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Session 83 - Elliptical Galaxies.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[83.05] Horizontal Branch Morphology andthe UV Upturn Phenomenon in the Elliptical Galaxies

S. Yi, P. Demarque, A. A. Oemler Jr. (Yale U.)

Recent HUT observation of the FUV flux of several elliptical galaxies has been interpreted as due to the very low-mass, modestly metal-rich (about 1-2 solar), helium burning (horizontal branch - HB) stars, with a narrow range of temperature (Brown, Ferguson, amp; Davidsen 1995, ApJ, 454, L15). It is interesting to see if population synthesis models based purely on standard stellar evolution theory can explain these observational suggestions. The discovery of several analogous hot stars in the old, metal-rich open cluster NGC 6791 also suggests that higher mass loss rates should be considered for more metal-rich stars (Liebert, Saffer, amp; Green, 1994, AJ, 107, 1408). We have constructed new helium burning phase evolutionary tracks using improved physics. Based on the tracks, synthetic HB models were built under the assumption of \DeltaY/\DeltaZ=2 and 3 and a gaussian mass distribution on the HB with various mass loss efficiency parameters \eta in the Reimers' formula. The mass distribution on the HB strongly affects the resulting composite UV spectra. We found that there is more than one solution which reproduces a similar magnitude of UV upturn. However, we prefer the modestly metal-rich populations with high mass loss rates as a solution because they can explain not only the magnitude of the UV flux but also strength of the absorption lines suggested by the HUT data, in a reasonable time scale (15-20 Gyr). Meanwhile, the physics which causes the unusually high mass loss for the metal-rich stars remains to be understood.

Program listing for Wednesday