On the Possibility of Observing Interstellar Aluminium.

Of the interstellar ions which might be expected to be observed, only Al0, Co+, and Ni+ remain undetected. Theoretical estimates of the equivalent widths of X 39 of Al I in the spectra of x2 Ori and Per have been made, using a photoionization cross-section derived by the quantum-defect method. The predicted width in Per is between 0 9 and 2.7 mA, depending on the importance of autoionixation; these values are not inconsistent with a recent observational upper limit. An appendix treats quantitatively the excitation of fine structure in interstellar space. While the results indicate that even very lowlying levels, such as that of Al0, are strongly depopulated, attention is drawn to the importance of excitation by collisions with H atoms.