No abstract

We discuss the chemical evolution of metal poor galaxies and conclude that their oxygen deficiency is not due to: the production of black holes by massive stars or a varying slope of the Initial Mass Function, IMF, at the high-mass end. A varying IMF at the low-mass end alone or in combination with: (a) an outflow of oxygen-rich material, (b) an outflow of well-mixed material, and (c) the presence of dark matter that does not participate in the chemical evolution process, is needed to explain their oxygen deficiency.Outflow of material rich in O helps to account for the large ∆Y /∆O values derived from these objects, but it works against explaining the (Z − C − O)/O and C/O values.

Emission lines from H II regions and planetary nebulae are contaminated by shock waves produced by stellar winds and supernova remnants. This effect is studied and it is found that it could be responsible for: (a) the difference between the O/H abundances derived from the O nebular lines coupled with H II region models and those derived from the observed I (4363)// (5007) ratio, (b) the large t 2 values observed in the Orion nebula and M 8, (c) the difference between the stellar and the H II region O/H abundances in the solar neighborhood, (d) the large [S n]/Ha ratios observed in H II galaxies, and (e) the low O/H ratios and the N/O versus O/H anticorrelation found in Type I PN and shell nebulae around Population I stars.

We study energy relaxation in thermalized one-dimensional nonlinear arrays of the Fermi-Pasta-Ulam type. The ends of the thermalized systems are placed in contact with a zero-temperature reservoir via damping forces. Harmonic arrays relax by sequential phonon decay into the cold reservoir, the lower-frequency modes relaxing first. The relaxation pathway for purely anharmonic arrays involves the degradation of higher-energy nonlinear modes into lower-energy ones. The lowest-energy modes are absorbed by the cold reservoir, but a small amount of energy is persistently left behind in the array in the form of almost stationary low-frequency localized modes. Arrays with interactions that contain both a harmonic and an anharmonic contribution exhibit behavior that involves the interplay of phonon modes and breather modes. At long times relaxation is extremely slow due to the spontaneous appearance and persistence of energetic high-frequency stationary breathers. Breather behavior is further ascertained by explicitly injecting a localized excitation into the thermalized arrays and observing the relaxation behavior.

The propagation of a pulse in a nonlinear array of oscillators is influenced by the nature of the array and by its coupling to a thermal environment. For example, in some arrays a pulse can be speeded up while in others a pulse can be slowed down by raising the temperature. We begin by showing that an energy pulse ͑one dimension͒ or energy front ͑two dimensions͒ travels more rapidly and remains more localized over greater distances in an isolated array ͑microcanonical͒ of hard springs than in a harmonic array or in a soft-springed array. Increasing the pulse amplitude causes it to speed up in a hard chain, leaves the pulse speed unchanged in a harmonic system, and slows down the pulse in a soft chain. Connection of each site to a thermal environment ͑canonical͒ affects these results very differently in each type of array. In a hard chain the dissipative forces slow down the pulse while raising the temperature speeds it up. In a soft chain the opposite occurs: the dissipative forces actually speed up the pulse, while raising the temperature slows it down. In a harmonic chain neither dissipation nor temperature changes affect the pulse speed. These and other results are explained on the basis of the frequency vs energy relations in the various arrays. ͓S1063-651X͑99͒11411-9͔