zhanwen han zhanwenhan [email protected] 1 the role of binary stars in evolutionary population...

Zhanwen Han www.ynao.ac.cn/~zhanwenhan [email protected] 1

The role of binary stars in

evolutionary population synthesis studies of galaxies

Zhanwen HANF. Zhang, Y. Zhang, X. Kang, X. Chen

Yunnan Observatory, China

Z. LiDali College, China

Ph. Podsiadlowski, A. Lynas-GrayOxford University, UK

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?

The evolution is “Hot to cold”

Hot spectra -> YoungCold spectra -> old

3. How a binary evolves?

Most stars are in binaries!


Stable RLOFDynamically unstable RLOF

CE

CE ejection Merger


Type Ia supernovaeX-ray binariesShort gamma-ray burstsMillisecond Pulsars….….Hot subdwarfsBlue stragglers…..


•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)


•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!!!


4. History of EPS studies

• Tinsley (1968)

• Rapid progress since 90’sBruzual & 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)

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, 449Han et al., 2003, MNRAS, 341, 669Han, 2008, A&A, 484, L31


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


Han, et al., 2007, MNRAS, 380, 1098


Brown et al. 2003


Virgo Cluster

• No dichotomy between stellar population properties of dwarfs and giants.

Lisker et al., 2008, ApJ, 680, 1042

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.


• 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 !

UV-upturn is explained naturally !

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

2010 中国天文学会 24

Stellar mass


Magnitudes

26

UV flux


Mass-light ratio


SED

black—with binary interactionsRed — without binary interactions


colourssolid—with binariesdotted— without binaries


Lick/IDS colour indices

black—with binariesred— without binaries

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)

33

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 .

34

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

Case B recombination

zhanwen han zhanwenhan [email protected] 1 the role of binary stars in evolutionary population...

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