The recent discovery of planets orbiting stars other than the sun has revitalized the fields of dynamics and planet formation. One of the planets has been observed to transit its star, with the result that we know the radius and mass of the planet---it is a gas giant similar to Jupiter. Many of the planets follow small but eccentric orbits; the former suggests that the planets have migrated inward from their birthplaces, while the latter suggests that two or more planets have interacted. In 2002 CITA researchers made some novel suggestions for how the mutual gravitational perturbations between planets, and between a companion star and a planet, have influenced the orbits of some planetary systems. Research has also continued on the tidal dissipation in exoplanets, on their heated atmospheres, and on the formation of exoplanets in dusty disks.
N. Murray, T. Thompson (Berkeley) and E. Quataert (Berkeley) investigated large-scale galactic winds driven by momentum deposition. Radiation can be produced by a starburst or AGN activity. They argued that momentum-driven winds are an efficient mechanism for feedback during the formation of galaxies. They showed that above a limiting luminosity, momentum deposition from star formation can expel a significant fraction of the gas in a galaxy. The limiting, Eddington-like luminosity is proportional to the fourth power of the stellar velocity dispersion. A starburst that attains this maximum luminosity moderates its star formation rate and its luminosity does not increase significantly further. They argue that ellipticals attain this limit during their growth at redshifts of order 1 to 4 and that this is the origin of the Faber-Jackson relation. They show that Lyman break galaxies and ultra-luminous infrared galaxies have luminosities near the limiting luminosity. Since these starbursting galaxies account for a significant fraction of the star formation at redshifts larger than one, this supports their hypothesis that much of the observed stellar mass in early type galaxies was formed during Eddington-limited star formation. Star formation is unlikely to efficiently remove gas from very small scales in galactic nuclei, i.e., scales much smaller than that of a nuclear starburst. This gas is available to fuel a central black hole (BH). They argue that a BH clears gas out of its galactic nucleus when the luminosity of the BH itself reaches the limiting luminosity described above. This shuts off the fuel supply to the BH and may also terminate star formation in the surrounding galaxy. As a result, the BH mass is fixed to be proportional to the fourth power of the stellar velocity dispersion. This limit is in accord with the observed black hole mass-stellar velocity dispersion relation.
T. Thompson (Berkeley), E. Quataert (Berkeley) and N. Murray considered the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This assumption is particularly appropriate when the disk is optically thick to its own infrared radiation, as in the central regions of Ultraluminous Infrared Galaxies (ULIRGs). They argue that because the disk radiates at its Eddington limit, the "Schmidt-law" for star formation changes in the optically-thick limit, with the star formation rate per unit area scaling as the gas surface density and inverseley with the mean opacity of the disk. Their calculations further show that optically thick starburst disks have a characteristic flux of $10^{13}$ solar luminosities per square kiloparsec , a star formation rate per unit area of 1000 solar masses per year per square kiloparsec, and dust effective temperature of 90 K. The model predictions are in good aggrement with observations of ULIRGs. They extend this model of starburst disks from many-hundred parsec scales to sub-parsec scales and address the problem of fueling active galactic nuclei (AGN). They consistently account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little or no fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN, although these disk solutions exhibit a broad mid- to far-infrared peak from star formation. They show that the vertical thickness of the starburst disk on parsec scales can approach , perhaps accounting for the nuclear obscuration in some Type 2 AGN. They also argue that the disk of young stars in the Galactic Center may be the remnant of such a compact nuclear starburst.
Levin has been collaborating with the group of Reinhard Genzel (MPE) in analysing of the kinematical data in the Galactic Center.
Together with the undegraduate summer student Sarah Nickerson, Levin is studying the dynamics of compact GC cluster IRS13E, in the hope of placing a limit on the cluster's putative Intermediate-Mass Black Hole.
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