population synthesis at the crossroads claus leitherer (stsci) and sylvia ekström (geneva obs.)
DESCRIPTION
Population Synthesis at the Crossroads Claus Leitherer (STScI) and Sylvia Ekström (Geneva Obs.). Cosmology. Galaxy Evolution. Stellar Populations. Stellar Astrophysics. Nuclear and Atomic Physics. Errors • Information Flow. Popularity • Mobility • Front Page News. - PowerPoint PPT PresentationTRANSCRIPT
9/5/2011Claus Leitherer:
Population Synthesis 1
Population Synthesis at the Crossroads
Claus Leitherer (STScI)and
Sylvia Ekström (Geneva Obs.)
9/5/2011Claus Leitherer:
Population Synthesis 2
Cosmology
Galaxy Evolution
Stellar Populations
Stellar Astrophysics Nuclear and Atomic Physics
Popularity • Mobility • Front Page News Errors • I
nformatio
n Flow
Evolutionary Population Synthesis: Basics
o Goal: model SEDs of nearby and distant galaxies
o Star formation law (IMF, SF history, clustering…)o Stellar evolution models (conversion of mass into
luminosity and introduction of the time-scale)o Spectral libraries (empirical and theoretical)o Other (dust, geometry…)
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Population Synthesis
Most widely used, public model packages:
o GALEV (Schulz et al. 2002; Kotulla et al. 2009)o GALEXEV (Bruzual & Charlot 1993; 2003)o PEGASE (Fioc & Rocca-Volmerange 1997; Le
Borgne et al. 2004) o STARBURST99 (Leitherer et al. 1999; Leitherer
& Chen 2009) generally good agreement
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Population Synthesis
Broadened scope beyond stellar populations:
o Panchromatic SEDs including stellar and gaseous contributions (Dopita et al. 2005; 2006a; 2006b; 2008)
o SEDs with self-consistent inclusion of dust (Dwek & Scalo 1980; Dwek 2009)
o Chemical evolution due to stars and galactic outflows and infall (Matteucci 2009; 2010)
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Population Synthesis
New Developments: the New Normal
o Stochasticityo Rapid eruptive evolutionary phaseso Stellar multiplicityo Convective overshootingo Stellar rotation
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Population Synthesis
Stochasticityo Bruzual (2002):o Cluster simulations of
sparsely populated evolutionary phases due to stochastic fluctuations of the IMF
o Cerviño et al. (2002):o Relevant for clusters with
M < 105 M⊙, depending on wavelength
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Population Synthesis
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Population Synthesis 8
• SLUG synthesizes stellar populations using a Monte Carlo technique with stochastic sampling, including the effects of clustering, the stellar IMF, star formation history, stellar evolution, and cluster disruption
Da Silva et al. (2011): SLUG – Stochastically Light Up Galaxies
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Population Synthesis 9
o Application: comparison with sample of 300 star-forming galaxies within 11 Mpc (Lee et al. 2009)
o SFR(H)/SFR(UV) declines for small SFR(UV)o Photon leakage?o Eldridge (2011): can also be accounted for by stochastic effects
Eldridge (2011): BPASS – Binary population and spectral synthesis
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Population Synthesis 10
o Four stars have masses between 150 and 300 M⊙
o These stars account for ~50% of the ionizing photon output of R136
o Correspondingly, they account for ~10% of the ionizing luminosity of the entire LMC
Crowther et al. (2010): the most massive stars in R136 (LMC)
Rapid Eruptive Evolutionary Phases
o Maraston (2011)o Contribution to LBol of
SSP by TP-AGB starso Evolution/atmospheres
still poorly understood
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Population Synthesis
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Population Synthesis 12
o SPS code combines stellar evolution calculations with stellar spectral libraries to produce simple stellar populations
o Manually adjust stellar phases and estimate errors
o Allows estimate of uncertainties of, e.g., the AGB phase
Conroy & Gunn (2010): FSPS – flexible stellar population synthesis
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Population Synthesis 13
o Continuum in M(peak) from novae to SNe
o AGB in the red, vs. LBV in the blue
o AGB important for mass loss and luminosity
o LBV only important for mass loss
o LBVs are critical predecessors of WR stars
Smith et al. (2011): transient, eruptive phases in the upper HRD
Stellar Multiplicity
o Sana & Evans 2011: >50% of all massive stars in binaries; significant fraction in close binaries
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Population Synthesis
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Population Synthesis 15
1. On the main-sequence: spin-up and higher rotation; mixing and evolution to higher L and Teff.
2. Post-main-sequence: Roche-lobe overflow and mass transfer to secondary: “rejuvenation”.
de Mink (2010): evolution of a massive close binary
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Population Synthesis 16
o Evolution of Hβ equivalent width with time in SF galaxyo Solid: single stars with different Z; dashed: binaries with different Zo Hot, ionizing population appears after ~10 Myro Relevance: age spread of star formation? LINERs?
Eldridge & Stanway (2009): rejuvenation effect in SSP
Convective Overshooting
Energy production of massive stars in convective core
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Population Synthesis
Stellar Rotation
o Hunter et al. (2009): N/H and v sin i in LMC OB starso Significant N enrichment on the main sequence
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Population Synthesis
o Stellar winds to remove outer, stellar layers
o Mass loss and mass transfer in close binaries
o Enhanced convective overshooting
o Correlates with rotation rotationally induced mixing
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Population Synthesis
How to reproduce the N enhancement
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Population Synthesis 20
1. M < 2 M⊙: rotation negligible because of magnetic braking
2. 2 M⊙ < M < 15 M⊙: Teff decrease caused by centrifugal forces
3. M > 15 M⊙: larger convective core with higher Teff and L
Brott et al. (2011): evolutionary models with rotation
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Population Synthesis 21
o 0.8 M⊙ < M < 120 M⊙
o Z = Z⊙ (other Z in preparation)
o vrot = 0.4 vbreakup on ZAMS
o Calibrated extensively via local stars and star clusters
o Implemented in Starburst99
Ekström et al. (2012): full set of evolution models with rotation
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Population Synthesis 22
o Models with rotation are more luminous by ~0.4 mag because of the higher L/M and Teff of individual stars
MBol vs. time (SSP, 106 M⊙, Z⊙, Kroupa IMF)
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Population Synthesis 23
o The number ionizing photons increases by a factor of ~4 when hot, massive stars are present
Lyman photons vs. time (SSP, 106 M⊙, Z⊙, Kroupa IMF)
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Population Synthesis 24
o W(H) increases by ~0.2 dex. If used as an IMF indicator in late-type galaxies, the new models change the IMF exponent from, e.g, 2.3 to 2.6
H equivalent width vs. time (continuous SF, Z⊙, Kroupa IMF)
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Population Synthesis 25
o Applied to IR-luminous galaxies: models with rotation lead to, e.g., SFR = 100 M⊙ yr-1, whereas models without give SFR = 175 M⊙ yr-1.
Br luminosity vs. time (SFR = 100 M⊙ yr-1, Z⊙, Kroupa IMF)
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Population Synthesis 26
Take-Away Pointso Replacemement of traditional population synthesis
models by a new generation of modelso Ready for quantitative testing, beyond mere
concept studieso Stochastic effects, stellar multiplicity, rotationo Significant decrease of M/L, and therefore revision
of IMF and SF rateso The new models need to be tested by comparison
with galaxy properties
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Population Synthesis 27
Cosmology
Galaxy Evolution
Stellar Populations
Stellar Astrophysics Nuclear and Atomic Physics
Inform
ation Flow