Photodetachment as destruction mechanism for CN- and C3N- anions in circumstellar envelopes

TL;DR

This study measures the photodetachment cross sections of CN- and C3N- anions and uses these data in models to show that UV photodetachment is a key destruction process for these molecules in circumstellar environments.

Contribution

The paper provides the first experimental measurements of photodetachment cross sections for CN- and C3N- and applies these to astrophysical models of circumstellar envelopes.

Findings

Photodetachment cross sections are approximately 1.2 and 1.4 x 10^-17 cm^2.

UV photodetachment is the dominant destruction mechanism in certain regions.

Photodetachment significantly influences anion abundances in circumstellar environments.

Abstract

Absolute photodetachment cross sections of two anions of astrophysical importance CN- and C3N- were measured to be (1.18 +- (0.03)_stat (0.17)_sys) * 10^-17 cm^2 and (1.43 +- (0.14)_stat (0.37)_sys) * 10^-17 cm^2 respectively at the ultraviolet wavelength of 266 nm (4.66 eV). These relatively large values of the cross sections imply that photodetachment can play a major role in the destruction mechanisms of these anions particularly in photon-dominated regions. We have therefore carried out model calculations using the newly measured cross sections to investigate the abundance of these molecular anions in the cirumstellar envelope of the carbon-rich star IRC+10216. The model predicts the relative importance of the various mechanisms of formation and destruction of these species in different regions of the envelope. UV photodetachment was found to be the major destruction mechanism for both CN- and C3N- anions in those regions of the envelope, where they occur in peak abundance. It was also found that photodetachment plays a crucial role in the degradation of these anions throughout the circumstellar envelope.