Yonsei Evolutionary Population Synthesis (YEPS): Effects of Super-Helium-Rich Populations Chul Chung, Young-Wook Lee, and Suk-Jin Yoon Department of Astronomy, Yonsei University, Seoul, South Korea Abstract The recent discoveries of multiple stellar populations in massive Milky Way globular clusters (GCs) have provided the evidence that Helium abundance anomalies are primordial in the GCs. In order to keep pace with the recent discoveries, we have constructed enhanced Helium abundance Yonsei Evolutionary Population Synthesis (YEPS) model based on most up-to-date Yonsei-Yale stellar evolution tracks. We present integrated spectro-photometric quantities of enhanced He abundance populations with special care in dealing with the morphologies of horizontal branch stars (HBs) with respect to metallicity, age & Helium contents. Our new models provide completely new insights not only on the strong FUV flux of extra galactic GCs but also on the enhanced Balmer-lines of local and intermediate redshift early type galaxies.

Observational evidence of multiple Stellar Populations in Galactic Globular Clusters Observation Ferraro et al Model Lee et al Model Lee et al HST snapshot Obs. Piotto et al Bedin et al. 2004

Only YEPS Can Do It! EPS models Photometric evolution Spectral evolution (alpha-element enhancement) Synthetic CMD Horizontal Branch Bruzual & Charlot OO (X) △ Red Clump Worthey O O ( △ ) XRed Clump Thomas Maraston Bender OO (O)XRed Clump Pegase OO (X)XX Yonsei EPS Model OO(O)O All HB Types O

Construction of Helium Enhanced YEPS model Yonsei-Yale stellar evolution model BaSeL 3.1 flux library (Westera et al. 2002) Johnson & Cousins SDSS, Galex, HST, Spitzer, STIS [Fe/H], Teff, log L, log g, [α/Fe] Fitting functions Korn+05 α-element response functions YEPS Photometric evolution model YEPS Spectroscopic evolution model Worthey+ 94, 97 Schiavon 07 Various passbands Helium enhanced Stellar Evolution Model

The effects of age & metallicity on SSPs. Left two panels are showing 10, 12 Gyr HB morphologies in CMD from [Fe/H]=-2.5 to-0.3. Three lines in middle top panel correspond to 10Gyr to 14Gyr by 2Gyr (red, blue and green). Middle bottom panels are the most representative MWGC age pair-M3 and M13-observations(Rey et al. 2001) and YEPS CMD models. Right panels are the plots of YEPS SSP integrated colors and spectral indices model reflecting the HB type in the middle panel. The effects of BHBs in the old age are the crucial part of determining & measuring the ages and the metallicities of stellar populations. BlueRed 12Gyr10Gyr [Fe/H]=-0.3 [Fe/H]=-0.9 [Fe/H]=-1.7 [Fe/H]=-2.5 Rey et al The effect of Age & Metallicity on Simple Stellar Population (SSP)

The effect of Helium on SSPs. All models in the left panels are the same age of 12 Gyr with different primodal Helium contents(Y=0.28 & Y=0.34) from [Fe/H]=-0.3 to [Fe/H]=-2.5. Blue solid line in middle panel is 12Gyr YEPS model prediction for HB type vs. metallicity, and the other lines (orange, pink & violet) in the same plot are for enhanced Helium populations from 10 to 50%. Right panels are YEPS model predictions for enhanced Helium populations in spectral indices and broadband colors. BHBs from enhanced Helium population, like the BHBs from the old stellar populations, make integrated color bluer & Balmer lines stronger. Our SHR population model also shows good agreement with FUV-V color of M87 GCs (Sohn et al. 2006). Y=0.34 Y=0.29 [Fe/H]=-0.3 [Fe/H]=-0.9 [Fe/H]=-1.7 [Fe/H]=-2.5 The effect of Helium on SSP Data from Sohn et al. 2006

The effect of Helium on Composite Stellar Population (CSP) Helium effects on composite stellar populations(CSP). Left top panel shows metallicity distribution function(MDF) for composite stellar population model. Left bottom panel is the fraction of SHR population used in YEPS SHR composite model. Right panel is the model prediction for CSP with two MDFs (simple, infall z=0.02). As shown in right panel, SHR CSP model(blue lines) results predict even more enhanced HδF line from small to intermediate look-back-times compared to normal Helium CSP models(red lines). All these results are coming from BHBs in Helium rich subpopulations. From this, we conclude with precaution that enhanced Balmer lines in local area are not from the recent star formation but from the old age Helium enhanced stellar populations. Formation Epoch Look-Back-Time 13Gyr Schiavon et al. 2006

References Bruzual & Charlot 2003, MNRAS, 344, 1000B Ferraro et al. 2004, ApJ, 603L, 81F Fioc & Rocca-Volmerange 1997, A&A, 326,950F Korn et al. 2005, A&A, 438, 685K Lee et al. 1994, ApJ, 423, 248L Lee et al. 2005, ApJ, 621L, 57L Lee et al. 2007, ApJ, 661, 49 Peng et al. 2006, ApJ, 639, 95 Piotto et al. 2007, ApJ, 661L, 53P Puzia et al. 2002, A&A, 395, 45P Rey et al. 2001, AJ, 122, 3219R Schiavon et al. 2006, ApJ, 651L, 93S Schiavon 2007, ApJS, 171, 146S Sohn et al. 2006, AJ, 131, 866S Thomas et al. 2003, MNRAS, 339, 897T Westera et al A&A, 389, 761 Worthey et al. 1994, ApJS 94, 687W Yoon et al. 2006, Science 311, 1129Y