Aromatic emission from the ionised mane of the Horsehead nebula

TL;DR

This study investigates the survival and properties of aromatic infrared band emitters, particularly PAHs, in the ionised environment of the Horsehead nebula's HII region, revealing conditions that allow their persistence.

Contribution

It provides new spectral mapping observations showing the presence and characteristics of PAHs in the HII region, highlighting their survival under moderate radiation conditions.

Findings

Detection of a strong 11.3 micron AIB in the HII region

Correlation between 11.3 micron band intensity and ionized gas indicators

PAHs can survive and remain neutral in moderate radiation fields

Abstract

We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across the illuminated edge of the Horsehead nebula and especially their survival and properties in the HII region. We present spectral mapping observations taken with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong AIB at 11.3 microns is detected in the HII region, relative to the other AIBs at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be correlated with the intensity of the [NeII] at 12.8 microns and of Halpha, which shows that the emitters of the 11.3 microns band are located in the ionised gas. The survival of PAHs in the HII region could be due to the moderate intensity of the radiation field (G0 about 100) and the lack of photons with energy above about 25eV. The enhancement of the intensity of the 11.3 microns band in the HII region, relative to the other AIBs can be explained by the presence of neutral PAHs. Our observations highlight a transition region between ionised and neutral PAHs observed with ideal conditions in our Galaxy. A scenario where PAHs can survive in HII regions and be significantly neutral could explain the detection of a prominent 11.3 microns band in other Spitzer observations.