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Star Formation in Clusters. John Bally. Center for Astrophysics and Space Astronomy Department of Astrophysical and Planetary Sciences University of Colorado, Boulder. Outline Most stars form in clusters:
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Star Formation in Clusters John Bally Center for Astrophysics and Space Astronomy Department of Astrophysical and Planetary Sciences University of Colorado, Boulder
Outline • Most stars form in clusters: • - Transient clusters => T, OB associations • > 90% of stars:t < few x tcross ~ r / s • - Open clusters: • few % of stars:t ~ 10 - 103 x tcross • - Globular clusters: • <<1% of stars: t >> 103 x tcross • Dissipation by cores, envelopes, disks: • - Collapse, interactions, IMF, mergers (?) • Formation of clusters: Feedback from massive stars • GMC: Vescape < CII ~ 10 km/s => Transient? • Vescape > CII ~ 10 km/s => Open ? • Vescape >> CII ~ 10 km/s => SSC => Globular ?
Cluster Formation I • Turbulent Giant Molecular Clouds: • - Dissipation • - Shocks => transient clumps • - Occasionally, clumps bound by gravity • - Graviational collapse: Dr ~ 107, Dr ~ 1021 • - Fragmentation => Cluster • Star formation: • - Competitive accretion: • - dM/dt a M, dM/dt high in dense core • => Range of masses • - Feedback: Outflows, UV, supernovae (SNe) • - Interactions: => IMF, binaries, mergers
Cluster Formation II • Interactions: • - Facilitated by disks, proto-star envelopes • - Capture formed binaries • Binary single star • Binary binary • - Stellar mergers (?) => high mass stars, GRBs? • Ejection of star(s) : Hardening of surviving binary • - High-velocity runaway stars (V > 50 km s-1) • - Intermediate-velocity runaways (10 <V < 50 km s-1) • - Field multiple star distribution? • Mass-segregation • Initial Mass Function • - Ejection => stop accretion => final stellar mass • - Determined by interactions in N-body system?
The Orion/Eridanus Bubble (Ha): d=180 to 500pc; l > 300 pc Orion OB1 Association: ~40 > 8 M stars: ~20 SN in 10 Myr l Ori (< 3 Myr) 1a (8 - 12 Myr; d ~ 350 pc)) 1b (3 -6 Myr; d ~ 420 pc) 1c (2 - 6 Myr; d ~ 420 pc) 1d (<2 Myr; d ~ 460 pc) Eridanus Loop Barnards's Loop
AE Aur 150 km/s Orion
Orion Molecular Clouds 13 Orion B 2.6 mm CO Orion Nebula Orion A
NGC 2024 (OB1 d) Horsehead Nebula s Orionis (OB 1 c) NGC 1981 NGC 1977 Orion Nebula i Ori NGC1980: Source ofm Col + AE Aur ; V ~ 150 km/s runaways, 2.6 Myr ago Orion below the Belt: Ori OB1c Ori OB1d
CO (Bally et al.) 2MASS stars(Carpenter et al.)
Northern part of Orion A 850 mm dust continuum SCUBA Trapezium
OMC 1 Outflow (H2 t = 3,000 yr) Orion Nebula BNKL Trapezium (L = 105 Lo t << 105 yr) (L = 105 Lo t < 105 yr) OMC1-S (L = 104 Lo , t < 105 yr)
Trapezium cluster Proper motions: Van Altena et al. 88 Vesc ~ 6 km s-1 2.6 1.8 2.5 5
YSOs with disks and envelopes are common: Facilitate interactions? d253-535 in M43
M = 20 m = 5 Mdisk= 1 Close encounters Moeckel & Bally 05
Retrograde Prograde Close encounters Moeckel & Bally 05
Orion BN/KL H2 OMC1-S Jets CO HH NICFPS APO 3.5 m First light 21 Nov 04 HH 202 Zapata jet + HH 625 HH 529 HH 269 HH 530 Schmid-Burgk jet HH 203/204 HH 528
104 AU 11.7 mm Gemini S TReCS
104 AU 0.5 – 2.2 mm
104 AU 11.7 mm
High-velocity stars:source I, BN (Rodriguez et al. 2005) BN: ~ 30 km s-1 I: ~ 13 km s-1 i ~ 24o t ~ 500 yrs
Arches Cluster • Galactic Center • Age ~ 2 Myr • ~ 50 OB stars • 103-4 stars (?) • 3 X 105 stars pc-3 • Stolte et al. (2005) • ApJ, 628, L113
Shallow, broken IMF (Arches) Stolte et al. (2005) • Mass segregation • Low M cut-off, bias towards massive stars • Dynamical evolution? Background GSalpeter = -1.35
Mass segregation in the Arches core annulus
Massive Stars: HII, SNe & SFE • Ionization (HII): • - Photo-ionization => Cs ~ 10 km/s • - Cs > Vescape => Fast blow-out of gas • =>OB star stops star formation • - If SFE < 0.3, tblow-out < tcross • => Unbound association • - Cs < Vescape => Slow removal of gas • => Open cluster • Supernovae (SN) • - MGMC Vesc < Meject Veject • => SN stops star formation=> Open cluster • - MGMC Vesc > Meject Veject (supermassive core) • =>Globular cluster
Conclusions • Most stars form in transient clusters: • - Transient T / OB associations • Circumstellar gas: • - Dissipation • - Mass segregation • - Capture formed binaries • - High-velocity stars • - Mergers • Impact of Massive Star UV, SN: • - Vescape < CII ~ 10 km/s => Transient association • - Vescape > CII ~ 10 km/s => Open Cluster • - Vescape >> CII ~ 10 km/s => SSC => Globular Cluster