Kiyoaki Wajima

Associating Sites of Methanol Maser Emissions at 6.7 GHz

We have carried out VLBI observations of some methanol maser sources at 6.7 GHz with the Japanese VLBI Network (JVN) using phase-referencing techniques. We superposed the methanol masers on water, hydroxyl masers, radio continuum, and various molecular line emissions with an accuracy of a milliarcsecond (mas) scale, which is enough for investigating a positional relation with a central or exciting star. In this paper, we report results of the superposition in massive star-forming regions (SFRs) Cepheus A (Cep A) and Onsala 1 (ON 1). The methanol maser spots of Cep A showed an arched structure with a scale of 1400 AU. This structure was roughly perpendicular to a radio continuum jet, having an exciting star near the center of the arch, and was coincident with the CH3CN, NH3 disks. It is easy to explain that the Cep A methanol masers are associated with a circumstellar disk around a massive young stellar object (YSO). The methanol maser spots of ON 1 formed two main clusters isolated 1800 AU from each other, which corresponded to a redand a blue-shifted spectral feature, respectively. The direction of the elongation of the isolated clusters coincided with the H 13CO+, SiO molecular outflows, although each spatial size is quite different. Also, the position-velocity diagram along the direction of the molecular outflows roughly coincided. The ON 1 methanol masers may trace an outflow ejected from a massive YSO, while it is also thought to be possible that the masers of this source may trace an expansion of an Ultra-compact (UC) H II region by superposing on the hydroxyl masers.