Internal Proper Motions of Methanol Masers at 6.7GHz in Massive Star-Forming Region Onsala 1

We present the internal proper motions of 6.7 GHz methanol masers in a massive star-forming region, Onsala 1 (ON 1), measured using the Japanese very long baseline interferometry network (JVN) at three epochs spanning 778 days. The methanol masers were clearly distinct from water masers. The methanol masers surrounded an ultra-compact (UC) H II region, and their distribution was similar to that of hydroxyl masers. The internal motions of the methanol masers were clearly detected; they show outward motions in roughly the north-south direction with a relative velocity of ∼5 km s^-1. Their motion is similar to that of hydroxyl masers, which showed expansion at ∼5 km s^-1, and these two types of masers seem to trace the expanding UC H II region. Another possibility is that the methanol masers are associated with a molecular outflow observed by H¹³CO⁺ and SiO lines, because the direction and velocity of the methanol masers were similar to those of the molecular lines. Regarding the relative phases of water and methanol masers in the evolutionary stage of massive young stellar objects, we found that the former was earlier than the latter in ON 1 by comparing the sites associated with the masers. The water masers were associated with dust emission at submillimeter wavelengths, whereas the methanol masers were associated with an UC H II region at centimeter wavelengths, which appears at a later evolutionary phase than the dust core.