The NH$_2$D hyperfine structure revealed by astrophysical observations
  The 1$_{11}$-1$_{01}$ lines of ortho and para--NH$_2$D (o/p-NH$_2$D),
respectively at 86 and 110 GHz, are commonly observed to provide constraints on
the deuterium fractionation in the interstellar medium. In cold regions, the
hyperfine structure due to the nitrogen ($^{14}$N) nucleus is resolved. To
date, this splitting is the only one which is taken into account in the NH$_2$D
column density estimates. We investigate how the inclusion of the hyperfine
splitting caused by the deuterium (D) nucleus affects the analysis of the
rotational lines of NH$_2$D. We present 30m IRAM observations of the above
mentioned lines, as well as APEX o/p-NH$_2$D observations of the
1$_{01}$-0$_{00}$ lines at 333 GHz. The hyperfine spectra are first analyzed
with a line list that only includes the hyperfine splitting due to the $^{14}$N
nucleus. We find inconsistencies between the line widths of the
1$_{01}$-0$_{00}$ and 1$_{11}$-1$_{01}$ lines, the latter being larger by a
factor of $\sim$1.6$\pm0.3$. Such a large difference is unexpected given the
two sets of lines are likely to originate from the same region. We next employ
a newly computed line list for the o/p-NH$_2$D transitions, where the hyperfine
structure induced by both nitrogen and deuterium nuclei is included. With this
new line list, the analysis of the previous spectra leads to linewidths which
are compatible. Neglecting the hyperfine structure owing to D leads to
overestimate the linewidths of the o/p-NH$_2$D lines at 3 mm. The error for a
cold molecular core is about 50\%. This error propagates directly to the column
density estimate. It is therefore recommended to take into account the
hyperfine splittings caused by both the $^{14}$N and D nuclei in any analysis
relying on these lines.
