NGC 7538 IRS1—an O Star Driving an Ionized Jet and Giant N-S Outflow

Abstract

NGC 7538 IRS 1 is a very young embedded O star driving an ionized jet and accreting mass with an accretion rate >10^-4 ${M}_{\odot }$ yr^-1, which is quenching the hypercompact H II region. We use SOFIA GREAT data, Herschel PACS and SPIRE archive data, SOFIA FORCAST archive data, Onsala 20 m and CARMA data, and JCMT archive data to determine the properties of the O star and its outflow. IRS 1 appears to be a single O star with a bolometric luminosity >1 × 10⁵ ${L}_{\odot }$ , i.e., spectral type O7 or earlier. We find that IRS 1 drives a large molecular outflow with the blueshifted northern outflow lobe extending to ∼280″ or 3.6 pc from IRS 1. Near IRS 1 the outflow is well aligned with the ionized jet. The dynamical timescale of the outflow is ∼1.3 × 10⁵ yr. The total outflow mass is ∼130 ${M}_{\odot }$ . We determine a mass outflow rate of 1.0 × 10^-3 ${M}_{\odot }$ yr^-1, roughly consistent with the observed mass accretion rate. We observe strong high-velocity [C II] emission in the outflow, confirming that strong UV radiation from IRS 1 escapes into the outflow lobes and is ionizing the gas. Many O stars may form like low-mass stars, but with a higher accretion rate and in a denser environment. As long as the accretion stays high enough to quench the H II region, the star will continue to grow. When the accretion rate drops, the H II region will rapidly start to expand.