COEVAL INTERMEDIATE-MASS STAR FORMATION IN N4W

Abstract

Protostars are mostly found in star-forming regions, where the natal molecular gas still remains. At 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 of T d ≳ 28 K , with the colder one being from the embedded compact submillimeter source SMM1. N4W’s mass is estimated to be ∼ 2.5 × 10 3 M ⊙ , and the mass of SMM1 is ∼ 5.5 × 10 2 M ⊙ at T d = 15 K , calculated from the CO 1 − 0 emission and 870 μ m dust continuum emission, respectively. Based on the estimates of the 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 ∼0.5 Myr in N4W. This small age spread implies that the intermediate-mass star formation processes happening in N4W are almost coeval.