Upper limits to interstellar NH⁺and para-NH₂⁻abundances

Abstract

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 2 Π 1/2 lower spin ladder, with fine-structure transitions at 1013 and 1019 GHz, and the para-NH in the Sgr B2 (M) molecular envelope and in the G10.6−0.4 molecular cloud, respectively. The searches are, however, complicated by the fact that the 1 013 GHz transition lies only −2.5 km s −1 from a CH 2 NH line, which is seen in absorption in Sgr B2 (M), and that the hyperfine structure components in the 1019 GHz transition are spread over 134 km s −1 . Searches for the so far undetected NH − 2 anion turned out to be unfruitful towards G10.6−0.4, while the para-NH − 2 1 1,1 −0 0,0 transition was tentatively detected towards Sgr B2 (M) at a velocity of 19 km s −1 . Assuming that the absorption occurs at the nominal source velocity of +64 km s −1 , 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-NH − 2 ) < ∼ 4 × 10 11 cm −2 , which implies an abundance of < ∼ 8 × 10 −13 in the Sgr B2 (M) molecular envelope. 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, NH 2 , and NH 3 towards both sources, and the abundance limits are < ∼ 2−4 × 10 −11 . An updated pseudo time-dependent chemical model with constant physical conditions, including both gas-phase and surface chemistry, 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 NH − 2 abundance is predicted to be several orders of magnitudes lower than our observed limits, hence not supporting our tentative detection. Thus, while NH − 2 may be very difficult to detect in interstellar space, it could, on the other hand, be possible to detect NH + in regions where the ionisation rates of H 2 and N are greatly enhanced.

Ortho-to-para ratio of NH2

“…This is in contrast to many other species, for example, CH (Gerin et al 2010), H 2 O (Flagey et al 2013) and HF (Sonnentrucker et al 2010), which trace lower density interstellar gas than NH 2 (see e.g. chemical models for the nitrogen hydrides in different physical conditions in Persson et al 2014). We did, however, find redshifted absorption in W51, at V LSR ∼ 68 km s −1 , tracing a dense clump in a filament interacting with W51, also detected in C 3 , HDO and NH 3 (Mookerjea et al 2014).…”

“…Once formed, NH + 4 undergoes dissociative recombination with electrons resulting in the production of NH 3 and NH 2 (see Fig. 3 in Persson et al 2014). NH 2 can also form through the dissociative recombination of NH + 3 .…”

Rotational Spectroscopy of a Triatomic Molecular Anion

“…With our improved line positions we can test the tentative assignment of an observed transition towards the middle part of the giant molecular cloud Sagittarius B2 (SgrB2(M)) to the NH − 2 anion by Persson et al [21]. Their observation with the Heterodyne Instrument for the Far-Infrared (HIFI) on board the Herschel Space Observatory was attributed to a transition with a rest frequency of between 933973 and 934009 GHz.…”

“…Recently, NH − 2 , a molecular anion with a similarly small electron binding energy as H − , has been tentatively assigned to a previously unidentified absorption feature near 934 GHz [21]. However, astrochemical model calculations by the same authors predict an expected abundance of NH − 2 far below the detection limit.…”

First detection of [N II] 205μm absorption in interstellar gas

“…Photoionised nebulae with densities n e 0.1 cm −3 can be ruled out because the 205 μm line would appear strongly in emission. Models of neutral, partly molecular gas at low temperature, T K 100 K, are unable to explain the observed absorption because the N + abundance in neutral gas is too low at the adopted ionisation rate and the observed abundance would imply an amount of NH + in conflict with observed upper limits (Persson et al 2012(Persson et al , 2014. The total cooling provided by [N ii] in our models is dominated by the 6548 Å and 6583 Å transitions (86%), while the 205 μm and 122 μm fine-structure transitions account for 12%.…”