300 research outputs found
Effects of XUV radiation on circumbinary planets
Several circumbinary planets have recently been discovered. The orbit of a
planet around a binary stellar system poses several dynamic constraints. The
effects that radiation from the host stars may have on the planet atmospheres
must be considered. Because of the configuration of a close binary system,
these stars have a high rotation rate, which causes a permanent state of high
stellar activity and copious XUV radiation. The accumulated effects are
stronger than for exoplanets around single stars, and cause a faster
evaporation of their atmospheres. We evaluate the effects that stellar
radiation has on the evaporation of exoplanets around binary systems and on the
survival of these planets. We considered the XUV spectral range to account for
the photons that are easily absorbed by a planet atmosphere that is mainly
composed of hydrogen. A more complex atmospheric composition is expected to
absorb this radiation more efficiently. We used direct X-ray observations to
evaluate the energy in the X-rays range and coronal models to calculate the
(nondetectable) EUV part of the spectrum. The simulations show that exoplanets
in a close orbit will suffer strong photoevaporation that may cause a total
loss of atmosphere in a short time. A binary system of two solar-like stars
will be highly efficient in evaporating the atmosphere of the planet. These
systems will be difficult to find, even if they are dynamically stable. Still,
planets may orbit around binary systems of low mass stars for wider orbits.
Currently known circumbinary planets are not substantially affected by thermal
photoevaporation processes, unless Kepler-47 b has an inflated atmosphere. The
distribution of the orbital periods of circumbinary planets is shifted to much
longer periods than the average of Kepler planets, which supports a scenario of
strong photoevaporation in close-in circumbinary planets.Comment: Accepted by A&A. 8 pages, 5 figure
The EUVE point of view of AD Leo
All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo, totalling
1.1 Ms of exposure time, have been employed to analyze the corona of this
single M dwarf. The light curves show a well defined quiescent stage, and a
distribution of amplitude of variability following a power law with a ~-2.4
index. The flaring behavior exhibits much similarity with other M active stars
like FK Aqr or YY Gem, and flares behave differently from late type active
giants and subgiants. The Emission Measure Distribution (EMD) of the summed
spectrum, as well as that of quiescent and flaring stages, were obtained using
a line-based method. The average EMD is dominated by material at log T(K)~6.9,
with a second peak around log T(K)~6.3, and a large increase in the amount of
material with log T(K)>~7.1 during flares, material almost absent during
quiescence. The results are interpreted as the combination of three families of
loops with maximum temperatures at log T(K)~6.3, ~6.9 and somewhere beyond log
T(K)>~7.1. A value of the abundance of [Ne/Fe]=1.05+-0.08 was measured at log
T(K)~5.9. No significative increment of Neon abundance was detected between
quiescence and flaring states.Comment: Full PS version can be found also at
http://www.astropa.unipa.it/~jsanz/papers0002.htm
Eclipsed X-ray flares in binary stars: geometrical constraints on the flare's location and size
The observation of eclipses during X-rays flares taking place in active cool
stars binaries allows us to calculate the position and size of the flares. This
information cannot be derived by analyzing the decay of the flares, a
frequently used approach in the literature that requires the assumption of a
physical model. We make use of the eclipsing light curve to constrain the set
of possible solutions, from the geometrical point of view, in two flares of
Algol, and one flare in VW Cep. We make use of a technique developed with the
system SV Cam (i~90 deg) and generalize it to binary systems with arbitrary
inclination. The method simulates all possible geometrical situations that can
produce the times of the four contacts of the eclipse. As an approximation we
assume that the emitting region has a spherical shape that remains unchanged
during the eclipse. We however show that this is a good approximation for the
problem. The solutions observed indicate that in two of the three cases the
flare cannot be polar (lat<55 deg) and in a third one the flare can be placed
either near the pole or at other latitudes. The emitting regions must have a
small size (0.002-0.5 R_*), but if interpreted as the apex of coronal loops,
their length could actually be up to 3.1 R_* for one of the Algol flares. These
measurements imply a lower limit to the electron density in the emitting region
between log n_e(cm^-3) 10.4 and 14.0, and a magnetic field between 70 and 3500
G. Similar results are found if the emitting region is assumed to be
loop-shaped.Comment: 8 pages (17 in online version), 9 figures (18 in online version).
Figure 12 is a set of animated GIF, available also at
http://laeff.esa.es/users/jsanz/Papers/Eclipses/Videosweb . Accepted for
publication in A&
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 â â Eridani, AD Leonis and AU Microscopii â 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 starâs 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 planetâs 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)
Active cool stars and He I 10830 \AA: the coronal connection
The mechanism of formation of the He I 10830 A triplet in cool stars has been
subject of debate for the last 30 years. A relation between the X-ray
luminosity and the He I 10830 A flux was found in cool stars, but the dominant
mechanism of formation in these stars (photoionization by coronal radiation
followed by recombination and cascade, or collisional excitation in the
chromosphere), has not yet been established. We use modern instrumentation
(NOT/SOFIN) and a direct measurement of the EUV flux, which photoionizes He I,
to investigate the formation mechanism of the line for the most active stars
which are frequently excluded from analysis. We have observed with an
unprecedented resolution (R~170,000) the He I 10830 A triplet in a set of 15
stars that were also observed with the Extreme Ultraviolet Explorer (EUVE) in
order to compare the line strengths with their EUV and X-ray fluxes. Active
dwarf and subgiant stars do not exhibit a relation between the EUV flux and the
equivalent width of the He I 10830 A line. Giant stars however, show a positive
correlation between the strength of the He I 10830 A absorption and the EUV and
X-ray fluxes. The strength of the C IV 1550 A emission does not correlate with
coronal fluxes in this sample of 15 stars. Active dwarf stars may have high
chromospheric densities thus allowing collisional excitation to dominate
photoionization/recombination processes in forming the He I 10830 A line.
Active giant stars possess lower gravities, and lower chromospheric densities
than dwarfs, allowing for photoexcitation processes to become important.
Moreover, their extended chromospheres allowfor scattering of infrared
continuum radiation, producing strong absorption in He I and tracing wind
dynamics.Comment: 9 pages, 12 figures. Accepted by A&A (June 2008
Estimation of the XUV radiation onto close planets and their evaporation
Context: The current distribution of planet mass vs. incident stellar X-ray
flux supports the idea that photoevaporation of the atmosphere may take place
in close-in planets. Integrated effects have to be accounted for. A proper
calculation of the mass loss rate due to photoevaporation requires to estimate
the total irradiation from the whole XUV range. Aims: The purpose of this paper
is to extend the analysis of the photoevaporation in planetary atmospheres from
the accessible X-rays to the mostly unobserved EUV range by using the coronal
models of stars to calculate the EUV contribution to the stellar spectra. The
mass evolution of planets can be traced assuming that thermal losses dominate
the mass loss of their atmospheres. Methods: We determine coronal models for 82
stars with exoplanets that have X-ray observations available. Then a synthetic
spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination
of the EUV stellar flux, calibrated with real EUV data, allows us to calculate
the accumulated effects of the XUV irradiation on the planet atmosphere with
time, as well as the mass evolution for planets with known density. Results: We
calibrate for the first time a relation of the EUV luminosity with stellar age
valid for late-type stars. In a sample of 109 exoplanets, few planets with
masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense
photoevaporation takes place in a short period of time, as previously found in
X-rays. The scenario is also consistent with the observed distribution of
planet masses with density. The accumulated effects of photoevaporation over
time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr.
Conclusions: Coronal radiation produces rapid photoevaporation of the
atmospheres of planets close to young late-type stars. More complex models are
needed to explain fully the observations.Comment: Accepted by A&A. 10 pages, 8 figures, 7 Tables (2 online). Additional
online material includes 7 pages, 6 figures and 6 tables, all include
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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