3,368 research outputs found
XUV-driven mass loss from extrasolar giant planets orbiting active stars
Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmospheres of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. In this study, we focus on Extrasolar Giant Planets (EPGs) orbiting K and M dwarf stars. XUV spectra for three different stars – ∊ Eridani, AD Leonis and AU Microscopii – are constructed using a coronal model. Neutral density and temperature profiles in the upper atmosphere of hypothetical EGPs orbiting these stars are then obtained from a fluid model, incorporating atmospheric chemistry and taking atmospheric escape into account. We find that a simple scaling based solely on the host star’s X-ray emission gives large errors in mass loss rates from planetary atmospheres and so we have derived a new method to scale the EUV regions of the solar spectrum based upon stellar X-ray emission. This new method produces an outcome in terms of the planet’s neutral upper atmosphere very similar to that obtained using a detailed coronal model of the host star. Our results indicate that in planets subjected to radiation from active stars, the transition from Jeans escape to a regime of hydrodynamic escape at the top of the atmosphere occurs at larger orbital distances than for planets around low activity stars (such as the Sun)
Magnetic phases of one-dimensional lattices with 2 to 4 fermions per site
We study the spectral and magnetic properties of one-dimensional lattices
filled with 2 to 4 fermions (with spin 1/2) per lattice site. We use a
generalized Hubbard model that takes account all interactions on a lattice
site, and solve the many-particle problem by exact diagonalization. We find an
intriguing magnetic phase diagram which includes ferromagnetism, spin-one
Heisenberg antiferromagnetism, and orbital antiferromagnetism.Comment: 8 pages, 6 figure
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&
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