From emission to absorption: the FAST observation of the OH 18-cm lines from the Comet C2025/A6

TL;DR

This study reports the first detection of OH 18-cm lines from comet C/2025 A6 using FAST, revealing increasing water expansion velocities and OH production rates as the comet approached the Sun.

Contribution

First observation of OH lines from comet C/2025 A6 with FAST, providing new data on water dynamics and production rates in this comet.

Findings

Water expansion velocity increased from 1.5 to 3.0 km/s as the comet approached.

OH production rates showed a significant upward trend over the observation period.

Detected water-related emissions for the first time in this comet with FAST.

Abstract

We observed comet C/2025 A6 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) equipped with the ultra-wideband receiver from 2025 October 23 to November 8, and it was the first detection for this comet with FAST.Through trapezoidal fitting of the OH line profiles, we derived the expansion velocities of the water which showed an increase from 1.5$\pm$0.3 km s$^{-1}$ at the heliocentric distance of 0.65 AU to 3.0$\pm$0.9 km s$^{-1}$ at 0.54 AU. Based on these results, we estimated the OH production rates of C/2025 A6 for October 23, October 26, November 4 and November 5 which were (1.0$\pm$0.1)$\times$10$^{29}$, (1.2$\pm$0.1)$\times$10$^{29}$, (1.4$\pm$0.3)$\times$10$^{29}$, and (1.5$\pm$0.4)$\times$10$^{29}$ s$^{-1}$ respectively. The results show a significant upward trend.