2005ApJ...621..839S

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''x6'' 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. 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]≥6x10^–8.