Bulletin of the American Physical Society
83rd Annual Meeting of the APS Southeastern Section
Volume 61, Number 19
Thursday–Saturday, November 10–12, 2016; Charlottesville, Virginia
Session G1: Astrophysics II |
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Chair: Phil Arras, University of Virginia Room: West Ballroom |
Friday, November 11, 2016 10:45AM - 10:57AM |
G1.00001: Polarized radiation from disks Haifeng Yang, Zhi-Yun Li Magnetic field plays an important role in the dynamics and evolution of protoplanetary disks. Submm/mm wavelength radiation has been employed to detect magnetic field in disks. However, polarimetry observations from such disks on small scales show unphysical magnetic field structure together with some other problems. We will discuss scattering by dust grains as an alternative mechanism in producing submm/mm wavelength polarized radiation and related physics like grain growth. [Preview Abstract] |
Friday, November 11, 2016 10:57AM - 11:09AM |
G1.00002: Direct comparison of hydrodynamic and kinetic models for thermal escape problems Alexey Volkov Thermal escape is the process of mass loss of a planetary atmosphere by gas molecules who leave the atmosphere with translational energies exceeding the gravitational binding energy. Application of hydrodynamic models to thermal escape problem is not straightforward, because they require boundary conditions in the exosphere, where flow is non-equilibrium. The goal of the present work is to study the properties of the hydrodynamic model of atmospheric escape and to establish the domain of its validity by means of systematic comparison of results of hydrodynamic and kinetic simulations. The consideration is limited by Parker's model, which implies the search of solutions of one-dimensional hydrodynamic equations with a critical point and vanishing temperature far from the source. Thermal escape described by Parker's model is found to occur in the low-density regime, when escape is dominated by heat conduction, and the high-density (HD) regime, when escape is dominated by adiabatic expansion. The comparison of results of hydrodynamic and kinetic simulations shows that Parker's model is capable of describing escape only in the HD regime. Based on this finding, a criterion of validity of Parker's model is developed in terms of the source Knudsen number and Jeans escape parameter. [Preview Abstract] |
Friday, November 11, 2016 11:09AM - 11:21AM |
G1.00003: A Model of the H$\alpha$ Transmission Spectrum of HD 189733b Chenliang Huang, Phil Arras, Duncan Christie, Zhi-Yun Li The hot gas in the upper thermosphere of hot Jupiter sets the boundary condition for understanding the rate of gas escape. Among current detections, H$\alpha$ transmission spectrum may play an important role in understanding the conditions in the planet's thermosphere. I present a detailed atmosphere model and comparison of H$\alpha$ model transmission spectra to the data, with the goal of constraining the temperature and particle densities in the region where the absorption line is formed. A hydrostatic atmosphere is constructed. Ionization equilibrium and balance of heating and cooling processes are enforced at each level of the atmosphere. The Ly$\alpha$ radiation intensity is computed using a Monte-Carlo code which includes resonant scattering. Both the incident stellar Ly$\alpha$ and internal sources due to recombination cascade and collisional excitation are included. The atomic hydrogen level population is computed including both collisional and radiative transition rates. The model transmission spectra are in broad agreement with the HD 189733b observation data by Jensen et al and Cauley et al. The base layer of thermosphere is optically thick to H$\alpha$, and temperature is in the range 3000 $\sim$ 6000 K. [Preview Abstract] |
Friday, November 11, 2016 11:21AM - 11:33AM |
G1.00004: Combing the Brown Dwarf Desert with APOGEE Nicholas Troup While numerous stellar and planetary-mass companions have been found, there has been a paucity of brown dwarf (BD) companions orbiting Sun-like stars, a phenomenon known as the BD desert. However, more recent work has shown that this desert might be limited in extent, only existing for small separation ($a<3$ AU) companions, and may not be as dry as initially thought. The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has compiled a catalog of 382 of its most compelling stellar and substellar companion candidates. Among these, 112 have a derived companion mass in the BD regime ($13-80 M_{Jup}$), which is a significant increase compared to the number of known small separation ($a<1$ AU) BD companions. Our sample appears to manifest the BD desert, but only for $a<0.2$ AU rather than the previously held 3. This is explained by one of the unique qualities of our sample when compared to previous companions surveys: Two-thirds of the BD candidates in our sample are orbiting evolved stars, most of which were F dwarfs during their main sequence lifetime, consistent with the notion of an F dwarf oasis. Using this sample, we further test this hypothesis by constraining the formation mechanisms of BD companions, and exploring their orbital evolution as their host evolves. [Preview Abstract] |
Friday, November 11, 2016 11:33AM - 11:45AM |
G1.00005: Tidal Interaction among Red Giants Close Binary Systems in APOGEE Database Meng Sun, Phil Arras, Nick Troup, Steve Majewski, Nevin Weinberg Motivated by the newly discovered close binary systems in the Apache Point Observatory Galactic Evolution Experiment (APOGEE-1), the tidal evolution of binaries containing a red giant branch (RGB) star with a stellar or substellar companion was investigated. The tide raised by the companion in the RGB star leads to exchange of angular momentum between the orbit and the stellar spin, causing the orbit to contract. The tidal dissipation rate is computed using turbulent viscosity acting on the equilibrium tidal flow, where careful attention is paid to the effects of reduced viscosity for close-in companions. Evolutionary models for the RGB stars, from the zero-age main sequence to the present, were acquired from the MESA code. "Standard" turbulent viscosity gives rise to such a large orbital decay that many observed systems have decay times much shorter than the RGB evolution time. Several theories for "reduced" turbulent viscosity are investigated, and reduce the number of systems with uncomfortably short decay times. [Preview Abstract] |
Friday, November 11, 2016 11:45AM - 11:57AM |
G1.00006: Long-term radio observations of varying orbital parameters in six eclipsing pulsar binaries. Brian Prager, Scott Ransom, Jason Hessels, Ingrid Stairs, Paulo Freire, Ryan Lynch, Phil Arras We present our results from the long-term radio observations of six eclipsing pulsars with low-mass companion stars. Studies of eclipsing pulsars have been rife with difficulties in explaining why some sources experience large stochastic changes in their orbital properties over the timescale of a few years. A possible explanation was put forth in the early 1990s by Applegate, proposing that gravitational coupling between the orbit and the shape of the magnetically active star could drive this effect. We investigate the applicability of the Applegate model to our sources by performing a power spectrum analysis of the varying orbital properties in order to detect time-variation in the gravitational force exerted by the companion star. Using 15 years of pulsar timing data, we discuss the energetics of the required mass rearrangements in the context of the Applegate model for each source. We also calculate a lower-limit on the Rossby numbers assuming tidal-locking of the orbit for the low-mass stars that make up our sample. Finally, we discuss the importance of these results in the context of the greater body of works focused on understanding the internal properties of low-mass stars, for which our sources represent some of the lowest masses studied to date. [Preview Abstract] |
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