Figure 2.1.2 - Galactic map of the ²⁶Al - 1809 keV line as seen by COMPTEL. A tracer of recent (10⁶yr) nucleosynthesis.

In addition to the galactic longitude and latitude distribution of the 1.809 MeV line emission, information on the origin of the ²⁶Al can also be obtained from studies of the shape of the line. Surprisingly, large Doppler broadening of the line has recently been observed with a balloon-borne gamma-ray instrument GRIS, implying velocities of about 400 km/sec, much larger than that expected from galactic rotation. This suggests that the emitting ²⁶Al may be in high-velocity grains precipitated from supernova ejecta. Future observations by INTEGRAL should provide a much better understanding of the origin of the radioactive aluminum and of the chemical evolution of the galaxy. The same models that agree with the observed ²⁶Al signal also predict a strong signal from ⁶⁰Fe, another long-lived nucleus (J1/2 = 1.5x10⁶ y) made in Type II supernovae. The mass of 60Fe predicted is 1.7 ± 0.9 M_o implying a signal about 15% as strong as ²⁶Al. This is on the edge of what can be currently detected but should be visible to INTEGRAL. The diffuse galactic 0.511 MeV line from positron annihilation, which has been extensively observed from the galactic center region, is the most luminous gamma-ray line in the galaxy. The positrons responsible for this emission are most likely from the decay of the radionuclei ⁵⁶Co, ⁴⁴Ti and ²⁶Al resulting from various processes of galactic nucleosynthesis.

2.1.3 INTERSTELLAR PROCESSES The recent discovery of gamma-ray emission lines from the Orion giant molecular cloud complex has revealed exciting new particle acceleration processes in this nearest region of recent star formation. This has very important implications for light element nucleosynthesis. Gamma-ray line emission in the 3 to 7 MeV range was observed from the Orion complex with COMPTEL. The radiation shows emission peaks near 4.4 and 6.1 MeV, consistent with the de-excitation of excited states in ¹²C and ¹⁶O produced by accelerated particle interactions. Moreover, the intensity of these lines is roughly two orders of magnitude greater than that expected from irradiation by low-energy cosmic rays with energy density equal to that of the local galactic cosmic rays.
This emission requires that the ambient matter in Orion, both gas and dust, is undergoing bombardment by an unexpectedly intense, locally accelerated, population of energetic particles. The present rate of energy dissipation of the particles in Orion is about 5x10³⁸ erg s^-1. The most likely source of this energy is the ~80,000-year old supernova which is thought to be responsible for the Orion-Eridanus bubble. If such particle fluxes also exist in other massive star formation regions, their interactions could be the major source of light element (⁶Li, Be & B) nucleosynthesis in our galaxy.