2,213 research outputs found
On the environment surrounding close-in exoplanets
Exoplanets in extremely close-in orbits are immersed in a local
interplanetary medium (i.e., the stellar wind) much denser than the local
conditions encountered around the solar system planets. The environment
surrounding these exoplanets also differs in terms of dynamics (slower stellar
winds, but higher Keplerian velocities) and ambient magnetic fields (likely
higher for host stars more active than the Sun). Here, we quantitatively
investigate the nature of the interplanetary media surrounding the hot Jupiters
HD46375b, HD73256b, HD102195b, HD130322b, HD179949b. We simulate the
three-dimensional winds of their host stars, in which we directly incorporate
their observed surface magnetic fields. With that, we derive mass-loss rates
(1.9 to 8.0 /yr) and the wind properties at the
position of the hot-Jupiters' orbits (temperature, velocity, magnetic field
intensity and pressure). We show that these exoplanets' orbits are
super-magnetosonic, indicating that bow shocks are formed surrounding these
planets. Assuming planetary magnetic fields similar to Jupiter's, we estimate
planetary magnetospheric sizes of 4.1 to 5.6 planetary radii. We also derive
the exoplanetary radio emission released in the dissipation of the stellar wind
energy. We find radio fluxes ranging from 0.02 to 0.13 mJy, which are
challenging to be observed with present-day technology, but could be detectable
with future higher sensitivity arrays (e.g., SKA). Radio emission from systems
having closer hot-Jupiters, such as from tau Boo b or HD189733b, or from nearby
planetary systems orbiting young stars, are likely to have higher radio fluxes,
presenting better prospects for detecting exoplanetary radio emission.Comment: 15 pages, 5 figures, accepted to MNRA
Rotationally Modulated X-ray Emission from T Tauri Stars
We have modelled the rotational modulation of X-ray emission from T Tauri
stars assuming that they have isothermal, magnetically confined coronae. By
extrapolating surface magnetograms we find that T Tauri coronae are compact and
clumpy, such that rotational modulation arises from X-ray emitting regions
being eclipsed as the star rotates. Emitting regions are close to the stellar
surface and inhomogeneously distributed about the star. However some regions of
the stellar surface, which contain wind bearing open field lines, are dark in
X-rays. From simulated X-ray light curves, obtained using stellar parameters
from the Chandra Orion Ultradeep Project, we calculate X-ray periods and make
comparisons with optically determined rotation periods. We find that X-ray
periods are typically equal to, or are half of, the optical periods. Further,
we find that X-ray periods are dependent upon the stellar inclination, but that
the ratio of X-ray to optical period is independent of stellar mass and radius.Comment: 10 pages, 8 figures, accepted for publication in MNRA
M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets
We perform three-dimensional numerical simulations of stellar winds of
early-M dwarf stars. Our simulations incorporate observationally reconstructed
large-scale surface magnetic maps, suggesting that the complexity of the
magnetic field can play an important role in the angular momentum evolution of
the star, possibly explaining the large distribution of periods in field dM
stars, as reported in recent works. In spite of the diversity of the magnetic
field topologies among the stars in our sample, we find that stellar wind
flowing near the (rotational) equatorial plane carries most of the stellar
angular momentum, but there is no preferred colatitude contributing to mass
loss, as the mass flux is maximum at different colatitudes for different stars.
We find that more non-axisymmetric magnetic fields result in more asymmetric
mass fluxes and wind total pressures (defined as the sum of
thermal, magnetic and ram pressures). Because planetary magnetospheric sizes
are set by pressure equilibrium between the planet's magnetic field and , variations of up to a factor of in (as found in the
case of a planet orbiting at several stellar radii away from the star) lead to
variations in magnetospheric radii of about 20 percent along the planetary
orbital path. In analogy to the flux of cosmic rays that impact the Earth,
which is inversely modulated with the non-axisymmetric component of the total
open solar magnetic flux, we conclude that planets orbiting M dwarf stars like
DT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric field
components, should be the more efficiently shielded from galactic cosmic rays,
even if the planets lack a protective thick atmosphere/large magnetosphere of
their own.Comment: 16 pages, 9 figures, to appear in MNRA
Slingshot prominences above stellar X-ray coronae
We present a new model for the coronal structure of rapidly rotating
solar-type stars. The presence of prominences trapped in co-rotation 2 to 5
stellar radii above the stellar surface has been taken as evidence that the
coronae of these stars must be very extended. The observed surface magnetic
fields, however, cannot contain X-ray emitting gas out to these distances. We
present an alternative model: that these prominences are trapped in long thin
loops embedded not in the X-ray corona, but in the wind. Above coronal helmet
streamers, oppositely-directed wind-bearing field lines reconnect to form
closed loops which then fill up with gas that was originally part of the wind.
We demonstrate that static equilibria exist for these loops at a range of
pressures and temperatures. The maximum loop height falls as the rotation rate
increases, but rises as the loop temperature decreases. For a solar-mass star
with rotation period 0.5 days, whose X-ray corona extends 1stellar radius above
the surface, loops at temperatures of 10, 000 K can extend out to 5 stellar
radii.Comment: 9 pages, 8 figure
Exoplanet Transit Variability: Bow Shocks and Winds Around HD 189733b
By analogy with the solar system, it is believed that stellar winds will form
bow shocks around exoplanets. For hot Jupiters the bow shock will not form
directly between the planet and the star, causing an asymmetric distribution of
mass around the exoplanet and hence an asymmetric transit. As the planet orbits
thorough varying wind conditions, the strength and geometry of its bow shock
will change, thus producing transits of varying shape. We model this process
using magnetic maps of HD 189733 taken one year apart, coupled with a 3D
stellar wind model, to determine the local stellar wind conditions throughout
the orbital path of the planet. We predict the time-varying geometry and
density of the bow shock that forms around the magnetosphere of the planet and
simulate transit light curves. Depending on the nature of the stellar magnetic
field, and hence its wind, we find that both the transit duration and ingress
time can vary when compared to optical light curves. We conclude that
consecutive near-UV transit light curves may vary significantly and can
therefore provide an insight into the structure and evolution of the stellar
wind.Comment: 9 Pages, 7 figures. Accepted for publication in Monthly Notices of
The Royal Astronomical Societ
Prospects for Detection of Exoplanet Magnetic Fields Through Bow-Shock Observations During Transits
An asymmetry between the ingress and egress times was observed in the near-UV
light curve of the transit planet WASP-12b. Such asymmetry led us to suggest
that the early ingress in the UV light curve of WASP-12b, compared to the
optical observations, is caused by a shock around the planet, and that shocks
should be a common feature in transiting systems. Here, we classify all the
transiting systems known to date according to their potential for producing
shocks that could cause observable light curve asymmetries. We found that 36/92
of known transiting systems would lie above a reasonable detection threshold
and that the most promising candidates to present shocks are: WASP-19b,
WASP-4b, WASP-18b, CoRoT-7b, HAT-P-7b, CoRoT-1b, TrES-3, and WASP-5b. For
prograde planets orbiting outside the co-rotation radius of fast rotating
stars, the shock position, instead of being ahead of the planetary motion as in
WASP-12b, trails the planet. In this case, we predict that the light curve of
the planet should present a late-egress asymmetry. We show that CoRoT-11b is a
potential candidate to host such a behind shock and show a late egress. If
observed, these asymmetries can provide constraints on planetary magnetic
fields. For instance, for a planet that has a magnetic field intensity similar
to Jupiter's field (~ 14 G) orbiting a star whose magnetic field is between 1
and 100G, the stand-off distance between the shock and the planet, which we
take to be the size of the planet's magnetosphere, ranges from 1 to 40
planetary radii.Comment: 7 pages (including the complete version of Table 1), 2 Tables, 3
Figures. Accepted by MNRAS Letter
The relation between stellar magnetic field geometry and chromospheric activity cycles - I. The highly variable field of ɛ Eridani at activity minimum
The young and magnetically active K dwarf Epsilon Eridani exhibits a chromospheric activity cycle of about 3 years. Previous reconstructions of its large-scale magnetic field show strong variations at yearly epochs. To understand how Epsilon Eridani's large-scale magnetic field geometry evolves over its activity cycle we focus on high cadence observations spanning 5 months at its activity minimum. Over this timespan we reconstruct 3 maps of Epsilon Eridani's large-scale magnetic field using the tomographic technique of Zeeman Doppler Imaging. The results show that at the minimum of its cycle, Epsilon Eridani's large-scale field is more complex than the simple dipolar structure of the Sun and 61 Cyg A at minimum. Additionally we observe a surprisingly rapid regeneration of a strong axisymmetric toroidal field as Epsilon Eridani emerges from its S-index activity minimum. Our results show that all stars do not exhibit the same field geometry as the Sun and this will be an important constraint for the dynamo models of active solar-type stars
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