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 <v(Birthline)> ~ 0.15 v(esc)

15

15.5

16

16.5

17

17.5

18

18.5

19

-1 -0.5 0 0.5 1 1.5 2

Log M/M

log Jsini/M

v (esc)

Single formation mechanism: 0.2-30 Msun

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/MSun < 5

0

5

10

15

20

25

30

0-25 26-50 51-75 76-100 101-150 151-200 201-250 251-300 301-350 >350

vsini (km/sec)

Probability Density x 100

Unevolved B Stars in h and chi Per

4 < M/MSun < 5

0

5

10

15

20

25

30

0-25 26-50 51-75 76-100 101-150 151-200 201-250 251-300 301-350 >350

vsini (km/sec)

Probability Density x 100

Cumulative Distribution of vsini MWG Clusters and Field Stars: 6-12 Msun

0

0.2

0.4

0.6

0.8

1

1.2

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

log vsini

Cumulative fraction

Bound

Field

Field

Bound Clusters

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

0

0.2

0.4

0.6

0.8

1

1.2

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

log vsini

Cumulative fraction

Bound

open

Field

Associations

R136The Challenge: Source Confusion

Selecting the Sample

R 136 Observations11 O Stars; 15 B Stars

HR Diagram for R136

-7

-6

-5

-4

-3

-2

-1

0

1

44.14.24.34.44.54.64.74.8

log Teff

Mv Series1

Series2

Zams LMC

25 Msun

12 Msun

5 Msun

25 Msun

12 Msun

5 Msun

Key Results for B stars

• R136: 15 B Stars (6-12 MSun)

– Results consistent with studies of regions in Milky Way– B stars in R 136 lack cohort of slow rotators

• R 136: <vsini> = 233 +- 19 km/sec• LMC Field: <vsini> = 105 +- 8 km/sec• LMC Clusters: <vsini> = 147 +- 14 km/sec

Rotation at Higher Masses15-30 Msun

LMC clusters(Hunter et al. 2008)

R136 O stars

Rotation at Higher Masses: Key Results

• R136: 11 O Stars (15-30 MSun)

• R 136: <vsini> = 189 +- 23 km/sec• LMC Clusters: <vsini> = 129 +- 13 km/sec

Environment or Something Else?

– Decrease in vsini During Main Sequence Evolution• 6-12 Msun: vsini constant during first 12-14 Myr of

evolution away from ZAMS (Wolff et al.; Huang and Gies)

– Metallicity

– Binarity

15-30 Msun: Evolutionary Effects Appear Negligible During Most of MS Evolution

x 3.2 <log g < 4

log g > 4

Hunteret al.2008

Metallicity: N(vsini): 6-12 Msun

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?: Differences 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 ~ (Macc/dt)3/7 B-6/7

Star and disk ‘locked’ at the co-rotation radius where Pdyn = Pmagnetic

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 Msun 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 < 105 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 106 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 (dMacc/dt)

• Conditions in dense, bound clusters should favor formation of

– Stars that rotate rapidly owing to high dMacc/dt