Blue Luminescence and the Presence of Small PAHs in the ISM

TL;DR

This study detects blue luminescence in various reflection nebulae, indicating that small PAH molecules responsible for BL are common in the interstellar medium, not limited to special environments like the Red Rectangle.

Contribution

It provides the first evidence of blue luminescence in ordinary reflection nebulae, expanding the understanding of small PAH molecules' ubiquity in the ISM.

Findings

BL detected in multiple reflection nebulae.

BL spatially correlates with aromatic emission features.

Suggests in-situ formation of small grains and molecules.

Abstract

Blue Luminescence (BL) was first discovered in a proto-planetary nebula, the Red Rectangle (RR) surrounding the post-AGB star HD 44179. BL has been attributed to fluorescence by small, 3-4 ringed neutral polycyclic aromatic hydrocarbon (PAH) molecules, and was thought to be unique to the RR environment where such small molecules are actively being produced and shielded from the harsh interstellar radiation by a dense circumstellar disk. In this paper we present the BL spectrum detected in several ordinary reflection nebulae illuminated by stars having temperatures between 10,000 -- 23,000 K. All these nebulae are known to also exhibit the infrared emission features called aromatic emission features (AEFs) attributed to large PAHs. We present the spatial distribution of the BL in these nebulae. In the case of Ced~112, the BL is spatially correlated with mid-IR emission structures attributed to AEFs. These observations provide evidence for grain processing and possibly for in-situ formation of small grains and large molecules from larger aggregates. Most importantly, the detection of BL in these ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR.