
Winter break
Rolf Chini (CAMK, Warsaw, Araucaria Project)
The project deals with the rotational velocity (v sin i) of 238 southern O stars. The sample contains 130 spectroscopic single stars (C), 36 single-lined binaries (SB1), and 72 SB2 systems (including eight triples). The overall v sin i statistics peaks at slow rotators (40–100 km/s) with a tail towards medium (100–200 km/s) and fast rotators (200–400 km/s). Binaries, on average, show increased rotation, which differs for close (Porb < 10 d) and wide binaries (10 d < Porb < 3700 d), and for primaries and secondaries. The spin-up of close binaries is well explained by the superposition of spin-orbit synchronization and mass transfer via Roche-lobe overflow. The increased rotation of wide binaries, however, can be caused by various spin-up mechanisms. Timescale arguments lead us to favor a scenario where wide O binaries are spun-up by a combination of cloud or disk fragmentation, which lays the basis of triple and multiple stars, and the subsequent merging or swallowing of low-mass by higher-mass stars or proto-stars.