Molecular Clouds around a Runaway O Star, ζ Ophiuchi

Molecular clouds around a runaway O star, ζ Oph, have been surveyed with the NANTEN telescope, and their streaming motion caused by ζ Oph has been detected. ζ Oph is the earliest (O9.5 V) member of the Sco OB 2 association and is rapidly moving accompanied by an H II region, S 27. We detected 2 major filamentary cloud complexes, L 156 and L 204 complexes. Their total masses traced by the ¹²CO emission are 520 M⊙ and 1110 M⊙, respectively. Denser molecular cloud cores detected in C¹⁸O are locally distributed on the near side of the L 204 complex to ζ Oph, and lower density gas spreads toward the opposite side. Both complexes have radial velocity shifts that are correlated with the gas density. It is suggested that the low-density gas is accelerated outward away from ζ Oph relative to the embedded dense cores. These density and velocity structures indicate a dynamical interaction between the H II region and the molecular clouds. We calculated the kinetic energies and momenta for seven individual clouds, and examined the effects of the stellar wind and photo-evaporation by the UV field of ζ Oph. It is found that the stellar wind can hardly input the momentum during the crossing time of the rapid movement of ζ Oph. UV radiation seems to be a more likely origin of the streaming gas motion.