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Astronomy Colloquium
Pulsating White Dwarfs and Orbital Decay in Binaries
Date: Thursday, October 18th
Time: 3:30 pm - 5:00 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM. Talk Begins at 3:45 PM
Speaker: Meng Sun, UW Astronomy Department
Abstract: Motivated by the discovery of a handful of pulsating, extremely low-mass white dwarfs in compact binaries, a formation model was developed for these systems. Evolutionary models are constructed using the MESA code. Magnetic braking torques are used to shrink the orbit and remove the envelope before the helium core can significantly grow in size. The resulting models for the stellar structure are used to understand the properties of g and p-mode oscillations.

WASP-12b is a hot Jupiter with an orbital period of only 1.1 days, making it one of the shortest-period giant planets known. Recent transit timing observations measure the orbital period to decrease on a 3.2 Myr timescale. These observations imply that a Gyr-old planet is now about to be destroyed by its star over the next few Myr. One mechanism to produce orbital decay is through tidal friction. Calculations are presented for the dynamical tide excitation of internal gravity waves by the tidal force. I show that sufficient tidal friction to explain the observations may be possible if the star is near the end of its main sequence lifetime.

Indirect evidence of orbital decay in binaries comes from the lack of binaries with close orbital separations, as they have already suffered orbital decay and merged. A broad parameter study of orbital decay due to tides is presented for a range of primary and secondary stars as well as orbital separation. The focus is on the red giant branch phase of the primary star. Both the dynamical tide, damped by nonlinear wave breaking or radiative diffusion, and the equilibrium tide, damped by the turbulent viscosity in the convective envelope, are included in the calculations of tidal friction. The calculations of orbital decay are compared to the sample of close APOGEE binaries with red giant branch primaries and substellar companions, as well as the sample of exoplanet host stars.
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