Yonsei Evolutionary Population Synthesis (YEPS):

Effects of Super-Helium-Rich Populations

Chul Chung, Young-Wook Lee, and Suk-Jin YoonDepartment 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 ClustersObservationFerraro et al. 2004

ModelLee et al. 2005

ModelLee et al. 2005HST snapshot Obs.

Piotto et al. 2007

Bedin et al. 2004

Only YEPS Can Do It!EPS models

Photometricevolution

Spectral evolution(alpha-element enhancement)

SyntheticCMD

Horizontal Branch

Bruzual & Charlot

O O (X) △ Red Clump

Worthey O O (△ ) X RedClump

ThomasMaraston Bender

O O (O) X RedClump

Pegase O O (X) X X

Construction of Helium Enhanced YEPS model

Yonsei-Yale stellar evolution

model

BaSeL 3.1 flux library(Westera et al. 2002)

Johnson & CousinsSDSS, 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.

Blue Red

12Gyr10Gyr

[Fe/H]=-0.3

[Fe/H]=-0.9

[Fe/H]=-1.7

[Fe/H]=-2.5

Rey et al. 2001

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=0.34Y=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

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