Rossiter-McLaughlin effect

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The Rossiter-McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's secondary star or an extrasolar planet is seen to transit across the face of the primary or parent star. As a main star rotates on its axis, one quadrant of the photosphere will be seen to be coming towards the viewer, and the other quadrant seen to be moving away. These produce differing redshifts in the star's spectrum, usually observed as a broadening of the spectral lines. When the secondary star or planet transits the primary, it blocks off part of the solar disc, obscuring some of the red- or blue-shifted light from reaching the observer. This causes the observed mean redshift of the primary star as a whole to vary from its previous value. As the transiting object moves across to the other side of the star's disc, so the redshift anomaly will switch from being negative to being positive, or vice versa.

Illustration showing the effect. The viewer is situated at the bottom. Light from the anticlockwise-rotating star is blue-shifted on the approaching side, and red-shifted on the receding side. As the planet passes in front of the star it variously blocks off blue- or red-shifted light, causing the star's apparent radial velocity to change when it in fact does not.

[edit] Further reading

• Y. Ohta, A. Taruya & Y. Suto; The Rossiter-McLaughlin Effect and Analytic Radial Velocity Curves for Transiting Extrasolar Planetary Systems; ApJ, v. 622, part 1 (2005), pp. 1118–1135