National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Random matrix physics: Spectrum and strength fluctuations

Abstract: It now appears that the general nature of the deviations from uniformity in the spectrum of a complicated nucleus is essentially the same in all regions of the spectrum and over the entire Periodic Table. This behavior, moreover, is describable in terms of standard Hamiltonian ensembles which could be generated on the basis of simple information-theory concepts, and which give also a good account of fluctuation phenomena of other kinds and, apparently, in other many-body systems besides nuclei. The main departures from simple behavior are ascribable to the moderation of the level repulsion by effects due to symmetries and collectivities, for the description of which more complicated ensembles are called for. One purpose of this review is to give a self-contained account of the theory, using methods: sometimes approximate: which are consonant with the usual theory of stochastic processes. Another purpose is to give a proper foundation for the use of ensemble theory, to make clear the origin of the simplicities in the observable fluctuations, and to derive other general fluctuation results. In comparing theory and experiment, the authors give an analysis of much of the nuclear-energy-level data, as well as an extended discussion of observable effects in nuclear transitionsmore » and reactions and in the low-temperature thermodynamics of aggregates of small metallic particles.« less

Accretion and the Evolution of T Tauri Disks

Abstract: Using results and calibrations from a previous paper (Gullbring et al. 1997), we estimate disk accretion rates for pre-main-sequence stars in the Taurus and Chamaeleon I molecular cloud complexes. The median accretion rate for T Tauri stars of age ~1 Myr is ~10-8 M☉ yr-1; the intrinsic scatter at a given age may be as large as 1 order of magnitude. There is a clear decline of mass accretion rates with increasing age t among T Tauri stars. Representing this decline as tη, we estimate 1.5 η 2.8; the large uncertainty is due to the wide range of accretion rates at a given age, the limited age range of the sample, and errors in estimating stellar ages and accretion luminosities. Adopting values of η near the low end of this range, which are more likely given probable errors and the neglect of birthline age corrections, masses accreted during the T Tauri phase are roughly consistent with disk masses estimated from millimeter-wave dust emission. Similarity solutions for evolving, expanding disks are used to investigate observational constraints on disk properties employing a minimum of parameters. For an assumed power-law form of the disk viscosity with radius ν Rγ, η 1.5 corresponds to γ 1. The limit γ ~ 1 corresponds to a roughly constant α in the Shakura-Sunyaev (1973) viscosity parameterization; using current observed disk sizes, we estimate α ~ 10-2 (on scales ~10-100 AU). Much of the observed variation in mass accretion rates can be accounted for by varying initial disk masses between 0.01 and 0.2 M☉, but this result may be strongly affected by the presence of binary companion stars. These results emphasize the need for older samples of stars for studying disk evolution.