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

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 and determine whether such OB associations help or hinder star formation. In this work, we examined the Cepheus OB4 association, an excellent region in which to evaluate star formation models. It is relatively close for an OB association (∼1100 pc), located off of the densest area of the Galactic plane, and has a clear circular geometry. We used Spitzer, WISE, and 2MASS photometry to identify 763 YSOs, and used the slope of their spectral energy distributions (SEDs) to classify them, finding 57 Class I, 548 Class II, 62 Class III, and 96 flat spectrum YSOs. We conducted a clustering analysis of the Cep OB4 YSOs and found 5 clusters - 3 in the Be 59 region, 1 larger cluster in the north of the association, and 1 small cluster near a set of pillars along the rim of the bubble. We found many young Class I objects distributed around the outskirts of the OB association, indicating that the expansion of the H II region likely exposed but did not trigger star formation on the bubble’s border. In fact, our findings indicate that the shock-wave of the expanding H II region may have quenched ongoing star formation inside the bubble. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-1852268, and by the Smithsonian Institution.