FAST detection of OH emission in the carbon-rich planetary nebula NGC 7027

TL;DR

This paper reports the first detection of OH emission in the planetary nebula NGC 7027 using FAST's new ultra-wideband receiver, revealing insights into chemical processes and demonstrating FAST's observational capabilities.

Contribution

It presents the first detection of ground-state OH emission in NGC 7027 with FAST, providing new data on molecular emissions in carbon-rich planetary nebulae.

Findings

First detection of OH emission at 1612 MHz in NGC 7027.

OH lines observed in absorption at 1665, 1667, and 1721 MHz.

OH absorption likely from the outer shell, with implications for chemical models.

Abstract

We present the first detection of the ground-state OH emission line at 1612 MHz toward the prototypical carbon-rich planetary nebula (PN) NGC 7027, utilizing the newly installed ultra-wideband (UWB) receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This emission is likely to originate from the interface of the neutral shell and the ionized region. The other three ground-state OH lines at 1665, 1667, and 1721 MHz are observed in absorption and have velocities well matched with that of HCO$^+$ absorption. We infer that the OH absorption is from the outer shell of NGC 7027, although the possibility that they are associated with a foreground cloud cannot be completely ruled out. All the OH lines exhibit a single blue-shifted component with respect to the central star. The formation of OH in carbon-rich environments might be via photodissociation-induced chemical processes. Our observations offer significant constraints for chemical simulations, and they underscore the potent capability of the UWB receiver of FAST to search for nascent PNe.