Two explanations exist for the short-lived radionuclides (T1/2 ≤ 5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7Be is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of 7Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if 7Be and 10Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F ~ 2 × 1010 cm-2 s-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced 60Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.
Recent satellite detections of 0 VI absorption and soft X-ray emission leave little doubt that, locally , a substantial fraction of the volume of space between interstellar clouds must be filled with rarified and highly ionized gas at temperatures ranging from 2 × 10 5 to > 10 6 K. (See the reviews of Spitzer and Jenkins 1975, and Kraushaar 1977.) The physical state of this gas contrasts sharply with the theoretical picture of a largely neutral, warm, intercloud medium at ∼ 10 4 K developed by Pikel'ner (1967), Field, Goldsmith, and Habing (1969), and Spitzer and Scott (1969). My purpose here is to review the evidence, observational and theoretical, concerning how extensive hot gas at ∼ 10 6 K might be.
The most numerous among bright galaxies and the largest in the universe are spirals and represent some of the most beautiful and spectacular phenomena due to the presence of their remarkable spiral arms, which denote young stars and starforming regions. We investigate the phenomenon of well-defined spiral arms through the basics of galactic dynamics and stellar orbits. Instead of the material arms the Lin-Shu density wave theory is introduced as a well accepted theory, which deals with a small-amplitude orbital perturbations and closed orbits in a noninertial frame of reference. We explain the corotation and ultraharmonic resonances of epicycles and mapping of our spiral Milky Way Galaxy.
This paper re-examines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution of centrally condensed molecular cloud cores, a process that has been termed the gravomagneto catastrophe. Our calculation applies in the geometric limit of a highly flattened core and allows for a semi-analytic treatment of the full problem, although physical fixes are required to resolve a poor representation of the central region. A noteworthy feature of the overall formulation is that the solutions for the ambipolar diffusion portion of the evolution for negative times ($t < 0$) match smoothly onto the collapse solutions for positive times ($t > 0$). The treatment shows that the resulting cores display non-zero, but sub-magnetosonic, inward velocities at the end of the diffusion epoch, in agreement with current observations. Another important result is the derivation of an analytic relationship between the dimensionless mass to flux ratio $λ_0\equiv f_0^{-1}$ of the central regions produced by runaway core condensation and the dimensionless measure of the rate of ambipolar diffusion $ε$. In conjunction with previous work showing that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., that the effective value of $ε$ can be enhanced by turbulence, the resultant theory provides a viable working hypothesis for the formation of isolated molecular-cloud cores and their subsequent collapse to form stars and planetary systems.
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