The gravities of K giant stars determined from [O I] and OH features

The oxygen abundances of four K giant stars, including Arcturus, are deduced from [O I] and OH line data. The OH lines used are vibration-rotation lines of the $$\Delta\upsilon = 2$$ and $$\Delta\upsilon = 1$$ sequences. Surface gravities are then determined for the stars, adopting a temperature and overall abundance and then solving for the surface gravity by requiring the oxygen abundance derived from OH lines to match that derived from [O I] lines. In addition, an analysis of the $$\Delta\upsilon = 2$$ OH lines in the solar spectrum is carried out in order to obtain the solar oxygen abundance and thereby check the transition probabilities used. We find that more accurate values of the OH transition probabilities would offer another means of choosing which solar models better represent the Sun. The wavelengths of the $$\Delta\upsilon = 2$$ lines are located in a region in which the H ^– opacity is a minimum, suggesting that these lines are formed deep in the stellar (and solar) atmospheres, and are therefore less likely to be subject to non-LTE effects. They are also much weaker than the $$\Delta\upsilon = 1$$ lines, and their equivalent widths are less dependent on microturbulent velocity. We also find that the microturbulent velocity is different for lines of the two sequences in α Tau, presumably because of the different depths of formation.