Cometary molecular clouds around RNO 6

On-going star formation near the double cluster h and χ Persei

IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain

Corresponding author: R. Bachiller bachiller@oan.es

Received: 29 August 2001
Accepted: 18 October 2001

Abstract

We present molecular line observations of the star-forming cloud around RNO 6 along with a newly discovered nearby molecular cloud that we name RNO 6 NW. Both clouds display striking similarities in their cometary structures and overall kinematics. By using ¹³CO line observations, we estimate that these clouds have similar sizes (~4.5 pc) and masses (~200 ). Both molecular clouds RNO 6 and RNO 6 NW are active in star formation. From new high resolution near-IR narrowband images, we confirm that RNO 6 hosts an embedded IR cluster that includes a Herbig Be star. A conspicuous H₂ filament is found to delineate the dense cometary head of the globule. RNO 6 NW hosts at least two IR sources and a bipolar molecular outflow of ~0.9 pc of length and ~0.5  of mass. We show that the cometary structure of both clouds has been created by the UV radiation from numerous OB stars lying ~1.5° to the north. Such OB stars are associated with the double cluster h and χ Persei, and are probably members of the Per OB1 association. Thus star formation inside these clouds has been very likely triggered by the Radiation Driven Implosion (RDI) mechanism. From comparison to RDI theoretical models, we find that the similar kinematics and morphology of both clouds is well explained if they are at a re-expansion phase. Triggered sequential star formation also explains the observed spatial distribution of the members of the near-IR cluster inside the RNO 6 cloud, and the morphology of the H₂ filament. We conclude that the RNO 6 and RNO 6 NW clouds are high-mass counterparts to the cometary globules of smaller masses which have been studied up to now. Thus our observations demonstrate that the RDI mechanism can produce, not only low mass stars in small globules, but also intermediate mass stars and clusters in massive clouds.