Potassium Abundances in Red Giants of Mildly to Very Metal-Poor Globular Clusters

A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 angstrom was carried out for 15 red giants belonging to three globular clusters of different metallicity (M 4, M 13, and M 15) along with two reference early-K giants (p Boo and a Boo), in order to check whether the K abundances are uniform within a cluster, and to investigate the behavior of the [K/Fe] ratio at the relevant metallicity range of -2.5 [Fe/H] less than or similar to -1. We confirmed that [K/H] (as well as [Fe/H]) is almost homogeneous within each cluster to a precision of less than or similar to 0.1 dex, though dubiously large deviations are exceptionally seen for two peculiar stars showing signs of considerably increased turbulence in the upper atmosphere. The resulting [K/Fe] ratios are mildly supersolar by a few tenths of dex for three clusters, tending to gradually increase from similar to 0.1-0.2 at [Fe/H] similar to -1 to similar to +0.3 at [Fe/H] similar to -2.5. This result connects reasonably well with the [K/Fe] trend of disk stars (-1 less than or similar to [Fe/H]) and that of extremely metal-poor stars (-4 less than or similar to [Fe/H.] less than or similar to -2.5). That is, [K/Fe] appears to continue a gradual increase from [Fe/H] similar to 0 toward a lower metallicity regime down to [Fe/H] similar to -3, where a broad maximum of [K/Fe] similar to +0.3-0.4 is attained, possibly followed by a slight downturn at [Fe/H] less than or similar to -3.