Michael J. Newman

s-Process Studies: Branching and the Time Scale

The theory of S-process heavy-element formation is reformulated to allow for competition between beta decay and neutron capture at various nuclei along the path. Solutions to the resulting branching network equations are presented (under the assumption of constant temperature and neutron flux) that do not require steady flow for the neutron current. Using the exponential exposure distribution rho (tau) =G exp(-tau/tau/sub 0/) and recently calculated temperature-dependent beta-decay rates, comparison of several key branches yields the following average conditions for the solar-system S-process environment: Tapprox. =3.1 x 10/sup 8/ K, n/subn/approx. =1.6 x 10/sup 7/ neutrons cm/sup -3/. For tau/sub 0/=0.25 n mb/sup -1/ we find that about 4.8 neutron captures per exposed iron seed are required over a time of the order of a few thousand years for synthesis of the bulk of the solarsystem S-process material, with an average neutron capture time approx.10 years (for sigmaapprox.500 mb). (AIP)

Solar Models of Low Neutrino-Counting Rate: The Central Black Hole

Partial evolutionary sequences have been calculated for several solar models with central black holes of order 10

Solar models of low neutrino-counting rate - The depleted Maxwellian tail

sup -5

S-process studies - The effects of a pulsed neutron flux

M/sub sun/. If these are assumed to radiate their Eddington limiting luminosity, the central temperature is depressed to the extent that the predicted count rate for the

Interstellar Cloud Material: Contribution to Planetary Atmospheres

sup 37

ENCOUNTERS WITH INTERSTELLAR CLOUDS: THE EFFECT ON STELLAR WINDS AND THE POSSIBILITIES FOR ACCRETION*

Cl solar neutrino experiment nears the current upper limit of 1 SNU; this occurs when the auxiliary energy source provides about half of the solar luminosity. Count rates below this limit would result from an even larger black-hole luminosity. Consequences for stellar evolution of the occasional presence of black holes inside normal stars are discussed. (AIP)

Implications of Solar Evolution for the Earth's Early Atmosphere

Evolutionary sequences for the sun are presented which confirm that the Cl-37 neutrino counting rate will be greatly reduced if the high-energy tail of the Maxwellian distribution of relative energies is progressively depleted. Thermonuclear reaction rates and pressure are reevaluated for a distribution function modified by the correction factor suggested by Clayton (1974), and the effect of the results on solar models calculated with a simple Henyey code is discussed. It is shown that if the depletion is characterized by a certain exponential dependence on the distribution function, the counting rate will fall below 1 SNU for a distribution function of not less than 0.01. Suggestions are made for measuring the distribution function in the sun by means of neutrino spectroscopy and photography.

Constraints on the gravitational constant at large distances

Recent investigations of the astrophysical site of the s-process strongly suggest the helium-burning shell of the helium-shell-flashing stars of intermediate mass as the most likely site for the s-process event which produced the solar-system abundances. Previous analyses of branching in the s-process have been in terms of a constant temperature and density environment. The occurrence of periodic thermal instabilities seems to make this assumption inappropriate. The current effort is to reformulate the mathematics of the branched s-process to allow for the influence of thermal pulses. The effect of pulses on the unique isobars between branches is generally small; the largest effect of pulses is at those branch points where the branching ratio for ..beta..-decay during a pulse is small, but the ..beta..-decay lifetime during the interpulse period is shorter than the time between pulses. It is shown that the decay of the unstable branch nuclei between neutron irradiations allows the mean s-process neutron flux responsible for the solar-system abundances in such a pulsed environment to be considerably larger than that for a single continuous-exposure event.

Climatic effects during passage of the Solar System through interstellar clouds

A statistical analysis of the properties of dense interstellar clouds indicates that the solar system has encountered at least a dozen clouds of sufficient density to cause planets to accumulate nonnegligible amounts of some isotopes. The effect is most pronounced for neon. This mechanism could be responsible for much of the neon in Earths atmosphere. For Mars, the predicted amount of neon added by cloud encounters greatly exceeds the present abundance.

The extraordinary composition of U Aquarii

McCrea’ has recalled the suggestion’ that terresterial Ice Ages may be produced by accretion of interstellar matter by the Sun. Subsequently several a ~ t h o r s j ~ have discussed further consequences o f encounters between stars and dense interstellar clouds. I n this report we summarize some results of an analysis that will be presented i n more detail e l ~ e w h e r e . ~ TABLE I presents an analysis of data from the literature concerning the properties of clouds i n the present interstellar medium. For each of several samples we display the number density n of hydrogen nuclei, the number nc of such clouds per cubic parsec in the disk of the Galaxy, the fraction f, of the disk of the Galaxy occupied by clouds of that class, the mean time 7 between encounters, and the number ofencounters N , with such an object expected for a disk star of solar age. A more detailed discussion of these and related quantities will appear elsewherc.’ The undertainties involved are such that the sample labeled 21 cm may represent the same class of object as Spitzer’s “standard cloud”; t h e CO observations and the more dense Lynds’ nebulae, however, appear to represent objects substantially diferent from the lower density clouds. I t is seen that while the less dense clouds occupy a fairly substantial fraction of the galactic disk, and encounters with such objects should be relatively routine for long-lived stars like the Sun , the very dense clouds occupy a much smaller fraction of the disk, and the Solar System could not be expected to have encountered many of them. This information has been summarized by the definition of the two integral distributions shown in FIGURE 1 . Here f ( 2 n ) is the fraction of the volume of the disk of the Galaxy occupied by clouds with density exceeding n , and N , (2 n ) is the number of encounters expected during 4.6 x 10’ yr with clouds of density greater than or equal to n. I t is not known whether the break at n lo3 (3m-j is real or due to some observational selection eKmt operating against the very dense clouds; i f the darkest clouds are underrepresented due to observational difficulties, their role may be considerably more important than is indicated by this analysis.