63,321 research outputs found
Dynamics of Circumstellar Disks III: The case of GG Tau A
(abridged) We present 2-dimensional hydrodynamic simulations using the
Smoothed Particle Hydrodynamic (SPH) code, VINE, to model a self-gravitating
binary system similar to the GG Tau A system. We simulate systems configured
with semi-major axes of either ~AU (`wide') or ~AU (`close'), and
with eccentricity of either or . Strong spiral structures are
generated with large material streams extending inwards. A small fraction
accretes onto the circumstellar disks, with most returning to the torus.
Structures also propagate outwards, generating net outwards mass flow and
eventually losing coherence at large distances. The torus becomes significantly
eccentric in shape. Accretion onto the stars occurs at a rate of a few
\msun/yr implying disk lifetimes shorter than ~yr,
without replenishment. Only wide configurations retain disks by virtue of
robust accretion. In eccentric configurations, accretion is episodic, occurs
preferentially onto the secondary at wrates peaked near binary periapse. We
conclude that the \ggtaua\ torus is strongly self gravitating and that a major
contribution to its thermal energy is shock dissipation. We interpret its
observed features as manifestations of spiral structures and the low density
material surrounding it as an excretion disk created by outward mass flux. We
interpret GG Tau A as a coplanar system with an eccentric torus, and account
for its supposed mutual inclination as due to degeneracy between the
interpretation of inclination and eccentricity. Although the disks persist for
long enough to permit planet formation, the environment remains unfavorable due
to high temperatures. We conclude that the GG Tau A system is in an eccentric,
~AU orbit.Comment: Accepted for publication in the Astrophysical Journa
The Role of the Unconscious in the Perception of Risks
Dr. Fritzsche argues that our world is too rational and that the psychology of the unconscious, as developed by Jung, can be key to understanding responses to hazards and to resolving conflicts that arise in the political management of risks
The Moral Dilemma in the Social Management of Risks
Dr. Fritzsche offers data seen as demonstrating that irrational fears can lead to grotesque imbalances in social efforts devoted to preventing fatalities
Sex investment ratios in eusocial Hymenoptera support inclusive fitness theory
Inclusive fitness theory predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry (relative relatedness to females and males) of the individuals controlling sex allocation. In monogynous ants (with one queen per colony), assuming worker control, the theory therefore predicts female-biased sex investment ratios, as found in natural populations. Recently, E.O. Wilson and M.A. Nowak criticized this explanation and presented an alternative hypothesis. The Wilson–Nowak sex ratio hypothesis proposes that, in monogynous ants, there is selection for a 1 : 1 numerical sex ratio to avoid males remaining unmated, which, given queens exceed males in size, results in a female-biased sex investment ratio. The hypothesis also asserts that, contrary to inclusive fitness theory, queens not workers control sex allocation and queen–worker conflict over sex allocation is absent. Here, I argue that the Wilson–Nowak sex ratio hypothesis is flawed because it contradicts Fisher’s sex ratio theory, which shows that selection on sex ratio does not maximize the number of mated offspring and that the sex ratio proposed by the hypothesis is not an equilibrium for the queen. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain ‘split’ (bimodal) sex ratios or data showing queen and worker control and ongoing queen–worker conflict. By contrast, these phenomena match predictions of inclusive fitness theory. Hence, the Wilson–Nowak sex ratio hypothesis fails both as an alternative hypothesis for sex investment ratios in eusocial Hymenoptera and as a critique of inclusive fitness theory
- …