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We present the results of broadband near-infrared spectroscopic observationsof the recently discovered mysterious stellar objects in the young supernovaremnant G54.1+0.3. These objects, which show significant mid-infrared excessemission, are embedded in a diffuse loop structure of ~1 arcmin in radius.Their near-infrared spectra reveal characteristics of late O- or early B-typestars with numerous H and He I absorption lines, and we classify their spectraltypes to be between O9 and B2 based on an empirical relation derived herebetween the equivalent widths of the H lines and stellar photospherictemperatures. The spectral types, combined with the results of spectral energydistribution 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 fromthe spectroscopic analyses, and the extinction distributions indicate a localenhancement of matter in the western part of the loop. If these objectsoriginate via triggered formation by the progenitor star of G54.1+0.3, thentheir formations likely began during the later evolutionary stages of theprogenitor, although a rather earlier formation may still be possible. If theobjects and the progenitor belong to the same cluster of stars, then ourresults constrain the progenitor mass of G54.1+0.3 to be between 18 and ~35M_sun and suggest that G54.1+0.3 was either a Type IIP supernova or, with arelatively lower possibility, Type Ib/c from a binary system.

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