control physiological and behavioral rhythms. Light may affect such overt rhythms in two ways: (1) by entraining the clock circuits and (2) via clock-independent molecular pathways. In this study we examine the relationship between autonomous transcript oscillations and light-driven transcript responses. Transcript profiles of wild-type and arrhythmic mutant Drosophila were recorded both in the presence of an environmental photocycle and in constant darkness. Systematic autonomous oscillations in the 12- to 48-h period range were detectable only in wild-type flies and occurred preferentially at the circadian period length. However, an extensive program of light-driven expression was confirmed in arrhythmic mutant flies. Many light-responsive transcripts are preferentially expressed in the compound eyes and the phospholipase C component of phototransduction, NORPA (no receptor potential), is required for their light-dependent regulation. Although there is evidence for the existence of multiple molecular clock circuits in cyanobacteria, protists, plants, and fungi, Drosophila appears to possess only one such system. The sustained photic expression responses identified here are partially coupled to the circadian clock and may reflect a mechanism for flies to modulate functions such as visual sensitivity and synaptic transmission in response to seasonal changes in photoperiod
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