We present the results of a study aimed at assessing the differences in the distribution of rotation speeds , N ( v \sin i ) among young ( 1-15 Myr ) B stars spanning a range of masses 6 < M/M _ { \sun } < 12 and located in different environments : 7 low density ( \rho < 1 M _ { \sun } / pc ^ { 3 } ) ensembles that are destined to become unbound stellar associations , and 8 high density ( \rho \gg 1 M _ { \sun } / pc ^ { 3 } ) ensembles that will survive as rich , bound stellar clusters for ages well in excess of 10 ^ { 8 } years .
Our results demonstrate ( 1 ) that independent of environment , the rotation rates for stars in this mass range do not change by more than 0.1 dex over ages t \sim 1 to t \sim 15 Myr ; and ( 2 ) that stars formed in high density regions lack the cohort of slow rotators that dominate the low density regions and young field stars .
We suggest that the differences in N ( v \sin i ) between low and high density regions may reflect a combination of initial conditions and environmental effects : ( 1 ) the higher turbulent speeds that characterize molecular gas in high density , cluster-forming regions ; and ( 2 ) the stronger UV radiation fields and high stellar densities that characterize such regions .
Higher turbulent speeds may lead to higher time averaged accretion rates during the stellar assembly phase .
In the context of stellar angular momentum regulation via “ disk-locking , ” higher accretion rates lead to both higher initial angular momenta and evolution-driven increases in surface rotation rates as stars contract from the birthline to the Zero Age Main Sequence .
Stronger UV radiation fields and higher densities may lead to shorter disk lifetimes in cluster-forming regions .
If so , B stars formed in dense clusters are more likely to be “ released ” from their disks early during their PMS lifetimes and evolve into rapid rotators as they conserve angular momentum and spin up in response to contraction .
By contrast , the majority of their brethren in low density , association forming regions can retain their disks for much or all of their PMS lifetimes , are “ locked ” by their disks to rotate at constant angular speed , and lose angular momentum as they contract toward the ZAMS , and thus arrive on the ZAMS as relatively slowly rotating stars .