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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 Presentation
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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…)
9/5/2011 3Claus Leitherer:
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
9/5/2011 4Claus Leitherer:
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)
9/5/2011 5Claus Leitherer:
Population Synthesis
New Developments: the New Normal
o Stochasticityo Rapid eruptive evolutionary phaseso Stellar multiplicityo Convective overshootingo Stellar rotation
9/5/2011 6Claus Leitherer:
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
9/5/2011 7Claus Leitherer:
Population Synthesis
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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
9/5/2011 11Claus Leitherer:
Population Synthesis
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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
9/5/2011 14Claus Leitherer:
Population Synthesis
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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
9/5/2011 17Claus Leitherer:
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
9/5/2011 18Claus Leitherer:
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
9/5/2011 19Claus Leitherer:
Population Synthesis
How to reproduce the N enhancement
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
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)
9/5/2011Claus Leitherer:
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)
9/5/2011Claus Leitherer:
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)
9/5/2011Claus Leitherer:
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)
9/5/2011Claus Leitherer:
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
9/5/2011Claus Leitherer:
Population Synthesis 27
Cosmology
Galaxy Evolution
Stellar Populations
Stellar Astrophysics Nuclear and Atomic Physics
Inform
ation Flow