The ‘Reloaded RM’: spatially-resolving stellar spectrums, determining 3D spin-orbit geometries, and characterising planetary atmospheres

Wednesday 4 July, 09:50

Stellar surface phenomena (spots, faculae, granulation etc.) alter the observed stellar spectra and can inject spurious signals into a variety of planet detection and characterisation methods. For these reasons, we ‘Reloaded’ the Rossiter-McLaughlin effect using transiting planets to directly probe stellar surfaces. For ground-based data, we must normalise the spectra, first relative to the continuum and second relative to a known reference light curve; this allows us to make a direct subtraction of the in- from out-of-transit data. Hence, we isolate the starlight behind the planet without making any assumptions on the local absorption line profiles, or underlying stellar radial velocities (RVs).  Thus, we spatially resolve the stellar spectrum along the transit chord, and can search for signs of stellar activity in the local profile shape and/or net RVs. Moreover, by measuring the local RV, and allowing for stellar surface differential rotation, we can determine the planets’ 3D trajectory and gain insight into its dynamical history. We have successfully shown this for HD 189733, as well as for Wasp-8, where we found previous results may have been biased. We have also shown this is an effective tool even for the coolest and slowest rotating stars, by determining the first obliquity for a (Neptune-mass) planet around a M dwarf. Moreover, for echelle spectrographs, we will show that with one reference light curve we can apply the Reloaded RM to specific passbands and utilise the colour-independent stellar rotation to determine wavelength dependent changes in the planet radius and thereby characterise its atmosphere.

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