Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023

TL;DR

This study uses Herschel observations to analyze gas morphology and energetics in the PDR of NGC 7023, revealing the origins of [CII] and PAH emissions and constraining surface energetics.

Contribution

It provides new insights into PDR surface energetics by combining Herschel-HIFI data with PDR modeling, focusing on [CII] and PAH emissions in NGC 7023.

Findings

[CII] and PAH emissions originate from the transition zone between atomic and molecular gas.

Good agreement between observed and modeled [CII] intensities using the Meudon PDR code.

HIFI data constrains the energetics at the surface of PDRs.

Abstract

We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Using Herschel-HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by HIFI and provide information on the emitting gas. We show that both the [CII] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [CII] intensities. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [OI] and use of a new PDR model that includes PAH-related species.