149 research outputs found
The contribution of starspots to coronal structure
Significant progress has been made recently in our understanding of the
structure of stellar magnetic fields, thanks to advances in detection methods
such as Zeeman-Doppler Imaging. The extrapolation of this surface magnetic
field into the corona has provided 3D models of the coronal magnetic field and
plasma. This method is sensitive mainly to the magnetic field in the bright
regions of the stellar surface. The dark (spotted) regions are censored because
the Zeeman signature there is suppressed. By modelling the magnetic field that
might have been contained in these spots, we have studied the effect that this
loss of information might have on our understanding of the coronal structure.
As examples, we have chosen two stars (V374 peg and AB Dor) that have very
different magnetograms and patterns of spot coverage. We find that the effect
of the spot field depends not only on the relative amount of flux in the spots,
but also its distribution across the stellar surface. For a star such as AB Dor
with a high spot coverage and a large polar spot, at its greatest effect the
spot field may almost double the fraction of the flux that is open (hence
decreasing the spindown time) while at the same time increasing the X-ray
emission measure by two orders of magnitude and significantly affecting the
X-ray rotational modulation.Comment: 10 pages, 10 figure
A small survey of the magnetic fields of planet-host stars
Using spectropolarimetry, we investigate the large-scale magnetic topologies
of stars hosting close-in exoplanets. A small survey of ten stars has been done
with the twin instruments TBL/NARVAL and CFHT/ESPaDOnS between 2006 and 2011.
Each target consists of circular-polarization observations covering 7 to 22
days. For each of the 7 targets in which a magnetic field was detected, we
reconstructed the magnetic field topology using Zeeman-Doppler imaging.
Otherwise, a detection limit has been estimated. Three new epochs of
observations of Tau Boo are presented, which confirm magnetic polarity
reversal. We estimate that the cycle period is 2 years, but recall that a
shorter period of 240 days can not still be ruled out. The result of our survey
is compared to the global picture of stellar magnetic field properties in the
mass-rotation diagram. The comparison shows that these giant planet-host stars
tend to have similar magnetic field topologies to stars without detected
hot-Jupiters. This needs to be confirmed with a larger sample of stars.Comment: Accepted for publication in Monthly Notices of The Royal Astronomical
Societ
The multipolar magnetic fields of accreting pre-main-sequence stars:B at the inner disk, B along the accretion flow, and B at the accretion shock
Zeeman-Doppler imaging studies have revealed the complexity of the large-scale magnetic fields of accreting pre-main-sequence stars. All have multipolar magnetic fields with the octupole component being the dominant field mode for many of the stars studied thusfar. Young accreting stars with fully convective interiors often feature simple axisymmetric magnetic fields with dipole components of order a kilo-Gauss (at least those of mass ), while those with substantially radiative interiors host more complex non-axisymmetric magnetic fields with dipole components of order a few 0.1 kilo-Gauss. Here, via several simple examples, we demonstrate that i). in most cases, the dipole component alone can be used to estimate the disk truncation radius (but little else); ii) due the presence of higher order magnetic field components, the field strength in the accretion spots is far in excess of that expected if a pure dipole magnetic field is assumed. (Fields of 6 have been measured in accretion spots.); iii) if such high field strengths are taken to be representative of the polar strength of a dipole magnetic field, the disk truncation radius would be overestimated. The effects of multipolar magnetic fields must be considered in both models of accretion flow and of accretion shocks...
Telluric-line subtraction in high-accuracy velocimetry: a PCA-based approach
Optical velocimetry has led to the detection of more than 500 planets to date
and there is a strong effort to push m/s velocimetry to the near-infrared to
access cooler and lighter stars. The presence of numerous telluric absorption
lines in the nIR brings an important challenge. As the star's barycentric
velocity varies through the year, the telluric absorption lines effectively
varies in velocity relative to the star's spectrum by the same amount leading
to important systematic RV offsets. We present a novel principal component
analysis-based approach for telluric line subtraction and demonstrated its
effectiveness with archival HARPS data for GJ436 and {\tau} Ceti, over parts of
the R-band that contain strong telluric absorption lines. The main results are:
1) a better RV accuracy with excluding only a few percentage of the domain, 2)
better use of the entire spectrum to measure RV and 3) a higher telescope time
efficency by using A0V telluric standard from telescope archive.Comment: Presented at SPIE Astronomical Telescopes + Instrumentation 201
A search for starlight reflected from HD 75289b
We have used a Doppler tomographic analysis to conduct a deep search for the starlight reflected from the planetary companion to HD 75289. In four nights on VLT(UT2)/UVES in 2003 January, we obtained 684 high-resolution echelle spectra with a total integration time of 26 h. We establish an upper limit on the geometric albedo of the planet p < 0.12 (to the 99.9 per cent significance level) at the most probable orbital inclination i≃ 60°, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp= 1.6 RJup. We are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud deck
Coronal Structure of Low-Mass Stars
We investigate the change in stellar magnetic topology across the
fully-convective boundary and its effects on coronal properties. We consider
both the magnitude of the open flux that influences angular momentum loss in
the stellar wind and X-ray emission measure. We use reconstructed maps of the
radial magnetic field at the stellar surface and the potential-field source
surface method to extrapolate a 3D coronal magnetic field for a sample of
early-to-mid M dwarfs. During the magnetic reconstruction process it is
possible to force a solution towards field geometries that are symmetric or
antisymmetric about the equator but we demonstrate that this has only a modest
impact on the coronal tracers mentioned above. We find that the dipole
component of the field, which governs the large-scale structure, becomes
increasingly strong as the stellar mass decreases, while the magnitude of the
open (wind-bearing) magnetic flux is proportional to the magnitude of the
reconstructed magnetic flux. By assuming a hydrostatic and isothermal corona we
calculate X-ray emission measures (in magnitude and rotational modulation) for
each star and, using observed stellar densities as a constraint, we reproduce
the observed X-ray saturation at Ro < 0.1. We find that X-ray rotational
modulation is not a good indicator of magnetic structure as it shows no trend
with Rossby number but can be useful in discriminating between different
assumptions on the field geometry.Comment: 12 pages, 10 figures, Published in MNRA
Portraying the hosts: Stellar science from planet searches
Information on the full session can be found on this website: https://sites.google.com/site/portrayingthehostscs18/We present a compendium of the splinter session on stellar science from planet searches that was organized as part of the Cool Stars 18 conference. Seven speakers discussed techniques to infer stellar information from radial velocity, transit and microlensing data, as well as new instrumentation and missions designed for planet searches that will provide useful for the study of the cool stars
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