A Massive-Star-forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post-Main-Sequence Triggered Star Formation?

Abstract

We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least 11 young stellar objects (YSOs) embedded in a ringlike diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of Class II YSOs in [3.6] - [5.8] but significantly redder in [8] - [24], i.e., 0 < [3.6] - [5.8] < 1.2 and 5 < [8] - [24] < 9. Most of them have near-infrared counterparts in the 2MASS JHK_s images, and some of them have an optical counterpart too. Their JHK_s colors and magnitudes indicate that the YSOs are massive (gtrsim10 M_⊙) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to early Herbig Be stars, which are early B type pre-main-sequence stars with inner disks that have been destroyed. The confinement to a loop structure indicates that the YSOs are young, i.e., lesssim2 Myr. We propose that their formation is triggered by the progenitor star of G54.1+0.3, which has a mass of lesssim15 M_⊙. The triggering must have occurred near the end of the progenitor's life, possibly after it had evolved off the main sequence.