The gas temperature in the surface layers of protoplanetary disks

Models for the structure of protoplanetary disks have so far been based on the assumption that the gas and the dust temperature are equal. The gas temperature, an essential ingredient in the equations of hydrostatic equilibrium of the disk, is then determined from a continuum radiative transfer calculation, in which the continuum opacity is provided by the dust. It has been long debated whether this assumption still holds in the surface layers of the disk, where the dust infrared emission features are produced. In this paper we compute the temperature of the gas in the surface layers of the disk in a self-consistent manner. The gas temperature is determined from a heating-cooling balance equation in which processes such as photoelectric heating, dissociative heating, dust-gas thermal heat exchange and line cooling are included. The abundances of the dominant cooling species such as CO, C, C+ and O are determined from a chemical network based on the atomic species H, He, C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find that in general the dust and gas temperature are equal to withing 10% for A_V >~ 0.1, which is above the location of the `super-heated surface layer' in which the dust emission features are produced (e.g. Chiang & Goldreich 1997). High above the disk surface the gas temperature exceeds the dust temperature and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as high as 600 K at a radius of 100 AU. This is a region where CO has fully dissociated, but a significant fraction of hydrogen is still in molecular form. The densities are still high enough for non-negligible H_2 emission to be produced.....(see paper for full abstract)Comment: 28 pages, 8 figures, accepted for publication in Ap

Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation, and development of topography in the central Southern Alps, New Zealand

Apatite and zircon (U-Th)/He and fission track ages were obtained from ridge transects across the central Southern Alps, New Zealand. Interpretation of local profiles is difficult because relationships between ages and topography or local faults are complex and the data contain large uncertainties, with poor reproducibility between sample duplicates. Data do form regional patterns, however, consistent with theoretical systematics and corroborating previous observations: young Neogene ages occur immediately southeast of the Alpine Fault (the main plate boundary structure on which rocks are exhumed); partially reset ages occur in the central Southern Alps; and older Mesozoic ages occur further toward the southeast. Zircon apparent ages are older than apatite apparent ages for the equivalent method. Three-dimensional thermokinematic modeling of plate convergence incorporates advection of the upper Pacific plate along a low-angle detachment then up an Alpine Fault ramp, adopting a generally accepted tectonic scenario for the Southern Alps. The modeling incorporates heat flow, evolving topography, and the detailed kinetics of different thermochronometric systems and explains both complex local variations and regional patterns. Inclusion of the effects of radiation damage on He diffusion in detrital apatite is shown to have dramatic effects on results. Geometric and velocity parameters are tuned to fit model ages to observed data. Best fit is achieved at 9 mm a−1 plate convergence, with Pacific plate delamination on a gentle 10°SE dipping detachment and more rapid uplift on a 45–60° dipping Alpine Fault ramp from 15 km depth. Thermokinematic modeling suggests dip-slip motion on reverse faults within the Southern Alps should be highest ∼22 km from the Alpine Fault and much lower toward the southeast

AGRICULTURAL POLICY DEVELOPMENTS IN THE UNITED STATES: 1991-93

Agricultural and Food Policy,

TRENDS AND DEVELOPMENTS IN UNITED STATES AGRICULTURAL POLICY: 1993-1995

A number of factors including budget pressures, emphasis on environmentally sensitive agriculture, emphasis on finding agricultural export markets, and anti-agricultural program sentiment have fueled a climate for change in United States agricultural policy. Whether significant changes will occur depends on the political strength of agricultural interest groups and on the compromises which can be reached between them. Several notable achievements have been accomplished in recent domestic agricultural policy legislation. The 1995 farm bill will define the commodity and conservation programs for the next five years. In addition to domestic developments, there have been history-setting accomplishments in reducing barriers to international agricultural trade. The tri-partite North American Free Trade Agreement became effective January 1, 1994. The Uruguay Round Agreement of the General Agreement on Tariffs and Trade was completed in late 1994 and will become effective in 1995.Agricultural and Food Policy,

Are Giant Planets Forming Around HR 4796A?

We have obtained FUSE and HST STIS spectra of HR 4796A, a nearby 8 Myr old main sequence star that possesses a dusty circumstellar disk whose inclination has been constrained from high resolution near-infrared observations to be ~17 deg from edge-on. We searched for circumstellar absorption in the ground states of C II at 1036.3 A, O I at 1039.2 A, Zn II at 2026.1 A, Lyman series H2, and CO (A-X) and failed to detect any of these species. We place upper limits on the column densities and infer upper limits on the gas masses assuming that the gas is in hydrostatic equilibrium, is well-mixed, and has a temperature, Tgas ~ 65 K. Our measurements suggest that this system possesses very little molecular gas. Therefore, we infer an upper limit for the gas:dust ratio (<4.0) assuming that the gas is atomic. We measure less gas in this system than is required to form the envelope of Jupiter.Comment: 10 pages, 3 figures (including 1 color figure), accepted for publication in Ap