A differential least-squares deconvolution method for high precision spectroscopy of stars and exoplanets - I. Application to obliquity measurements of HARPS observations of HD189733b

High precision measurements of stellar spectroscopic line profiles and their changes over time contain very valuable information about the physics of the stellar photosphere (stellar activity) and can be used to characterize extrasolar planets via the Rossiter-McLaughlin effect or from reflected light from the planet. In this paper, we present a new method for measuring small changes in the mean line profile of a spectrum by performing what we call differential least-squares deconvolution (dLSD). The method consists in finding the convolution function (or kernel) required to transform a high-signal-to-noise ratio template of the star into each observed spectrum. Compared to similar techniques, the method presented here does not require any assumptions on the template spectrum (e.g. no line-list or cross-correlation mask required). We show that our implementation of dLSD is able to perform at least as good as other techniques by applying it to star-planet obliquity measurements of exoplanet HD183799 during its transit. 1 Among other things, the method should enable model-independent detection of light reflected by an exoplanet.