The11B/10B Ratio of Local Interstellar Diffuse Clouds

The isotopic ratio 11 B/ 10 B of gas in diffuse interstellar clouds toward ζ Oph, κ Ori, and δ Sco is measured using HST /Goddard High Resolution Spectrograph echelle spectra of the B II 1362 Å line. To within the errors of measurement, the three lines of sight give identical results for a mean value of 11 B/ 10 B = 3.4 ± 0.7, a value quite similar to the solar system ratio of 4.05. These results show that the latter value is not highly anomalous and that a ratio higher than 2.5, as predicted for boron produced from spallation reactions controlled by high-energy cosmic rays, most probably requires a general explanation. The observed ratio is consistent with additional boron production either by spallation by low-energy ("Orion") cosmic rays or by neutrino-induced spallation of carbon in Type II supernovae. The total abundance of B in the diffuse clouds is a factor of 5 less than the meteoritic value. This depletion of B is consistent with that found for Cu and Ga, two elements with a condensation temperature similar to B.