Upper limits to interstellar NH+ and para-NH2- abundances. Herschel-HIFI
  observations towards Sgr B2 (M) and G10.6-0.4 (W31C)
  The understanding of interstellar nitrogen chemistry has improved
significantly with recent results from the Herschel Space Observatory. To set
even better constraints, we report here on deep searches for the NH+ ground
state rotational transition J=1.5-0.5 of the ^2Pi_1/2 lower spin ladder, with
fine-structure transitions at 1013 and 1019 GHz, and the para-NH2- 1_1,1-0_0,0
rotational transition at 934 GHz towards Sgr B2(M) and G10.6-0.4 using
Herschel-HIFI. No clear detections of NH+ are made and the derived upper limits
are <2*10^-12 and <7*10^-13 in Sgr B2(M) and G10.6-0.4, respectively. The
searches are complicated by the fact that the 1013 GHz transition lies only
-2.5 km/s from a CH2NH line, seen in absorption in Sgr B2(M), and that the
hyperfine structure components in the 1019 GHz transition are spread over 134
km/s. Searches for the so far undetected NH2- anion turned out to be unfruitful
towards G10.6-0.4, while the para-NH2- 1_1,1-0_0,0 transition was tentatively
detected towards Sgr B2(M) at a velocity of 19 km/s. Assuming that the
absorption occurs at the nominal source velocity of +64 km/s, the rest
frequency would be 933.996 GHz, offset by 141 MHz from our estimated value.
Using this feature as an upper limit, we found N(p-NH2-)<4*10^11 cm^-2. The
upper limits for both species in the diffuse line-of-sight gas are less than
0.1 to 2 % of the values found for NH, NH2, and NH3 towards both sources.
Chemical modelling predicts an NH+ abundance a few times lower than our present
upper limits in diffuse gas and under typical Sgr B2(M) envelope conditions.
The NH2- abundance is predicted to be several orders of magnitudes lower than
our observed limits, hence not supporting our tentative detection. Thus, while
NH2- may be very difficult to detect in interstellar space, it could, be
possible to detect NH+ in regions where the ionisation rates of H2 and N are
greatly enhanced.
