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Stellar Rotation the Missing Piece in Stellar Physics

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Stellar Rotation the Missing Piece in Stellar Physics Stellar rotation the missing piece in Stellar physics Collaborators: Richard de Grijs (MQ), Licai Deng (NAOC), Chengyuan Li (SYSU), Michael Albrow (UC), Petri Vaisanen (SAAO), Zara Randriamanakoto (SAAO) Weijia Sun (PKU) 07/14/2021 Why stellar rotation is important? Why stellar rotation is important? • Dynamo-driven magnetic activity • Stellar winds • Surface abundances • Chemical yields • Internal structure • External structure Why stellar rotation is important? • Dynamo-driven magnetic activity • Stellar winds • Surface abundances • Chemical yields • Internal structure • External structure Matt & Pudritz 2005 Why stellar rotation is important? • Dynamo-driven magnetic activity • Stellar winds • Surface abundances • Chemical yields • Internal structure • External structure Maeder & Meynet 2011 Why stellar rotation is important? • Dynamo-driven magnetic activity • Stellar winds • Surface abundances • Chemical yields • Internal structure • External structure Rivinius+2013 extended MSTO and split MS Found in Magellanic Clouds clusters rMS bMS NGC 1846 NGC 1856 1.5 Gyr 350 Myr Mackey et al. 2008 Milone et al. 2015 Not only in MC clusters Niederhofer et al. 2015 But also in Galactic OCs Pattern NGC 5822 0.9 Gyr Sun et al. 2019a Cordoni et al. 2018 What causes eMSTO and split MS? • Extended star formation history (eSFH) • Variability • A wide range of stellar rotations What causes eMSTO and split MS? Stellar rotation 300 0.0 250 Gravity darkening 0.5 1.0 200 (mag) 1.5 150 G M 2.0 100 2.5 50 pole-on (i = 0) 3.0 0.2 0.4 0.6 G G (mag) BP ° RP edge-on (i = 90) It’s mainly v sin i that affects the locus of a star in the CMD Georgy et al. 2014 How well is stellar rotation model? 1. Rotation detection through photometry Be stars NGC 1850 80 Myr Milone et al. 2018 How well is stellar rotation model? 2. Rotation detection through spectroscopy NGC 5822 0.9 Gyr South Africa Large Telescope (SALT) Multi-object Spectroscopy (MOS) R ~ 4000 Sun et al. 2019a How well is stellar rotation model? 2. Rotation detection through spectroscopy NGC 5822 0.9 Gyr • The loci of the main-sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities • Fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side How well is stellar rotation model? 2. Rotation detection through spectroscopy A well-separated double MS NGC 2287 200 Myr Sun et al. 2019b How well is stellar rotation model? 2. Rotation detection through spectroscopy • The mean projected rotational velocity for bMS and rMS stars are 111 km s−1 and 255 km s−1, respectively. • Rapidly rotating stars are generally redder than slowly or non- rotating stars Sun et al. 2019b How well is stellar rotation model? 2. Rotation detection through spectroscopy NGC 1846 1.5 Gyr Kamann et al. 2020 From to v sin i veq Knowledge from Asteroseismology NGC 6791 8.3 Gyr NGC 6819 2.4 Gyr Corsaro et al. 2017 From to v sin i veq Evidence against Spin alignment Spin alignment Bimodal rotational distribution NGC 2287 200 Myr Sun et al. 2019b Stellar rotation in star clusters How to unravel the stellar rotation distribution 0 10 MSTO NGC 3960 NGC 6134 8 IC 4756 ~ 1 Gyr NGC 5822 • Five Galactic NGC 2818 OCs that have 1 6 similar 4 chronological 2 2 (~ 1 Gyr) and 0 0.4 0.6 0.8 1.0 (mag) dynamical ages ¢(GBP GRP) (mag) G ° M MS NGC 3960 3 NGC 6134 15 IC 4756 • Four clusters NGC 5822 were observed NGC 2818 10 with SALT 4 NGC 3960 NGC 6134 5 IC 4756 NGC 5822 NGC 2818 5 0 0.25 0.00 0.25 0.50 0.75 1.00 1.25 0.2 0.1 0.0 0.1 0.2 0.3 ° ° ° G G (mag) ¢(G G ) (mag) BP ° RP BP ° RP Sun et al. 2020 Stellar rotation in star clusters Binary-driven stellar rotation evolution 0.8 NGC 3960 • NGC 6134 Nslow/Ntot IC 4756 0.7 NGC 5822 • A tight correlation NGC 2818 between the number 0.6 ratio of slow rotators and tot 0.5 the clusters’ binary /N fractions slow N 0.4 0.3 0.2 0.1 0.2 0.3 0.4 0.5 fb Stellar rotation in star clusters Binary-driven stellar rotation evolution NGC 3960 • 0.7 NGC 6134 Nslow/Ntot IC 4756 NGC 5822 NGC 2818 • A tight correlation 0.6 between the number ratio of slow rotators and tot 0.5 the clusters’ binary /N fractions slow N 0.4 • The correlation remains the same regardless of 0.3 the choices of defining subsamples 0.2 0.2 0.3 0.4 0.5 fb Stellar rotation in star clusters Binary-driven stellar rotation evolution NGC 3960 NGC 6134 • N /N IC 4756 slow tot 0.7 NGC 5822 • A tight correlation NGC 2818 between the number 0.6 ratio of slow rotators and tot the clusters’ binary /N 0.5 fractions slow N 0.4 • The correlation remains the same regardless of 0.3 the choices of defining subsamples 0.2 0.2 0.3 0.4 0.5 fb Binary-driven stellar rotation evolution Does it exist in Magellanic Clouds? 0.8 NGC 3960 • Magellanic Clouds NGC 6134 clusters have IC 4756 0.7 NGC 5822 approximately constant NGC 2818 number ratios (25%— 0.6 45%) tot 0.5 /N • Their binary fractions slow are around 0.3 N 0.4 0.3 0.2 0.1 0.2 0.3 0.4 0.5 fb Binary-driven stellar rotation evolution Does it exist in Magellanic Clouds? 0.8 NGC 3960 • Magellanic Clouds NGC 6134 clusters have IC 4756 0.7 NGC 5822 approximately constant NGC 2818 NGC 1866 (200 Myr) number ratios (25%— 0.6 45%) tot 0.5 /N • Their binary fractions slow are around 0.3 N 0.4 0.3 0.2 0.1 0.2 0.3 0.4 0.5 fb Binary-driven stellar rotation evolution Does it exist in Magellanic Clouds? 0.8 NGC 3960 • Magellanic Clouds NGC 6134 clusters have IC 4756 0.7 NGC 5822 approximately constant NGC 2818 NGC 1866 (200 Myr) number ratios (25%— 0.6 NGC 1818 (40 Myr) 45%) NGC 330 (40 Myr) tot 0.5 /N Their binary fractions slow • 0.4 are around 0.3 N 0.3 • Young clusters evolve toward this correlation 0.2 through dynamical 0.1 evolution 0.2 0.3 0.4 0.5 fb What causes the correlation? What causes the correlation? Scenario A • Within the first few Myr: Slow rotators have been able to retain their circumstellar discs throughout their PMS lifetimes while rapid rotators may have lost the discs destroyed by binaries (Bastian et al. 2020) Disk-locking • Binaries may be expected to destroy discs around the individual stars Higher -> • fb Shorter disk lifetime -> Less slow rotators • In conflict with our results Kraus et al. 2012 What causes the correlation? Scenario B • a few tens of millions of years: bMS in young clusters might be the outcome of braking of the rapidly rotating population. The deceleration might be due to interaction between close binaries through magnetic-wind braking or tidal torques (D’Antona et al. 2015 & 2017) Tidal-locking • Higher binary fraction -> More slow rotators • Only close binaries may become tidally locked What causes the correlation? Scenario B Synchronization timescale NGC 2287 200 Myr Sun et al. 2019b What causes the correlation? Scenario B • a few tens of millions of years: bMS in young clusters might be the outcome of braking of the rapidly rotating population. The deceleration might be due to interaction between close binaries through magnetic-wind braking or tidal torques (D’Antona et al. 2015 & 2017) Tidal-locking 0.8 NGC 3960 NGC 6134 IC 4756 Higher binary fraction -> 0.7 NGC 5822 • NGC 2818 More slow rotators NGC 1866 (200 Myr) 0.6 NGC 1818 (40 Myr) NGC 330 (40 Myr) • Only close binaries may become tot 0.5 tidally locked /N slow 0.4 N is comparable to in 0.3 • Nslow/Ntot fb our result 0.2 The slope is greater than unity 0.1 • 0.2 0.3 0.4 0.5 fb Do we know rotation in the field? • LAMOST MRS DR7, with v sin i down to a few km/s • Line indices of Mg Ib and Hα • Stellar LAbel Machine (SLAM) Scatters for , , • Teff log g [M/H], and v sin i are ∼ 75 K, 0.06 dex, 0.05 dex, and 3.5 km s−1 Sun+2021a, submitted to ApJS Do we know rotation in the field? Do we know rotation in the field? 3.5 3.0 5.0 2.0 2.5 3.6 ) 1.5 Ø 2.0 2.8 L/L 2.4 log ( 1.0 1.5 2.0 1.7 1.0 1.5 0.5 0.5 0.0 4.2 4.1 4.0 3.9 3.8 3.7 log (TeÆ /K) Do we know rotation in the field? • The largest catalog (40034) of late-B and A- type main sequence stars from LAMOST MRS DR7 • We can statistically rectify the projection effect and the error distribution • Contamination is important (binary, chemical peculiar stars, periodic variables, cluster members) Sun+2021b, accepted by ApJ Do we know rotation in the field? Chemical peculiar stars • early-type MS stars exhibiting anomalous chemical abundances • metallic line (Am), magnetically peculiar (mAp), stars with enhanced Hg ii and Mn ii (HgMn), and He-weak stars • CP stars tend to be slow rotators Do we know rotation in the field? How to quantify rotation? • v (M, ωinit, t/tMS) • v changes as a function of time, and also depends on the stellar mass is almost constant • v/vcrit over its MS lifetime Do we know rotation in the field? 3.75 2.0 3.50 3.25 1.5 ) 3.00 Ø M ( 2.75 M 1.0 2.50 2.25 0.5 2.00 0.0 0.0 0.2 0.4 0.6 0.8 1.0 v/vcrit Do we know rotation in the field? 3.75 2.0 3.50 3.25 1.5 ) 3.00 Ø M ( 2.75 M 1.0 2.50 2.25 0.5 2.00 0.0 0.0 0.2 0.4 0.6 0.8 1.0 v/vcrit Do we know rotation in the field? 3.75 2.0 3.50 3.25 1.5 ) 3.00 Ø M ( 2.75 M 1.0 2.50 2.25 0.5 2.00 0.0 0.0 0.2 0.4 0.6 0.8 1.0 v/vcrit Do we know rotation in the field? Dependence on metallicity Do we know rotation in the field? Bimodality in late-type MS stars • K2 targets with Gaia DR 2 • A gap in the rotation period-color diagram 0.57M⊙ < M < 0.76M⊙ • Departure from Skumanich spin down law rather than a bimodal star formation history Gordon+2021 Take-home message • Differences in stellar rotation rates are a key driver of extended MSTOs and split MSs in star clusters.
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