98 research outputs found
Magnetic moment and plasma environment of HD 209458b as determined from Ly observations
Transit observations of HD 209458b in the stellar Lyman- (Ly)
line revealed strong absorption in both blue and red wings of the line
interpreted as hydrogen atoms escaping from the planet's exosphere at high
velocities. The following sources for the absorption were suggested:
acceleration by the stellar radiation pressure, natural spectral line
broadening, charge exchange with stellar wind. We reproduce the observation by
means of modelling that includes all aforementioned processes. Our results
support a stellar wind with a velocity of kms at
the time of the observation and a planetary magnetic moment of Am.Comment: This is the author's version of the work. It is posted here by
permission of the AAAS for non-commercial research use only. The definitive
version was published in Science, vol. 346, p. 981, 21 November 2014, DOI:
10.1126/science.1257829. 3 pages, 3 figure
Extreme hydrodynamic atmospheric loss near the critical thermal escape regime
By considering martian-like planetary embryos inside the habitable zone of
solar-like stars we study the behavior of the hydrodynamic atmospheric escape
of hydrogen for small values of the Jeans escape parameter , near
the base of the thermosphere, that is defined as a ratio of the gravitational
and thermal energy. Our study is based on a 1-D hydrodynamic upper atmosphere
model that calculates the volume heating rate in a hydrogen dominated
thermosphere due to the absorption of the stellar soft X-ray and extreme
ultraviolet (XUV) flux. We find that when the value near the
mesopause/homopause level exceeds a critical value of 2.5, there exists a
steady hydrodynamic solution with a smooth transition from subsonic to
supersonic flow. For a fixed XUV flux, the escape rate of the upper atmosphere
is an increasing function of the temperature at the lower boundary. Our model
results indicate a crucial enhancement of the atmospheric escape rate, when the
Jeans escape parameter decreases to this critical value. When
becomes 2.5, there is no stationary hydrodynamic transition from subsonic
to supersonic flow. This is the case of a fast non-stationary atmospheric
expansion that results in extreme thermal atmospheric escape rates.Comment: 6 pages, 3 figure
Stellar wind induced soft X-ray emission from close-in exoplanets
In this paper, we estimate the X-ray emission from close-in exoplanets. We
show that the Solar/Stellar Wind Charge Exchange Mechanism (SWCX) which
produces soft X-ray emission is very effective for hot Jupiters. In this
mechanism, X-ray photons are emitted as a result of the charge exchange between
heavy ions in the solar wind and the atmospheric neutral particles. In the
Solar System, comets produce X-rays mostly through the SWCX mechanism, but it
has also been shown to operate in the heliosphere, in the terrestrial
magnetosheath, and on Mars, Venus and Moon. Since the number of emitted photons
is proportional to the solar wind mass flux, this mechanism is not very
effective for the Solar system giants. Here we present a simple estimate of the
X-ray emission intensity that can be produced by close-in extrasolar giant
planets due to charge exchange with the heavy ions of the stellar wind. Using
the example of HD~209458b, we show that this mechanism alone can be responsible
for an X-ray emission of ~erg~s, which is times
stronger than the emission from the Jovian aurora. We discuss also the
possibility to observe the predicted soft X-ray flux of hot Jupiters and show
that despite high emission intensities they are unobservable with current
facilities.Comment: 5 pages, 1 figure, published in The Astrophysical Journal Letters,
799:L15 (5pp), 2015 January 3
Effective induction heating around strongly magnetized stars
Planets that are embedded in the changing magnetic fields of their host stars
can experience significant induction heating in their interiors caused by the
planet's orbital motion. For induction heating to be substantial, the planetary
orbit has to be inclined with respect to the stellar rotation and dipole axes.
Using WX~UMa, for which the rotation and magnetic axes are aligned, as an
example, we show that for close-in planets on inclined orbits, induction
heating can be stronger than the tidal heating occurring inside Jupiter's
satellite Io; namely, it can generate a surface heat flux exceeding
2\,W\,m. An internal heating source of such magnitude can lead to
extreme volcanic activity on the planet's surface, possibly also to internal
local magma oceans, and to the formation of a plasma torus around the star
aligned with the planetary orbit. A strongly volcanically active planet would
eject into space mostly SO, which would then dissociate into oxygen and
sulphur atoms. Young planets would also eject CO. Oxygen would therefore be
the major component of the torus. If the O{\sc i} column density of the torus
exceeds 10\,cm, the torus could be revealed by detecting
absorption signatures at the position of the strong far-ultraviolet O{\sc i}
triplet at about 1304\,\AA. We estimate that this condition is satisfied if the
O{\sc i} atoms in the torus escape the system at a velocity smaller than
1--10\,km\,s. These estimates are valid also for a tidally heated
planet.Comment: 8 pages, 6 figures, accepted for publication in Ap
Stellar Driven Evolution of Hydrogen-Dominated Atmospheres from Earth-Like to Super-Earth-Type Exoplanets
In the present chapter we present the results of evolutionary studies of
exoplanetary atmospheres. We mostly focus on the sub- to super-Earth domain,
although these methods are applicable to all types of exoplanets. We consider
both thermal and nonthermal loss processes. The type of thermal loss mechanism
depends on so-called escape parameter , which is the ratio of the
gravitational energy of a particle to its thermal energy. While is
decreasing, an exoplanet switches from classical Jeans to modified Jeans and
finally to blow-off escape mechanisms. During blow-off the majority of the
atmospheric particles dispose of enough energy to escape the planet's gravity
field. This leads to extreme gas losses.
Although nonthermal losses never exceed blow-off escape, they are of
significant importance for planets with relatively weak Jeans-type escape. From
the diversity of nonthermal escape mechanisms, in the present chapter we focus
on ion pickup and discuss the importance of other loss mechanisms. The general
conclusion of the chapter is, that escape processes strongly shape the
evolution of the exoplanets and determine, if the planet loses its atmosphere
due to erosion processes or, on the contrary, stays as mini-Neptune type body,
which can probably not be considered as a potential habitat as we know it.Comment: Originally published by: Springer International Publishing
Switzerland 2015 H. Lammer, M. Khodachenko (eds.), Characterizing Stellar and
Exoplanetary Environments, Astrophysics and Space Science Library 41
Probing the Blow-Off Criteria of Hydrogen-Rich "Super-Earths"
The discovery of transiting "super-Earths" with inflated radii and known
masses such as Kepler-11b-f, GJ 1214b and 55 Cnc e, indicates that these
exoplanets did not lose their nebula-captured hydrogen-rich, degassed or
impact-delivered protoatmospheres by atmospheric escape processes. Because
hydrodynamic blow-off of atmospheric hydrogen atoms is the most efficient
atmospheric escape process we apply a time-dependent numerical algorithm which
is able to solve the system of 1-D fluid equations for mass, momentum, and
energy conservation to investigate the criteria under which "super-Earths" with
hydrogen-dominated upper atmospheres can experience hydrodynamic expansion by
heating of the stellar XUV (soft X-rays and extreme ultraviolet) radiation and
thermal escape via blow-off. Depending on orbit location, XUV flux, heating
efficiency and the planet's mean density our results indicate that the upper
atmospheres of all "super-Earths" can expand to large distances, so that
besides of Kepler-11c all of them experience atmospheric mass-loss due to Roche
lobe overflow. The atmospheric mass-loss of the studied "super-Earths" is one
to two orders of magnitude lower compared to that of "hot Jupiters" such as HD
209458b, so that one can expect that these exoplanets cannot lose their
hydrogen-envelopes during their remaining lifetimes.Comment: 11 pages, 4 figures, accepted for publication in MNRA
Thermal mass loss of protoplanetary cores with hydrogen-dominated atmospheres: The influences of ionization and orbital distance
We investigate the loss rates of the hydrogen atmospheres of terrestrial
planets with a range of masses and orbital distances by assuming a stellar
extreme ultraviolet (EUV) luminosity that is 100 times stronger than that of
the current Sun. We apply a 1D upper atmosphere radiation absorption and
hydrodynamic escape model that takes into account ionization, dissociation and
recombination to calculate hydrogen mass loss rates. We study the effects of
the ionization, dissociation and recombination on the thermal mass loss rates
of hydrogen-dominated super-Earths and compare the results to those obtained by
the energy-limited escape formula which is widely used for mass loss evolution
studies. Our results indicate that the energy-limited formula can to a great
extent over- or underestimate the hydrogen mass loss rates by amounts that
depend on the stellar EUV flux and planetary parameters such as mass, size,
effective temperature, and EUV absorption radius.Comment: 8 pages, 2 figures, 1 table, submitted to MNRA
Origin and Stability of Exomoon Atmospheres - Implications for Habitability
We study the origin and escape of catastrophically outgassed volatiles
(HO, CO) from exomoons with Earth-like densities and masses of
, and orbiting an extra-solar gas
giant inside the habitable zone of a young active solar-like star. We apply a
radiation absorption and hydrodynamic upper atmosphere model to the three
studied exomoon cases. We model the escape of hydrogen and dragged dissociation
products O and C during the activity saturation phase of the young host star.
Because the soft X-ray and EUV radiation of the young host star may be up to
100 times higher compared to today's solar value during the first 100 Myr
after the system's origin, an exomoon with a mass located
in the HZ may not be able to keep an atmosphere because of its low gravity.
Depending on the spectral type and XUV activity evolution of the host star,
exomoons with masses between may evolve to Mars-like
habitats. More massive bodies with masses , however, may
evolve to habitats that are a mixture of Mars-like and Earth-analogue habitats,
so that life may originate and evolve at the exomoon's surface.Comment: 27 pages, 3 figure
XUV exposed non-hydrostatic hydrogen-rich upper atmospheres of terrestrial planets. Part I: Atmospheric expansion and thermal escape
The recently discovered low-density "super-Earths" Kepler-11b, Kepler-11f,
Kepler-11d, Kepler-11e, and planets such as GJ 1214b represent most likely
planets which are surrounded by dense H/He envelopes or contain deep H2O oceans
also surrounded by dense hydrogen envelopes. Although these "super-Earths" are
orbiting relatively close to their host stars, they have not lost their
captured nebula-based hydrogen-rich or degassed volatile-rich steam
protoatmospheres. Thus it is interesting to estimate the maximum possible
amount of atmospheric hydrogen loss from a terrestrial planet orbiting within
the habitable zone of late main sequence host stars. For studying the
thermosphere structure and escape we apply a 1-D hydrodynamic upper atmosphere
model which solves the equations of mass, momentum and energy conservation for
a planet with the mass and size of the Earth and for a "super-Earth" with a
size of 2 R_Earth and a mass of 10 M_Earth. We calculate volume heating rates
by the stellar soft X-ray and EUV radiation and expansion of the upper
atmosphere, its temperature, density and velocity structure and related thermal
escape rates during planet's life time. Moreover, we investigate under which
conditions both planets enter the blow-off escape regime and may therefore
experience loss rates which are close to the energy-limited escape. Finally we
discuss the results in the context of atmospheric evolution and implications
for habitability of terrestrial planets in general.Comment: 50 pages, 9 figures, 4 tables, submitted to Astrobiolog
Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating
Low-mass M stars are plentiful in the Universe and often host small, rocky
planets detectable with the current instrumentation. Recently, seven small
planets have been discovered orbiting the ultracool dwarf
TRAPPIST-1\cite{Gillon16,Gillon17}. We examine the role of electromagnetic
induction heating of these planets, caused by the star's rotation and the
planet's orbital motion. If the stellar rotation and magnetic dipole axes are
inclined with respect to each other, induction heating can melt the upper
mantle and enormously increase volcanic activity, sometimes producing a magma
ocean below the planetary surface. We show that induction heating leads the
three innermost planets, one of which is in the habitable zone, to either
evolve towards a molten mantle planet, or to experience increased outgassing
and volcanic activity, while the four outermost planets remain mostly
unaffected.Comment: Published in Nature Astronomy;
https://www.nature.com/articles/s41550-017-0284-
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