572 research outputs found
Searching for Star-Planet interactions within the magnetosphere of HD 189733
HD 189733 is a K2 dwarf, orbited by a giant planet at 8.8 stellar radii. In
order to study magnetospheric interactions between the star and the planet, we
explore the large-scale magnetic field and activity of the host star.
We collected spectra using the ESPaDOnS and the NARVAL spectropolarimeters,
installed at the 3.6-m Canada-France-Hawaii telescope and the 2-m Telescope
Bernard Lyot at Pic du Midi, during two monitoring campaigns (June 2007 and
July 2008).
HD 189733 has a mainly toroidal surface magnetic field, having a strength
that reaches up to 40 G. The star is differentially rotating, with latitudinal
angular velocity shear of domega = 0.146 +- 0.049 rad/d, corresponding to
equatorial and polar periods of 11.94 +- 0.16 d and 16.53 +- 2.43 d
respectively. The study of the stellar activity shows that it is modulated
mainly by the stellar rotation (rather than by the orbital period or the beat
period between the stellar rotation and the orbital periods). We report no
clear evidence of magnetospheric interactions between the star and the planet.
We also extrapolated the field in the stellar corona and calculated the
planetary radio emission expected for HD 189733b given the reconstructed field
topology. The radio flux we predict in the framework of this model is time
variable and potentially detectable with LOFAR
Refurbishing Voyager 1 & 2 Planetary Radio Astronomy (PRA) Data
Voyager/PRA (Planetary Radio Astronomy) data from digitized tapes archived at
CNES have been reprocessed and recalibrated. The data cover the Jupiter and
Saturn flybys of both Voyager probes. We have also reconstructed
goniopolarimetric datasets (flux and polarization) at full resolution. These
datasets are currently not available to the scientific community, but they are
of primary interest for the analysis of the Cassini data at Saturn, and the
Juno data at Jupiter, as well as for the preparation of the JUICE mission. We
present the first results derived from the re-analysis of this dataset.Comment: Accepted manuscript for PRE8 (Planetary Radio Emission VIII
conference) proceeding
A magnetic field evolution scenario for brown dwarfs and giant planets
Very little is known about magnetic fields of extrasolar planets and brown
dwarfs. We use the energy flux scaling law presented by Christensen et al.
(2009) to calculate the evolution of average magnetic fields in extrasolar
planets and brown dwarfs under the assumption of fast rotation, which is
probably the case for most of them. We find that massive brown dwarfs of about
70 M_Jup can have fields of a few kilo-Gauss during the first few hundred
Million years. These fields can grow by a factor of two before they weaken
after deuterium burning has stopped. Brown dwarfs with weak deuterium burning
and extrasolar giant planets start with magnetic fields between ~100G and ~1kG
at the age of a few Myr, depending on their mass. Their magnetic field weakens
steadily until after 10Gyr it has shrunk by about a factor of 10. We use
observed X-ray luminosities to estimate the age of the known extrasolar giant
planets that are more massive than 0.3M_Jup and closer than 20pc. Taking into
account the age estimate, and assuming sun-like wind-properties and radio
emission processes similar to those at Jupiter, we calculate their radio flux
and its frequency. The highest radio flux we predict comes out as 700mJy at a
frequency around 150MHz for Boob, but the flux is below 60mJy for the
rest. Most planets are expected to emit radiation between a few Mhz and up to
100MHz, well above the ionospheric cutoff frequency.Comment: 7 pages, accepted by A&
The Search for Signatures Of Transient Mass Loss in Active Stars
The habitability of an exoplanet depends on many factors. One such factor is
the impact of stellar eruptive events on nearby exoplanets. Currently this is
poorly constrained due to heavy reliance on solar scaling relationships and a
lack of experimental evidence. Potential impacts of Coronal Mass Ejections
(CMEs), which are a large eruption of magnetic field and plasma from a star,
are space weather and atmospheric stripping. A method for observing CMEs as
they travel though the stellar atmosphere is the type II radio burst, and the
new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15
hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star,
taken in LOFAR's beam-formed observation mode for the purposes of measuring
transient frequency-dependent low frequency radio emission. The observations
utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for
five three-hour observation periods. In this data set, there were no confirmed
type II events in this frequency range. We explore the range of parameter space
for type II bursts constrained by our observations Assuming the rate of shocks
is a lower limit to the rate at which CMEs occur, no detections in a total of
15 hours of observation places a limit of shocks/hr for YZ CMi due to the stochastic nature of the events and
limits of observational sensitivity. We propose a methodology to interpret
jointly observed flares and CMEs which will provide greater constraints to CMEs
and test the applicability of solar scaling relations
Stoics against stoics in Cudworth's "A Treatise of Freewill"
In his 'A Treatise of Freewill', Ralph Cudworth argues against Stoic determinism by drawing on what he takes to be other concepts found in Stoicism, notably the claim that some things are ‘up to us’ and that these things are the product of our choice. These concepts are central to the late Stoic Epictetus and it appears at first glance as if Cudworth is opposing late Stoic voluntarism against early Stoic determinism. This paper argues that in fact, despite his claim to be drawing on Stoic doctrine, Cudworth uses these terms with a meaning first articulated only later, by the Peripatetic commentator Alexander of Aphrodisias
Numerical simulations of unbounded cyclotron-maser emissions
Numerical simulations have been conducted to study the spatial growth rate and emission topology of the cyclotron-maser instability responsible for stellar/planetary auroral magnetospheric radio emission and intense non-thermal radio emission in other astrophysical contexts. These simulations were carried out in an unconstrained geometry, so that the conditions existing within the source region of some natural electron cyclotron masers could be more closely modelled. The results have significant bearing on the radiation propagation and coupling characteristics within the source region of such non-thermal radio emissions
Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR
LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz
frequencies each orbital cycle of 26.5 d and a superorbital
modulation with a period of 4.6 yr. We have performed a detailed
study of the low-frequency radio emission of LS I +61 303 by analysing all the
archival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR
observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected
the source for the first time at 150 MHz, which is also the first detection of
a gamma-ray binary at such a low frequency. We have obtained the light-curves
of the source at 150, 235 and 610 MHz, all of them showing orbital modulation.
The light-curves at 235 and 610 MHz also show the existence of superorbital
variability. A comparison with contemporaneous 15-GHz data shows remarkable
differences with these light-curves. At 15 GHz we see clear outbursts, whereas
at low frequencies we see variability with wide maxima. The light-curve at 235
MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of
0.5 orbital phases in the maxima. We model the shifts between the maxima
at different frequencies as due to changes in the physical parameters of the
emitting region assuming either free-free absorption or synchrotron
self-absorption, obtaining expansion velocities for this region close to the
stellar wind velocity with both mechanisms.Comment: 12 pages, 10 figures, accepted for publication in MNRA
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