1,168 research outputs found
Constraining photon dispersion relations from observations of the Vela pulsar with H.E.S.S
Some approaches to Quantum Gravity (QG) predict a modification of photon
dispersion relations due to a breaking of Lorentz invariance. The effect is
expected to affect photons near an effective QG energy scale. This scale has
been constrained by observing gamma rays emitted from variable astrophysical
sources such as gamma-ray bursts and flaring active galactic nuclei. Pulsars
exhibit a periodic emission of possibly ms time scale. In 2014, the H.E.S.S.
experiment reported the detection down to 20 GeV of gamma rays from the Vela
pulsar having a periodicity of 89 ms. Using a likelihood analysis, calibrated
with a dedicated Monte-Carlo procedure, we obtain the first limit on QG energy
scale with the Vela pulsar. In this paper, the method and calibration procedure
in use will be described and the results will be discussed.Comment: 7 pages, 4 figures, In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague (The Netherlands
The full spectral radiative properties of Proxima Centauri
The discovery of Proxima b, a terrestrial temperate planet, presents the
opportunity of studying a potentially habitable world in optimal conditions. A
key aspect to model its habitability is to understand the radiation environment
of the planet in the full spectral domain. We characterize the X-rays to mid-IR
radiative properties of Proxima with the goal of providing the
top-of-atmosphere fluxes on the planet. We also aim at constraining the
fundamental properties of the star. We employ observations from a large number
of facilities and make use of different methodologies to piece together the
full spectral energy distribution of Proxima. In the high-energy domain, we pay
particular attention to the contribution by rotational modulation, activity
cycle, and flares so that the data provided are representative of the overall
radiation dose received by the atmosphere of the planet. We present the full
spectrum of Proxima covering 0.7 to 30000 nm. The integration of the data shows
that the top-of-atmosphere average XUV irradiance on Proxima b is 0.293 W m^-2,
i.e., nearly 60 times higher than Earth, and that the total irradiance is
877+/-44 W m^-2, or 64+/-3% of the solar constant but with a significantly
redder spectrum. We also provide laws for the XUV evolution of Proxima
corresponding to two scenarios. Regarding the fundamental properties of
Proxima, we find M=0.120+/-0.003 Msun, R=0.146+/-0.007 Rsun, Teff=2980+/-80 K,
and L=0.00151+/-0.00008 Lsun. In addition, our analysis reveals a ~20% excess
in the 3-30 micron flux of the star that is best interpreted as arising from
warm dust in the system. The data provided here should be useful to further
investigate the current atmospheric properties of Proxima b as well as its past
history, with the overall aim of firmly establishing the habitability of the
planet.Comment: 12 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Tidal dissipation in rotating low-mass stars and implications for the orbital evolution of close-in planets I. From the PMS to the RGB at solar metallicity
Star-planet interactions must be taken into account in stellar models to
understand the dynamical evolution of close-in planets. The dependence of the
tidal interactions on the structural and rotational evolution of the star is of
peculiar importance and should be correctly treated. We quantify how tidal
dissipation in the convective envelope of rotating low-mass stars evolves from
the pre-main sequence up to the red-giant branch depending on the initial
stellar mass. We investigate the consequences of this evolution on planetary
orbital evolution. We couple the tidal dissipation formalism described in
Mathis (2015) to the stellar evolution code STAREVOL and apply it to rotating
stars with masses between 0.3 and 1.4 M. In addition, we generalize the
work of Bolmont & Mathis (2016) by following the orbital evolution of close-in
planets using the new tidal dissipation predictions for advanced phases of
stellar evolution. On the PMS the evolution of tidal dissipation is controlled
by the evolution of the internal structure of the contracting star. On the MS
it is strongly driven by the variation of surface rotation that is impacted by
magnetized stellar winds braking. The main effect of taking into account the
rotational evolution of the stars is to lower the tidal dissipation strength by
about four orders of magnitude on the main-sequence, compared to a normalized
dissipation rate that only takes into account structural changes. The evolution
of the dissipation strongly depends on the evolution of the internal structure
and rotation of the star. From the pre-main sequence up to the tip of the
red-giant branch, it varies by several orders of magnitude, with strong
consequences for the orbital evolution of close-in massive planets. These
effects are the strongest during the pre-main sequence, implying that the
planets are mainly sensitive to the star's early history.Comment: 13 pages, 7 figures, accepted for publication in A&
Study of time lags in HETE-2 Gamma-Ray Bursts with redshift: search for astrophysical effects and Quantum Gravity signature
The study of time lags between spikes in Gamma-Ray Bursts light curves in
different energy bands as a function of redshift may lead to the detection of
effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts
with measured redshift, detected by the HETE-2 mission between 2001 and 2006 in
order to measure time lags related to astrophysical effects and search for
Quantum Gravity signature in the framework of an extra-dimension string model.
The use of photon-tagged data allows us to consider various energy ranges.
Systematic effects due to selection and cuts are evaluated. No significant
Quantum Gravity effect is detected from the study of the maxima of the light
curves and a lower limit at 95% Confidence Level on the Quantum Gravity scale
parameter of 3.2x10**15 GeV is set.Comment: 4 pages, 5 figures. v3: Error corrected in Eq. 1. Results updated.
Proceedings of the 30th ICRC, Merida, Mexico (2007
First exploration of the runaway greenhouse transition with a GCM
Even if their detection is for now challenging, observation of small
terrestrial planets will be easier in a near future thanks to continuous
improvements of detection and characterisation instruments. In this quest,
climate modeling is a key step to understand their characteristics, atmospheric
composition and possible history. If a surface water reservoir is present on
such a terrestrial planet, an increase in insolation may lead to a dramatic
positive feedback induced by water evaporation: the runaway greenhouse. The
resulting rise of global surface temperature leads to the evaporation of the
entire water reservoir, separating two very different population of planets: 1)
temperate planets with a surface water ocean and 2) hot planets with a puffed
atmosphere dominated by water vapor. In this work we use a 3D General
Circulation Model (GCM), the Generic-PCM, to study the runaway greenhouse
transition, linking temperate and post-runaway states. Our simulations are made
of two steps. First, assuming initially a liquid surface ocean, an evaporation
phase which enriches the atmosphere in water vapor. Second, when the ocean is
considered entirely evaporated, a dry transition phase for which the surface
temperature increases dramatically. Finally, it converges on a hot and stable
post-runaway state. By describing in detail the evolution of the climate during
these two steps, we show a rapid transition of the cloud coverage and of the
wind circulation from the troposphere to the stratosphere. By comparing our
result to previous studies using 1D models, we discuss the effect of
intrinsically 3D processes such as the global dynamics and the clouds, keys to
understand the runaway greenhouse. We also explore the potential reversibility
of the runaway greenhouse, limited by its radiative unbalance.Comment: 15 pages, 17 figures, accepted for publication in A&
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