Not-so-simple stellar populations in nearby, massive star clusters Young and middle-aged Richard de Grijs Macquarie University, Sydney, Australia Main collaborators: Chengyuan Li (Macquarie University, Australia), Licai Deng (NAOC, China), Yujiao Yang, Xiaohan Wu and Hao Zhang (PKU, China) Harris 2003, in: A decade of Hubble Space Telescope science, p. 78 Red giant branch Horizontal branch Simple stellar populations? log(Luminosity) Single age: sharp, narrow MSTO Single metallicity: narrow CMD Mass range given by the IMF “Temperature” … but for single stars only! “Blue stragglers” Binary systems MV = –2.5 log LV [L¤] + constant Main sequence at D = 10 pc (absolute magnitude) (V–I) = mV – mI = –2.5 log LV/LI Subscript “0”: corrected for the effects of extinction (dust) White dwarfs Colour–magnitude M3, M55, M68, (Hertzsprung–Russell) NGC 6397, NGC 2419 diagram Mackey et al. (2008) LMC NGC 1851 NGC 1783 Milone et al. (2008) m mF435W–mF814W F606W F814W m NGC 2808 NGC 2808 Villanova et al. NGC 1851 (2007); Piotto et al. (2007) mF475W–mF606W (Martocchia et al. 2018, MNRAS) NGC 1831 1–2 Gyr-old LMC clusters NGC 1868 17 “Blue” 18 “Red” 18 Broadening of the main sequence (turn-off) could be 19due to: 19 20 20 1. differences in helium abundances; 21 21 (mag) 2. binary populations; V 22 22 3. a range in stellar ages and/or metallicities; or … 23 23 4. a population of rapidly rotating stars 24 24 25 25 0 0.5 1 1.5 0 0.5 1 1.5 V – I (mag) NGC 1831 NGC 1868 17 3. Explore the effects 2. Constrain maximum of rapid rotation plausible age spread 18 18 19 19 20 20 21 21 (mag) 1. Determine global V binary fraction 22 22 23 23 24 24 25 25 0 0.5 1 1.5 0 0.5 1 1.5 V – I (mag) (Li, de Grijs , & Deng, 2014, NGC 1831 NGC 1868 550 Myr 850 Myr ApJ , 784, 157) 830 Myr 1.2 Gyr 17 ω = 0.55 stellar rotation for intermediate stellar populations: NGC 1831 18 19 20 21 (mag) 22 V 23 24 25 26 −0.5 0 0.5 1 1.5 2 V – I (mag) (Li, de Grijs, & Deng, 2014, ApJ, 784, 157) Rapid stellar rotation? (see Royer et al. 2007; Bastian & de Mink 2009) (Li, de Grijs, & Deng, 2014, ApJ, 784, 157) (Li, de Grijs, & Deng, 2014, ApJ, 784, 157) ~ (V–I) colour Zhang, de Grijs, Li & Wu, 2018, ApJ, 853, 186; cf. Martocchia et al. 2018) NGC 1651: An unexpected discovery (Li, de Grijs & Deng, 2014, Nature, 516, 367) Full SGB sample Width expected given the observational uncertainties (Li, de Grijs & Deng, 2014, Nature, 516, 367) Core SGB sample: R ≤ 15 arcsec (Li, de Grijs & Deng, 2014, Nature, 516, 367) Full SGB sample Core SGB sample Maximum plausible age range allowed by the SGB width: 80 Myr Bastian & Niederhofer 2015, MNRAS, 448, 1863 (Li, de Grijs, et al. 2016, NGC 411 MNRAS, 461, 3212) - Most extended MSTO known - Lowest escape velocity of an MSTO cluster 19.5 2 NGC 419 Rotational deceleration? 20 1.9 1.8 20.5 1.7 21 (mag) V 1.6 Wu, Li, de Grijs, & Deng, 21.5 2016, ApJL, 826, L14 1.5 22 0.6 1.4 22.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.5 V–I(mag) 0.4 0.3 P(N) 0.2 0.1 0 1.32 1.42 1.52 1.62 1.72 1.82 1.92 2.02 Age (Gyr) (Yang, Li, Deng, de Grijs, & Milone, 2018, ApJ, 859, 98) (Yang, Li, Deng, de Grijs, & Milone, 2018, ApJ, 859, 98) Take-home messages Gyr 1–3 • At intermediate ages, extended Main-Sequence Turn-Offs imply the presence of an age spread or a population of rapidly rotating MSTO stars. • A simple stellar population including rapidly rotating stars seems the “best” match to intermediate-age clusters • The presence of an extended MSTO does not necessarily imply an age spread • Our most recent results suggest that a major reassessment of the multiple stellar population paradigm is sorely needed! .