Rotation of young stars in Cepheus OB3b

We present a photometric study of I-band variability in the young association Cepheus OB3b. The study is sensitive to periodic variability on time-scales of less than a day, to more than 20 d. After rejection of contaminating objects using V, I, R and narrow-band Hα photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star-forming region. We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor. This yields a distance modulus of 8.8 ± 0.2 mag, corresponding to a distance of 580 ± 60 pc, and an age of 4–5 Myr. The rotation period distribution confirms the general picture of rotational evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrow-band Hα photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star-forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star-forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects.