Transcript Rotation Among High Mass Stars: A Link to Initial

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)
19
v (esc)
18.5
log Jsini/M
18
17.5
17
16.5
16
15.5
15
-1
-0.5
0
0.5
1
1.5
Log M/M
Single formation mechanism: 0.2-30 Msun
2
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
30
Probability Density x 100
25
20
15
10
5
0
0-25
26-50
51-75
76-100
101-150
151-200
vsini (km/sec)
201-250
251-300
301-350
>350
Unevolved B Stars in h and chi Per
4 < M/MSun < 5
30
Probability Density x 100
25
20
15
10
5
0
0-25
26-50
51-75
76-100
101-150
151-200
vsini (km/sec)
201-250
251-300
301-350
>350
Cumulative Distribution of vsini
MWG Clusters and Field Stars: 6-12 Msun
1.2
Cumulative fraction
1
0.8
Field
0.6
Bound
Field
0.4
Bound Clusters
0.2
0
1.2
1.4
1.6
1.8
2
log vsini
2.2
2.4
2.6
2.8
Cumulative Distribution of vsini
MWG Clusters, Field &Associations: 6-12 Msun
1.2
Cumulative fraction
1
0.8
Bound
0.6
open
Field
0.4
0.2
Associations
0
1.2
1.4
1.6
1.8
2
log vsini
2.2
2.4
2.6
2.8
R136
The Challenge: Source Confusion
Selecting the Sample
R 136 Observations
11 O Stars; 15 B Stars
HR Diagram for R136
-7
25 Msun
-6
-5
12 Msun
Mv
-4
Series1
-3
Series2
Zams LMC
5 Msun
-2
25 Msun
12 Msun
5 Msun
-1
0
1
4.8
4.7
4.6
4.5
4.4
log T eff
4.3
4.2
4.1
4
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 Masses
15-30 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
Hunter
et al.
2008
Metallicity: N(vsini): 6-12 Msun
LMC and MW Appear Similar
MWG
Field Stars
,
LMC
Clusters
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
Wdisk = Wstar
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