Hide and seek : radial-velocity searches for planets around active stars

Extrasolar planets--Detection;Stars--Magnetic fields;Stars--Motion in line of sight

Haywood, Raphaëlle D. ; Cameron, A. C. ; Cameron, A. C.

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Extrasolar planets--Detection;    Stars--Magnetic fields;    Stars--Motion in line of sight;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/7798/RaphaelleHaywoodPhDThesis.pdf?sequence=6&isAllowed=y

来源: DR-NTU

【 摘 要 】

The detection of low-mass extra-solar planets through radial-velocity searches is currently limited by the intrinsic magnetic activity of the host stars. The correlated noise that arises from their natural radial-velocity variability can easily mimic or conceal the orbital signals of super-Earth and Earth-mass extra-solar planets. I developed an intuitive and robust data analysis framework in which the activity-induced variations are modelled with a Gaussian process that has the frequency structure of the photometric variations of the star, thus allowing me to determine precise and reliable planetary masses.I applied this technique to three recently discovered planetary systems: CoRoT-7,Kepler-78 and Kepler-10. I determined the masses of the transiting super-EarthCoRoT-7b and the small Neptune CoRoT-7c to be 4.73 ± 0.95 M⊕ and 13.56 ±1.08 M⊕, respectively. The density of CoRoT-7b is 6.61 ± 1.72 g.cm⁻³, which iscompatible with a rocky composition. I carried out Bayesian model selection toassess the nature of a previously identified signal at 9 days, and found that it is bestinterpreted as stellar activity. Despite the high levels of activity of its host star, Idetermined the mass of the Earth-sized planet Kepler-78b to be 1.76 ± 0.18 M⊕.With a density of 6.2(+1.8:-1.4) g.cm⁻³, it is also a rocky planet. I found the masses of Kepler-10b and Kepler-10c to be 3.31 ± 0.32 M⊕ and 16.25 ± 3.66 M⊕, respectively. Their densities, of 6.4(+1.1:-0.7) g.cm⁻³ and 8.1 ± 1.8 g.cm⁻³, imply that they are both of rocky composition – even the 2 Earth-radius planet Kepler-10c!In parallel, I deepened our understanding of the physical origin of stellar radial-velocity variability through the study of the Sun, which is the only star whose surface can be imaged at high resolution. I found that the full-disc magnetic flux is an excellent proxy for activity-induced radial-velocity variations; this result may become key to breaking the activity barrier in coming years.I also found that in the case of CoRoT-7, the suppression of convective blueshift leads to radial-velocity variations with an rms of 1.82 m.s⁻¹, while the modulation induced by the presence of dark spots on the rotating stellar disc has an rms of 0.46 m.s⁻¹. For the Sun, I found these contributions to be 2.22 m.s⁻¹ and 0.14 m.s⁻¹, respectively. These results suggest that for slowly rotating stars, the suppression of convective blueshift is the dominant contributor to the activity-modulated radial-velocity signal, rather than the rotational Doppler shift of the flux blocked by starspots.