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dc.contributor.advisorCameron, A. C.
dc.contributor.authorHaywood, Raphaëlle D.
dc.description.abstractThe 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-Earth CoRoT-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 ± 1.72⁻³, which is compatible with a rocky composition. I carried out Bayesian model selection to assess the nature of a previously identified signal at 9 days, and found that it is best interpreted as stellar activity. Despite the high levels of activity of its host star, I determined 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)⁻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)⁻³ and 8.1 ± 1.8⁻³, 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.en_US
dc.publisherUniversity of St Andrews
dc.relation“Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system”, R. D. Haywood, A. Collier Cameron, D. Queloz, S.C.C. Barros, M. Deleuil, R. Fares, M. Gillon, A.F. Lanza, C. Lovis, C. Moutou, F. Pepe, D. Pollacco, A. Santerne, D. Segransan and Y. C. Unruh, 2014, Monthly Notices of the Royal Astronomy: 443, 2517en_US
dc.relation“The Sun as a planet-host star: Proxies from SDO images for HARPS radial-velocity variations”, R. D. Haywood, A. Collier Cameron, Y. C. Unruh, C. Lovis, A. F. Lanza, J. F. Llama, M. Deleuil, R. Fares, M. Gillon, C. Moutou, F. Pepe, D. Pollacco, D. Queloz, and D. Segransan, submitted for publication to Monthly Notices of the Royal Astronomyen_US
dc.rightsCreative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
dc.subjectExoplanet detectionen_US
dc.subjectStellar activity of Sun-like starsen_US
dc.subjectSunspots, faculae/plageen_US
dc.subjectExoplanet characterisationen_US
dc.subjectRadial-velocity techniqueen_US
dc.subject.lcshExtrasolar planets--Detectionen_US
dc.subject.lcshStars--Magnetic fieldsen_US
dc.subject.lcshStars--Motion in line of sighten_US
dc.titleHide and seek : radial-velocity searches for planets around active starsen_US
dc.contributor.sponsorScience and Technology Facilities Council (STFC)en_US
dc.type.qualificationnamePhD Doctor of Philosophyen_US
dc.publisher.institutionThe University of St Andrewsen_US

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