Monitoring Volatile Evolution in Disrupting Comet D/2021 A1 (Leonard) with NOEMA and APEX

Abstract

We report a pre-perihelion survey of volatile emissions from comet D/2021 A1 (Leonard) with the Northern Extended Millimeter Array (NOEMA; UT 2021 Nov. 5, 21, and Dec. 1) and the Atacama Pathfinder Experiment (APEX; UT 2021 Dec. 9-10), spanning heliocentric distances ($r_H$) from 1.3 to 0.80 au. We securely detected HCN and CS and place 3$\sigma$ upper limits on CH$_3$OH, H$_2$CO, and CO abundances. Line kinematics and NOEMA spatial constraints indicate that HCN was released at or near the nucleus (parent scale length $<300$ km), while CS showed higher gas expansion velocities and mixing ratios that increased with decreasing $r_H$ $-$ consistent with production from a distributed source. Across our campaign, CS mixing ratios relative to H$_2$O increased by a factor of $\sim$5, from $0.02 \pm 0.01\%$ at $r_H$ = 1.3 au to $0.10\pm0.02\%$ by $r_H$ = 0.80 au. HCN mixing ratios in our data rose modestly, from $0.04 \pm 0.02\%$ at $r_H$ = 1.3 au to $0.07 \pm 0.02\%$ by $r_H$ = 0.81 au. However, contemporaneous measurements from other facilities placed HCN consistently at a higher absolute level ($\sim\!0.08\%$) with additional variability. Once cross-facility measurements were included, the HCN abundance showed no statistically robust monotonic dependence on $r_H$. Variability in both species during the mid-December outbursts and fragmentation suggests that D/2021 A1's volatile evolution reflected not only solar insolation but also disruption processes, underscoring the value of multi-epoch, multi-instrument monitoring to capture rapid, species-dependent changes.