Norman Murray

Research Summary

[ Computational Astrophysics, Dynamics, General Relativity High Energy Astrophysics Interstellar Medium ]

Norman Murray carries out research on a variety of topics, including planet formation, radiative outflows in quasars and other objects, the physics of black holes and neutron stars, and stellar physics.

Research Projects:
(September 2002 - August 2003)

Mechanism for halting planet migration

Norm Murray, Isamu Matsuyama and Doug Johnstone (HIA) proposed a mechanism for halting migration in a gas disk. Photoevaporation of the disk by irradiation from the central star can produce a gap in the disk, preventing planets outside the gap from migrating down to the star. This would result in an excess of systems with planets at or just outside the photoevaporation radius. It may also halt migration in the inner disk, a possibility that becomes more likely when a massive star is located in the vicinity of the star-disk system.

Orbit of the extra-solar planet HD 80606

Norm Murrayand Yaqin Wu (University of Toronto) studied the origin of the highly eccentic (e=0.93) and tight (a=0.46 AU) orbit of the extra-solar planet HD 80606. The presence of a stellar companion to the host star suggests the possibility that the Kozai mechanism and tidal dissipation combined to draw the planet inward well after it formed. This implies that the initial planet orbit is highly inclined (nearly perpendicular) relative to the binary orbit. Wu and Murray investigated the likelihood of this scenario, as well as its consequences. Moreover, they showed that it is unlikely that the tide raised on the star by the planet can account for the velocity residual (after the motion induced by the planet is removed) observed on the host star. They also discussed the relevance of such a migration scenario for other planetary systems.

Detection of extrasolar dust particles

Norm Murray, Joe Weingartner and Chris Capobianco presented estimates of the minimum detectable particle sizes and collecting areas for ground based radar at Arecibo and New Zealand. Micron size extrasolar dust particles have been convincingly detected by satellites and Larger extrasolar meteoroids (5-35 microns) have most likely been detected by ground based radar at Arecibo and New Zealand. Murray and collaborators showed that particles larger than about 10 micron can propagate for tens of parsecs through the interstellar medium, opening up the possibility that ground based radar systems can detect AGB stars, young stellar objects such as T Tauri stars, and debris disks around Vega-like stars. They provide analytical and numerical estimates of the ejection velocity in the case of a debris disk interacting with a Jupiter mass planet. They also provide rough estimates of the flux of large micrometeoroids from all three classes of sources. Current radar systems are unlikely to detect significant numbers of meteors from debris disks such as Beta Pictoris. However, they suggest improvements to radar systems that should allow for the detection of multiple examples of all three classes.

Lithium-6 abundance

Using high-resolution, high quality spectra, Avi Mandell (Penn State), Jian Ge (Penn State) and Norman Murray investigated the presence of Lithium 6 in two lithium-poor stars that host extrasolar planetary systems. They present improved atomic and molecular line lists for the region in the vicinity of the lithium line at 6707.8 Angstroms, and produce an excellent fit to the solar spectrum. From line profle fitting, they find results consistent with no lithium 6 in either of the lithium-poor planet-bearing stars or in three comparison stars with and without planets, and 1-sigma upper limits of 0.04 for the isotopic ratios of the two lithium- poor stars give an upper limit of 0.3 Jupiter masses of material with primordial abundances that could have been recently deposited in their outer layers. These results suggest that post-main sequence accretion of planets or planetary material that is undepleted in lithium is uncommon.

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