441 research outputs found
Roche lobe effects on the atmospheric loss of "Hot Jupiters"
Observational evidence of a hydrodynamically evaporating upper atmosphere of
HD209458b (Vidal-Madjar et al. 2003; 2004) and recent theoretical studies on
evaporation scenarios of ``Hot Jupiters'' in orbits around solar-like stars
with the age of the Sun indicate that the upper atmospheres of short-periodic
exoplanets experience hydrodynamic blow-off conditions resulting in loss rates
of the order of about 10^10 - 10^12 g s^-1 (Lammer et al. 2003; Yelle 2004;
Baraffe et al. 2004; Lecavlier des Etangs et al. 2004; Jaritz et al. 2005, Tian
et al. 2005; Penz et al. 2007). By studying the effect of the Roche lobe on the
atmospheric loss from short-periodic gas giants we found, that the effect of
the Roche lobe can enhance the hydrodynamic evaporation from HD209458b by about
2 and from OGLE-TR-56b by about 2.5 times. For similar exoplanets which are
closer to their host star than OGLE-TR-56b, the enhancement of the mass loss
can be even larger. Moreover, we show that the effect of the Roche lobe raises
the possibility that ``Hot Jupiters'' can reach blow-off conditions at
temperatures which are less than expected (< 10000 K) due to the stellar X-ray
and EUV (XUV) heating.Comment: 4 pages, 2 figures, submitted to A&
Corrigendum to "The upper atmosphere of the exoplanet HD209458b revealed by the sodium D lines: Temperature-pressure profile, ionization layer and thermosphere" [2011, A&A, 527, A110]
An error was detected in the code used for the analysis of the HD209458b
sodium profile (Vidal-Madjar et al. 2011). Here we present an updated T-P
profile and briefly discuss the consequences.Comment: Published in Astronomy & Astrophysics, 533, C
New observations of the extended hydrogen exosphere of the extrasolar planet HD209458b
Atomic hydrogen escaping from the planet HD209458b provides the largest
observational signature ever detected for an extrasolar planet atmosphere.
However, the Space Telescope Imaging Spectrograph (STIS) used in previous
observational studies is no longer available, whereas additional observations
are still needed to better constrain the mechanisms subtending the evaporation
process, and determine the evaporation state of other `hot Jupiters'. Here, we
aim to detect the extended hydrogen exosphere of HD209458b with the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find
evidence for a hydrogen comet-like tail trailing the planet, which size would
depend on the escape rate and the amount of ionizing radiation emitted by the
star. These observations also provide a benchmark for other transiting planets,
in the frame of a comparative study of the evaporation state of close-in giant
planets. Eight HST orbits are used to observe two transits of HD209458b.
Transit light curves are obtained by performing photometry of the unresolved
stellar Lyman-alpha emission line during both transits. Absorption signatures
of exospheric hydrogen during the transit are compared to light curve models
predicting a hydrogen tail. Transit depths of (9.6 +/- 7.0)% and (5.3 +/-
10.0)% are measured on the whole Lyman-alpha line in visits 1 and 2,
respectively. Averaging data from both visits, we find an absorption depth of
(8.0 +/- 5.7)%, in good agreement with previous studies. The extended size of
the exosphere confirms that the planet is likely loosing hydrogen to space.
Yet, the photometric precision achieved does not allow us to better constrain
the hydrogen mass loss rate.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 3
figure
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b
Lyman-alpha (Lya) emission of neutral hydrogen (1215.67 Angstr\"om) is the
main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It
is also the main light source used in studies of the evaporating upper
atmospheres of transiting extrasolar planets with ultraviolet transmission
spectroscopy. However, there are very few observations of the Lya emissions of
quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present
Lya observations of the hot-neptune host star GJ436 with the Hubble Space
Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the
first resolved and high quality spectrum of a quiet M dwarf at Lya. Using an
energy diagram for exoplanets and an N-body particle simulation, this detection
enables the possible exospheric signature of the hot neptune to be estimated as
a ~11% absorption in the Lya stellar emission, for a typical mass-loss rate of
10^10 g/s. The atmosphere of the planet GJ436b is found to be stable to
evaporation, and should be readily observable with HST. We also derive a
correlation between X-ray and Lya emissions for M dwarfs. This correlation will
be useful for predicting the evaporation signatures of planets transiting other
quiet M dwarfs.Comment: 8 pages, 8 figures, 2 tables. Accepted for publication in Astronomy &
Astrophysic
A scenario of planet erosion by coronal radiation
Context: According to theory, high-energy emission from the coronae of cool
stars can severely erode the atmospheres of orbiting planets. No observational
tests of the long term effects of erosion have yet been made. Aims: To analyze
the current distribution of planetary mass with X-ray irradiation of the
atmospheres in order to make an observational assessment of the effects of
erosion by coronal radiation. Methods: We study a large sample of
planet-hosting stars with XMM-Newton, Chandra and ROSAT; make a careful
identification of X-ray counterparts; and fit their spectra to make accurately
measurements of the stellar X-ray flux. Results: The distribution of the
planetary masses with X-ray flux suggests that erosion has taken place: most
surviving massive planets, (M_p sin i >1.5 M_J), have been exposed to lower
accumulated irradiation. Heavy erosion during the initial stages of stellar
evolution is followed by a phase of much weaker erosion. A line dividing these
two phases could be present, showing a strong dependence on planet mass.
Although a larger sample will be required to establish a well-defined erosion
line, the distribution found is very suggestive. Conclusions: The distribution
of planetary mass with X-ray flux is consistent with a scenario in which planet
atmospheres have suffered the effects of erosion by coronal X-ray and EUV
emission. The erosion line is an observational constraint to models of
atmospheric erosion.Comment: A&A 511, L8 (2010). 4 pages, 3 figures, 1 online table (included).
Language edited; corrected a wrong unit conversion (g/s -> M_J/Gyr);
corrected values in column 12 of Table 1 (slightly underestimated in first
version), and Figure 2 updated accordingl
Upper atmospheres and ionospheres of planets and satellites
The upper atmospheres of the planets and their satellites are more directly
exposed to sunlight and solar wind particles than the surface or the deeper
atmospheric layers. At the altitudes where the associated energy is deposited,
the atmospheres may become ionized and are referred to as ionospheres. The
details of the photon and particle interactions with the upper atmosphere
depend strongly on whether the object has anintrinsic magnetic field that may
channel the precipitating particles into the atmosphere or drive the
atmospheric gas out to space. Important implications of these interactions
include atmospheric loss over diverse timescales, photochemistry and the
formation of aerosols, which affect the evolution, composition and remote
sensing of the planets (satellites). The upper atmosphere connects the planet
(satellite) bulk composition to the near-planet (-satellite) environment.
Understanding the relevant physics and chemistry provides insight to the past
and future conditions of these objects, which is critical for understanding
their evolution. This chapter introduces the basic concepts of upper
atmospheres and ionospheres in our solar system, and discusses aspects of their
neutral and ion composition, wind dynamics and energy budget. This knowledge is
key to putting in context the observations of upper atmospheres and haze on
exoplanets, and to devise a theory that explains exoplanet demographics.Comment: Invited Revie
On the Age of Stars Harboring Transiting Planets
Results of photometric surveys have brought to light the existence of a
population of giant planets orbiting their host stars even closer than the hot
Jupiters (HJ), with orbital periods below 3 days. The reason why radial
velocity surveys were not able to detect these very-hot Jupiters (VHJ) is under
discussion. A possible explanation is that these close-in planets are
short-lived, being evaporated on short time-scales due to UV flux of their host
stars. In this case, stars hosting transiting VHJ planets would be
systematically younger than those in the radial velocity sample. We have used
the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of
5 faint stars hosting transiting planets, namely, OGLE-TR-10, 56, 111, 113 and
TrES-1. Previously obtained CORALIE spectra of HD189733, and published data on
the other transiting planet-hosts were also used. The immediate objective is to
estimate ages via Li abundances, using the Ca II activity-age relation, and
from the analysis of the stellar rotational velocity. For the stars for which
we have spectra, Li abundances were computed as in Israelian et al. (2004)
using the stellar parameters derived in Santos et al. (2006). The chromospheric
activity index was built as the ratio of the flux within the core of
the Ca II H & K lines and the flux in two nearby continuum regions. The index
was calibrated to Mount Wilson index allowing the computation
of the Ca II H & K corrected for the photospheric contribution. These values
were then used to derive the ages by means of the Henry et al. (1996)
activity-age relation. Bearing in mind the limitations of the ages derived by
Li abundances, chromospheric activity, and stellar rotational velocities, none
of the stars studied in this paper seem to be younger than 0.5 Gyr.Comment: Accepted for publication in A&
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Atmospheric escape has been detected from the exoplanet HD 209458b through
transit observations of the hydrogen Lyman-alpha line. Here we present
spectrally resolved Lyman-alpha transit observations of the exoplanet HD
189733b at two different epochs. These HST/STIS observations show for the first
time, that there are significant temporal variations in the physical conditions
of an evaporating planetary atmosphere. While atmospheric hydrogen is not
detected in the first epoch observations, it is observed at the second epoch,
producing a transit absorption depth of 14.4+/-3.6% between velocities of -230
to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from
radiation pressure alone and require an additional acceleration mechanism, such
as interactions with stellar wind protons. The observed absorption can be
explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9
g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the
second-epoch observation supports the idea that the observed changes within the
upper atmosphere of the planet can be caused by variations in the stellar wind
properties, or by variations in the stellar energy input to the planetary
escaping gas (or a mix of the two effects). These observations provide the
first indication of interaction between the exoplanet's atmosphere and stellar
variations.Comment: To be published in A&A Letters, June 28, 201
Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?
Planets less massive than about 10 MEarth are expected to have no massive
H-He atmosphere and a cometary composition (50% rocks, 50% water, by mass)
provided they formed beyond the snowline of protoplanetary disks. Due to inward
migration, such planets could be found at any distance between their formation
site and the star. If migration stops within the habitable zone, this will
produce a new kind of planets, called Ocean-Planets. Ocean-planets typically
consist in a silicate core, surrounded by a thick ice mantle, itself covered by
a 100 km deep ocean. The existence of ocean-planets raises important
astrobiological questions: Can life originate on such body, in the absence of
continent and ocean-silicate interfaces? What would be the nature of the
atmosphere and the geochemical cycles ?
In this work, we address the fate of Hot Ocean-Planets produced when
migration ends at a closer distance. In this case the liquid/gas interface can
disappear, and the hot H2O envelope is made of a supercritical fluid. Although
we do not expect these bodies to harbor life, their detection and
identification as water-rich planets would give us insight as to the abundance
of hot and, by extrapolation, cool Ocean-Planets.Comment: 47 pages, 6 Fugures, regular paper. Submitted to Icaru
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