Evidence for Ubiquitous Carbon Grain Destruction in Hot Protostellar Envelopes

Abstract

Earth is deficient in carbon and nitrogen by up to ∼4 orders of magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich) refractory organics in the high-temperature planet-forming regions could explain this deficiency. Assuming a refractory cometary composition for these grains, their destruction enhances nitrogen-containing, oxygen-poor molecules in the hot gas (≳300 K) after the initial formation and sublimation of these molecules from oxygen-rich ices in the warm gas (∼150 K). Using observations of 37 high-mass protostars with the Atacama Large Millimeter/submillimeter Array, we find that oxygen-containing molecules (CH3OH and HNCO) systematically show no enhancement in their hot component. In contrast, nitrogen-containing, oxygen-poor molecules (CH3CN and C2H3CN) systematically show an enhancement of a factor ∼5 in their hot component, pointing to additional production of these molecules in the hot gas. Assuming only thermal excitation conditions, we interpret these results as a signature of destruction of refractory organics, consistent with the cometary composition. This destruction implies a higher C/O and N/O in the hot gas than the warm gas, while the exact values of these ratios depend on the fraction of grains that are effectively destroyed. This fraction can be found by future chemical models that constrain C/O and N/O from the abundances of minor carbon, nitrogen, and oxygen carriers presented here.

Identification of Hot Gas around Low-mass Protostars

“…An enhancement of CH 3 CN in hot gas was also found by Nazari et al (2023) for high-mass protostars in the ALMAGAL survey. These authors determined the relative contribution of warm and hot gas by dividing the column density derived from transitions with low and high (>400 K) upper-level energies.…”

“…A similar result was found for the nitrogen-bearing species C 2 H 3 CN, while HNCO was similar to CH 3 OH. A different distribution for CH 3 CN and CH 3 OH, with CH 3 CN enhanced in hot gas, has now thus been found using two different analysis techniques and toward both low-mass (this work) and high-mass sources (Nazari et al 2023), suggesting that this may be a common feature of protostellar chemistry.…”

Chemistry of Complex Organic Molecules in the V883 Ori Disk Revealed by ALMA Band 3 Observations

“…This apparent deficiency of nitrogen-bearing COMs may indicate that the nitrogen is locked into less volatile components, e.g., refractory carbonaceous grains (van 't Hoff et al 2020) and/or ammonium salt (Altwegg et al 2020b;Poch et al 2020). This scenario is supported by the recent observations of protostellar envelopes, which report a difference in the spatial distributions between nitrogen-and oxygenbearing COMs in protostellar envelopes (Csengeri et al 2019;Okoda et al 2021;Lee et al 2022;Okoda et al 2022;Nazari & Tabone 2023). While the gaseous nitrogen-bearing COMs may exist in the inner hot regions of the V883 Ori disk, their emission may be beam-diluted and/or hidden by the intense dust continuum emission as discussed in Section 4.2 in the present observations.…”