Herschel observations of ortho- and para-oxidaniumyl (H2O+) in spiral
  arm clouds toward Sgr B2(M)
  H2O+ has been observed in its ortho- and para- states toward the massive star
forming core Sgr B2(M), located close to the Galactic center. The observations
show absorption in all spiral arm clouds between the Sun and Sgr B2. The
average o/p ratio of H2O+ in most velocity intervals is 4.8, which corresponds
to a nuclear spin temperature of 21 K. The relationship of this spin
temperature to the formation temperature and current physical temperature of
the gas hosting H2O+ is discussed, but no firm conclusion is reached. In the
velocity interval 0 to 60 km/s, an ortho/para ratio of below unity is found,
but if this is due to an artifact of contamination by other species or real is
not clear.
