Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session X5: Plasmas, Planets and Protostars |
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Sponsoring Units: GPAP DPP DAP Chair: Hantao Ji, Princeton Plasma Physics Laboratory Room: Royal AB |
Tuesday, May 3, 2011 10:45AM - 11:21AM |
X5.00001: The Structure and Composition of Protoplanetary Disks Invited Speaker: The Spitzer Space Telescope observed all but a few percent of the 2500 protoplanetary disks within 500 parsecs of the Solar System, at the infrared wavelengths at which these objects produce most of their luminosity. The sample covers completely the mass range of hydrogen-burning stars, and penetrates well into the domain of brown dwarfs, as young as several hundred thousand years. It includes disks subjected to an extremely large range of environmental conditions, from the high-density, high-UV regions of massive star formation like the Orion A cloud, to the quiet expanses of exclusively low-mass star formation in the Taurus-Auriga cloud. Here we will offer a brief review of these results, with an eye to noting new constraints which the observations can place on the gas and plasma dynamics of the objects. In particular, the observations include demonstrations of dust settling and gap formation in protoplanetary disks, of grain growth and mineralization of the dust, and of the presence of prebiotic molecules, all on scales similar to the range of planetary orbits in our own Solar system. The results bear on the physics of formation of terrestrial and giant planets, and on radial transport of gas and dust in both the partially-ionized upper layers and in the neutral ``dead zone'' beneath. [Preview Abstract] |
Tuesday, May 3, 2011 11:21AM - 11:57AM |
X5.00002: Planet Formation in Magnetized Accretion Disks Invited Speaker: Stars form by the flow of matter through an accretion disk. Inside these disks, solids particles suspended in the gas grow to form terrestrial planets and giant planet cores. I will review the physical processes of early planet growth, with an emphasis on the strong aerodynamic coupling between gas and dust (as well as larger solids). Turbulence in the gas disk is a crucial issue for these interactions. The magneto rotational instability (MRI) is the leading candidate to drive turbulent momentum transport in disks. I will briefly summarize the current status of MRI turbulence in weakly magnetized circumstellar disks. Then I will describe how MRI turbulence affects the formation of planets. By vigorously mixing small solids, turbulence generally tends to oppose the accumulation into planets. Yet somehow planets form. MRI turbulence has the tendency to support long-lived, axisymmetric zonal flows. These super- and sub-Keplerian flows surround a pressure maximum which efficiently accumulates centimeter to meter scale solids. These solids are further subject to a strong aerodynamic clumping mechanism driven by the streaming instability (Youdin \& Goodman, 2005). Dense clumps of small solids can then collapse gravitationally into 100 km-scale solid planetesimals. Theories of early planet formation are recorded in the asteroid and Kuiper belts of our Solar System, the debris disks surrounding other stars and in magnetized meteorite fragments that fall to Earth. [Preview Abstract] |
Tuesday, May 3, 2011 11:57AM - 12:33PM |
X5.00003: X-rays Flares and Protoplanetary Disks Invited Speaker: X-ray observations of star forming regions show that magnetic reconnection flares are powerful and frequent in pre-main sequence solar-type stars. Well-defined samples in the Orion Nebula Cluster and Taurus clouds exhibit flares with peak X- ray luminosities $L_x \sim 10^{29} - 10^{32}$ erg/s, orders of magnitude stronger and more frequent than contemporary solar flares. X-rays are emitted in magnetic loops extending 0.1-10 R $_*$ above the stellar surface and thus have a favorable geometry to irradiate the protoplanetary disk. Several lines of evidence - fluorescent iron X-ray emission line, forbidden [NeII] infrared line, and excited molecular bands - support X-ray irradiation of cold material in some young systems. Several astrophysical consequences of X-ray irradiation are outlined. As ionization fractions need only reach $10^{-12}$ to induce the magnetorotational instability and associated turbulence, X-rays may be the principal determinant of the extent of the viscous ``active zone'' and laminar ``dead zone'' in the layered accretion disk. X-ray irradiation may thus play a major role in planet formation processes: particle settling; meter-size inspiral; protoplanetary migration; and dissipation of the gaseous disk. [Preview Abstract] |
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