Herschel HIFI Observations of the Sgr A +50 km/s Cloud. Deep Searches
  for O2 in Emission and Foreground Absorption
  To date O2 has definitely been detected in only two sources, namely rho Oph A
and Orion, reflecting the extremely low abundance of O2 in the interstellar
medium. One of the sources in the HOP program is the +50 km/s Cloud in the Sgr
A Complex in the centre of the Milky Way. The Herschel HIFI is used to search
for the 487 and 774 GHz emission lines of O2. No O2 emission is detected
towards the Sgr A +50 km/s Cloud, but a number of strong emission lines of
methanol (CH3OH) and absorption lines of chloronium (H2Cl+) are observed. A 3
sigma upper limit for the fractional abundance ratio of (O2)/(H2) in the Sgr A
+50 km/s Cloud is found to be X(O2) less than 5 x 10(-8). However, since we can
find no other realistic molecular candidate than O2 itself, we very tentatively
suggest that two weak absorption lines at 487.261 and 487.302 GHz may be caused
by the 487 GHz line of O2 in two foreground spiral arm clouds. By considering
that the absorption may only be apparent, the estimated upper limit to the O2
abundance of less than (10-20) x 10(-6) in these foreground clouds is very
high. This abundance limit was determined also using Odin non-detection limits.
If the absorption is due to a differential Herschel OFF-ON emission, the O2
fractional abundance may be of the order of (5-10) x 10(-6). With the
assumption of pure absorption by foreground clouds, the unreasonably high
abundance of (1.4-2.8) x 10(-4) was obtained. The rotation temperatures for
CH3OH-A and CH3OH-E lines in the +50 km/s Cloud are found to be 64 and 79 K,
respectively, and the fractional abundance of CH3OH is approximately 5 x
10(-7).
