What’s Behind the Elephant’s Trunk? Identifying Young Stellar Objects on the Outskirts of IC 1396*

Abstract

Empirically, the estimated lifetime of a typical protoplanetary disk is <5–10 Myr. However, the disk lifetimes required to produce a variety of observed exoplanetary systems may exceed this timescale. Some hypothesize that this inconsistency is due to estimating disk fractions at the cores of clusters, where radiation fields external to a star–disk system can photoevaporate the disk. To test this, we have observed a field on the western outskirts of the IC 1396 star-forming region with XMM-Newton to identify new Class III YSO cluster members. Our X-ray sample is complete for YSOs down to 1.8 M ⊙ . We use a subset of these X-ray sources that have near- and mid-infrared counterparts to determine the disk fraction for this field. We find that the fraction of X-ray-detected cluster members that host disks in the field we observe is 17 − 7 + 10 % (1 σ ), comparable with the 29 − 3 + 4 % found in an adjacent field centered on the cometary globule IC 1396A. We reevaluate YSO identifications in the IC 1396A field using Gaia parallaxes compared to previous color-cut-only identifications, finding that incorporating independent distance measurements provides key additional constraints. Given the existence of at least one massive star producing an external radiation field in the cluster core, the lack of a statistically significant difference in disk fraction in each observed field suggests that disk lifetimes remain consistent as a function of distance from the cluster core.