Stars, Planets, and ISM Seminar

Nathanael Burns-Watson, The University of Texas at Austin

Determining Host Stars for Planets in Binary Star Systems Using Asterodensity Profiling

Abstract:
A defining characteristic of exoplanet populations is the lack of planets between 1.8-2.0 Earth radii known as the radius gap. The radius gap is thought to distinguish between terrestrial planets with little to no atmosphere and gaseous planets with substantial atmospheres. The radius gap is now thought to be absent among planets hosted in binary star systems. However, this assumes that all of the planets were orbiting the brighter primary star. In many cases, the radius of the planet would be significantly larger if it were orbiting the companion star. We are determining which star these planets are orbiting and revising their radius values in turn. We used data from the Kepler mission and transit refitting to compare transit and spectroscopic densities to determine the host stars and new planetary radii. We selected a sample of planets that would fall in the radius gap if they were orbiting the primary star, but would fall above the radius gap as sub-Neptunes if they were orbiting the secondary star. Some of these planets are in multiplanet systems, providing the opportunity to determine whether these planets are all orbiting the same star or some combination of both stars. We find evidence that more of the planets in our sample are orbiting the secondary star than initially expected. We also find that at least some of the planets in 2-planet systems seem to be orbiting different stars. Our results suggest that there are fewer planets in the canonical radius gap, though its depth is not yet clear. The results also suggest that a non-trivial fraction of planets in binaries are hosted by the secondary star.

Carlos Jurado, The University of Texas at Austin

The Effects of Natal Kicks in the Galactic Center

Abstract:
Most galaxies, including the Milky Way, harbor a central supermassive black hole (SMBH) weighing millions to billions of solar masses. Surrounding these SMBHs are dense regions of stars and stellar remnants, such as neutron stars (NSs) and black holes (BHs). NSs and possibly BHs receive large natal kicks at birth on the order of hundreds of kilometers per second. The natal kicks that occur in the vicinity of an SMBH may redistribute the orbital configuration of the compact objects and alter their underlying density distribution. We model the effects of natal kicks on a Galactic center (GC) population of massive stars and stellar binaries with different initial density distributions. Using observational constraints from stellar orbits near the GC, we place an upper limit on the steepness of the initial stellar profile and find it to be core-like. In addition, we predict that 30%–70% of compact objects become unbound from the SMBH due to their kicks and will migrate throughout the Galaxy.