An expanding system of molecular clouds surrounding Lambda Orionis

Abstract

Observations of the J = 1-0 line of CO, taken from the Columbia University survey of Orion, reveal a ring of molecular gas which is 10 deg (69 pc) in diameter and centered on the H II region S264 and its ionizing O8 star, Lambda Ori. A kinematical model for the expanding ring has been fitted to the positions and velocities of the observed molecular peaks. It suggests that the ring is tilted by 36 deg with respect to the line of sight and is expanding at 14 km/s. The molecular clouds, which lie on a well-defined ellipse just outside the edge of the H II region, coincide with a ring of dust clouds and diffuse IR emission. The distribution and velocity of ionized, atomic, and molecular gases suggest that Lambda Ori formed at the center of a flattened molecular cloud which was disrupted by a combination of the gas pressure within the H II region created by Lambda Ori and the rocket effect. A model is presented for the evolution of a flattened or oblate molecular cloud at whose center an O star forms. The parameters of the model compare favorably with the observed characteristics of the Lambda Ori system, with the exception of the velocity of Lambda Ori itself. Possible reasons for this discrepancy are noted.