[Astronomy seminar] Astronomy seminar Friday 23/11 at 10:30 in FC61

[Astronomy seminar announcements]

Speaker: Heather Cegla (Geneva Observatory)

When/where:  Friday November 23 at 10:30 in FC61 (Astronomy corridor, 
6th floor)

Title: Probing the surfaces of Sun-like stars using and 3D 
magnetohydrodynamical simulations and transiting planets

Abstract:
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. 
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. 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).

All welcome!

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.su.se/pipermail/seminars-announce.astro-at-su.se/attachments/20181119/36d25b2a/attachment.html>