Neutral Oxygen in Late-Type Stars

Abstract

We have identified the forbidden absorption nebular and auroral lines of oxygen in the spectra of about seventy G and K stars, mostly giants. While weak, they appear in a relatively uncrowded spectral region and can be studied at high dispersion. Coming from the ground state of a dominantly neutral element, the nebular lines are insensitive to differences of model and non-equilibrium effects in line formation. They are formed over a wide range of optical depth and are therefore insensitive to the detailed structure near the boundary temperature. Approximate model atmospheres predict the expected changes in the [0 i] line strength as compared with its strength in the Sun; the models use a T(r) relation scaled from that in the Sun and opacity tables. The depletion of the free atomic oxygen is caused mostly by CO formation which must be included in detail. We have measured equivalent widths of forbidden oxygen lines in about one-third of the stars and estimated them in the other two-thirds. The measured strengths, from 12 to 140 mA, together with theoretical calculations, lead to the following conclusions: 1. Most stars have a solar abundance ratio of 0/C, near 1 6, and the solar absolute abundances of both these elements. 2. The forbidden lines are observed to be strengthened in the stars with weak CN and high velocity, such as a Boo. These stars are mildly metal deficient (by a factor of 2 ), but have a larger 0/metals ratio than does the Sun. The 0/C ratio and 0/H ratios are…