Transit spectroscopy is one of the most commonly used techniques for exoplanet atmosphere characterisation. The centre-to-limb variation (CLV) of the stellar lines across the stellar disk is an important effect for transmission spectroscopy, since it results in a change of stellar line depth when the planet transits different parts of the stellar disk.

The CLV effect describes the stellar line profile change across the stellar disk. For example, the following figure shows the solar Na line profile at solar disk centre and limb:

During exoplanet transit, the CLV effect causes a change of the observed stellar depth:

We analyzed the HARPS data of HD 189733b’s transit and the CLV effect of Na D lines are clearly detected (see the figure below). The CLV modifies the planetary sodium absorption signal. This demonstrates the importance of the CLV effect on transmission spectroscopy. We further simulated the CLV with different stellar and planetary parameters to evaluate its effect on various exoplanet systems.

Publication: Yan et al. 2017, A&A, 603, A73.

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