793 research outputs found
Heating efficiency in hydrogen-dominated upper atmospheres
Context. The heating efficiency is defined as the ratio of the net local
gas-heating rate to the rate of stellar radiative energy absorption. It plays
an important role in thermal-escape processes from the upper atmospheres of
planets that are exposed to stellar soft X-rays and extreme ultraviolet
radiation (XUV). Aims. We model the thermal-escape-related heating efficiency
of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the
extrasolar gas giant HD 209458b. The model result is then compared with
previous thermal-hydrogen-escape studies which assumed heating efficiency
values between 10-100%. Methods. The photolytic and electron impact processes
in the thermosphere were studied by solving the kinetic Boltzmann equation and
applying a Direct Simulation Monte Carlo model. We calculated the energy
deposition rates of the stellar XUV flux and that of the accompanying primary
photoelectrons that are caused by electron impact processes in the H2 to H
transition region in the upper atmosphere. Results. The heating by XUV
radiation of hydrogen-dominated upper atmospheres does not reach higher than
20% above the main thermosphere altitude, if the participation of photoelectron
impact processes is included. Conclusions. Hydrogen-escape studies from
exoplanets that assume heating efficiency values that are >= 20 % probably
overestimate the thermal escape or mass-loss rates, while those who assumed
values that are < 20% probably produce more realistic atmospheric-escape rates.Comment: 7 pages, 4 figures, accepted to A&
The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats
Since the Archean, N2 has been a major atmospheric constituent in Earth's
atmosphere. Nitrogen is an essential element in the building blocks of life,
therefore the geobiological nitrogen cycle is a fundamental factor in the long
term evolution of both Earth and Earth-like exoplanets. We discuss the
development of the Earth's N2 atmosphere since the planet's formation and its
relation with the geobiological cycle. Then we suggest atmospheric evolution
scenarios and their possible interaction with life forms: firstly, for a
stagnant-lid anoxic world, secondly for a tectonically active anoxic world, and
thirdly for an oxidized tectonically active world. Furthermore, we discuss a
possible demise of present Earth's biosphere and its effects on the atmosphere.
Since life forms are the most efficient means for recycling deposited nitrogen
back into the atmosphere nowadays, they sustain its surface partial pressure at
high levels. Also, the simultaneous presence of significant N2 and O2 is
chemically incompatible in an atmosphere over geological timescales. Thus, we
argue that an N2-dominated atmosphere in combination with O2 on Earth-like
planets within circumstellar habitable zones can be considered as a
geo-biosignature. Terrestrial planets with such atmospheres will have an
operating tectonic regime connected with an aerobe biosphere, whereas other
scenarios in most cases end up with a CO2-dominated atmosphere. We conclude
with implications for the search for life on Earth-like exoplanets inside the
habitable zones of M to K-stars
- A tool for multiband light curve modeling of planetary transits and stellar spots
Several studies have shown that stellar activity features, such as occulted
and non-occulted starspots, can affect the measurement of transit parameters
biasing studies of transit timing variations and transmission spectra. We
present , which we designed to model multiband transit
light curves showing starspot anomalies, inferring both transit and spot
parameters. The code follows a pixellation approach to model the star with its
corresponding limb darkening, spots, and transiting planet on a two dimensional
Cartesian coordinate grid. We combine with an MCMC
framework to study and derive exoplanet transmission spectra, which provides
statistically robust values for the physical properties and uncertainties of a
transiting star-planet system. We validate 's performance
by analyzing eleven synthetic light curves of four different star-planet
systems and 20 transit light curves of the well-studied WASP-41b system. We
also investigate the impact of starspots on transit parameters and derive
wavelength dependent transit depth values for WASP-41b covering a range of
6200-9200 , indicating a flat transmission spectrum.Comment: 17 pages, 22 figures; accepted for publication in Astronomy &
Astrophysic
Young planets under extreme UV irradiation. I. Upper atmosphere modelling of the young exoplanet K2-33b
The K2-33 planetary system hosts one transiting ~5 R_E planet orbiting the
young M-type host star. The planet's mass is still unknown, with an estimated
upper limit of 5.4 M_J. The extreme youth of the system (<20 Myr) gives the
unprecedented opportunity to study the earliest phases of planetary evolution,
at a stage when the planet is exposed to an extremely high level of high-energy
radiation emitted by the host star. We perform a series of 1D hydrodynamic
simulations of the planet's upper atmosphere considering a range of possible
planetary masses, from 2 to 40 M_E, and equilibrium temperatures, from 850 to
1300 K, to account for internal heating as a result of contraction. We obtain
temperature profiles mostly controlled by the planet's mass, while the
equilibrium temperature has a secondary effect. For planetary masses below 7-10
M_E, the atmosphere is subject to extremely high escape rates, driven by the
planet's weak gravity and high thermal energy, which increase with decreasing
mass and/or increasing temperature. For higher masses, the escape is instead
driven by the absorption of the high-energy stellar radiation. A rough
comparison of the timescales for complete atmospheric escape and age of the
system indicates that the planet is more massive than 10 M_E.Comment: 11 pages, 7 figure
A search for flares and mass ejections on young late-type stars in the open cluster Blanco-1
We present a search for stellar activity (flares and mass ejections) in a
sample of 28 stars in the young open cluster Blanco-1. We use optical spectra
obtained with ESO's VIMOS multi-object spectrograph installed on the VLT. From
the total observing time of 5 hours, we find four H flares but
no distinct indication of coronal mass ejections (CMEs) on the investigated
dK-dM stars. Two flares show "dips" in their light-curves right before their
impulsive phases which are similar to previous discoveries in photometric
light-curves of active dMe stars. We estimate an upper limit of 4 CMEs per
day per star and discuss this result with respect to a semi- empirical
estimation of the CME rate of main-sequence stars. We find that we should have
detected at least one CME per star with a mass of 1-15 g
depending on the star's X-ray luminosity, but the estimated H fluxes
associated with these masses are below the detection limit of our observations.
We conclude that the parameter which mainly influences the detection of stellar
CMEs using the method of Doppler-shifted emission caused by moving plasma is
not the spectral resolution or velocity but the flux or mass of the CME.Comment: Accepted for publication in MNRAS, accepted 2014 June 10, received
2014 June 5, in original form 2014 March 24, 14 pages, 5 figure
A grid of upper atmosphere models for 1--40 MEARTH planets: application to CoRoT-7 b and HD219134 b,c
There is growing observational and theoretical evidence suggesting that
atmospheric escape is a key driver of planetary evolution. Commonly, planetary
evolution models employ simple analytic formulae (e.g., energy limited escape)
that are often inaccurate, and more detailed physical models of atmospheric
loss usually only give snapshots of an atmosphere's structure and are difficult
to use for evolutionary studies. To overcome this problem, we upgrade and
employ an already existing upper atmosphere hydrodynamic code to produce a
large grid of about 7000 models covering planets with masses 1 - 39 Earth mass
with hydrogen-dominated atmospheres and orbiting late-type stars. The modeled
planets have equilibrium temperatures ranging between 300 and 2000 K. For each
considered stellar mass, we account for three different values of the
high-energy stellar flux (i.e., low, moderate, and high activity). For each
computed model, we derive the atmospheric temperature, number density, bulk
velocity, X-ray and EUV (XUV) volume heating rates, and abundance of the
considered species as a function of distance from the planetary center. From
these quantities, we estimate the positions of the maximum dissociation and
ionisation, the mass-loss rate, and the effective radius of the XUV absorption.
We show that our results are in good agreement with previously published
studies employing similar codes. We further present an interpolation routine
capable to extract the modelling output parameters for any planet lying within
the grid boundaries. We use the grid to identify the connection between the
system parameters and the resulting atmospheric properties. We finally apply
the grid and the interpolation routine to estimate atmospheric evolutionary
tracks for the close-in, high-density planets CoRoT-7 b and HD219134 b,c...Comment: 21 pages, 4 Tables, 15 Figure
Transit Ly- signatures of terrestrial planets in the habitable zones of M dwarfs
We modeled the transit signatures in the Lya line of a putative Earth-sized
planet orbiting in the HZ of the M dwarf GJ436. We estimated the transit depth
in the Lya line for an exo-Earth with three types of atmospheres: a
hydrogen-dominated atmosphere, a nitrogen-dominated atmosphere, and a
nitrogen-dominated atmosphere with an amount of hydrogen equal to that of the
Earth. We calculated the in-transit absorption they would produce in the Lya
line. We applied it to the out-of-transit Lya observations of GJ 436 obtained
by the HST and compared the calculated in-transit absorption with observational
uncertainties to determine if it would be detectable. To validate the model, we
also used our method to simulate the deep absorption signature observed during
the transit of GJ 436b and showed that our model is capable of reproducing the
observations. We used a DSMC code to model the planetary exospheres. The code
includes several species and traces neutral particles and ions. At the lower
boundary of the DSMC model we assumed an atmosphere density, temperature, and
velocity obtained with a hydrodynamic model for the lower atmosphere. We showed
that for a small rocky Earth-like planet orbiting in the HZ of GJ436 only the
hydrogen-dominated atmosphere is marginally detectable with the STIS/HST.
Neither a pure nitrogen atmosphere nor a nitrogen-dominated atmosphere with an
Earth-like hydrogen concentration in the upper atmosphere are detectable. We
also showed that the Lya observations of GJ436b can be reproduced reasonably
well assuming a hydrogen-dominated atmosphere, both in the blue and red wings
of the Lya line, which indicates that warm Neptune-like planets are a suitable
target for Lya observations. Terrestrial planets can be observed in the Lya
line if they orbit very nearby stars, or if several observational visits are
available.Comment: 17 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
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