[X] My Assistant

[ljk4-1]

Astrophysics, abstract
astro-ph/0602217

From: Dimitar Sasselov [view email]

Date: Thu, 9 Feb 2006 21:42:02 GMT (35kb)

On the Location of the Snow Line in a Protoplanetary Disk

Authors: M. Lecar (1), M. Podolak (2), D. Sasselov (1), E. Chiang (3) ((1)Harvard-Smithsonian CfA, (2)Tel Aviv Univ., (3)UC Berkeley)

Comments: Accepted for publication in ApJ, 9 pages, 4 figures

In a protoplanetary disk, the inner edge of the region where the temperature falls below the condensation temperature of water is referred to as the 'snow line'. Outside the snow line, water ice increases the surface density of solids by a factor of 4. The mass of the fastest growing planetesimal (the 'isolation mass') scales as the surface density to the 3/2 power. It is thought that ice-enhanced surface densities are required to make the cores of the gas giants (Jupiter and Saturn) before the disk gas dissipates. Observations of the Solar System's asteroid belt suggest that the snow line occurred near 2.7 AU. In this paper we revisit the theoretical determination of the snow line. In a minimum-mass disk characterized by conventional opacities and a mass accretion rate of 10^-8 solar masses per year, the snow line lies at 1.6-1.8 AU, just past the orbit of Mars. The minimum-mass disk, with a mass of 0.02 solar, has a life time of 2 million years with the assumed accretion rate. Moving the snow line past 2.7 AU requires that we increase the disk opacity, accretion rate, and/or disk mass by factors ranging up to an order of magnitude above our assumed baseline values.

http://arxiv.org/abs/astro-ph/0602217

[ljk4-1]

Astrophysics, abstract
astro-ph/0602438

From: Mauro Sereno [view email]

Date: Mon, 20 Feb 2006 19:11:18 GMT (10kb)

Solar and stellar system tests of the cosmological constant

Authors: M. Sereno (Univ. Zurich), Ph. Jetzer (Univ. Zurich)

Comments: 4 pages; this is a preprint of an article accepted for publication in Physical Review D

Some tests of gravity theories - periastron shift, geodetic precession, change in mean motion and gravitational redshift - are applied in solar and stellar systems to constrain the cosmological constant. We thus consider a length scale range from 10^8 to 10^{15} km. Best bounds from the solar system come from perihelion advance and change in mean motion of Earth and Mars, Lambda < 10^{-36} km^{-2}. Such a limit falls very short to estimates from observational cosmology analyses but a future experiment performing radio ranging observations of outer planets could improve it by four orders of magnitude. Beyond the solar system, together with future measurements of periastron advance in wide binary pulsars, gravitational redshift of white dwarfs can provide bounds competitive with Mars data.

http://arxiv.org/abs/astro-ph/0602438

[ljk4-1]

EXO WORLDS

- Simulation Tracks Planetary Evolution

http://www.spacedaily.com/reports/Simulati..._Evolution.html

London, UK (SPX) Mar 21, 2006 - Two British astronomers have constructed a
computer simulation that tracks how giant protoplanets tend to form and migrate
inward toward their central star.