PI Total time #CoIs, team

1. Hot Stars in Helium-Enriched Globular Clusters Several globular clusters show evidence of being helium-enriched. At the same time these clusters show unusual populations of hot stars, whose evolutionary history is not yet fully understood. We want to analyse spectra of these stars in order to determine their status. PI Total time #CoIs, team Sabine Moehler 9.4 nights (42m) > 4 (stellar evolution, stellar atmospheres)

2. Background • Double main sequences discovered in w Cen and NGC 2808: Helium Enrichment • w Cen also shows large number of helium-core or helium atmosphere (DB) white dwarfs

3. White Dwarfs in w Cen

4. Background • Double main sequences discovered in w Cen and NGC 2808: Helium Enrichment • w Cen also shows large number of helium-core or helium atmosphere (DB) white dwarfs • Complete contradiction to white dwarf distribution in the field • Hottest HB stars too hot and helium-rich for canonical theory • Explanation predicts large numbers of helium-core white dwarfs

5. Background/Project • Helium enrichment could also explain blue HBs in metal-rich bulge GCs NGC 6388/NGC 6441 • Status of stars can only be determined by spectroscopy • Objective: Spectra of about 100 white dwarfs/ extreme HB stars per cluster down to Mv of +8 to +9 • Goal: determine spectral type, helium abundance, effective temperature, surface gravity, (mass)

6. ELT Justification: Faint blue targets in crowded fields with much brighter red objects; IQ of 0.8” definitely too bad, at most 0.2” needed Data Reduction: With reduced crowding no problem unless AO creates problems. Spectroscopy with R=1000, S/N > 50 @ 440nm, FoV 5’, 50mas/px, MP=100, GLAO to achieve IQ of at most 0.2”