A novel, robust approach to determine the strength of convective blueshift
F. Liebing, S.V. Jeffers, A. Reiners, M. Zechmeister
Institute for Astrophysics, University of Göttingen
With ever increasing instrumental resolutions and stabilities, the limiting element towards detecting low-mass exoplanets using the radial velocity approach has shifted towards the stars themselves. The noise floor is no longer dominated by instrumental effects but by phenomena originating on the stellar surface relating to the complex interaction between magnetic activity and convection. On its own, convection imposes a spectral blueshift due to flux imbalances within convective cells, an effect that gets suppressed and modulated by magnetically active regions rotating in and out of view. To accurately model this modulation, an essential part of future exoplanet detection algorithms, one must first understand the basic pattern of convection and how it changes not just from star to star but also from spectral line to spectral line. To measure the convective blueshift as a function of spectral type we have developed a novel approach, utilizing the universal nature of the third signature of granulation and scaling an empirical solar signature model, created based on an ultra-high resolution spectrum. We have applied this to HARPS observations of more than 400 stars that span the main sequence from early-M to late-F stars. The results show a well-defined relation between effective temperature and strength of convective blueshift and provide a way to calculate the shift for any spectral line in any star. This clears the way towards an accessible method of identifying the convective blueshift component in the search for small Earth-like exoplanets.