An Exploration into the Radial-Velocity Variability of the Sun

Abstract

The presence of starspots , faculae and granulation on the photosphere of a star induces quasi-periodic signals that can conceal and even mimic the Doppler signature of orbiting planets. This has resulted in several false detections (see Queloz et al. 2001; Bonfils et al. 2007; Huélamo et al. 2008; Boisse et al. 2009, 2011; Gregory 2011; Haywood et al. 2014; Santos et al. 2014; Robertson et al. 2014 and many others). Understanding the RV signatures of stellar activity, in particular those modulated by the stellar rotation, is essential to develop the next generation of more sophisticated activity models and further improve our ability to detect and characterise low-mass planets. The Sun is the only star whose surface can be directly resolved at high resolution, and therefore constitutes an excellent test case to explore the physical origin of stellar radial-velocity variability. In this chapter, I present HARPS observations of sunlight scattered off the bright asteroid 4/Vesta, from which I deduced the Sun’s activity-driven RV variations. In parallel, the HMI instrument onboard the Solar Dynamics Observatory provided me with simultaneous high spatial resolution magnetograms, dopplergrams, and continuum images of the Sun. I determined the RV modulation arising from the suppression of granular blueshift by magnetically active regions (sunspots and faculae) and the flux imbalance induced by dark spots. I confirm that the inhibition of convection is the dominant source of activity-induced RV variations at play. Finally, I find that the activity-driven RV variations of the Sun are strongly correlated with its full-disc magnetic flux, which could become a useful proxy for activity-related RV noise in future exoplanet searches.

This chapter uses material from, and is based on, Haywood et al., 2016, MNRAS, 457, 3637.