Astron. Astrophys. 361, 1095-1111 (2000)

6. HCN and HNC

The HNC and HCN isomers represent a very interesting chemical probe in interstellar sources. These two molecules have similar dipole moments and their different intensities ratios therefore reflect chemical changes rather than excitation effects. The HCN to HNC abundance ratio present a strong chemical temperature dependence. In the OMC-1 region, this ratio has been shown to vary significantly in regions with different kinetic temperatures (Schilke et al. 1992), being of 80 in Orion-KL and falling to 5 in cooler regions.

Recently, Hirota et al. (1998) have studied the HNC/HCN ratio in dark cloud cores and shown a clear temperature dependence of this ratio when comparing the available data on these molecules (see their Fig. 4).

When we observed the HCN(1-0) transitions (Fig. 15) we could see two strong self absorptions arising at different velocities. We could accurately determine these velocities thanks to the hyperfine structure, and found 43.0 km.s^-1 and 46.3 km.s^-1. The HCN(1-0) spectrum is very complicated but the absorption at 43.0 km.s^-1 seems roughly a factor of two stronger than the one observed at 46.3 km.s^-1.

Fig. 15. HCN(1-0) (top) and HNC(1-0) (bottom) spectra observed toward RAFGL7009S. The velocity scale for HCN has been adjusted for the 1-0, F2-1 transition at 88631.8473 MHz. HCN shows two strong triplet absorptions (indicated by the two sets of vertical tick lines) whereas only one absorption is seen for HNC. The vertical line in the HNC spectrum indicate the position where we would expect the emission of the second hyperfine component seen in absorption in the HCN spectrum.

When one looks at the HNC(1-0) spectrum measured at the same position, it is also strongly self absorbed and present a T_mb of the same order of magnitude. However, there is only one emission and self absorption, both centred at 43.0 km.s^-1, and nothing at 46.3 km.s^-1 (indicated by a vertical line in the Fig. 15).

Thus, although the spectra are too complicated to derive accurate numbers without observing the HC¹⁵N, H¹³CN and HN¹³C isotopes, we clearly sample two different physical regions.

The difference in opacity between the 43 and 46 km.s^-1 components is much more pronounced for HNC than for HCN. The most likely explanation is that the gas associated with the 46 km.s^-1 feature is warmer than the gas associated with the 43 km.s^-1, hence has a larger HCN/HNC abundance ratio by roughly one order of magnitude. The 43 km.s^-1 component is also seen in other species (HCO⁺, CS,...). It is typical of fairly cold gas.

Owing to the temperature behaviour of the HNC/HCN ratio, as described in Hirota et al. (1998), we can conclude that with the 43.0 km.s^-1 component we are probing a very cold phase (T25 K), whereas the component at 46.3 km.s^-1 must have been formed at temperatures above 40 K. The former is probably associated with the cold parts of the cloud (envelope) whereas the latter could trace the outflow observed in CO (Shepherd & Churchwell 1996). Indeed, a high HNC/HCN abundance ratio can be maintained in the gas phase if and only if both species have stayed in cold gas since their formation. A passage through a high temperature phase would remove the HNC isomer from the gas phase, and convert it to HCN. As we have observed for CS, the kinetic temperature of the 43.0 km.s^-1 component is quite low, in the 10-50K range. For the other component, we have no constraint on its rotational excitation temperature. Maps of these two isomers (and their respective isotopes to avoid opacity effects), should permit us to probe and decipher the geometry of the cold envelope versus the outflow.

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
helpdesk.link@springer.de