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<p>In addition to the astronomy seminar today at 10:30, there will
also be an astrobiology talk at 15:30 in FC61 (same location):
"Limits on the lifespan of habitability of M-star planets", by
Raymond T. Pierrehumbert (Oxford).<br>
</p>
On 2018-11-19 06:38, Alexis Brandeker wrote:<br>
<blockquote type="cite"
cite="mid:4a1d4060-0019-1851-b401-34ed6088b59b@astro.su.se">
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<div class="" style="margin:0px; line-height:normal"><span
class="" style="">Speaker: Heather Cegla (Geneva Observatory)<br>
<br>
When/where: Friday November 23 at 10:30 in FC61 (Astronomy
corridor, 6th floor)<br>
<br>
Title: Probing the surfaces of Sun-like stars using and 3D
magnetohydrodynamical simulations and </span>transiting panets</div>
<div class="" style="margin:0px; line-height:normal;
min-height:14px"><br>
Abstract:<br class="">
</div>
<div class="" style="margin:0px; text-align:justify;
line-height:normal"><span class="" style="">Inhomogeneities on
stellar surfaces pose the fundamental stumbling block on the
pathway to true Earth analogues. This is especially pertinent
as we enter the era of 10 cm/s radial velocity (RV) precision,
with spectrographs like ESPRESSO continuing to come online.
From a spectroscopic point of view, manifestations of stellar
activity (such as star-spots, plage/faculae, convective flows,
and oscillations) alter the observed stellar line profiles. In
turn, these time-variable line asymmetries can be mistakenly
interpreted as whole-sale Doppler shifts that mask or mimic
planetary signals. Here, I will focus on the impact of solar
surface oscillations and magnetoconvection, as these ‘noise’
sources are present on even the (magnetically) quietest
exoplanet host stars. I will demonstrate that we can bin down
the pressure-mode oscillations to ~10 cm/s with an exposure
time of just 5.4 minutes. Moreover, I will show how
exposure times slightly larger than this can actually increase
the noise level, and how even doubling the exposure time has
little impact. In addition, I will show how magnetoconvection
does not average out well over such timescales, and how its
centre-to-limb dependence can impact exoplanet measurements.
Using 3D solar MHD simulations as a backbone, I will explore
both the oscillation and convective induced line shape
changes, and demonstrate how these changes can be used
to track the remaining convective noise. Hence, in the era of
10 cm/s RV precision, I will show that we should we be
fine-tuning exposure times to our host star parameters, as
well as exploiting the line profile characteristics to
mitigate the astrophysical noise emanating from stellar
convective envelopes. </span><span class="" style="">Alongside
this, I will show how we can use transiting planets to probe
and spatially resolve stellar surfaces, which in turn helps us
to validate MHD simulations and determine 3D star-planet
trajectories — that ultimately feed into planet formation,
migration and evolution theories. </span>We have successfully
applied this new technique to HD 189733, as well as for Wasp-8,
where we found previous results may have been biased. We have
also shown this is an effective tool even for the coolest and
slowest rotating stars, by determining the first obliquity for a
(Neptune-mass) planet around a M dwarf (GJ 436).<br>
<br>
All welcome!<br>
<br>
</div>
</blockquote>
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