Rotation and Oxygen Line Strengths in Blue Horizontal-Branch Stars

Abstract

We have measured rotational velocities and oxygen abundances in 29 blue horizontal-branch (BHB) stars in M13, 22 in M3, and 16 in NGC 288 from high-resolution spectra of the O I triplet at 7771-7775 Å. Here we outline the behavior of rotation and oxygen line strength with stellar effective temperature T<SUB>eff</SUB> within each cluster and compare the average values of each from cluster to cluster. <P />The mean strength of the oxygen lines is greatest in NGC 288 and weakest in M3. Within each cluster, the oxygen abundances are nearly constant among stars of a given T<SUB>eff</SUB> but may decrease with increasing T<SUB>eff</SUB> Among the BHB stars cooler than 10,000 K, there is no sign of the counterparts to the super-oxygen-poor giants which dominate the bright end of the giant branch in M13. The stars hotter than T<SUB>eff</SUB> = 11,500 K show no oxygen lines at all. While these faint blue stars have poorly exposed spectra and may be hot enough to ionize oxygen, the abruptness of the disappearance of the O I lines suggests that either the oxygen abundances are truly low or diffusion sets in abruptly at this color. <P />No star in any cluster rotates more rapidly than 40 km s-<SUP>-1</SUP>.Thus &lt;υ sin i&gt; is much lower in BHBs than in the superficially similar Population I B8 V-A7 V main-sequence stars. In M13, there is no obvious dependence of υ sin i on horizontal-branch (HB) color. The distribution of υ sin i values in that cluster can be matched only with a bimodal or otherwise non-Gaussian distribution of V values. Rotational velocities are largest in M13, where six stars have υ sin i ≥ 30 km s<SUP>-1</SUP>, and are much smaller in M3 and NGC 288, with none at this level. Since both M 13 and NGC 288 have very blue HBs, this indicates that rotation and oxygen abundance are not enough by themselves to determine HB color, either alone or together; another factor such as age must also be at work. Thus, the "second-parameter" problem involves more than one such parameter, whose relative influence varies among clusters.