Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...510..915k&link_type=abstract
The Astrophysical Journal, Volume 510, Issue 2, pp. 915-929.
Astronomy and Astrophysics
Astronomy
60
Gamma Rays: Observations, Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Wolf-Rayet, Stars: Supernovae: General
Scientific paper
Recent analysis of COMPTEL data has revealed an extremely close correlation between 53 GHz microwave free-free and 1.8 MeV gamma-ray line emission. While microwave free-free emission arises from the ionized interstellar medium, 1.8 MeV gamma rays are emitted during the radioactive decay of ^26Al. We argue that the close correlation can only be understood if massive stars (M>~20 M_solar) are at the origin of Galactic ^26Al. Based on the measured proportionality factor, we estimate the ^26Al yield of an ``equivalent O7 V star'' to be (1.0+/-0.5)x10^-4 M_solar. Using an estimate for the total Galactic Lyman continuum luminosity of Q=3.5x10^53 photons s^-1, we derive the Galactic ^26Al mass to be 3.1+/-0.9 M_solar. The mass estimate is compared to theoretical nucleosynthesis predictions for ^26Al from core-collapse supernovae and Wolf-Rayet stars. We circumvent the problem of using a weakly constrained star formation rate for this comparison by determining the star formation rate self-consistently from our models, using the Galactic Lyman continuum luminosity. The effects of mass loss and metallicity are considered, and the uncertainties of predicted ^26Al production rates due to poorly known initial mass limits for the candidate sources are discussed. Assuming solar metallicity throughout the entire Galaxy, we predict a Galactic ^26Al mass of 1.6+/-0.3 M_solar, of which ~60% is produced by core-collapse supernovae, while ~40% originates from Wolf-Rayet stars. Taking the Galactic metallicity gradient into account increases the Galactic ^26Al mass to 2.2+/-0.4 M_solar, consistent with the observed value. The increase mainly arises from enhanced production by Wolf-Rayet stars in the metal-rich inner Galaxy; these contribute ~60% of the Galactic ^26Al budget. We predict that the metallicity gradient should produce an inner-to-outer Galaxy intensity contrast of ~30% between 1.8 MeV and Galactic free-free emission, which should be observable by the future gamma-ray spectrometer SPI on the International Gamma-Ray Astrophysics Laboratory (INTEGRAL).
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