2,856 research outputs found
Genetic studies of population history and contemporary microevolution in grayling (Thymallus: Salmonidae)
Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b
Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b
absorbs over 50% of the stellar Ly emission during transit. Given the
planet's atmospheric composition and energy-limited escape rate, this hydrogen
outflow is expected to entrain heavier atoms such as C and O. We searched for C
and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M
observations made during the planet's extended H I transit. These observations
show no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\
lines integrated over [-100, 100] km s, imposing 95% (2) upper
limits of 14% (C II) and 60% (Si III) depth on the transit of an opaque disk
and 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transit
similar to that of H I Ly. C is likely present in the outflow
according to a simulation we carried out using a spherically-symmetric,
photochemical-hydrodynamical model. This simulation predicts a 2% transit
over the integrated bandpass, consistent with the data. At line center, we
predict the C II transit depth to be as high as 19%. Our model predicts a
neutral hydrogen escape rate of g s (
g s for all species) for an upper atmosphere composed of hydrogen and
helium.Comment: 7 pages, 4 figures, 1 table; accepted to ApJ Letter
Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
Stellar flares are a frequent occurrence on young low-mass stars around which
many detected exoplanets orbit. Flares are energetic, impulsive events, and
their impact on exoplanetary atmospheres needs to be taken into account when
interpreting transit observations. We have developed a model to describe the
upper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. The
model simulates thermal escape from the upper atmospheres of close-in EGPs.
Ionisation by solar radiation and electron impact is included and photochemical
and diffusive transport processes are simulated. This model is used to study
the effect of stellar flares from the solar-like G star HD209458 and the young
K star HD189733 on their respective planets. A hypothetical HD209458b-like
planet orbiting the active M star AU Mic is also simulated. We find that the
neutral upper atmosphere of EGPs is not significantly affected by typical
flares. Therefore, stellar flares alone would not cause large enough changes in
planetary mass loss to explain the variations in HD189733b transit depth seen
in previous studies, although we show that it may be possible that an extreme
stellar proton event could result in the required mass loss. Our simulations do
however reveal an enhancement in electron number density in the ionosphere of
these planets, the peak of which is located in the layer where stellar X-rays
are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare
levels and enhanced electron densities last from about 3 to 10 hours after the
onset of the flare. The strength of the flare and the width of its spectral
energy distribution affect the range of altitudes that see enhancements in
ionisation. A large broadband continuum component in the XUV portion of the
flaring spectrum in very young flare stars, such as AU Mic, results in a broad
range of altitudes affected in planets orbiting this star.Comment: accepted for publication in A&
The stability of short-period extrasolar giant planets
A three-dimensional coupled thermosphere-ionosphere model for extrasolar giant planets (EXOTIM) has been developed. This is the first such model reported in the literature. This thesis contains an extensive description of the model and the methods adopted in modelling the different physical processes expected in the upper atmospheres and ionospheres of extrasolar giant planets. Modelling the upper atmosphere is important because the stability of the atmosphere against thermal evaporation is controlled by the conditions in the thermosphere. The thermosphere is heated by the absorption of EUV and X ray (XUV) radiation emitted by the host star. The radiation also ionises the neutral species in the upper atmosphere, which is expected to be composed mainly of molecular and atomic hydrogen, and atomic helium. Ionisation and subsequent photochemistry leads to the formation of the H^+, H^+_2 , H^+_3 , and He^+ ions (and small quantities of HeH^+). H^+_3 emits strongly in the infrared and may act as a significant coolant in gas giant thermospheres. Assuming photochemical equilibrium, the absorption of XUV radiation and ion photochemistry were modelled in a self-consistent fashion. The 3D model can also simulate strong winds affecting the upper atmosphere, and account for both advection and diffusion of the neutral species around the planet. The results indicate that within 1.0 AU from a solar-type host star, the upper atmospheres of Jupiter-type EGPs can be substantially cooler and more stable than implied by studies that ignore the possibility of radiative (H^+_3 ) cooling. In this context, a limiting distance, or a stability limit, was identified for such EGPs that depends on the composition of the upper atmosphere and ionosphere, and within which the atmospheres of the planets undergo hydrodynamic escape. Under restricted conditions, this limit is located around 0.15 AU from a Sun-like host star. The model was also used to simulate a newly found transiting planet HD17156b, which orbits its host star on a highly eccentric orbit.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Colorado Ultraviolet Transit Experiment Data Simulator
The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat
carrying on-board a low-resolution (R~2000--3000), near-ultraviolet (2500--3300
{\AA}) spectrograph. It has a rectangular primary Cassegrain telescope to
maximize the collecting area. CUTE, which is planned for launch in Spring 2020,
is designed to monitor transiting extra-solar planets orbiting bright, nearby
stars aiming at improving our understanding of planet atmospheric escape and
star-planet interaction processes. We present here the CUTE data simulator,
which we complemented with a basic data reduction pipeline. This pipeline will
be then updated once the final CUTE data reduction pipeline is developed. We
show here the application of the simulator to the HD209458 system and a first
estimate of the precision on the measurement of the transit depth as a function
of temperature and magnitude of the host star. We also present estimates of the
effect of spacecraft jitter on the final spectral resolution. The simulator has
been developed considering also scalability and adaptability to other missions
carrying on-board a long-slit spectrograph. The data simulator will be used to
inform the CUTE target selection, choose the spacecraft and instrument settings
for each observation, and construct synthetic CUTE wavelength-dependent transit
light curves on which to develop the CUTE data reduction pipeline.Comment: Accepted for publication in the Journal of Astronomical Telescopes,
Instruments and System
Contribution of non-work and work-related risk factors to the association between income and mental disorders in a working population: the Health 2000 Study
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