3D magneto-hydrodynamical simulations of stellar convective noise for improved exoplanet detection

S Sulis,D Mary,L Bigot

doi:10.1051/0004-6361/201937105

Abstract

Context. Convective motions at the stellar surface generate a stochastic colored noise source in the radial velocity (RV) data. This noise impedes the detection of small exoplanets. Moreover, the unknown statistics (amplitude, distribution) related to this noise make it difficult to estimate the false alarm probability (FAP) for exoplanet detection tests. Aims. In this paper, we investigate the possibility of using 3D magneto-hydrodynamical (MHD) simulations of stellar convection to design detection methods that can provide both a reliable estimate of the FAP and a high detection power. Methods. We tested the realism of 3D simulations in producing solar RV by comparing them with the observed disk integrated velocities taken by the GOLF instrument on board the SOHO spacecraft. We presented a new detection method based on periodograms standardized by these simulated time series, applying several detection tests to these standarized periodograms. Results. The power spectral density of the 3D synthetic convective noise is consistent with solar RV observations for short periods. For regularly sampled observations, the analytic expressions of FAP derived for several statistical tests applied to the periodogram standardized by 3D simulation noise are accurate. The adaptive tests considered in this work (Higher-Criticism, Berk-Jones), which are new in the exoplanet field, may offer better detection performance than classical tests (based on the highest periodogram value) in the case of multi-planetary systems and planets with eccentric orbits. Conclusions. 3D MHD simulations are now mature enough to produce reliable synthetic time series of the convective noise affecting RV data. These series can be used to access to the statistics of this noise and derive accurate FAP of tests that are a critical element in the detection of exoplanets down to the cm s−1 level.

Highlights

At the time of this writing, 880 extrasolar planets have been discovered so far by the radial velocity (RV) technique1
We tested the realism of 3D simulations in producing solar RV by comparing them with the observed disk integrated velocities taken by the Global Oscillation at Low Frequencies (GOLF) instrument on board the SOHO spacecraft
3D MHD simulations are mature enough to produce reliable synthetic time series of the convective noise affecting RV data. These series can be used to access to the statistics of this noise and derive accurate false alarm probability (FAP) of tests that are a critical element in the detection of exoplanets down to the cm s−1 level

Summary

Introduction

At the time of this writing, 880 extrasolar planets have been discovered so far by the radial velocity (RV) technique1 In this sample, 52% of these consist of planets that are more massive than Jupiter and 14% that have a mass inferior to 10 Earthmasses (M⊕). Since the detection of HD215152c (Mayor et al 2011), only 19 planets have been found with a mass ≤2 M⊕ and all of the latter are short-period (≤50 days) planets orbiting stars that are less massive than the Sun. detecting planets is easier around low-mass stars (as the ratio of the planet to stellar masses is higher) and, a first strategy consists in monitoring cool M dwarfs to increase the detection probability. New instruments such as ESPRESSO (Pepe et al 2010) and EXPRES (Jurgenson et al 2016) have been developed to ensure the longterm stability that is needed to detect signals of terrestrial planets orbiting main sequence G-dwarf stars (with an amplitude around 10–30 cm s−1)

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Discussion

Conclusion