Global VLBI Observations of the 6.0 GHz Hydroxyl Masers in Onsala 1
  We present global VLBI observations of the first-excited state OH masers in
the massive star-forming region Onsala 1 (ON 1). The 29 masers detected are
nearly all from the 6035 MHz transition, and nearly all are identifiable as
Zeeman pair components. The 6030 and 6035 MHz masers are coincident with
previously published positions of ground-state masers to within a few
milliarcseconds, and the magnetic fields deduced from Zeeman splitting are
comparable. The 6.0 GHz masers in ON 1 are always found in close spatial
association with 1665 MHz OH masers, in contrast to the situation in the
massive star-forming region W3(OH), suggesting that extreme high density OH
maser sites (excited-state masers with no accompanying ground-state maser, as
seen in W3(OH)) are absent from ON 1. The large magnetic field strength among
the northern, blueshifted masers is confirmed. The northern masers may trace an
outflow or be associated with an exciting source separate from the other
masers, or the relative velocities of the northern and southern masers may be
indicative of expansion and rotation. High angular resolution observations of
nonmasing material will be required to understand the complex maser
distribution in ON 1.
