Resolving distance ambiguities towards 6.7 GHz methanol masers

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: jpandian@mpifr-bonn.mpg.de
² Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA e-mail: emomjian@naic.edu
³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA e-mail: Paul.F.Goldsmith@jpl.nasa.gov

Received: 18 March 2008
Accepted: 14 May 2008

Abstract

Context. Distances to most star forming regions are determined using kinematics, through the assumption that the observed radial velocity arises from the motion of the source with respect to the Sun resulting from the differential rotation of Galaxy. The primary challenge associated with the application of this technique in the inner Galaxy is the kinematic distance ambiguity.

Aims. In this work, we aim to resolve the kinematic distance ambiguity towards a sample of 6.7 GHz methanol masers, which are signposts of the early stages of massive star formation.

Methods. We measured 21 cm H I absorption spectra using the Very Large Array in C and CnB configurations. A comparison of the maximum velocity of H I absorption with the source velocity and tangent point velocity was used to resolve the kinematic distance ambiguity.

Results. We resolved the distance ambiguity towards 41 sources. Distance determinations that are in conflict with previous measurements are discussed. The NE2001 spiral arm model is broadly consistent with the locations of the star forming complexes. We find that the use of vertical scale height arguments to resolve the distance ambiguity can lead to erroneous classifications for a significant fraction of sources.