Near-Infrared Spectroscopy of Infrared-Excess Stellar Objects in the Young Supernova Remnant G54.1+0.3

TL;DR

This study uses near-infrared spectroscopy to analyze stellar objects in supernova remnant G54.1+0.3, revealing their spectral types, distances, and implications for the remnant's progenitor star and supernova type.

Contribution

First spectroscopic classification of these objects, constraining their properties and providing insights into the supernova remnant's progenitor and explosion type.

Findings

Objects are late O- or early B-type stars with spectral types O9-B2.

Distance to objects is approximately 6.0 kpc.

Progenitor star mass estimated between 18 and 35 solar masses.

Abstract

We present the results of broadband near-infrared spectroscopic observations of the recently discovered mysterious stellar objects in the young supernova remnant G54.1+0.3. These objects, which show significant mid-infrared excess emission, are embedded in a diffuse loop structure of ~1 arcmin in radius. Their near-infrared spectra reveal characteristics of late O- or early B-type stars with numerous H and He I absorption lines, and we classify their spectral types to be between O9 and B2 based on an empirical relation derived here between the equivalent widths of the H lines and stellar photospheric temperatures. The spectral types, combined with the results of spectral energy distribution fits, constrain the distance to the objects to be 6.0 +/- 0.4 kpc. The photometric spectral types of the objects are consistent with those from the spectroscopic analyses, and the extinction distributions indicate a local enhancement of matter in the western part of the loop. If these objects originate via triggered formation by the progenitor star of G54.1+0.3, then their formations likely began during the later evolutionary stages of the progenitor, although a rather earlier formation may still be possible. If the objects and the progenitor belong to the same cluster of stars, then our results constrain the progenitor mass of G54.1+0.3 to be between 18 and ~35 M_sun and suggest that G54.1+0.3 was either a Type IIP supernova or, with a relatively lower possibility, Type Ib/c from a binary system.