Gregory P. Laughlin

A dying universe: the long-term fate and evolution of astrophysical objects

Astrophysical issues related to the long-term fate of the universe are outlined. The evolution of planets, stars, stellar populations, galaxies, and the universe itself over time scales that greatly exceed the current age of the universe are considered. Their discussion starts with new stellar evolution calculations which follow the future evolution of the low-mass (M-type) stars that dominate the stellar mass function. They derive scaling relations that describe how the range of stellar masses and lifetimes depends on forthcoming increases in metallicity. They then proceed to determine the ultimate mass distribution of stellar remnants, i.e., the neutron stars, white dwarfs, and brown dwarfs remaining at the end of stellar evolution; this aggregate of remnants defines the {open_quotes}final stellar mass function.{close_quotes} At times exceeding {approximately}1{endash}10 trillion years, the supply of interstellar gas will be exhausted, yet star formation will continue at a highly attenuated level via collisions between brown dwarfs. This process tails off as the galaxy gradually depletes its stars by ejecting the majority and driving a minority toward eventual accretion onto massive black holes. As the galaxy disperses, stellar remnants provide a mechanism for converting the halo dark matter into radiative energy. Posited weakly interacting massive particles are accretedmorexa0» by white dwarfs, where they subsequently annihilate with each other. Thermalization of the decay products keeps the old white dwarfs much warmer than they would otherwise be. After accounting for the destruction of the galaxy, the authors consider the fate of the expelled degenerate objects (planets, white dwarfs, and neutron stars) within the explicit assumption that proton decay is a viable process. The evolution and eventual sublimation of these objects is dictated by the decay of their constituent nucleons, and this evolutionary scenario is developed in some detail. (Abstract Truncated)«xa0less

Implications of White Dwarf Galactic Halos

Motivated by recent measurements which suggest that roughly half the mass of the galactic halo may be in the form of white dwarfs, we study the implications of such a halo. We first use current limits on the infrared background light and the galactic metallicity to constrain the allowed initial mass function (IMF) of the stellar population that produced the white dwarfs. The IMF must be sharply peaked about a characteristic mass scale

The End of the Main Sequence

M_C approx 2.3 M_odot

Spiral Mode Saturation in Self-Gravitating Disks

. Since only a fraction of the initial mass of a star is incorporated into the remnant white dwarf, we argue that the mass fraction of white dwarfs in the halo is likely to be 25% or less, and that 50% is an extreme upper limit. We use the IMF results to place corresponding constraints on the primordial initial conditions for star formation. The initial conditions must be much more homogeneous and skewed toward higher temperatures (

Possible stellar metallicity enhancements from the accretion of planets

T_{rm gas} sim

MACHOs, White Dwarfs, and the Age of the Universe

200 K) than the conditions which lead to the present day IMF. Next we determine the luminosity function of white dwarfs. By comparing this result with the observed luminosity function, we find that the age of the halo population must be greater than

CONSTRAINTS ON THE INTERGALACTIC TRANSPORT OF COSMIC RAYS

sim 16

The Frozen Earth

Gyr. Finally, we calculate the radiative signature of a white dwarf halo. This infrared background is very faint, but is potentially detectable with future observations.

The Future of the Universe

We present stellar evolution calculations for the lowest mass stars, i.e., those stars with masses in the range 0.08 M☉ ≤ M* ≤ 0.25 M☉. Our particular emphasis is on the post-main-sequence evolution of these objects. We establish a hydrogen-burning timescale of τH ~ 1.0 × 1013 years for the minimum-mass main-sequence star. This timescale determines the duration over which the light of our Galaxy is dominated by a conventional stellar contribution. We find that for masses M* < 0.25 M☉, stars remain fully convective for a significant fraction of the duration of their evolution. The maintenance of full convection precludes the development of large composition gradients and allows the entire star to build up a large helium mass fraction. We find that stars with masses M < 0.20 M☉ will never evolve through a red giant stage. After becoming gradually brighter and bluer for trillions of years, these late M dwarfs of today will develop radiative-conductive cores and mild nuclear shell sources; these stars then end their lives as helium white dwarfs. Our work has significant bearing on the general question of why stars become red giants. The fact that the lowest mass stars grow neither red nor giant as they evolve provides an important insight into this problem. Through both analytical and numerical arguments, we have determined that the development of low-mass red giants requires a combination of (1) increasing core luminosity, (2) the existence of molecular weight gradients between the core and the envelope, and (3) the presence of an atmospheric opacity which is an increasing function of temperature. Finally, we discuss the implications of our results with regards to the long-term fate and evolution of the Galaxy.

Limitations of the Transport of Cosmic Ray Antimatter from Distant Galaxies

This paper examines a second-order nonlinear mechanism that appears to bear responsibility for (1) eliciting transport of mass and angular momentum in self-gravitating gaseous disks and (2) inducing mode saturation that can preclude the onset of disk fragmentation. Our analysis indicates in quantitative detail how torques arising from gravitationally unstable spiral modes can lead to disk accretion. We begin by performing a linear global stability analysis on an idealized model equilibrium disk that is prone to a single , rapidly growing two-armed spiral. We compare the linearized predictions with the full hydrodynamical evolution of the disk provided by numerical simulations. We then retain second-order terms in a perturbative reanalysis of the hydrodynamic governing equations. We derive equations that describe how mass and angular momentum are redistributed in the disk. Then we solve these equations numerically and compare the results with the simulations. We conclude with a discussion of how nonlinear mode interactions and self-interactions are responsible for mode saturation in the disk and the development of steady mass accretion.