Exploring the Origins of Earth's Nitrogen

Abstract

The origins of the ingredients necessary for life on Earth – such as water, carbon, and nitrogen – are unknown, with nitrogen’s origins proving to be particularly elusive. By far the most abundant carrier of nitrogen in star-forming regions and protoplanetary disks is molecular nitrogen (N2), which is too volatile to contribute directly to planetesimal formation in any solid form, is not disposed to chemical processing into more refractory forms, and is essentially invisible to astronomical observation. In this dissertation, I investigate the disposition of other nitrogen carriers, especially nitrogen-bearing organics, in a variety of protostellar environments to constrain how nitrogen makes its way into the building blocks of the Earth. In Chapter 2, I use Herschel observations to compare the abundance of nitrogen-bearing organics in a high-mass and a low-mass protostellar hot core, finding that HCN is the dominant carrier of nitrogen in organics. Its abundance relative to water (HCN/H2O) can be compared to N/H2O ratios in comets. Through this comparison, I rule out organic molecular ices as the primary contributor to cometary nitrogen, but identify that they are a likely important donor of the 15N isotopic enrichment which is seen in the Earth, comets, and other Solar System terrestrial bodies. I find that refractory forms of nitrogen are the likely source of the majority of cometary nitrogen, but their abundance is difficult to directly constrain or characterize. In Chapter 3, I describe an observational survey using the IRAM 30m and NOEMA observatories to measure HCN towards five additional low-mass protostars, to explore possible variations in the results described in Chapter 2. Here, I use models of the physical and chemical structure of the protostellar envelopes to jointly interpret the single-dish and interferometric observations. I find that on small scales, the brightness of HCN strongly depends on the bolometric luminosity of each protostar, leading to a strong detection of HCN on small scales in only one of the five protostars. Its abundance is close to the value measured for the low-mass protostar in Chapter 2, but the non-detections towards the other four protostars prevent a broader exploration of variation in HCN abundances in low-mass protostellar hot cores. In Chapter 4, I use high-resolution ALMA observations to better-characterize the physical and chemical properties of one protostar studied in Chapter 3. By comparing high-resolution CS, CH3OH, and H13CN emission, I observe a velocity gradient that I identify as associated with a tentative protostellar disk. NOEMA measurements of the sensitive temperature probe CH3CN indicate hot gas (T ≈ 170 K) coincident with this disk, supporting a scenario of active disk accretion. In Chapter 5, I summarize the conclusions of each chapter, and outline several important paths forward for the study of the elusive nitrogen carriers that were the original source of the Earth’s nitrogen. Overall, I find evidence supporting a common refractory-centric inheritance of nitrogen among the Galaxy’s terrestrial worlds, and I advance the observational study of nitrogen carriers in the planet-forming zones of young protostellar systems.