2018ApJ...866..156R

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 toward two protostellar sources: the high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422. Toward Orion KL, our analysis of the molecular inventory of Crockett et al. 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 toward IRAS 16293-2422, which is considered a solar analog. Toward 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_in = 5.9 ± 0.7 x 10^–8 and an outer-envelope HCN abundance X_out = 1.3 ± 0.1 x 10^–9. We also find the sublimation temperature of HCN to be T_jump = 71 ± 3 K; this measured T_jump enables us to predict an HCN binding energy E_B/k = 3840 ± 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.