62 research outputs found
Star-planet interactions: I. Stellar rotation and planetary orbits
Context. As a star evolves, the planet orbits change with time due to tidal
interactions, stellar mass losses, friction and gravitational drag forces, mass
accretion and evaporation on/by the planet. Stellar rotation modifies the
structure of the star and therefore the way these different processes occur.
Changes of the orbits, at their turn, have an impact on the rotation of the
star.
Aims. Models accounting in a consistent way for these interactions between
the orbital evolution of the planet and the evolution of the rotation of the
star are still missing. The present work is a first attempt to fill this gap.
Methods. We compute the evolution of stellar models including a comprehensive
treatment of rotational effects together with the evolution of planetary
orbits, so that the exchanges of angular momentum between the star and the
planetary orbit are treated in a self-consistent way. The evolution of the
rotation of the star accounts for the angular momentum exchange with the planet
and also follows the effects of the internal transport of angular momentum and
chemicals.
Results. We show that rotating stellar models without tidal interactions can
well reproduce the surface rotations of the bulk of the red giants. However,
models without any interactions cannot account for fast rotating red giants in
the upper part of the red giant branch, where, such models, whatever the
initial rotation considered on the ZAMS, always predict very low velocities.
For those stars some interaction with a companion is highly probable and the
present rotating stellar models with planets confirm that tidal interaction can
reproduce their high surface velocities. We show also that the minimum distance
between the planet and the star on the ZAMS that will allow the planet to avoid
engulfment and survive is decreased around faster rotating stars. [abridged]Comment: 14 pages, abstract abridged for arXiv submission, accepted for
publication in Astronomy & Astrophysic
The effects of stellar winds on the magnetospheres and potential habitability of exoplanets
Context: The principle definition of habitability for exoplanets is whether
they can sustain liquid water on their surfaces, i.e. that they orbit within
the habitable zone. However, the planet's magnetosphere should also be
considered, since without it, an exoplanet's atmosphere may be eroded away by
stellar winds. Aims: The aim of this paper is to investigate magnetospheric
protection of a planet from the effects of stellar winds from solar-mass stars.
Methods: We study hypothetical Earth-like exoplanets orbiting in the host
star's habitable zone for a sample of 124 solar-mass stars. These are targets
that have been observed by the Bcool collaboration. Using two wind models, we
calculate the magnetospheric extent of each exoplanet. These wind models are
computationally inexpensive and allow the community to quickly estimate the
magnetospheric size of magnetised Earth-analogues orbiting cool stars. Results:
Most of the simulated planets in our sample can maintain a magnetosphere of ~5
Earth radii or larger. This suggests that magnetised Earth analogues in the
habitable zones of solar analogues are able to protect their atmospheres and is
in contrast to planets around young active M dwarfs. In general, we find that
Earth-analogues around solar-type stars, of age 1.5 Gyr or older, can maintain
at least a Paleoarchean Earth sized magnetosphere. Our results indicate that
planets around 0.6 - 0.8 solar-mass stars on the low activity side of the
Vaughan-Preston gap are the optimum observing targets for habitable Earth
analogues.Comment: 8 pages, 3 figures, accepted to Astronomy and Astrophysic
Evolution of helium triplet transits of close-in gas giants orbiting K-dwarfs
Atmospheric escape in exoplanets has traditionally been observed using
hydrogen Lyman- and H- transmission spectroscopy, but more
recent detections have utilised the metastable helium triplet at 1083nm.
Since this feature is accessible from the ground, it offers new possibilities
for studying atmospheric escape. Our goal is to understand how the
observability of escaping helium evolves during the lifetime of a highly
irradiated gas giant. We extend our previous work on 1-D self-consistent
hydrodynamic escape from hydrogen-only atmospheres as a function of planetary
evolution to the first evolution-focused study of escaping hydrogen-helium
atmospheres. Additionally, using these novel models we perform helium triplet
transmission spectroscopy. We adapt our previous hydrodynamic escape model to
now account for both hydrogen and helium heating and cooling processes and
simultaneously solve for the population of helium in the triplet state. To
account for the planetary evolution, we utilise evolving predictions of
planetary radii for a close-in 0.3 gas giant and its received
stellar flux in X-ray, hard and soft EUV, and mid-UV wavelength bins assuming a
K dwarf stellar host. We find that the helium triplet signature diminishes with
evolution. Our models suggest that young (~Myr), close-in gas
giants ( to ) should produce helium 1083nm transit
absorptions of or , for a slow or fast-rotating K dwarf,
respectively, assuming a 2 helium abundance.Comment: 20 pages, 13 figures, 4 tables; accepted for publication in MNRA
Radio masers on WX UMa : hints of a Neptune-sized planet, or magnetospheric reconnection?
RDK acknowledges funding received from the Irish Research Council (IRC) through the Government of Ireland Postgraduate Scholarship Programme. RDK and AAV acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 817540, ASTROFLOW). We acknowledge the provisions of the Space Weather Modelling Framework (SWMF) code from the Center for Space Environment Modeling (CSEM) at the University of Michigan, and the computational resources of the Irish Centre for High End Computing (ICHEC), both of which were utilised in this work.The nearby M dwarf WX UMa has recently been detected at radio wavelengths with LOFAR. The combination of its observed brightness temperature and circular polarisation fraction suggests that the emission is generated via the electron-cyclotron maser instability. Two distinct mechanisms have been proposed to power such emission from low-mass stars: either a sub-Alfvénic interaction between the stellar magnetic field and an orbiting planet, or reconnection at the edge of the stellar magnetosphere. In this paper, we investigate the feasibility of both mechanisms, utilising the information about the star’s surrounding plasma environment obtained from modelling its stellar wind. Using this information, we show that a Neptune-sized exoplanet with a magnetic field strength of 10 – 100 G orbiting at ∼0.034 au can accurately reproduce the observed radio emission from the star, with corresponding orbital periods of 7.4 days. Due to the stellar inclination, a planet in an equatorial orbit is unlikely to transit the star. While such a planet could induce radial velocity semi-amplitudes from 7 to 396 m s−1, it is unlikely that this signal could be detected with current techniques due to the activity of the host star. The application of our planet-induced radio emission model here illustrates its exciting potential as a new tool for identifying planet-hosting candidates from long-term radio monitoring. We also develop a model to investigate the reconnection-powered emission scenario. While this approach produces less favourable results than the planet-induced scenario, it nevertheless serves as a potential alternative emission mechanism which is worth exploring further.PostprintPeer reviewe
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