Rotation Among High Mass Stars: A Link to the Star Formation Process? S. Wolff and S. Strom National Optical Astronomy Observatory

Initial Rotation vs Mass ~ 0.15 v(esc) v (esc) Single formation mechanism: M sun

Early Hints: Distribution of Rotational Velocities Depends on Environment Wolff, Edwards & Preston observations of Orion B stars –1982 paper shows that the bound ONC cluster exhibits Much higher median rotation speed Lack of slow rotators compared to stars distributed in the surrounding unbound association Guthrie (1982) study of late B stars showed that on average field stars rotated more slowly than B stars in clusters

Unevolved Field B Stars 4 < M/M Sun < 5

Unevolved B Stars in h and chi Per 4 < M/M Sun < 5

Cumulative Distribution of vsini MWG Clusters and Field Stars: 6-12 M sun Field Bound Clusters

Cumulative Distribution of vsini MWG Clusters, Field &Associations: 6-12 M sun Associations

R136 The Challenge: Source Confusion

Selecting the Sample

R 136 Observations 11 O Stars; 15 B Stars 25 M sun 12 M sun 5 M sun

N(vsini): 6-12 M sun LMC Field R 136

Key Results for B stars R136: 15 B Stars (6-12 M Sun ) –Results consistent with studies of regions in Milky Way –B stars in R 136 lack cohort of slow rotators R 136: = km/sec LMC Field: = km/sec LMC Clusters: = km/sec

Rotation at Higher Masses M sun LMC clusters (Hunter et al. 2008) R136 O stars

Rotation at Higher Masses: Key Results R136: 11 O Stars (15-30 M Sun ) R 136: = km/sec LMC Clusters: = km/sec

Environment or Something Else? –Decrease in vsini During Main Sequence Evolution 6-12 M sun : vsini constant during first Myr of evolution away from ZAMS (Wolff et al.; Huang and Gies) –Metallicity –Binarity

15-30 M sun : Evolutionary Effects Appear Negligible During Most of MS Evolution x 3.2 <log g < 4 log g > 4 Hunter et al. 2008

Metallicity: N(vsini): 6-12 M sun LMC and MW Appear Similar Field Stars Clusters MWG, LMC

Binarity??? Analysis includes all stars in each type of environment independent of knowledge of binary properties Do binary properties depend on environment? Does rotation depend on binary properties?

Why should birth in a cluster or the field matter? Nature?: D ifferences in star-forming core initial conditions Nuture?: Environmental conditions (radiation field; stellar density) We argue that differences in initial conditions dominate

Physical Mechanism Responsible for Rotation Low Mass Stars (Disk-locking): Ω ~ (M acc /dt) 3/7 B -6/7 Star and disk ‘locked’ at the co-rotation radius where P dyn = P magnetic  disk =  star

Potential Effects of Environment For a low-mass star, the lifetime of the disk plays a major role in determining rotation rate on the main sequence –Stars deposited on a “birthline” well above the ZAMS on PMS convective tracks –Stars that lose their disks will spin up more as they contract toward the main sequence and will become rapid rotators –Stars that remained locked to their disks until contraction is nearly complete will be slow rotators Cluster environments are more conducive to early disk loss In cluster regions containing a number of early-type stars, external uv radiation fields can erode disks rapidly via photoevaporation

Observational Tests of the Effects of Environment vs Initial Conditions Difficult for low mass stars because initial rotation speeds on birthline altered during subsequent evolution But for typical accretion rates, stars with M > 8 M sun are already on the main sequence when the main accretion phase ends –Initial speed not altered by subsequent additional contraction But what about variations in disk lifetime?

Variations in Disk Lifetime Unlikely to Account for Distribution of O & B Star Rotation Rates Disk lifetimes are short (t < 10 5 yr) – Rapid disk disruption driven by photoevaporation from the forming star – No evidence of disks among B0-B3 stars among rich, young clusters with ages t ~ 1 Myr Photoevaporation by external sources requires much longer – In the ONC, photoevaporation by external sources of a disk of 0.1 Msun (relatively low for a B star disk) would require 10 6 years

Rotation could reflect differences in initial conditions in star-forming core –Cluster-forming molecular clumps appear to have higher turbulent speeds (Plume et al. 1997) –If higher turbulent speeds also characterize the star-forming cores, then higher initial densities are required in order that self gravity can overcome the higher turbulent pressures –Higher core densities lead to shorter collapse times and higher high time-averaged accretion rates (McKee & Tan 2003) –In the context of ‘disk-locking’, higher time-averaged accretion rates lead to higher initial rotation speeds

Needed Observations –Differences in turbulence between individual cores not yet established –Direct measurements of infall rates are needed –Requirements: A list of massive stars still embedded within their natal cores Measurements of infall rates –Ultimately from ALMA –In the near-term, from high resolution mid-IR spectroscopy –The observations of BN by Kleinmann et al (1983) provide an example –8-10 m telescopes can make a start on this problem

Summary –N(vsini) differs between cluster & field Higher median rotation in dense, cluster-forming regions Near absence of slow rotators in cluster-forming regions – Rotation differences likely result from differences in initial conditions

Summary –Initial conditions -- specifically higher turbulent speeds and resulting higher time-averaged accretion rates -- can account for differences in rotation speeds between cluster & field –Direct measurements of infall rates for individual cores in cluster- and association- forming regions will provide an important test of the ‘hints’ provided by the results of stellar rotation studies

Turbulence in Clusters vs Field Gas turbulent velocities in these regions are high (e.g. Plume et al. 1997) High turbulent velocities lead to: –rapid protostellar collapse times and – high time-averaged accretion rates (dM acc /dt) Conditions in dense, bound clusters should favor formation of –Stars that rotate rapidly owing to high dM acc /dt