Download The role of binary stars in evolutionary population synthesis studies

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
[email protected]
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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?
Stable RLOF
Dynamically unstable 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!!!
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
[email protected]
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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
[email protected]
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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
[email protected]
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Han, et al., 2007, MNRAS, 380, 1098
Zhanwen Han
www.ynao.ac.cn/~zhanwenhan
[email protected]
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Brown et al. 2003
Zhanwen Han
www.ynao.ac.cn/~zhanwenhan
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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 FUVwww.ynao.ac.cn/~zhanwenhan
Zhanwen
Han
NUV occurs
naturally when the formation of hot subdwarfs
[email protected]
through binary evolution is taken into account.
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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
[email protected]
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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
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Magnitudes
2010 中国天文学会
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UV flux
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Mass-light ratio
2010 中国天文学会
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SED
black—with binary interactions
Red — without binary interactions
2010 中国天文学会
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colours
solid—with binaries
dotted— without binaries
2010 中国天文学会
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Lick/IDS colour indices
black—with binaries
red— without binaries
2010 中国天文学会
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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 .
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SFR-L(Ha), decreases by ~0.2dex
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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 farUV 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
[email protected]
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Case B recombination