Bulletin of the American Physical Society
19th Annual Meeting of the APS Northwest Section
Volume 63, Number 6
Thursday–Saturday, May 31–June 2 2018; Tacoma, Washington
Session B1: Astronomy and Cosmology I |
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Chair: Bennett Link, Montana State University Room: Thompson Hall 175 |
Friday, June 1, 2018 1:30PM - 2:00PM |
B1.00001: Binary Neutron Star Mergers and Short Gamma-Ray Bursts: the case of GW170817 Invited Speaker: Davide Lazzati The propagation of a relativistic jet through a static ambient medium drives a bow shock that feeds a cocoon around the jet itself. As the jet breaks out, the cocoon is released, adding a mildly relativistic, wide angle component to the outflow. In this talk I will discuss the dynamics and radiative properties of the ensuing structured jet, considering both radiation from the outflow itself (the so-called prompt emission) and radiation produced by the interaction of the outflow with the interstellar medium (the so-called afterglow). The resulting electromagnetic transients will be discussed as possible interpretations for the observations of GW170817 as well as possible counterparts of future binary merger detections. [Preview Abstract] |
Friday, June 1, 2018 2:00PM - 2:12PM |
B1.00002: Stars that Fission Rebecka Tumblin, James Imamura, Kathryn Hadley, William Dumas, Erik Keever Binary stars are estimated to comprise upwards of half of all main-sequence stars, yet very little is known about their formation process. Binary stars are thought to form via four possible scenarios: fission, capture, cloud fragmentation, and disk fragmentation. In the Fission scenario, as the protostar contracts toward the main-sequence, rotational energy becomes increasingly important. If the star spin-up reaches a break-up velocity, a short-period close binary pair could form. Based on previous nonlinear studies of solitary differentially rotating polytropic stars, it is generally believed that the fission process can not form binary pairs. We modeled the nonlinear evolution of $3D$ differentially rotating polytropic stars surrounded by inviscid polytropic disks and found that, in some cases, rapidly rotating stars unstable to the bar-mode instability fission in the presence of a circumstellar disk. We find that circumstellar material tends to reduce the inward force of gravity, thereby allowing the rotational energy to dominate the system. In systems with sufficient flattening, an $m=2$ bar mode develops, and the system develops two central objects which rotate on independent axes while orbiting a common center of mass. [Preview Abstract] |
Friday, June 1, 2018 2:12PM - 2:24PM |
B1.00003: Segue 1 and 3: Captured Dwarf Galaxies and Stray Star Clusters Joanne Hughes, Rose Marie Haynes, Eileen Flesher Along with globular clusters and field stars, the Milky Way's outer halo contains individual star systems, both dwarf galaxies and stellar clusters, which are not native to our Galaxy. Amongst the wealth of data produced by recent sky-surveys, SEGUE (Sloan Extension for Galactic Understanding and Exploration) 1 was discovered in 2006, and thought to be either a dwarf galaxy or a faint globular cluster. Follow-up observations of stellar velocities found the system to be highly dark-matter dominated; all its stars appear to be very old ($>12$ Gyr) and metal-poor. Segue 3 was discovered in 2010, and was originally considered to be an ancient and metal-poor system, although not as extreme as Segue 1. Several groups, including ours, confirmed that Segue 3 was younger and more metal-rich, and a $3$ Gyr-old globular cluster could not have formed in the Milky Way today. Segue 3 was likely accreted from a dwarf galaxy, similar to the Magellanic Clouds; its former host was cannibalized by the Milky Way in the recent past. We explore observational tests, using databases and ground-based telescopes, that can be applied to these sparse interlopers to determine their origin and star formation history. [Preview Abstract] |
Friday, June 1, 2018 2:24PM - 2:36PM |
B1.00004: Cooling and fragmentation of narrow self-gravitating disks William Dumas, Kathryn Hadley, James Imamura, Erik Keever, Rebecka Tumblin We perform nonlinear hydrodynamic simulations to investigate the evolution and possible fragmentation of narrow, self-gravitating, radiating disks surrounding a central point mass protostar. As initial conditions we use equilibrium models of axisymmetric polytropic disks and hold the ratio of the inner to outer edge of the disk at $\frac{r_-}{r_+}=0.6$ and vary the star to disk mass ratio $\frac{M_*}{M_d}=0.1,1,2$, and $5$. For each model we use a parameterized cooling function with weak, moderate and strong cooling rates. For weakly cooled models, the growth of gravitational instabilities (GIs) lead to nonaxisymmetric overdensities in the disk. Previous work has shown that, for low $\frac{M_*}{M_d}$, these cause the disk to fragment into isolated clumps, even in the absence of cooling. For high $\frac{M_*}{M_d}$, collapse of the overdense regions is apparently disrupted as arms develop in lieu of isolated clumps. When $M_*$ is equal to $M_d$, the equilibrium model evolves towards a system with both a massive clump and several arms which contain a significant portion of the systems mass. We discuss the possible role of GIs in disks as a mechanism for the formation of binary and multiple stellar systems, such as the triple protostar system L1448 IRS3B. [Preview Abstract] |
Friday, June 1, 2018 2:36PM - 2:48PM |
B1.00005: Gravity, Simple As Possible, But Not Simpler Jerry Witt All points are universal center as gravity bends space and time about itself, Hawking. All is in motion relative to all other points center, or relative to the observer, Einstein (Al). Entropic Derivation of f=ma in Circular Motion. Essentially, the universe is made up of energy, motion, time, space, and gravity. And, by using the formula for a sphere's volume as Gravity's naturalest, most natural shape in the spatial universe is a sphere. Well, I'd like to explain and show the math that accounts for gravity being the singularity, or the non cancelled 4th physical dimension being the singularity was never cancelled, we it's energy, and gravity it's singularly connected proof. Well, at least to a better that 99\% accuracy. [Preview Abstract] |
Friday, June 1, 2018 2:48PM - 3:00PM |
B1.00006: Evolution of the NANOGrav 11yr dataset and limits on the Gravitational Wave Stochastic Background Jeffrey Hazboun The North American Nanohertz Observatory for Gravitational Waves is a collaboration of scientists that monitor an array of millisecond pulsars as a galactic scale gravitational wave observatory. The timing precision of these pulsars is such (∼ 100 ns) that one can measure the Hellings and Down correlations between pulsars to look for the signature of ultra-low frequency gravitational waves. Since the maximum power of the stochastic background of gravitational waves from super- massive black hole binaries is thought to be in frequencies smaller than 1/yr, our signal is expected to grow slowly as we are able to observe lower frequencies in the power spectrum and as we add more pulsars to our array. With the most recent data release, we have undertaken an investigation into the evolution of our signal by slicing the dataset in time and running our statistical analyses on its subsets. We will demonstrate how this technique can be used to assess various characteristics of our pulsar timing array, including the effects of the interstellar medium, how removal of our best timed pulsar effects our limits and expectations about the evolution of our signal in the future. The investigation into unexpected features of the evolution will also be discussed. [Preview Abstract] |
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