Al-26 and circumstellar envelopes

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Abundance, Aluminum Isotopes, Carbon Stars, Cosmochemistry, Gamma Ray Spectra, Radioactive Decay, Stellar Envelopes, Beta Particles, Gas Ionization, Grains, Magnesium Isotopes, Meteoritic Microstructures

Scientific paper

The effects of the radioactive decay of Al-26 on the circumstellar envelopes of asymptotic giant branch stars are analyzed. The gamma-rays emitted by the product nucleus Mg-26 escape most envelopes, but the beta-decay positrons are stopped and can ionize and heat the gas. The ionization may produce observable effects in C-rich circumstellar envelopes, particularly if the photospheric Al-26 abundance is as large as inferred from measurements of live Al-26 in the primitive solar nebula or the observations of interstellar 1.8 MeV gamma-rays. For the nearby carbon star IRC +10216, the measured abundance of the molecular ion HCO(+) provides an upper limit of about 4 x 10-3 for the photospheric Al-26/Al-27 ratio, consistent with presolar SiC grains with about the same C-12/C-13 ratio.

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