Blue Straggler Stars: Lessons from Open Clusters Aaron M. Geller NSF AAPF Fellow Northwestern University CIERA The Adler Planetarium Why Open Clusters • Rich in binaries • Dynamically active! • Cover wide parameter space • Small enough for direct N-body models • Near enoughenough for for very very detailed detailedobservations observations Open vs. Globular Clusters Characteristic Open Globular 2 3 4 6 Mass Mc ~ 10 – 10 M! Mc ~ 10 – 10 M! 3 -3 4 6 -3 Density !0 ~ 10 – 10 M! pc !0 ~ 10 – 10 M! pc Age " ~ 1 Myr – few Gyr " ~ 10 – 12 Gyr Velocity Dispersion #0 ~ 0.1-1 km/s #0 ~ 1-10 km/s Multiplicity fb ~ 10-50% fb ~ 1-10% ft ~ 2-10% ft ~ ??? Encounters 3 $ N $ 6? 7? 8? 3 $ N $ 4 Exotica BSs, CVs, qLMXBs? BSs, CVs, LMXBs, MSPs !"#$%&'(&)*++*#& Table borrowed from Nathan Leigh Open vs. Globular Clusters Do dynamics help or hurt BSS production? 23,4()"%*+,'*( 23,4()"%*+,'( -.&'/(!&"&01,*( -.&'/(!&"&01,*( !"#$%"&'()"%*+,'*( !"#$%"&'()"%*+,'*( !"#$%&'(&)*++*#& Momany et al. (2015) Outline “Deep Dive” into 2 Open Clusters 1. NGC 188 (6-7 Gyr, solar metallicity, ~1500 M! today) • Spoiler: lots of evidence for mass transfer! • Some BSS from dynamics(?) 2. M67 (4 Gyr, solar metallicity, ~2100 M! today) • Interesting individuals • (Sub-subgiants) Skim Surface of NGC 6819 (2.5 Gyr), NGC 6791 (8 Gyr) !"#$%&'(&)*++*#& NGC 188 A Story About Mass Transfer (and some dynamics) NGC 188 Blue Stragglers Binary properties reveal formation channel(s) 60 16.0 1.0 50 13.3 0.8 40 10.6 0.6 30 8.0 0.4 eccentricity 20 5.3 0.2 Blue Straggler Frequency (%) 10 2.7 Main Sequence Frequency (%) 0 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 1 2 3 4 M ( M ) log( Period [days]) 2 Geller & Mathieu (2009, 2011) Observed MS and BSS !"#$%&'(&)*++*#&M2, log(P) and e distributions NGC 188 Blue Stragglers Binary properties reveal formation channel(s) 60 16.0 7 35 50 13.3 6 30 5 25 40 10.6 4 20 30 8.0 3 15 20 5.3 2 10 10 Blue Straggler Frequency (%) 10 2.7 Main Sequence Frequency (%) MT Blue Straggler Frequency (%) MT Blue Straggler Frequency (%) Coll Blue Straggler Frequency (%) Coll Blue Straggler Frequency (%) 1 5 0 0.0 0 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 M ( M ) M ( M ) 2 2 Geller & Mathieu (2009, 2011) N-body model of NGC 188 : BSS!"#$%&'(&)*++*#& from collisions and mass transfer NGC 188 Blue Stragglers Binary properties reveal formation channel(s) 607 16.035 • Mass transfer mechanism predicts 506 13.330 white – dwarf companions 5 25 40 10.6 4 20 30 8.0 3 15 20 5.3 2 10 Blue Straggler Frequency (%) 10 2.7 Main Sequence Frequency (%) MT Blue Straggler Frequency (%) Coll Blue Straggler Frequency (%) 1 5 0 0.00 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 M ( M ) 2 Geller & Mathieu (2009, 2011) N-body model of NGC 188 : BSS!"#$%&'(&)*++*#& from collisions and mass transfer NGC 188 Blue Stragglers Binary properties reveal formation channel(s) • Mass transfer mechanism predicts white – dwarf companions • HST fUV photometry confirms! • HST COS spectra analysis underway! Gosnell et al. (2014, 2015) !"#$%&'(&)*++*#& NGC 188 Blue Stragglers Wow, some are very young! Gosnell et al. (2015) !"#$%&'(&)*++*#& NGC 188 Blue Stragglers Wow, some are very young! ,$-*#&.&/$0%1*#& • BSS near ZAMS are mostly young • Young brighter BSS move redward • Single (collisions/mergers?) BSS are bright • !2/3 of NGC 188 BSS formed through mass transfer! et al. (2015) Gosnell NGC 188 Blue Stragglers Short-P BSS from Dynamics? 1.0 ID: 5078 • Note non-zero eccentricity 0.8 • Recent dynamical encounter? • Kozai from a triple companion? 0.6 0.4 eccentricity 0.2 0.0 0 1 2 3 4 log( Period [days]) Geller et al. (2009, 2012) NGC 188 Blue Stragglers Short-P BSS from Dynamics? ID: 5078 • Secondary Teff ! M2 " 1 M! 13 • q = 0.68 ! M1 " 1.5 M! • But L and Teff ! M1 " 1.2 M! 1.5 M • Underluminous BSS, like S1082 in M67 14 1.4 M 1.3 M V 1.2 M 15 16 0.4 0.6 0.8 1.0 1.2 Geller et al. (2012) (B-V) NGC 188 Blue Stragglers Short-P BSS from Dynamics? 1.0 ID: 7782 • q ~ 1 0.8 • Two blue stragglers! • Difficult for N-body models 0.6 0.4 eccentricity 0.2 0.0 0 1 2 3 4 log( Period [days]) Geller et al. (2009, 2012) NGC 188 Blue Stragglers 11 NGC 188 Short-P BSSRotation from Dynamics?Rotation 12 • Rotating faster than normal MS stars, but 13 V not very rapidly 14 • Need spin-down mechanism 15 • Could also use rotation rate to constrain the 16 BSS ages 11 NGC 6819 12 13 V 14 legend 15 10 km/s 20 km/s 30 km/s 16 40 km/s 50 km/s 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Mathieu & Geller (2009) (B-V) !"#$%&'(&)*++*#& 0 NGC 188 Blue Stragglers Short-SpatialP BSS Distributionfrom Dynamics? R (core radii) • Reminiscent of GCs 0.0 2.1 4.3 6.4 8.5 10.6 12.8 • No obvious 1.0 difference between core and halo BSS 0.8 • Likely not because of 2 distinct 0.6 formation channels 0.4 Single MS Stars Single Giant Stars Velocity Variables Fraction of Population 0.2 Blue Straggler Stars 0.0 0 5 10 15 20 25 30 R (arcmin) !"#$%&'(&)*++*#& Geller et al. (2008) NGC 188 Blue Stragglers Short-ProblemsP BSS with from the Dynamics? models • Mass transfer models do not 1.0 produce non-zero eccentricities 0.8 – Need “eccentricity pumping”? – Triples? 0.6 – Wind RLOF? • NGC 188 N-body models don’t 0.4 produce enough MT BSS or eccentricity bimodal radial distribution 0.2 • Models predict much higher rotation rates after formation (all 0.0 0 1 2 3 4 mechanisms) log( Period [days]) !"#$%&'(&)*++*#& NGC 188 Blue Stragglers M67: Latham (2007) 1.0 0.8 0.6 0.4 eccentricity 0.2 0.0 0 1 2 3 4 log( Period [days]) NGC 188: Geller et al. (2009, 2012) Field: Carney et al.(2001) M67 A Long History of Observations (and still coming! + K2) M67 Blue Stragglers 8 S1040 “Yellow Giant”: Cluster Member d Velocity Variable : • P = 42.8 ; e = 0 with Solution (BM) without Solution (BLM+BU) • He WD 0.22 M!, Teff = 16,900K, log(g) = 7.0 10 • Mass Transfer product V 12 14 0.0 0.5 1.0 1.5 Landsman et al. (1997) Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S1040 “Yellow Giant”: Cluster Member d Velocity Variable : • P = 42.8 ; e = 0 with Solution (BM) without Solution (BLM+BU) • He WD 0.22 M!, Teff = 16,900K, log(g) = 7.0 10 • Mass Transfer product V 12 14 0.0 0.5 1.0 1.5 K2 data c/o Andrew Vandenburg Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S1082 Blue Straggler(s): Cluster Member d d Velocity Variable : • Triple: P =1.07 , P =1190 with Solution (BM) 1 2 without Solution (BLM+BU) Inner : MA=2.5M!, MB=1.6M! 10 V 12 et al. (2003) 14 Sandquist 0.0 0.5 1.0 1.5 Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S1082 Blue Straggler(s): Cluster Member d d Velocity Variable : • Triple: P =1.07 , P =1190 with Solution (BM) 1 2 without Solution (BLM+BU) Inner : MA=2.5M!, MB=1.6M! 10 V 12 14 K2 data c/o Andrew Vandenburg 0.0 0.5 1.0 1.5 Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S1082 Blue Straggler(s): Cluster Member d d Velocity Variable : • Triple: P =1.07 , P =1190 with Solution (BM) 1 2 without Solution (BLM+BU) Inner : MA=2.5M!, MB=1.6M!" 10 • But… turnoff = 1.3M!" • And… secondary appears underluminous for its mass C V (recall 5078 in NGC 188) 12 • And… tertiary is also a A blue straggler! B 14 0.0 0.5 1.0 1.5 Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S1082 Blue Straggler(s): Cluster Member d d Velocity Variable : • Triple: P =1.07 , P =1190 with Solution (BM) 1 2 without Solution (BLM+BU) Inner : MA=2.5M!, MB=1.6M!" 10 • But… turnoff = 1.3M!" • And… secondary appears underluminous for its mass C V (recall 5078 in NGC 188) 12 • And… tertiary is also a A blue straggler! B 14 0.0 0.5 1.0 1.5 Geller et al. (2015) (B-V) M67 Blue Stragglers 8 S977 (F81) Blue Straggler: Cluster Member Velocity Variable : • Rapid rotator with Solution (BM) without Solution (BLM+BU) • Spotted (but why? so blue) 10 • ~9d rotation period V 12 14 0.0 0.5 1.0 1.5 K2 data c/o Andrew Vandenburg Geller et al. (2015) (B-V) Sub-Subgiants 8 What is a sub-subgiant (a.k.a. Cluster Member Velocity Variable : red straggler)? with Solution (BM) without Solution (BLM+BU) • Redder than MS, fainter than giants 10 30-31 -1 • Most Lx~10 erg s • Half with photometric and/or RV periods < 10 days • Wide range of companion types: V MS, NS, BH, WD? 12 • ~70 sources in 16 clusters SG Mag SG Coll (6 OC, 10 GC) SG MT MS Coll 14 • HST Theory project ; modeling et al.