Protostars and Outflows in the NGC 7538 IRS 9 Cloud Core

Abstract

New high-resolution observations of HCO+ J = 1 → 0, H13CN J = 1 → 0, SO (22 → 11), and continuum with BIMA at 3.4 mm show that the NGC 7538 IRS 9 cloud core is a site of active, ongoing star formation. Our observations reveal at least three young bipolar molecular outflows, all ~10,000-20,000 yr old. IRS 9 drives a bipolar extreme high-velocity outflow observed nearly pole-on. South of IRS 9 we find a cold protostellar condensation with a size of ~14'' × 6'' and a mass of more than 250 M☉. This is the center of one of the outflows and shows deep redshifted self-absorption in HCO+, suggesting that there is a protostar embedded in the core, still in a phase of active accretion. This source is not detected in the far-infrared, suggesting that the luminosity is less than 104 L☉; yet, the mass of the outflow is ~60 M☉. The redshifted HCO+ self-absorption profiles observed toward the southern protostar and IRS 9 predict accretion rates of a few times 10-4 to 10-3 M☉ yr-1. Deep VLA continuum observations at 3.6 cm show that IRS 9 coincides with a faint thermal VLA source, but no other young star in the IRS 9 region has any detectable free-free emission at a level of ~60 μJy at 3.6 cm. The HCO+ abundance is significantly enhanced in the hot IRS 9 outflow. A direct comparison of mass estimates from HCO+ and CO for the well-characterized redshifted IRS 9 outflow predicts an HCO+ enhancement of more than a factor of 30, or [HCO+/H2] ≥ 6 × 10-8.