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 [O_I] line strength as compared with its strength in the Sun; the models use a T(τ) 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 mÅ, 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 2-4), but have a larger 0/metals ratio than does the Sun. The 0/C ratio and 0/H ratios are probably the same as in the Sun. The observed weakness of CN may be caused primarily by a low N/H ratio. 3. Stars with strong CN such as a Ser have weak [O_I], an 0/C ratio of about 1.2, probably caused by a slight carbon abundance enhancement. The increased CN line strength in these stars is caused primarily by enriched nitrogen. We suggest that this originates during helium flash and mixing 4. Moderately metal-deficient stars, such as HD 148897, have a large 0/metals ratio. It could be that this quite weak-lined star has an oxygen abundance as high as the Sun. 5. The oxygen lines are not observed in the high-velocity, extremely metal-deficient star HD 122563. Oxygen is deficient, but perhaps not as much so as the metals. During the early phases of nucleosynthesis in our Galaxy, the oxygen content increased at a different rate than the metals and reached its present abundance much more rapidly. This may also be true for carbon, but studies of the CN and CH bands and detailed molecular equilibria will be required. Helium burning in massive red giants, or in supermassive stars, apparently proceeded more rapidly than iron-peak element synthesis. Evolution within a star that would lead to increases of surface 0 and C abundances is not probable except in highly evolved stars.