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Title:
Rotation and Oxygen Line Strengths in Blue Horizontal-Branch Stars
Authors:
Peterson, Ruth C.; Rood, Robert T.; Crocker, Deborah A.
Publication:
Astrophysical Journal v.453, p.214 (ApJ Homepage)
Publication Date:
11/1995
Origin:
APJ; KNUDSEN
Astronomy Keywords:
GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL NGC NUMBER: NGC 288, GALAXY: GLOBULAR CLUSTERS: GENERAL, STARS: ABUNDANCES, STARS: HORIZONTAL-BRANCH, STARS: ROTATION
DOI:
10.1086/176381
Bibliographic Code:
1995ApJ...453..214P

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 Teff within each cluster and compare the average values of each from cluster to cluster.

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 Teff but may decrease with increasing Teff 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 Teff = 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.

No star in any cluster rotates more rapidly than 40 km s--1.Thus <upsilo sin i> 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 upsilo sin i on horizontal-branch (HB) color. The distribution of upsilo 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 upsilo sin i >= 30 km s-1, 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.


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