Deriving stellar inclination of slow rotators using stellar activity

Abstract

Stellar inclination is an important parameter for many astrophysical studies. Although different techniques allow us to estimate stellar inclinationt for fast rotators, it becomes much more difficult when stars are rotating slower than $\sim2$-2.5 \kms. By using the new activity simulation SOAP 2.0 that can reproduce the photometric and spectroscopic variations induced by stellar activity, we are able to fit observations of solar-type stars and derive their inclination. For HD189733, we estimate the stellar inclination to be $i=84^{+6}_{-20}$ degrees, which implies a star-planet obliquity of $\psi=4^{+18}_{-4}$ considering previous measurements of the spin-orbit angle. For $\alpha$ Cen B, we derive an inclination of $i=45^{+9}_{-19}$, which implies that the rotational spin of the star is not aligned with the orbital spin of the $\alpha$ Cen binary system. In addition, assuming that $\alpha$ Cen Bb is aligned with its host star, no transit would occur. The inclination of $\alpha$ Cen B can be measured using 40 radial-velocity measurements, which is remarkable given that the projected rotational velocity of the star is smaller than $1.15\,km\,s^{-1}$.