ON THE MEASUREMENT OF THE DEUTERIUM ABUNDANCE AND THE INTERPRETATION OF THE LYMAN EDGE IN QSO ABSORPTION-LINE SYSTEMS

Abstract

ABSTRACT A detailed examination of the detectability of the Lyman series of deuterium in QSO absorption-line systems shows that observations of the high-order Lyman lines in clouds of very high column density and low velocity dispersion (such as "damped Ly-alpha systems") might produce efficient and precise measures of the deuterium to hydrogen ratio, although these systems typically also display heavy element absorption and therefore are not ideal for specifying the primordial [D/H] ratio. This work is complementary to the earlier work of Webb et al. (1991), who explored the measurement of the deuterium in systems with higher velocity dispersions. The dependence of the shape of the Lyman continuum cut-off on deuterium abundance, velocity dispersion and column denisty is also examined in detail, and low to moderate resolution synthetic spectra are presented. For clouds of Doppler dispersion sigma, the "effective" wavelength of the Lyman edge is shifted to ~911.76(1 + (sigma/c)2/3)A. for absorbing clouds of high column density, DI can make a significant contribution to the opacity close to the "effective" Lyman cut-off, where the Doppler broadening is comparable to the spacing between the high-order lines.