M1-78: a nitrogen-rich Galactic compact HII region beyond the Perseus arm

TL;DR

This study reveals that M1-78 is a nitrogen-rich compact HII region with complex physical and chemical variations, likely a combination of a typical HII region and ejecta from evolved massive stars, with UV fluorescence exciting surrounding H2 emission.

Contribution

It provides a detailed spectroscopic analysis confirming nitrogen enrichment and complex morphology, suggesting M1-78 is a hybrid of HII region and stellar ejecta, which was not previously established.

Findings

Nitrogen enrichment confirmed, especially in the NE blob.

Physical conditions vary significantly across the nebula.

H2 emission is excited by UV fluorescence over a large area.

Abstract

There is considerable controversy surrounding the nature of M1-78, a compact nebula located beyond the Perseus arm. It was first classified as a planetary nebula and is nowadays generally considered to be a compact HII region. To investigate the nature M1-78 further, we present a detailed spectroscopic study of M1-78 in the optical and near-infrared. M1-78 is a high-density nebula with substantial physical differences between its two main morphological zones: a bright arc to the SW and a blob of emission in the NE. Specifically, the blob in the NE has a higher electron temperature and visual extinction than the SW arc. The most important result, however, is the confirmation of a nitrogen enrichment in M1-78. This enrichment is stronger at the location of the NE blob and is correlated with a defficiency in the O abundance and a (dubious) He enrichment. Such an abundance pattern is typical of ejecta nebulae around evolved massive stars such as Wolf-Rayet and Luminous Blue Variable stars. The spatial variations in the physical conditions and chemical abundances and the presence of more than one possible ionizing source indicates, however, that M1-78 is better described as a combination of a compact HII region + ejecta. Finally, we detect H2 emission that extends over a large (~30 arcsec) area around the ionized nebula. Analysis of the near-infrared H2 lines indicates that the excitation mechanism is UV fluorescence.