Gamma Rays, Cosmic Rays, and Extinct Radioactivity in Molecular Clouds

Details Gamma Rays, Cosmic Rays, and Extinct Radioactivity in Molecular Clouds Gamma Rays, Cosmic Rays, and Extinct Radioactivity in Molecular Clouds

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...451..681c&link_type=abstract

Astrophysical Journal v.451, p.681

Astronomy and Astrophysics

Astronomy

37

Gamma Rays: Theory, Ism: Clouds, Ism: Cosmic Rays, Nuclear Reactions, Nucleosynthesis, Abundances, Solar System: General

Scientific paper

We investigate causal connection between two astonishingly big numbers: the very large 26Al concentration (5 × 10-5 of 27Al) in the early solar system and the very large nuclear excitation rate in Orion clouds. We present three separate pictures attributing 26Al within the early solar system and other molecular cloud cores to special cosmic-ray irradiation of those cloud cores. These pictures reinterpret the large 26Al/27Al ratio found in the early solar accretion disk, and seem not to be relevant to the present interstellar 1.5 Msun of 26Al. These three pictures of cosmic-ray irradiation of molecular clouds accounting for their high 26Al content are:
1. High flux of low-energy cosmic ray 0, Na, Mg, and Si nuclei stopping in the clouds with partial conversion to 26Al by nuclear interactions while they stop (Clayton 1994);
2. Stopping of low-energy galactic cosmic rays, which are known (at 100 MeV nucleon-1) to carry the very large activity 26Al/27Al = 0.1 and which we argue to be absorbed by cloud cores;
3. Stopping of newly synthesized particles accelerated from local ejecta of supernovae and W-R star winds, which carry activities as great as 26Al/27Al = 0.01 from those events.
In these pictures the cosmic rays may be very different in origin than the galactic cosmic rays. At low energy they are injected into clouds and stopped in the cloud cores. We normalize our expectations for massive clouds to the inelastic nuclear excitation rates of 12C*(4.43 MeV) and 16O*(6.13 MeV) gamma rays emerging from the clouds in Orion (Bloemen et al. 1994). Picture 1 is plagued by very large power requirements if the accelerated particles are predominantly hydrogen. Nonetheless, we show that several other extinct radioactivity concentrations that accompanied 26Al in the early solar system would be coproduced by ordinary cosmic-ray composition. Our most promising construction of picture 1 appears to be anomalous acceleration of 16O ions (as known from the solar wind) to several MeV nucleon-1, resulting in both 4.43 and 6.13 MeV gamma ray lines and 12C(16O, pn)26Al in the clouds. Those ions ease the risk of producing too much 53Mn, which plagues each picture unless proton bombardment is suppressed by the acceleration mechanism. We confirm that overabundance of 9Be in solar matter also plagues irradiations producing 26Al within it unless those energies are less than about 10 MeV nucleon-1.
We motivate each of these pictures by advancing a magnetized raisin-pudding model of molecular clouds and their embedded cores. The model suggests that both the nuclear interactions and the stopping of the ions occur preferentially within the dense cores of molecular clouds, which causes those cores to accumulate larger 26Al/27Al ratios than does the bulk of the molecular clouds. We argue that cosmic rays from OB associations drive the turbulence within molecular clouds near them. It is possible that these processes, rather than fresh radioactivity ejected from stars, cause star-forming regions to contain several of the extinct radioactivities near levels found in the early solar system. A new chapter in the relationship of OB associations and cosmic rays to the origins of solar systems has been opened by the COMPTEL detection.

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