Showing papers by "Alan P. Boss published in 1996"

A Road Map for the Exploration of Neighboring Planetary Systems (ExNPS)

Abstract: A brown dwarf star having only 20-50 times the mass of Jupiter is located below and to the left of the bright star GL 229 in this image from the Hubble Space Telescope At the 19 light year distance to GL 229, the 77-arcsec separation between the star and the brown dwarf corresponds to roughly the separation between Pluto and the Sun in our Solar System The goal of the program described in this report is to detect and characterize Earth-like planets around nearby stars where conditions suitable for life might be found For a star like the Sun located 30 light years away, the appropriate star-planet separation would be almost 100 times closer than seen here for GL 229B

Giants and dwarfs meet in the middle

Abstract: The discovery of two remarkable new objects by astronomers has blurred the distinction between stars and planets. It also raises hopes of finding planetary systems like our own in the near future.

Large scale processes in the solar nebula.

Abstract: Theoretical models of the structure of a minimum mass solar nebula should be able to provide the physical context to help evaluate the efficacy of any mechanism proposed for the formation of chondrules or Ca, Al-rich inclusions (CAI's). These models generally attempt to use the equations of radiative hydrodynamics to calculate the large-scale structure of the solar nebula throughout the planet-forming region. In addition, it has been suggested that chondrules and CAI's (=Ch&CAI's) may have been formed as a direct result of large-scale nebula processing such as passage of material through high-temperature regions associated with the global structure of the nebula. In this report we assess the status of global models of solar nebula structure and of various related mechanisms that have been suggested for Ch and CAI formation.

Nonaxisymmetry in the Solar Nebula: Disk Evolution or Giant Gaseous Protoplanet Formation?

Abstract: Radiative hydrodynamical calculations of the thermal structure of an axisymmetric (2D) protoplanetary disk with a mass of N 0.14Mo predict that the outer disk may be cool enough to become gravitationally unstable. The dynamical evolution of a 0.14Mo disk has been computed with a 3D hydrodynamics code in order to learn the outcome of gravitational instability in an intermediate-mass disk orbiting a solar-mass star. Growth of nonaxisymmetry occurs within a few rotation periods of the outer disk (Po),. and the nonaxisymmetry is large enough to result in disk evolution through gravitational torques within N l o 5 yrs. After about 10 Po, the dominant m = 2 (bar) mode begins to saturate at an amplitude greater than 1 by this time, two Jupiter-mass clumps of gas have formed around 8 AU and continue to gain mass. The hot inner disk remains nearly axisymmetric throughout this interval. The model suggests a "best of both worlds" scenario may be tenable: formation of terrestrial planets through collisional accumulation in the hot inner nebula, and rapid formation of giant gaseous protoplanets in the cool outer nebula through gravitational instability of the disk. INTRODUCTION. Forming the gas giant planets within the expected lifetime of the solar nebula (N lo5 to lo7 yrs [I]) is a longstanding problem for planetary formation by collisional accumulation. Possible solutions include a phase of runaway accretion in a nebula with a suitably high surface density in the outer planet region [2]. The alternative is rapid formation of giant gaseous protoplanets (GGPP) through gravitational inst ability of the gasous portion of the nebula [3]. The latter idea has not been pursued, in part because ice and rock are believed to be soluble in Jovian planet envelopes [4], thereby preventing formation of the rock/ice cores of the gas giant planets in a planet formed by gravitational instability [5]. However, km-sized or larger planetesimals should be able to reach the core of a GGPP [6], so this objection could be removed if Jupiter received its excess solids by ingesting planetesimals rather than dust grains. The possibility of a mixed scenario [7] with collisional accumulation of the terrestrial planets and gravitational instability of the gas disk forming the gas giant planets then becomes more attractive. The mixed scenario is also suggested by thermal profiles calculated for protoplanetary disks with a 2D radiative hydrodynamics code [8,9] surface densities in intermediatemass (0.14Mo) disks can be high enough in the cool outer disk to exceed Toomre's Q stability criterion [lo], implying gravitational instability. Gravitational instability could lead to GGPP formation, or at least to rapid disk evolution through gravitational torques [I 1,121, perhaps helping to solve the equally longstanding problem of disk evolution. METHODS. The temperature distribution from the 2D radiative transfer calculation was assumed to be appropriate throughout the 3D calculation. This approximation is necessary to allow the 3D calculation to be followed for N loPo. The inner disk rotation period Pi is 1 year, and 10Po z 200 yrs. The active computational volume (N, = 51, No = 23 in 7r/2 2 8 > 0, N+ = 64) extends from 1 AU to 10 AU, with boundary conditions at 10 AU chosen to absorb outward-moving velocity perturbations and to maintain constant density. The initial disk density is seeded with a ~ 0 . ~ 2 4 perturbation (amplitude a,=;! = 0.01) and with random noise (m = 1,2, ... 16 with a N 0.001). RESULTS. The m = 2 mode grows monotollically and dominates the disk throughout the evolution (see Figures). By loPo, the outer disk has formed two well-defined spiral arms with rapidly increasing density maxima the outer disk appears to be forilling two GGPP with masses of at least a Jupiter mass and average temperatures 50 I<. Gravitational torques associated with the nonaxisymmetry are large enough to transport all the disk's angular inomenturn in N lo5 yrs. The disk's surface density is beginning to pile-up at the inner edge ( N 7 AU) of the unstable region as a result of the torques.