Coeval intermediate-mass star formation in N4W

Abstract

Protostars are mostly found in star-forming regions, where the natal molecular gas still remains. In about 5' west of the molecular bubble N4, N4W is identified as a star-forming clump hosting three Class II (IRS\,1\,--\,3), and one Class I (IRS\,4) young stellar objects (YSOs), as well as a submillimeter source SMM1. The near-IR polarization imaging data of N4W reveal two infrared reflection nebulae close to each other, which are in favor of the outflows of IRS\,1 and IRS\,2. The bipolar mid-IR emission centered on IRS\,4 and the arc-like molecular gas shell are lying on the same axis, indicating a bipolar molecular outflow from IRS\,4. There are two dust temperature distributions in N4W. The warmer one is widely distributed and has a temperature $T_\mathrm{d}\gtrsim28\,\mathrm{K}$, with the colder one from the embedded compact submillimeter source SMM1. N4W's mass is estimated to be $\sim2.5\times10^3\,M_\odot$, and the mass of SMM1 is $\sim5.5\times10^2\,M_\odot$ at $T_\mathrm{d}=15\,\mathrm{K}$, calculated from the CO\,$1-0$ emission and $870\,μ$m dust continuum emission, respectively. Based on the estimates of bolometric luminosity of IRS\,1\,--\,4, these four sources are intermediate-mass YSOs at least. SMM1 is gravitationally bound, and is capable of forming intermediate-mass stars or even possibly massive stars. The co-existence of the IR bright YSOs and the submillimeter source represents potential sequential star formation processes separated by $\sim0.5$\,Myr in N4W. This small age spread implies that the intermediate-mass star formation processes happening in N4W are almost coeval.