The role of binary stars in evolutionary population synthesis studies of galaxies Zhanwen HAN F. Zhang, Y. Zhang, X. Kang, X. Chen Yunnan Observatory, China Z. Li Dali College, China Ph. Podsiadlowski, A. Lynas-Gray Oxford University, UK Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

1. What is EPS? Evolutionary Population Synthesis (EPS) is an approach to derive the physical parameters of stellar populations of galaxies from the Spectral Energy Distributions(SEDs) observed.

1. What is EPS? The ingredients of an EPS model: Star Formation Rate -> how many stars? Initial Mass Function -> how much mass of each star? Stellar Evolution Models -> how they evolve? Model Stellar Atmospheres -> how their spectra? Integrated SED (the sum of all the spectra of the stars)

2. How a single star evolves?

2. How a single star evolves?

2. How a single star evolves? The evolution is “Hot to cold” Hot spectra -> Young Cold spectra -> old

3. How a binary evolves? Most stars are in binaries!

3. How a binary evolves? Dynamically unstable RLOF Stable RLOF CE CE ejection Merger

3. How a binary evolves? Type Ia supernovae X-ray binaries Short gamma-ray bursts Millisecond Pulsars …. Hot subdwarfs Blue stragglers …..

3. How a binary evolves? RLOF removes stellar envelope →hot core exposed →(core can be ignited) A star grows in mass via accretion →rejuvenation (hotter) Coalescence of a binary → a more massive star (hotter)

3. How a binary evolves? Binary interactions rejuvenate stars! similar to cosmetics, to make look like

What we should do? To derive physical parameters properly from a given SED we should consider binary interactions!!!

www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn 4. History of EPS studies Tinsley (1968) Rapid progress since 90’s Bruzual & Charlot 2003, MNRAS, 344, 1000 (2923 citations) Binaries first included: Zhang et al. 2004, A&A, 415, 117 (Yunnan Model) Hot subdwarfs: Han et al. 2007, MNRAS, 380, 1098 (Yunnan Model) Blue stragglers: Chen et al. 2009, MNRAS, 395, 1822 (Yunnan Model) Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

5. Hot subdwarf stars and far-UV excess Elliptical galaxies: Old, cold stellar popultion, far-UV spectrum should be dark. 1969: OAO-2 discovered the far-UV radiation (UV-upturn, far-UV excess，UV rising-branch, UV rising flux, UVX) A puzzle for more than 30 years UV-upturn Originates from hot subdwarf stars.

What is hot subdwarf stars? Core-helium burning stars with very thin envelopes

Binary model for hot subdwarfs For both binary and single hot subdwarfs . Han et al., 2002, MNRAS, 336, 449 Han et al., 2003, MNRAS, 341, 669 Han, 2008, A&A, 484, L31 Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

Binary hot subdwarf model + BPS → hot subdwarf population + It would be “a priori” to apply binary hot subdwarf model to UV upturn problem Binary hot subdwarf model + BPS → hot subdwarf population + Energy flux from model stellar atmospheres of hot subdwarf stars ↓ UV flux Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn Han, et al., 2007, MNRAS, 380, 1098 Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn Brown et al. 2003 Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn Virgo Cluster Lisker et al., 2008, ApJ, 680, 1042 No dichotomy between stellar population properties of dwarfs and giants. FUV-NUV is reddest at about the dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue for both brighter and fainter luminosities. This behavior can be easily explained by the binary model with a continuous sequence of longer duration and later truncation of star formation at lower galaxy masses. Thus, in contrast to previous conclusions, the GALEX data do not require a dichotomy between the stellar population properties of dwarfs and giants. Their apparently opposite behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs through binary evolution is taken into account. Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

UV-upturn is explained naturally ! Despite its simplicity, our model can reproduce lots of observations UV-upturn is universal (from dwarf to giant ellipticals) The magnitude of UV-upturn does NOT depend much on metallicity or redshift. UV-upturn is NOT an age indicator ! Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

6. The Yunnan Model stellar evolutionary tracks (Cambridge stellar evolution code STAR) BaSeL spectral library Binary interactions (blue stragglers, hot subdwarf stars, rejuvenated stars, ……)

Isochrones

Stellar mass 2010 中国天文学会

Magnitudes 2010 中国天文学会

UV flux

Mass-light ratio 2010 中国天文学会

SED black—with binary interactions Red — without binary interactions 2010 中国天文学会

colours solid—with binaries dotted— without binaries 2010 中国天文学会

Lick/IDS colour indices black—with binaries red— without binaries 2010 中国天文学会

Summary of the Yunnan Model Stellar mass smaller； Magnitude bigger (fainter)； Colour bluer； Mass-light ratio bigger； UV flux higher； colour indices bluer。

7. Applications of the Yunnan Model SFR (Zhang et al. 2012, MNRAS, 421, 743) M33 (Kang et al. 2012, MNRAS, submitted)

SFR The luminosities of Ha are obtained by using EPS models and the assumptions for SFR and nebular emission. We assume that SFR satisfies the exponential decreasing form, We assume case B recombination, the electron temperature of ten-thousand (10000) degree and number density of one hundred per cubic centimeter .

SFR-L(Ha), decreases by ~0.2dex

Binary interactions on the chemical evolution of M33

Binary interactions on the chemical evolution of M33 Solid line- with binary (best fit) Dashed line-without binary (best fit)

With binary interactions, we found M* increases, infall decrease (fast gas infall process and high SF process in the early time of the galaxy evolution) with binaries: M*=5e9M_sun, infall =0.5r/rd+4; without binaries: M*=4e9M_sun, infall = r/rd+5.

8. Conclusions Binary interactions play a crucial role in the far-UV part of SED for an old passively evolving galaxy. Binary interactions are also important to derive physical parameters of stellar populations in galaxies.

Blue stragglers

HST colour differences: (with BS – without BS) Chen et al. 2009, MNRAS, 395, 1822 Zhanwen Han www.ynao.ac.cn/~zhanwenhan zhanwenhan@ynao.ac.cn

Case B recombination