A Pathway to Earth-like Worlds: Overcoming Astrophysical Noise due to Convection

Heather M. Cegla
Astrophysics Research Center, Queens University Belfast

One of the consequences of the plasma motions within the convective envelopes of low-mass main-sequence stars (i.e. potential planet hosting stars) are the radial velocity (RV) shifts due to variable stellar line profile asymmetries, known as astrophysical noise (or stellar jitter). This can pose a major problem for planet hunters because RV follow-up is mandatory for most planet confirmation and characterisation. Furthermore, as the net RV shifts produced from these photospheric convective motions are around the (sub) m/s level this is especially troublesome for confirmation of Earth-analogs that induce Doppler-wobbles on the cm/s level. The currently implemented noise removal technique for granulation rests on adapting observational strategies to average out such noise. However, this technique is extremely observationally intensive and does not provide information on the nature of jitter. My aim is to go beyond these previous techniques by understanding the physical processes involved in granulation and removing the actual RV signature from granulation. I outline a technique to characterise photospheric granulation as an astrophysical noise source. The backbone of this characterisation is a state-of-the-art 3D magnetohydrodynamic solar simulation, coupled with detailed wavelength-dependent radiative transfer. Due to the time-intensive nature of these simulations, I use a short time-series to parameterise the granulation signal. This parameterisation is then used to construct Sun-as-a-star observations to determine how the convective plasma motions on the stellar surface alter the disc-integrated profiles. The initial results of this study are presented here, including the identification of several correlations and significant noise reduction. In particular, it is shown that the velocity asymmetry (a measurement comparing the spectral information content of the blue wing to the red wing) and brightness measurements (as approximated by integrating the area under the model observation profiles) are the best-suited diagnostics for reducing granulation noise to a level sufficient for the confirmation of habitable, terrestrial-mass planets.

19 março 2014, 13:30

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