The Horizontal Branch ``Jump" in Broadband U

Details The Horizontal Branch ``Jump" in Broadband U The Horizontal Branch ``Jump" in Broadband U

May 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...19513101s&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #131.01; Bulletin of the American Astronomical Society, Vol. 32, p.878

Astronomy and Astrophysics

Astronomy

Scientific paper

Recent examinations of globular clusters have revealed a new phenomenon affecting the horizontal-branch (HB) morphology of Population II stars. Galactic globular clusters have been found to have a ``jump" in the (Strömgren) u, u-y diagram, which is best characterized as a systematic deviation of blue-HB stars from canonical zero-age HB (ZAHB) models---in the sense that the stars appear brighter and/or hotter than predicted by these models (Grundahl et al. 1999, ApJ, 524, 242). This deviation begins at T eff = 11,500 +/- 500 K and continues until T eff 20,000 K. The most likely mechanism for the enhancement of near-ultraviolet flux is radiative levitation of heavy elements in hot HB atmospheres. Accurate determination of the physical parameters of HB stars is crucial for a large number of reasons, including the formation history and age of our galaxy. Hence it is important to pose as many empirical constraints on the jump phenomenon as possible. We have examined the horizontal branches of a number of globular clusters and two dwarf spheroidal galaxies using broadband photometry in an effort to place additional empirical constraints on the jump phenomenon. In particular, we were able to address the following issues: (1) Does the jump occur in broadband U as well as in Strömgren u? (2) Is the jump present in a wider variety of objects than the inner halo globular clusters discussed in Grundahl et al. (1999)? (3) Does the presence of internal metallicity dispersions affect the presence and morphology of the jump? We confirm that the broadband U photometry of our studied clusters shows a departure from the canonical models at relatively high effective temperatures. We also confirm this departure in outer halo globulars clusters and, possibly, dwarf spheroidal galaxies with internal metallicity dispersions.

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