2001ApJ...551L.193G

We present new high spectral resolution observations of the N₂D⁺ J=2⟶1 and J=3⟶2 as well as N₂H⁺ J=3⟶2 lines toward dense molecular cores. The hyperfine structure of the N₂H⁺ and N₂D⁺ transitions is clearly resolved in these sources, in excellent agreement with quantum mechanical calculations. Optical depths are determined from the observed spectra, leading to D/H values on the order of 10%-30% for the N₂D⁺/N₂H⁺ system. There are a large number of hyperfine components, and in addition to providing satellites, they contribute to the broadening and asymmetric shapes of the N₂H⁺ and N₂D⁺ line profiles. It is therefore important to take into account the detailed hyperfine structure of the transitions when deducing astrophysical parameters from observed data.