1,061 research outputs found
Interactions of the magnetospheres of stars and close-in giant planets
Since the first discovery of an extrasolar planetary system more than a
decade ago, hundreds more have been discovered. Surprisingly, many of these
systems harbor Jupiter-class gas giants located close to the central star, at
distances of 0.1 AU or less. Observations of chromospheric 'hot spots' that
rotate in phase with the planetary orbit, and elevated stellar X-ray
luminosities,suggest that these close-in planets significantly affect the
structure of the outer atmosphere of the star through interactions between the
stellar magnetic field and the planetary magnetosphere. Here we carry out the
first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation
containing the two magnetic bodies and explore the consequences of such
interactions on the steady-state coronal structure. The simulations reproduce
the observable features of 1) increase in the total X-ray luminosity, 2)
appearance of coronal hot spots, and 3) phase shift of these spots with respect
to the direction of the planet. The proximate cause of these is an increase in
the density of coronal plasma in the direction of the planet, which prevents
the corona from expanding and leaking away this plasma via a stellar wind. The
simulations produce significant low temperature heating. By including dynamical
effects, such as the planetary orbital motion, the simulation should better
reproduce the observed coronal heating
Photospheric and chromospheric activity in four young solar-type stars
We present a photometric and spectroscopic study of four G-K dwarfs, namely
HD 166, epsilon Eri, chi1 Ori and kappa1 Cet. In three cases, we find a clear
spatial association between photospheric and chromospheric active regions. For
chi1 Ori we do not find appreciable variations of photospheric temperature, and
chromospheric Halpha emission. We applied a spot/plage model to the observed
rotational modulation of temperature and flux to derive spot/plage parameters
and to reconstruct a rough three-dimensional map of the outer atmosphere of
kappa1 Cet, HD 166 and epsilon Eri.Comment: 12 pages, 3 tables, 9 figures. Submitted to Ap
Persistent Magnetic Wreaths in a Rapidly Rotating Sun
When our Sun was young it rotated much more rapidly than now. Observations of
young, rapidly rotating stars indicate that many possess substantial magnetic
activity and strong axisymmetric magnetic fields. We conduct simulations of
dynamo action in rapidly rotating suns with the 3-D MHD anelastic spherical
harmonic (ASH) code to explore the complex coupling between rotation,
convection and magnetism. Here we study dynamo action realized in the bulk of
the convection zone for a system rotating at three times the current solar
rotation rate. We find that substantial organized global-scale magnetic fields
are achieved by dynamo action in this system. Striking wreaths of magnetism are
built in the midst of the convection zone, coexisting with the turbulent
convection. This is a surprise, for it has been widely believed that such
magnetic structures should be disrupted by magnetic buoyancy or turbulent
pumping. Thus, many solar dynamo theories have suggested that a tachocline of
penetration and shear at the base of the convection zone is a crucial
ingredient for organized dynamo action, whereas these simulations do not
include such tachoclines. We examine how these persistent magnetic wreaths are
maintained by dynamo processes and explore whether a classical mean-field
-effect explains the regeneration of poloidal field.Comment: 17 pages, 9 figures, 1 appendix, emulateapj format; published version
of sections 3-4, 7 and appendix from arXiv:0906.240
FK Comae Berenices, King of Spin: The COCOA-PUFS Project
COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast
spinning, heavily spotted, yellow giant FK Com (HD117555; G4 III). This single
star is thought to be a recent binary merger, and is exceptionally active by
measure of its intense ultraviolet and X-ray emissions, and proclivity to
flare. COCOA-PUFS was carried out with Hubble Space Telescope in the UV
(120-300 nm), using mainly its high-performance Cosmic Origins Spectrograph,
but also high-precision Space Telescope Imaging Spectrograph; Chandra X-ray
Observatory in the soft X-rays (0.5-10 keV), utilizing its High-Energy
Transmission Grating Spectrometer; together with supporting photometry and
spectropolarimetry in the visible from the ground. This is an introductory
report on the project.
FK Com displayed variability on a wide range of time scales, over all
wavelengths, during the week-long main campaign, including a large X-ray flare;
"super-rotational broadening" of the far-ultraviolet "hot-lines" (e.g., Si IV
139 nm (T~80,000 K) together with chromospheric Mg II 280 nm and C II 133 nm
(10,000-30,000 K); large Doppler swings suggestive of bright regions
alternately on advancing and retreating limbs of the star; and substantial
redshifts of the epoch-average emission profiles. These behaviors paint a
picture of a highly extended, dynamic, hot (10 MK) coronal magnetosphere around
the star, threaded by cooler structures perhaps analogous to solar prominences,
and replenished continually by surface activity and flares. Suppression of
angular momentum loss by the confining magnetosphere could temporarily postpone
the inevitable stellar spindown, thereby lengthening this highly volatile stage
of coronal evolution.Comment: to be published in ApJ
Cool Stars and Space Weather
Stellar flares, winds and coronal mass ejections form the space weather. They
are signatures of the magnetic activity of cool stars and, since activity
varies with age, mass and rotation, the space weather that extra-solar planets
experience can be very different from the one encountered by the solar system
planets. How do stellar activity and magnetism influence the space weather of
exoplanets orbiting main-sequence stars? How do the environments surrounding
exoplanets differ from those around the planets in our own solar system? How
can the detailed knowledge acquired by the solar system community be applied in
exoplanetary systems? How does space weather affect habitability? These were
questions that were addressed in the splinter session "Cool stars and Space
Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In
this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur
On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs
We investigate the detectability of Zeeman broadening in optical Stokes I
spectra of slowly rotating sun-like stars. To this end, we apply the LTE
spectral line inversion package SPINOR to very-high quality CES data and
explore how fit quality depends on the average magnetic field, Bf .
One-component (OC) and two-component (TC) models are adopted. In OC models, the
entire surface is assumed to be magnetic. Under this assumption, we determine
formal 3{\sigma} upper limits on the average magnetic field of 200 G for the
Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of ~
500 G is found for 59 Vir (G0V), and of ~ 1000 G for HD 68456 (F6V). A
distinction between magnetic and non-magnetic regions is made in TC models,
while assuming a homogeneous distribution of both components. In our TC
inversions of 59 Vir, we investigate three cases: both components have equal
temperatures; warm magnetic regions; cool magnetic regions. Our TC model with
equal temperatures does not yield significant improvement over OC inversions
for 59 Vir. The resulting Bf values are consistent for both. Fit quality is
significantly improved, however, by using two components of different
temperatures. The inversions for 59 Vir that assume different temperatures for
the two components yield results consistent with 0 - 450 G at the formal
3{\sigma} confidence level. We thus find a model dependence of our analysis and
demonstrate that the influence of an additional temperature component can
dominate over the Zeeman broadening signature, at least in optical data.
Previous comparable analyses that neglected effects due to multiple temperature
components may be prone to the same ambiguities.Comment: 18 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
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