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<div class="" style="margin:0px; line-height:normal"><span class=""
style="">Speaker: Heather Cegla (Geneva Observatory)<br>
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When/where: Friday November 23 at 10:30 in FC61 (Astronomy
corridor, 6th floor)<br>
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Title: Probing the surfaces of Sun-like stars using and 3D
magnetohydrodynamical simulations and </span>transiting planets</div>
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Abstract:<br class="">
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<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>
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All welcome!<br>
<br>
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