Exploring the Origins of Earth's Nitrogen: Astronomical Observations of
  Nitrogen-bearing Organics in Protostellar Environments
  It is not known whether the original carriers of Earth's nitrogen were
molecular ices or refractory dust. To investigate this question, we have used
data and results of Herschel observations towards two protostellar sources: the
high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422.
Towards Orion KL, our analysis of the molecular inventory of Crockett et al.
(2014) indicates that HCN is the organic molecule that contains by far the most
nitrogen, carrying $74_{-9}^{+5}\%$ of nitrogen-in-organics. Following this
evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar
analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra
of HCN, and fit these observations against "jump" abundance models of IRAS
16293-2422's protostellar envelope. We find an inner-envelope HCN abundance
$X_{\textrm{in}} = 5.9\pm0.7 \times 10^{-8}$ and an outer-envelope HCN
abundance $X_{\textrm{out}} = 1.3 \pm 0.1 \times 10^{-9}$. We also find the
sublimation temperature of HCN to be $T_{\textrm{jump}} = 71 \pm 3$~K; this
measured $T_{\textrm{jump}}$ enables us to predict an HCN binding energy
$E_{\textrm{B}}/k = 3840 \pm 140$~K. Based on a comparison of the HCN/H2O ratio
in these protostars to N/H2O ratios in comets, we find that HCN (and, by
extension, other organics) in these protostars is incapable of providing the
total bulk N/H2O in comets. We suggest that refractory dust, not molecular
ices, was the bulk provider of nitrogen to comets. However, interstellar dust
is not known to have 15N enrichment, while high 15N enrichment is seen in both
nitrogen-bearing ices and in cometary nitrogen. This may indicate that these
15N-enriched ices were an important contributor to the nitrogen in
planetesimals and likely to the Earth.
