1. Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
2. University of California, Department of Physics, Broida Hall, Santa Barbara, California 93106, USA
3. Maria Mitchell Observatory, 3 Vestal Street, Nantucket, Massachusetts 02554, USA
4. Present address: Colby College, Waterville, Maine 04901, USA
5. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA

Correspondence to: L. J. Greenhill¹ Correspondence and requests for materials should be addressed to L.J.G. (e-mail: Email: greenhill@cfa.harvard.edu).

Abstract

The BN/KL region in the Orion molecular cloud¹ is an archetype for the study of the formation of stars much more massive than the Sun². This region contains luminous young stars and protostars but, like most star-forming regions, is difficult to study in detail because of the obscuring effects of dust and gas. Our basic expectations are shaped to some extent by the present theoretical picture of star formation, the cornerstone of which is that protostars accrete gas from rotating equatorial disks and shed angular momentum by ejecting gas in bipolar outflows. The main source of the outflow in the BN/KL region^{3, 4, 5} may be an object known as radio source I (ref. 6), which is commonly believed to be surrounded by a rotating disk of molecular material^{7, 8, 9}. Here we report high-resolution observations of silicon monoxide (SiO) and water maser emission from the gas surrounding source I. We show that within 60 AUof the source (about the size of the Solar System), the region is dominated by a conical bipolar outflow, rather than the expected disk. A slower outflow, close to the equatorial plane of the protostellar system, extends to radii of 1,000 AU.