David S. P. Dearborn

The formation and early dynamical evolution of bound stellar systems

We present the results of numerical N-body calculations which simulate the dynamical evolution of young clusters as they emerge from molecular clouds. We follow the evolution of initially virialized stellar systems of 50 and, in some cases, 100 stars from the point in time immediately after the stars have formed in a cloud until a time long after all the residual star-forming gas has been dispersed from the system. By varying the star formation efficiency and the gas dispersal time for each model, we determined the combination of these parameters which result in the production of bound stellar groups after the gas not used in star formation is completely dispersed.

Deep Mixing of 3He: Reconciling Big Bang and Stellar Nucleosynthesis

Low-mass stars, ∼1 to 2 solar masses, near the Main Sequence are efficient at producing the helium isotope 3He, which they mix into the convective envelope on the giant branch and should distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the low observed cosmic abundance of 3He with the predictions of both stellar and Big Bang nucleosynthesis. Here we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell, where a nuclear reaction lowers the mean molecular weight slightly. Thus, we are able to remove the threat that 3He production in low-mass stars poses to the Big Bang nucleosynthesis of 3He.

Compulsory Deep Mixing of 3He and CNO Isotopes in the Envelopes of low-mass Red Giants

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the {sup 3}He({sup 3}He,2p){sup 4}He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of {sup 3}He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades standing. This mixing mechanism operates rapidly once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M{sub {circle_dot}} develop {sup 12}C/{sup 13}C ratios of 14.5 {+-} 1.5, while Pop II stars process the carbon to ratios of 4.0 {+-} 0.5. In stars less than 1.25 M{sub {circle_dot}}, this mechanism also destroys 90% to 95% of the {sup 3}He produced on the main sequence.

Three-dimensional Numerical Experimentation on the Core Helium Flash of Low-Mass Red Giants

We model the core helium flash in a low-mass red giant using Djehuty, a fully three-dimensional (3D) code. The 3D structures were generated from converged models obtained during the one-dimensional (1D) evolutionary calculation of a 1 M☉ star. Independently of which starting point we adopted, we found that after some transient relaxation the 3D model settled down with a briskly convecting He-burning shell that was not very different from what the 1D model predicted.

Diagnostics of stellar evolution: The oxygen isotopes

Abstract The oxygen isotope ratios provide a probe of the physics of stellar interiors. Among the phenomena that can be studied through the oxygen isotope ratios are the maximum depth of penetration of the convective envelopes on the giant branch, the efficiency of nonconvective mixing, as well as restrictions on the mass loss that occurs. The oxygen isotope ratios also provide an indicator of the nuclear processing that occurs during third dredge up and carbon star formation. In addition to the oxygen isotopes, we indicate the expected behavior of nitrogen and the light elements.

/sup 12/C//sup 13/C ratios in stars ascending the giant branch the first time

Stellar evolution calculations were carried out for 1 M/sub sun/ and 2 M/sub sun/ red giants in order to understand the observed /sup 12///sup 13/C ratios. Although very low values of 5 to 7 in some red supergiants can be understood in terms of thermally unstable helium shell burning models, there remains a problem of moderately low values (10 to 20) in stars not luminous enough for this process. In addition to the standard mixing at the base of the giant branch, which can give /sup 12/C/sup 13/C ratios in the region of 25 to 30, we consider: variation in initial composition; meridional mixing; mass loss; zero-age composition gradient; mixing during the helium flash; and instability of the hydrogen-burning shell. Of these, only the mass loss mechanism and the hydrogen shell instability produce values below approx.20. (AIP)

Main-sequence mass loss and the lithium dip

The significant dip in observed lithium abundances for Population I stars near M about 1.3 solar mass is discussed. It is noted that this dip occurs near where the instability strip crosses the main sequence on the lower edge of the Delta Scuti stars and that stellar pulsations are expected to give rise to mass loss. A total mass loss of 0.05 solar mass over the main-sequence lifetime of these stars would be sufficient to explain the observations of lithium depletion. The absence of a dip in the Pleiades and of significant depletion of beryllium in the Hyades places tight constraints on the rate of mass loss. These constraints make unlikely the high main-sequence mass-loss rates which would significantly affect globular cluster ages. 30 refs.

The Near-Infrared Spectrum of the Planetary Nebula IC 5117

Infrared spectroscopy from 0.8 to 2.5 μm is presented for the planetary nebula IC 5117. The emission lines of IC 5117 span a wide range of ionization that includes He II, [S III], [S II], [N I], and H2. The reddening measured from the hydrogen lines is E(B-V) = 0.79, most of which is probably interstellar in origin. The He/H abundance ratio is 0.113 ± 0.015, with approximately 10% of the helium being doubly ionized. Using our measurements of [S II] and [S III] lines and published observations of [S IV], we find a sulfur abundance, relative to hydrogen, of N(S)/N(H) = 7.8 × 10-6, or approximately half the solar value. Fluxes and flux limits for several lines of molecular hydrogen are presented. Measurements of 1–0 transitions, together with the limits on 2–1 transitions, indicate Tvib ~ Trot = 1900 K, suggesting a purely collisional excitation mechanism. The ortho-to-para ratio is ~3, a value that is also indicative of collisional excitation. The presence of [C I] λ9850 is consistent with previous studies of IC 5117 that indicated carbon is more abundant than oxygen. IC 5117 follows the trend of planetary nebulae that display bipolar outflows and H2 emission to be carbon-rich. We confirm the results of Zhang & Kwok, who reported infrared continuum emission substantially in excess of that produced by the ionized gas. This emission is most likely due to hot dust (T ~ 1300 K) and accounts for roughly half of the continuum between 1.5 and 2 μm.

Window for the dark matter solution to the solar neutrino problem

The existence of dark matter particles (cosmions) inside the Sun has been suggested as an explanation for the disagreement between measured and theorical solar neutrino fluxes. The new theorical results on cosmion capture, evaporation and energy tranport, as well as a modern solar evolution code, are used to estimate accuratly the range of cosmion masses and cross sections which can solve the solar neutrino problem. A range somewhat smaller than previously estimated is found. Including new results from a silicon direct detection experiment, only two extremely small regions of parameter space remain acceptable for the coherent spin-independent cross sections often discussed.

Glitter and Glints on Water

We present new observations of glitter and glints using short and long time exposure photographs and high frame rate videos. Using the sun and moon as light sources to illuminate the ocean and laboratory water basins, we found that (1) most glitter takes place on capillary waves rather than on gravity waves, (2) certain aspects of glitter morphology depend on the presence or absence of thin clouds between the light source and the water, and (3) bent glitter paths are caused by asymmetric wave slope distributions We present computer simulations that are able to reproduce the observations and make predictions about the brightness, polarization, and morphology of glitter and glints. We demonstrate that the optical catastrophe represented by creation and annihilation of a glint can be understood using both ray optics and diffraction theory.