Speaker:  Erica Nelson

Title:  Where stars form: inside-out growth and coherent star formation across the main sequence from HST Halpha maps at z~1

Abstract: 
Imaging surveys with HST have demonstrated that many galaxies attained their current forms at z~1. Key to understanding this process is a direct measurement of the distribution of star formation within galaxies at this crucial epoch. This is now possible with the WFC3 grism capability on HST, as it provides Hα maps of all galaxies at 0.7< z <1.5 in its field of view. Using Hα maps for ~2000 galaxies, we show where star formation is distributed in galaxies across the star formation - mass plane (the “main sequence”). We find that the disk scale length of Hα is larger than that of the stellar continuum emission, consistent with inside-out assembly of galactic disks. Across the main sequence, we find evidence for ‘coherent star formation’: in galaxies with higher than average star formation rates, Hα is enhanced throughout the disk; similarly, in galaxies with low star formation rates Hα is depressed throughout the disk. I discuss these results in the context of several proposed mechanisms for enhancing and quenching star formation. I also show first results of the spatial distribution of star formation at z~2-3.

Talk Host:  Rahul Kannan

Additional information about Erica Nelson

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Speaker:  Kento Masuda

Title: Obliquity and orbital precession of the Kepler-13A system probed with gravity-darkened transit light curves

Abstract:
The stellar obliquity, the angle between the stellar spin and planetary orbital axes, provides clues to the formation and subsequent orbital evolution of the planet. So far, obliquities have been measured mainly with spectroscopy for more than 80 transiting systems to reveal diverse angular momentum configurations in explanatory systems. Most of these measurements, however, are for giant planets on close-in orbits due to the detectability of the spectroscopic effect. Given the limitation, several new methods to measure stellar obliquities have recently been developed, which make use of the high-precision photometric data as provided by the Kepler space telescope and complement the traditional technique. This talk focuses on one of those alternatives based on the analysis of transit light curves of a rapidly rotating star, whose equator is darker than its pole due to the effect known as gravity darkening. We apply this method to the Kepler-13A system and show how the subtle feature in the light curve caused by the gravity darkening reveals a significant spin-orbit misalignment. We also discuss the secular orbital precession observed in this system, which is induced by the misalignment of the planetary orbit with respect to the stellar equator elongated owing to the rapid rotation. Our analysis predicts that the precession will cause the variations in the transit shape and obliquity detectable from the ground in near future, and thus our solution can be tested or even refined with follow-up observations.

Talk Host: Josh Winn

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