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

Timothy N. Proudkii, Nathan X. Roth, Jérémie Boissier, Dominique Bockelée-Morvan, Nicolas Biver, Steve Charnley, Stefanie Milam, Martin Cordiner, Mike A. DiSanti, Boncho P. Bonev, Neil Dello Russo

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$σ$ 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.

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 () from 1.3 to 0.80 au. We securely detected HCN and CS and place 3 upper limits on CHOH, HCO, and CO abundances. Line kinematics and NOEMA spatial constraints indicate that HCN was released at or near the nucleus (parent scale length km), while CS showed higher gas expansion velocities and mixing ratios that increased with decreasing consistent with production from a distributed source. Across our campaign, CS mixing ratios relative to HO increased by a factor of 5, from at = 1.3 au to by = 0.80 au. HCN mixing ratios in our data rose modestly, from at = 1.3 au to by = 0.81 au. However, contemporaneous measurements from other facilities placed HCN consistently at a higher absolute level () with additional variability. Once cross-facility measurements were included, the HCN abundance showed no statistically robust monotonic dependence on . 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.

Paper Structure

This paper contains 23 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Comets as Primitive Solar System Bodies
  3. Motivation and Study of Comet D/2021 A1 (Leonard)
  4. Observations
  5. NOEMA Observations
  6. Synergy Between NOEMA ON-OFF and Interferometric Data
  7. APEX Observations
  8. Data Reduction
  9. Radiative Transfer Modeling
  10. Model Geometry
  11. Visibility Modeling
  12. Spectral Line Modeling
  13. Results
  14. NOEMA Results
  15. APEX Results
  16. ...and 8 more sections

Figures (7)

  • Figure 1: Spectrally integrated flux maps for NOEMA HCN ($J$=3-2) detections on November 5 (left) and November 21 (right). In the bottom left of the map, we display the size and orientation of the synthesized beam. The lower right of the map indicates the comet's illumination, the Sun's orientation (S), and the dust trail (T), while the upper right panel displays the spectral line extracted from the brightest pixel. For November 5, the rms noise is 9.62 mJy beam$^{-1}$ km s$^{-1}$, with the contours drawn from 2$\sigma$ to 4$\sigma$. For November 21, the rms noise is 13.46 mJy beam$^{-1}$ km s$^{-1}$, with the contours drawn from 2$\sigma$ to 8$\sigma$.
  • Figure 2: The two panels show $\Delta\chi^{2}$ as a function of parent scale length ($L_p$) for November 5 (left) and 21 (right). The black line connects the discrete $\Delta\chi^{2}$ points to guide the eye. Dotted horizontal lines indicate the $\Delta \chi^2 = 1$ threshold (1$\sigma$, 68$\%$ confidence), while dashed horizontal lines mark the $\Delta \chi^2 = 6.63$ threshold (2.6$\sigma$, 99$\%$ confidence), with $\chi^2$ computed from the difference between the observed interferometric and synthetic complex visibilities (real and imaginary parts) and the ON-OFF spectrum providing the $uv = 0$ constraint.
  • Figure 3: (A) HCN ($J$=3-2) spectrum of A1 on UT 2021 November 5 (black). The top panel shows the ON-OFF spectrum, while the lower two panels display spectra extracted over different baseline ranges (i.e., angular scales). The spectra are shown at a frequency resolution of 125 kHz (velocity resolution 0.15 km s$^{-1}$). The best-fit model is overplotted in red. (B) Real part of the observed visibility amplitude as a function of projected baseline length for HCN on UT 2021 November 5. The parent model and the upper-limit distributed source model are overplotted for comparison. The uv-distance is given in units of k$\lambda$. (C-D) HCN spectra for November 21, with traces and labels as in (A-B).
  • Figure 4: Evolution of the HCN spectral line in comet A1. The black line depicts the extracted spectra, while the red line represents our modeled data. NOEMA spectra shown here are from the ON-OFF observations.
  • Figure 5: Evolution of the CS spectral line in comet A1. The black line depicts the extracted spectra, while the red line represents our modeled data. NOEMA spectra shown here are from the ON-OFF observations.
  • ...and 2 more figures