Welcome to Seattle
Quite by accident, I arrived at my hotel in Seattle with a bus full of astronomers–mostly other graduate students, two undergrads, and a tenth person of unknown origin. My first impression of Seattle, then, was one of poster tubes and like-minded people. This wasn’t a coincidence: I’m here for the winter meeting of the American Astronomical Society (AAS), which started last night with a kick-off event. This is the first conference I’ve been able to attend, and I’m quite excited to be here.
The AAS hosts two large meetings each year where professional astronomers, students and those in related disciplines meet for five days of presentations, posters, and networking. The main hall is filled with posters, far more than I could ever look at (and they change each day) and talks are usually 5 minutes + questions, and a half dozen (at least) sessions run concurrently, all of which makes it impossible to see a tenth of what’s being presented.
In my posts on AAS, I will report on some of the talks I’ve attended. Most of them will be on exoplanets, since this is the field I am beginning research in.
The first session I attended this morning was Kepler 1, the one of several sessions devoted to results from NASA’s Kepler mission. Two talks especially caught my attention. William F. Welsh from San Diego State University talked about KOI-54 (KOI stands for Kepler Object of Interest). This object is actually a binary star system. The two stars are on highly eccentric orbits, which bring them quite close together: the distance at periastron (or closest approach) is just six stellar radii. This results in extreme distortion and irradiation of the stars. Interestingly, the stellar pulsations are harmonics (integer multiples) of the orbital frequency. The 90th and 91st harmonics are strongest, but many others are seen as well.
Althea Moorhead (U. Florida) presented her work on determining the orbital eccentricities (deviation from a circular orbit) of Kepler candidate planets. It’s difficult to measure the eccentricities of each planet individually from transits, so the authors have taken a different approach. They assume that there is some distribution of eccentricities underlying their planet sample, then simulate the resulting distribution of transit properties. Specifically, they look at transit durations (TD), which are sensitive to eccentricity. For example, they could assume that all planets are on perfectly circular obits, and would get one resulting distribution of TDs. Then they could assume a distribution of eccentricities that obeys some function, and would get another distribution of TDs. The authors simulate the TD distribution for several different eccentricity distributions and compare their models to the data; the eccentricity distribution resulting in the best match to the data is most likely to be representative of the true underlying distribution. One thing they find is that data show an excess of long-duration transits relative to any of their simulations.
Super-Earths and terrestrial planets: latest results from the Kepler Mission
First, an overview of the Kepler Mission, as discussed by William J. Borucki (NASA Ames) at this session. Kepler is a NASA discovery mission dedicated to the discovery of habitable, Earth-like planets. The instrument is a photometer with a wide field of view, capable of monitoring 170 000 stars at 30 minute cadence and up to 512 stars at 1 minute cadence. Long cadence data is the main way Kepler will find new transiting exoplanets. The short cadence data is primarily for astroseismology and transit timing variations (stay tuned for further posts on these methods–I’m sure an interesting paper will be forthcoming).
One of the most exciting results to come out of Kepler is the discovery of the smallest known exoplanet, Kepler 10b, which was announced this morning. Dimitar Sasselov (Harvard U.) discussed the composition of this planet, as well as the general difficulties of determining planetary compositions. When you observed a transiting planet, you typically can determine its radius and mass: two parameters. But, a high density planet–like Kepler 10b–could be composed of varying amounts of rock, iron and water: three parameters. If you know the planet is just rock and iron, you can determine how much of each material is present: two unknowns, two constraints. With the possibility of water, there is a degeneracy between the unknowns which is unlikely to be resolved without more information. For Kepler 10b, however, we are met with a little bit of good luck. All the models that fit the data have a quite low water content, and the team is able to determine its composition to be ~75% iron.