Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions

TL;DR

This study uses mid-infrared emission analysis to investigate how Very Small Grains evaporate into PAHs in photo-dissociation regions, revealing a correlation between UV radiation and grain destruction.

Contribution

We developed the PAHTAT fitting tool to quantify eVSG and PAH contributions, establishing a method to trace UV radiation intensity in PDRs based on grain evaporation.

Findings

eVSG fraction decreases with increasing UV radiation

PAHTAT effectively traces UV field strength in dense PDRs

Supports photo-destruction scenario of eVSGs in UV-rich environments

Abstract

Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains (VSGs) by the impinging UV radiation field from a nearby star. Aims. We investigate quantitatively the transition zone between evaporating Very Small Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fit the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derive the fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of the local UV radiation field. Results. We show a clear decrease of f_eVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including non resolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.