OH$^+$ emission from cometary knots in planetary nebulae

TL;DR

This study models molecular emissions from cometary knots in planetary nebulae, highlighting the importance of EUV radiation and star temperature in matching observed OH$^+$ and other molecular emissions.

Contribution

It introduces combined photoionization and PDR models that incorporate EUV flux to better match observed molecular emissions in planetary nebulae.

Findings

EUV radiation is crucial for reproducing observed OH$^+$ brightness.

Models predict undetected molecules like ArH$^+$ and HeH$^+$ in PNe.

OH$^+$ brightness correlates with central star temperature above 100 kK.

Abstract

We model the molecular emission from cometary knots in planetary nebulae (PNe) using a combination of photoionization and photodissociation region (PDR) codes, for a range of central star properties and gas densities. Without the inclusion of ionizing extreme ultraviolet (EUV) radiation, our models require central star temperatures $T_*$ to be near the upper limit of the range investigated in order to match observed H$_2$ and OH$^+$ surface brightnesses consistent with observations - with the addition of EUV flux, our models reproduce observed OH$^+$ surface brightnesses for $T_* \ge 100 \, {\rm kK}$. For $T_* < 80 \, {\rm kK}$, the predicted OH$^+$ surface brightness is much lower, consistent with the non-detection of this molecule in PNe with such central star temperatures. Our predicted level of H$_2$ emission is somewhat weaker than commonly observed in PNe, which may be resolved by the inclusion of shock heating or fluorescence due to UV photons. Some of our models also predict ArH$^+$ and HeH$^+$ rotational line emission above detection thresholds, despite neither molecule having been detected in PNe, although the inclusion of photodissociation by EUV photons, which is neglected by our models, would be expected to reduce their detectability.