Laboratory spectroscopic study of the $^{15}$N isotopomers of cyanamide,
  H$_2$NCN, and a search for them toward IRAS 16293$-$2422 B
  Cyanamide is one of the few interstellar molecules containing two chemically
different N atoms. It was detected recently toward the solar-type protostar
IRAS 16293-2422 B together with H$_2$N$^{13}$CN and HDNCN in the course of the
Atacama Large Millemeter/submillimeter Array (ALMA) Protostellar
Interferometric Line Survey (PILS). The detection of the 15N isotopomers or the
determination of upper limits to their column densities was hampered by the
lack of accurate laboratory data at the frequencies of the survey. We wanted to
determine spectroscopic parameters of the $^{15}$N isotopomers of cyanamide
that are accurate enough for predictions well into the submillimeter region and
to search for them in the PILS data. We investigated the laboratory rotational
spectra of H$_2^{15}$NCN and H$_2$NC$^{15}$N in the selected region between 192
and 507~GHz employing a cyanamide sample in natural isotopic composition.
Additionally, we recorded transitions of H$_2$N$^{13}$CN. We obtained new or
improved spectroscopic parameters for the three isotopic species. Neither of
the $^{15}$N isotopomers of cyanamide were detected unambiguously in the PILS
data. Two relatively clean lines can be tentatively assigned to H$_2^{15}$NCN.
If confirmed, their column densities would imply a low $^{14}$N/$^{15}$N ratio
for cyanamide toward this source. The resulting line lists should be accurate
enough for observations up to about 1 THz. More sensitive observations,
potentially at different frequencies, may eventually lead to the astronomical
detection of these isotopic species.
