Testing Models of Triggered Star Formation with Young Stellar Objects in Cepheus OB4

Abstract

OB associations are home to newly formed massive stars, whose turbulent winds and ionizing flux create H ii regions rich with star formation. Studying the distribution and abundance of young stellar objects (YSOs) in these ionized bubbles can provide essential insight into the physical processes that shape their formation, allowing us to test competing models of star formation. In this work, we examined one such OB association, Cepheus OB4 (Cep OB4)—a well-suited region for studying YSOs due to its Galactic location, proximity, and geometry. We created a photometric catalog from Spitzer/Infrared Array Camera (IRAC) mosaics in bands 1 (3.6 μ m) and 2 (4.5 μ m). We supplemented the catalog with photometry from the Wide-field Infrared Survey Explorer, the Two Micron All Sky Survey, IRAC bands 3 (5.8 μ m) and 4 (8.0 μ m), MIPS 24 μ m, and MMIRS near-infrared data. We used color–color selections to identify 821 YSOs, which we classified using the IR slope of the YSOs’ spectral energy distributions, finding 67 Class I, 103 flat spectrum, 569 Class II, and 82 Class III YSOs. We conducted a clustering analysis of the Cep OB4 YSOs and fit their spectral energy distributions. We found many young Class I objects distributed in the surrounding shell and pillars as well as a relative age gradient of unclustered sourcesin the bubble surrounding the OB association, with YSOs generally decreasing in age with distance from the central cluster. Both of these results indicate that the expansion of the H ii region may have triggered star formation in CepOB4.