The PAH emission spectra of Large Magellanic Cloud HII regions

Abstract

A set of ISOPHOT spectra from a sample of HII regions in the Large Magellanic Cloud (LMC) is presented. In all the spectra, emission bands arising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearly present. These features are observed to vary considerably in relative strength to each other from source to source and even within 30 Doradus. The LMC spectra have been compared with ISO-SWS spectra from Galactic H II regions and with the ISOCAM observation towards a quiescent molecular cloud in the SMC (Reach et al. 2000). A correlation is found between the I7.7/I11.2 versus I6.2/I11.2 and the I8.6/I11.2 versus I6.2/In11.2 ratios. A segregation between the sources in the different types of environment (Milky Way - LMC - SMC) is present. Furthermore, within the LMC observations, a clear distinction between 30 Doradus and non-30 Doradus pointings is found. We discuss the variations in the relative strength of the PAH features in view of the different physical environments and highlight the relation with the PAH/dust ratio and the extinction curve. We conclude that 1) the same conditions responsible for the observed trends in the relative PAH-feature strengths also affect the carrier of the 2175 Å bump leading to the differences in strength of the latter, and 2) the molecular structure is the major cause of the observed variations in the relative strength of the PAH features. In the SMC and 30 Doradus compact PAH species dominate, while PAHs with an open, uneven structure are the dominant ones in Galactic HII regions and the non-30 Dor LMC sources.