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

Melodie, a grad student

Hot Jupiter

On Thursday, Melodie Kao and I had lunch together. We met and chatted about her undergrad at MIT and her research, here at LIGO and in Chile.

Melodie is a new first year grad student in Professor Johnson's lab! She's newly from MIT, that 'other school' as Techers see it. She's writing a proposal for an NSF grant for work on the Cold Friends of Hot Jupiters project, which I was lucky enough to work a little on earlier this summer. 

At age nine, she had already decided that she was going to be an architect. (This is so cool.) She got into many schools with amazing architecture programs, but luckily chose MIT over Cornell or CMU. Luckily, as she discovered that she missed the math and the science involved in...science, so she changed to aerospace engineering. Engineering was not her favorite; she mentioned that she hated coming up with an answer, testing it, implementing it, discovering that it didn't work, fixing it, and then redoing it all over again, over and over. As an astronomer, she didn't have to deal with that, not with the people who work at telescopes and understand them so much. 

Side note: this is so true! When I sat in on the observing nights, the lady and other people in charge knew every in and out of every problem and could resolve it. I had the most utterly unshakable faith in their abilities. (end of digression)

I love architecture. [1] [2] [3] [4]

Amazingly, she switched into Physics with a concentration in architecture, and was able to finish her requirements early. She did prior research (including at LIGO, trying to reduce the noise involved in the measurements by decreasing the thermoelastic deformation of the mirrors), but did something amazing her senior year. For the southern hemisphere's summer, she worked in Chile somewhere for 5-6 months, resolving a large problem in determining the mass of galaxy clusters. (This part is beyond cool.) Because the normal method to determine the mass of a cluster of galaxies is highly dependent on the  assumption that the system in is dynamic equilibrium (enough time has passed since any galaxies/objects disturbed the cluster) it didn't work for anything but old clusters. At first, she had to find which galaxies were part of the cluster, instead of simply being in front of or passing by her cluster (which I forgot the name of, unfortunately).  

My personal favorite, M13, a globular cluster in
Hercules. It was one of the first things I saw through
a telescope, in the middle of  Joshua Tree. 

(This part is possibly uncool, i.e., wrong.) It might have something to do with the virial equation and timescales to equilibrium, which this post implies is going to be taught! :D Cool. 

But by mapping the parameter space of individual galaxy velocity relative to the cluster and the displacement between the galaxy and the center of mass? a spacial center of the cluster? (a radius, I think) she could get a very good estimate of the shape of the cluster's age, and better calculate the mass.

When I say the shape, she was able to take account of differences in age of the galaxies and each massed according to a formula-function of time, and thus mass the the entire cluster much better!

I'm happy she'll be working with Professor Knutson!

See also

• her abstract for ligo: Numerical Thermal Noise Modeling
• her presentation at the 2010 Summer Student Symposium on the Star Formation History of the Dwarf Galaxy Holmberg IX
• My blog for this summer: Searching for Cold Friends of Hot Jupiters