7,639 research outputs found
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
The magnetic fields of forming solar-like stars
Magnetic fields play a crucial role at all stages of the formation of low
mass stars and planetary systems. In the final stages, in particular, they
control the kinematics of in-falling gas from circumstellar discs, and the
launching and collimation of spectacular outflows. The magnetic coupling with
the disc is thought to influence the rotational evolution of the star, while
magnetised stellar winds control the braking of more evolved stars and may
influence the migration of planets. Magnetic reconnection events trigger
energetic flares which irradiate circumstellar discs with high energy particles
that influence the disc chemistry and set the initial conditions for planet
formation. However, it is only in the past few years that the current
generation of optical spectropolarimeters have allowed the magnetic fields of
forming solar-like stars to be probed in unprecedented detail. In order to do
justice to the recent extensive observational programs new theoretical models
are being developed that incorporate magnetic fields with an observed degree of
complexity. In this review we draw together disparate results from the
classical electromagnetism, molecular physics/chemistry, and the geophysics
literature, and demonstrate how they can be adapted to construct models of the
large scale magnetospheres of stars and planets. We conclude by examining how
the incorporation of multipolar magnetic fields into new theoretical models
will drive future progress in the field through the elucidation of several
observational conundrums.Comment: 55 pages, review article accepted for publication in Reports on
Progress in Physics. Astro-ph version includes additional appendice
Estimation of the infinitesimal generator by square-root approximation
For the analysis of molecular processes, the estimation of time-scales, i.e.,
transition rates, is very important. Estimating the transition rates between
molecular conformations is -- from a mathematical point of view -- an invariant
subspace projection problem. A certain infinitesimal generator acting on
function space is projected to a low-dimensional rate matrix. This projection
can be performed in two steps. First, the infinitesimal generator is
discretized, then the invariant subspace is approxi-mated and used for the
subspace projection. In our approach, the discretization will be based on a
Voronoi tessellation of the conformational space. We will show that the
discretized infinitesimal generator can simply be approximated by the geometric
average of the Boltzmann weights of the Voronoi cells. Thus, there is a direct
correla-tion between the potential energy surface of molecular structures and
the transition rates of conformational changes. We present results for a
2d-diffusion process and Alanine dipeptide
Magnetometry of the classical T Tauri star GQ Lup: non-stationary dynamos & spin evolution of young Suns
We report here results of spectropolarimetric observations of the classical T
Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii
Telescope (CFHT) in the framework of the "Magnetic Protostars and Planets"
(MaPP) programme, and obtained at 2 different epochs (2009 July & 2011 June).
From these observations, we first infer that GQ Lup has a photospheric
temperature of 4,300+-50\^A K and a rotation period of 8.4+-0.3 d; it implies
that it is a 1.05+-0.07 Msun star viewed at an inclination of ~30deg, with an
age of 2-5 Myr, a radius of 1.7+-0.2 Rsun, and has just started to develop a
radiative core.
Large Zeeman signatures are clearly detected at all times, both in
photospheric lines & in accretion-powered emission lines, probing longitudinal
fields of up to 6 kG and hence making GQ Lup the cTTS with the strongest
large-scale fields known as of today. Rotational modulation of Zeeman
signatures is clearly different between our 2 runs, demonstrating that
large-scale fields of cTTSs are evolving with time and are likely produced by
non-stationary dynamo processes.
Using tomographic imaging, we reconstruct maps of the large-scale field, of
the photospheric brightness & of the accretion-powered emission of GQ Lup. We
find that the magnetic topology is mostly poloidal & axisymmetric; moreover,
the octupolar component of the large-scale field (of strength 2.4 & 1.6 kG in
2009 & 2011) dominates the dipolar component (of strength ~1 kG) by a factor of
~2, consistent with the fact that GQ Lup is no longer fully-convective.
GQ Lup also features dominantly poleward magnetospheric accretion at both
epochs. The large-scale dipole of GQ Lup is however not strong enough to
disrupt the surrounding accretion disc further than about half-way to the
corotation radius, suggesting that GQ Lup should rapidly spin up like other
similar partly-convective cTTSs (abridged).Comment: MNRAS, in press (17 pages, 10 figures, 1 table
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
Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha
We have produced brightness and magnetic field maps of the surfaces of CV Cha
and CR Cha: two actively accreting G and K-type T Tauri stars in the Chamaeleon
I star-forming cloud with ages of 3-5 Myr. Our magnetic field maps show
evidence for strong, complex multi-polar fields similar to those obtained for
young rapidly rotating main sequence stars. Brightness maps indicate the
presence of dark polar caps and low latitude spots -- these brightness maps are
very similar to those obtained for other pre-main sequence and rapidly rotating
main sequence stars.
Only two other classical T Tauri stars have been studied using similar
techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field
patterns that are significantly more complex than those recovered for BP Tau, a
fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the
complexity of the stellar magnetic field is related to the convection zone;
with more complex fields being found in T Tauri stars with radiative cores
(V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct
magnetic field studies of T Tauri star systems, exploring a wide range of
stellar parameters in order to establish how they affect magnetic field
generation, and thus how these magnetic fields are likely to affect the
evolution of T Tauri star systems as they approach the main sequence.Comment: Accepted for publication by MNRAS: 15 pages, 11 figure
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
From observations collected with the ESPaDOnS spectropolarimeter, we report
the discovery of magnetic fields at the surface of the mildly accreting
classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in
photospheric lines and in the emission lines formed at the base of the
accretion funnels linking the disc to the protostar, and monitored over the
whole rotation cycle of V2129 Oph. We observe that rotational modulation
dominates the temporal variations of both unpolarized and circularly polarized
line profiles. We reconstruct the large-scale magnetic topology at the surface
of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to
be rather complex, with a dominant octupolar component and a weak dipole of
strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to
the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar
radial field spots located at high latitudes and coinciding with cool dark
polar spots at photospheric level. This large-scale field geometry is unusually
complex compared to those of non-accreting cool active subgiants with moderate
rotation rates. As an illustration, we provide a first attempt at modelling the
magnetospheric topology and accretion funnels of V2129 Oph using field
extrapolation. We find that the magnetosphere of V2129 Oph must extend to about
7R* to ensure that the footpoints of accretion funnels coincide with the
high-latitude accretion spots on the stellar surface. It suggests that the
stellar magnetic field succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of V2129
Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes
typical of those observed in cTTSs.Comment: MNRAS, in press (18 pages, 17 figures
Modeling X-ray emission from stellar coronae
By extrapolating from observationally derived surface magnetograms of
low-mass stars we construct models of their coronal magnetic fields and compare
the 3D field geometry with axial multipoles. AB Dor, which has a radiative
core, has a very complex field, whereas V374 Peg, which is completely
convective, has a simple dipolar field. We calculate global X-ray emission
measures assuming that the plasma trapped along the coronal loops is in
hydrostatic equilibrium and compare the differences between assuming isothermal
coronae, or by considering a loop temperature profiles. Our preliminary results
suggest that the non-isothermal model works well for the complex field of AB
Dor, but not for the simple field of V374 Peg.Comment: 4 pages, proceedings of Cool Stars 15, St Andrews, July 2008, to be
published in the Conference Proceedings Series of the American Institute of
Physic
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