Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.2609c&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #26.09; Bulletin of the American Astronomical Society, V
Astronomy and Astrophysics
Astronomy
Scientific paper
We present a detailed study of the horizontal branch (HB) of the core region of the globular cluster M15 (NGC 7078), using archival HST (Hubble Space Telescope) data covering out to 2.2 arcminutes from the center of M15. The HB distribution in I-band was determined and compared with synthetic models of evolved stars tabulated by Cassisi et al. 2004. We focus on the ratio R2 = NAGB/NHB, which is sensitive to semiconvection in the cores of HB stars and to the rate of the 12C(α, γ)16O reaction in the latter stages of the HB phase. The R2 value from the M15 distribution was computed to be 0.123 ± 0.019, which is in agreement with the value predicted by appropriate synthetic models of both the primary and secondary HB peaks (0.0110 ± 0.011). This result suggests either that extreme blue HB stars produce AGB stars at approximately the same rate as redder HB stars, or that HB stars just bluer than the instability strip are relatively more effective at producing AGB stars and therefore partially compensate for the lack of AGB stars produced by extreme blue HB stars. We will also discuss the cluster AGB-manqué stars to attempt to constrain where it is on the HB that stars begin to avoid a subsequent AGB stage.
Castora Jessica E.
Sandquist Eric L.
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