[Astronomy seminar] Astronomy seminar TODAY at 10:30 in FC61

[Astronomy seminar announcements]

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).

On 2018-11-19 06:38, Alexis Brandeker wrote:
> 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 panets
>
> 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!
>

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