Triggered Star Formation and Young Stellar Population in Bright-rimmed Cloud SFO 38

Abstract

We have investigated the young stellar population in and around SFO 38, one of the massive globules located in the northern part of the Galactic H II region IC 1396, using the Spitzer IRAC and MIPS observations (3.6-24 μm), and followed up with ground-based optical photometric and spectroscopic observations. Based on the IRAC and MIPS colors and Hα emission, we identify ~45 young stellar objects (Classes 0/I/II) and 13 probable pre-main-sequence candidates. We derive the spectral types (mostly K- and M-type stars), effective temperatures, and individual extinction of the relatively bright and optically visible Class II objects. Most of the Class II objects show variable Hα emission as well as optical and near-infrared photometric variability, which confirm their "youth." Based on optical photometry and theoretical isochrones, we estimate the spread in stellar ages to be between 1 and 8 Myr with a median age of 3 Myr and a mass distribution of 0.3-2.2 M sun with a median value around 0.5 M sun. Using the width of the Hα emission line measured at 10% peak intensity, we derive the mass accretion rates of individual objects to be between 10–10 and 10–8 M sun yr–1. From the continuum-subtracted Hα line image, we find that the Hα emission of the globule is not spatially symmetric with respect to the O-type ionizing star HD 206267, and the interstellar extinction toward the globule is also anomalous. We clearly detect an enhanced concentration of YSOs closer to the southern rim of SFO 38 and identify an evolutionary sequence of YSOs from the rim to the dense core of the cloud, with most of the Class II objects located at the bright rim. The YSOs appear to be aligned along two different directions toward the O6.5V type star HD 206267 and the B0V type star HD 206773. This is consistent with the Radiation Driven Implosion (RDI) model for triggered star formation. Further, the apparent speed of sequential star formation is consistent with the speed of propagation of shocks in dense globules as derived from numerical simulations of RDI.