Rossiter-McLaughlin effect

Heya peeps.

Has it ever struck you that a greeting is quite a revealing reflection? Or perhaps not. Maybe it's just me, maybe just some slight inability to suppress introspection and self-evaluation; although it's a gnawing suspicion in my head that almost every Techer is highly suspect to this trait. But I can recall exactly why I say 'sup to some peoples, name the two unrelated, separated-by-thousands-of-miles persons who called Howdy on a regular basis, and explain my tendency to salute some others.

We should find some sort of formalism for this. Does that even make any sense? Eh.

Radial velocity measurements (in km/s) of the transit
 of CoRoT-2 b, around an active G star. The
spin-orbit misalignment angle is +7.2 ± 4.5 degrees.

Anyways, the point. We've all heard of this Rossiter-McLaughlin effect and its usefulness, but I didn't really get it until I realized it allows for some cool sciencey stuff to be done. These are a kind of unique signature, a kind of hello or greeting unique and individualized to the extrasolar planet in question, which grants observers a lot more inferred information than could be hoped for. For instance, the asymmetry in the revealed stellar spectrum due to the spin of the star lets us extract the projected angle between the planetary orbit axis and the stellar spin axis, as well as the stellar spin velocity (which is useful for calculating the predicted precision of an observing run -- the faster the star spins, the more line broadening occurs, and the less precise measurements will be).

Yeah.

In a sentance, the Rossiter-McLaughlin effect is the change in the observed radial velocity/mean redshift of a star due to an eclipsing binary's secondary star or an extrasolar planet during transit.

A star's rotation means that at any time, one quadrant of its photosphere will be seen coming towards the viewer, and one quadrant moving away. These motions produce blueshifts and redshifts, respectively, which we observe only as spectral line broadening. However, during transit, the orbiting object blocks part of the disk, preventing some of the shifted light from reaching the observer and changing the observed mean redshift, resulting in a positive-to-negative anomaly if the orbit is prograde, and vice versa if the orbit is retrograde.

The view is situated at the bottom. The light is blueshifted on the approaching side and redshifted on the receding side. As the planet passes in front of the star it causes the star's apparent radial velocity to change.

This effect has been used to show that as many as 25% of hot Jupiters are orbiting in a retrograde direction with respect to their parents stars, strongly suggesting that dynamical interactions, rather than planetary migration, produce these objects. For cool stuff on misaligned orbits of hot Jupiters, see this.

Actually, I'll overview the link a bit. ESO claimed that "Most hot Jupiters are misaligned...the histogram of projected obliquities matches closely the theoretical distributions of using Kozai cycles and tidal friction...most hot Jupiters are formed by this very mechanism without the need to use type I or II migration." Greg Laughlin, a professor at UCSC, discusses this and comes to the conclusion that Kozai-migration, well understood for HD80606 (and explained very nicely in the post), "plays a larger role is sculpting the planet distribution than previously believed."

These transits are quite amazing bits of work. With the knowledge of the effect and the subsequent radial velocity measurements, we can better understand the fundamental formation scenarios and dynamical processes that bring the companion, including the hot jupiter, into the observed orbital state (semi-major axis/orbit, the inclination, eccentricity).

See also

• Paper on the math behind the Rossiter-McLaughlin effect
• Paradigm upended, an importnat reference for this post