5,066 research outputs found
Predicting radio emission from the newborn hot Jupiter V830 Tau and its host star
Magnetised exoplanets are expected to emit at radio frequencies analogously
to the radio auroral emission of Earth and Jupiter. We predict the radio
emission from V830 Tau b, the youngest (2 Myr) detected exoplanet to date. We
model the host star wind using 3DMHD simulations that take into account its
surface magnetism. With this, we constrain the local conditions around V830 Tau
b that we use to then compute its radio emission. We estimate average radio
flux densities of 6 to 24mJy, depending on the assumed radius of the planet
(one or two Rjupiter). These radio fluxes are present peaks that are up to
twice the average values. We show here that these fluxes are weakly dependent
(a factor of 1.8) on the assumed polar planetary magnetic field (10 to 100G),
opposed to the maximum frequency of the emission, which ranges from 18 to
240MHz. We also estimate the thermal radio emission from the stellar wind. By
comparing our results with VLA and VLBA observations of the system, we
constrain the stellar mass-loss rate to be <3e-9 Msun/yr, with likely values
between ~1e-12 and 1e-10 Msun/yr. The frequency-dependent extension of the
radio-emitting wind is around ~ 3 to 30 Rstar for frequencies in the range of
275 to 50MHz, implying that V830 Tau b, at an orbital distance of 6.1 Rstar,
could be embedded in the regions of the host star's wind that are optically
thick to radio wavelengths, but not deeply so. Planetary emission can only
propagate in the stellar wind plasma if the frequency of the cyclotron emission
exceeds the stellar wind plasma frequency. For that, we find that for planetary
radio emission to propagate through the host star wind, planetary magnetic
field strengths larger than ~1.3 to 13 G are required. The V830 Tau system is a
very interesting system for conducting radio observations from both the
perspective of radio emission from the planet as well as from the host star's
wind.Comment: A&A, in pres
Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR 1099
(V711 Tau) secured from 1998 February to 2002 January with the
spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic
du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness
and magnetic surface topologies of the K1 primary subgiant of the system, at
five different epochs. We confirm the presence of large, axisymmetric regions
where the magnetic field is mainly azimuthal, providing further support to the
hypothesis that dynamo processes may be distributed throughout the whole
convective zone in this star. We study the short-term evolution of surface
structures from a comparison of our images with observations secured at
close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We
conclude that the small-scale brightness and magnetic patterns undergo major
changes within a timescale of 4 to 6 weeks, while the largest structures remain
stable over several years. We report the detection of a weak surface
differential rotation (both from brightness and magnetic tracers) indicating
that the equator rotates faster than the pole with a difference in rotation
rate between the pole and the equator about 4 times smaller than that of the
Sun. This result suggests that tidal forces also impact the global dynamic
equilibrium of convective zones in cool active stars.Comment: accepted by MNRA
Radial dependence of line profile variability in seven O9--B0.5 stars
Massive stars show a variety of spectral variability: presence of discrete
absorption components in UV P-Cygni profiles, optical line profile variability,
X-ray variability, radial velocity modulations. Our goal is to study the
spectral variability of single OB stars to better understand the relation
between photospheric and wind variability. For that, we rely on high spectral
resolution, high signal-to-noise ratio optical spectra collected with the
spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We
investigate the variability of twelve spectral lines by means of the Temporal
Variance Spectrum (TVS). The selected lines probe the radial structure of the
atmosphere, from the photosphere to the outer wind. We also perform a
spectroscopic analysis with atmosphere models to derive the stellar and wind
properties, and to constrain the formation region of the selected lines. We
show that variability is observed in the wind lines of all bright giants and
supergiants, on a daily timescale. Lines formed in the photosphere are
sometimes variable, sometimes not. The dwarf stars do not show any sign of
variability. If variability is observed on a daily timescale, it can also (but
not always) be observed on hourly timescales, albeit with lower amplitude.
There is a very clear correlation between amplitude of the variability and
fraction of the line formed in the wind. Strong anti-correlations between the
different part of the temporal variance spectrum are observed. Our results
indicate that variability is stronger in lines formed in the wind. A link
between photospheric and wind variability is not obvious from our study, since
wind variability is observed whatever the level of photospheric variability.
Different photospheric lines also show different degrees of variability.Comment: 13 pages, 9 figures + appendix. A&A accepted. Figures degraded for
arxiv submissio
Temporal fluctuations in the differential rotation of cool active stars
This paper reports positive detections of surface differential rotation on
two rapidly rotating cool stars at several epochs, by using stellar surface
features (both cool spots and magnetic regions) as tracers of the large scale
latitudinal shear that distorts the convective envelope in this type of stars.
We also report definite evidence that this differential rotation is different
when estimated from cool spots or magnetic regions, and that it undergoes
temporal fluctuations of potentially large amplitude on a time scale of a few
years. We consider these results as further evidence that the dynamo processes
operating in these stars are distributed throughout the convective zone rather
than being confined at its base as in the Sun. By comparing our observations
with two very simple models of the differential rotation within the convective
zone, we obtain evidence that the internal rotation velocity field of the stars
we investigated is not like that of the Sun, and may resemble that we expect
for rapid rotators. We speculate that the changes in differential rotation
result from the dynamo processes (and from the underlying magnetic cycle) that
periodically converts magnetic energy into kinetic energy and vice versa. We
emphasise that the technique outlined in this paper corresponds to the first
practical method for investigating the large scale rotation velocity field
within convective zones of cool active stars, and offers several advantages
over asteroseismology for this particular purpose and this specific stellar
class.Comment: 14 pages, 4 figure
Magnetic activity on AB Doradus: Temporal evolution of starspots and differential rotation from 1988 to 1994
Surface brightness maps for the young K0 dwarf AB Doradus are reconstructed
from archival data sets for epochs spanning 1988 to 1994. By using the
signal-to-noise enhancement technique of Least-Squares Deconvolution, our
results show a greatly increased resolution of spot features than obtained in
previously published surface brightness reconstructions. These images show that
for the exception of epoch 1988.96, the starspot distributions are dominated by
a long-lived polar cap, and short-lived low to high latitude features. The
fragmented polar cap at epoch 1988.96 could indicate a change in the nature of
the dynamo in the star. For the first time we measure differential rotation for
epochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). These
measurements show variations on a timescale of at least one year, with the
strongest surface differential rotation ever measured for AB Dor occurring in
1994.86. In conjunction with previous investigations, our results represent the
first long-term analysis of the temporal evolution of differential rotation on
active stars.Comment: accepted by MNRAS 18 pages 18 figure
The evolution of surface magnetic fields in young solar-type stars
The surface rotation rates of young solar-type stars decrease rapidly with
age from the end of the pre-main sequence though the early main sequence. This
suggests that there is also an important change in the dynamos operating in
these stars, which should be observable in their surface magnetic fields. Here
we present early results in a study aimed at observing the evolution of these
magnetic fields through this critical time period. We are observing stars in
open clusters and stellar associations to provide precise ages, and using
Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented
here are results for six stars, three in the in the beta Pic association (~10
Myr old) and three in the AB Dor association (~100 Myr old).Comment: To appear in the proceedings of IAU symposium 302: Magnetic fields
throughout stellar evolution. 2 pages, 3 figure
Magnetic cycles of the planet-hosting star tauBootis
We have obtained new spectropolarimetric observations of the planet-hosting
star tauBootis, using the ESPaDOnS and NARVAL spectropolarimeters at the
Canada-France-Hawaii Telescope and Telescope Bernard-Lyot. With this data set,
we are able to confirm the presence of a magnetic field at the surface of
tauBoo and map its large-scale structure over the whole star. The overall
polarity of the magnetic field has reversed with respect to our previous
observation (obtained a year before), strongly suggesting that tauBoo is
undergoing magnetic cycles similar to those of the Sun. This is the first time
that a global magnetic polarity switch is observed in a star other than the
Sun; we speculate that the magnetic cycle period of tauBoo is much shorter than
that of the Sun.
Our new data also allow us to confirm the presence of differential rotation
from the latitudinal shearing that the magnetic structure is undergoing. The
differential rotation surface shear that tauBoo experiences is found to be 6 to
10 times larger than that of the Sun. We propose that the short magnetic cycle
period is due to the strong level of differential rotation. With a rotation
period of 3.0 and 3.9 d at the equator and pole respectively, tauBoo appears as
the first planet-hosting star whose rotation (at intermediate latitudes) is
synchronised with the orbital motion of its giant planet (period 3.3 d).
Assuming that this synchronisation is not coincidental, it suggests that the
tidal effects induced by the giant planet can be strong enough to force the
thin convective enveloppe (though not the whole star) into corotation and thus
to play a role in the activity cycle of tauBoo.Comment: MNRAS, in pres
Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields
Disk accretion to rotating stars with complex magnetic fields is investigated
using full three-dimensional magnetohydrodynamic (MHD) simulations. The studied
magnetic configurations include superpositions of misaligned dipole and
quadrupole fields and off-centre dipoles. The simulations show that when the
quadrupole component is comparable to the dipole component, the magnetic field
has a complex structure with three major magnetic poles on the surface of the
star and three sets of loops of field lines connecting them. A significant
amount of matter flows to the quadrupole "belt", forming a ring-like hot spot
on the star. If the maximum strength of the magnetic field on the star is
fixed, then we observe that the mass accretion rate, the torque on the star,
and the area covered by hot spots are several times smaller in the
quadrupole-dominant cases than in the pure dipole cases. The influence of the
quadrupole component on the shape of the hot spots becomes noticeable when the
ratio of the quadrupole and dipole field strengths , and
becomes dominant when . In the case of an off-centre dipole
field, most of the matter flows through a one-armed accretion stream, forming a
large hot spot on the surface, with a second much smaller secondary spot. The
light curves may have simple, sinusoidal shapes, thus mimicking stars with pure
dipole fields. Or, they may be complex and unusual. In some cases the light
curves may be indicators of a complex field, in particular if the inclination
angle is known independently. We also note that in the case of complex fields,
magnetospheric gaps are often not empty, and this may be important for the
survival of close-in exosolar planets.Comment: 13 pages, 21 figures, accepted for publication in MNRA
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