The Effects of Horizontal-Branch Stars on the Strength of the Hβ Line and the Ages of Old Stellar Systems

Details The Effects of Horizontal-Branch Stars on the Strength of the Hβ Line and the Ages of Old Stellar Systems The Effects of Horizontal-Branch Stars on the Strength of the Hβ Line and the Ages of Old Stellar Systems

Jan 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...19312408l&link_type=abstract

American Astronomical Society, 193rd AAS Meeting, #124.08; Bulletin of the American Astronomical Society, Vol. 31, p.669

Astronomy and Astrophysics

Astronomy

Scientific paper

The strength of the Hβ line for integrated globular cluster spectra has been computed from the evolutionary population synthesis models. These models include, for the first time, the systematic variations of horizontal-branch (HB) morphology with metallicity and age. Our models suggest that the strength of the Hβ line is significantly affected by the presence of blue HB stars. The Hβ line strength does not monotonically decrease as metallicity increases, instead it reaches maximum when the distribution of HB stars is centered around 9500 K, the temperature where the Hβ line is most prominent. Our models indicate that the strength of the Hβ line increases as much as 0.7 { Angstroms} due to the presence of HB stars. We note that the relations between Hβ line index and Mg line indices of the globular clusters in M87 and in NGC 1399 are systematically shifted to higher metallicity compared to the Galactic counterparts. Our models suggest that this systematic difference is understood if the globular clusters in giant elliptical galaxies are, in the mean, several billion years older than those in our Galaxy. Further observations from the large ground based telescopes and space UV facilities will help to clarify whether this difference is indeed due to the age differences or not.

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