Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA

Abstract

NGC 602 is a young, low-metallicity star cluster in the "Wing" of the Small Magellanic Cloud. We reveal the recent evolutionary past of the cluster through analysis of high-resolution ($\sim$0.4 pc) Atacama Large Millimeter/submillimeter Array observations of molecular gas in the associated $\textrm{H}\scriptstyle\mathrm{II}$ region N90. We identify 110 molecular clumps ($R <$ 0.8 pc) traced by CO emission, and study the relationship between the clumps and associated young stellar objects (YSOs) and pre-main-sequence (PMS) stars. The clumps have high virial parameters (typical $\alpha_{\rm{vir}} = $ 4-11) and may retain signatures of a collision in the last $\lesssim$8 Myr between $\textrm{H}\scriptstyle\mathrm{I}$ components of the adjacent supergiant shell SMC-SGS 1. We obtain a CO-bright-to-H$_2$ gas conversion factor of $X_{CO,B} = (3.4 \pm 0.2) \times 10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$, and correct observed clump properties for CO-dark H$_2$ gas to derive a total molecular gas mass in N90 of $16,600 \pm 2,400 \ M_\odot$. We derive a recent ($\lesssim 1$ Myr) star formation rate of $130 \pm 30 \ M_{\odot}$ Myr$^{-1}$ with an efficiency of 8 $ \pm$ 3\% assessed through comparing total YSO mass to total molecular gas mass. Very few significant radial trends exist between clump properties or PMS star ages and distance from NGC 602. We do not find evidence for a triggered star formation scenario among the youngest ($\lesssim$2 Myr) stellar generations, and instead conclude that a sequential star formation process in which NGC 602 did not directly cause recent star formation in the region is likely.