Discovery of a Photoionized Bipolar Outflow towards the Massive Protostar G45.47+0.05

TL;DR

This paper reports the discovery of a photoionized bipolar outflow from a massive protostar G45.47+0.05, revealing insights into star formation feedback mechanisms through high-resolution ALMA and VLA observations.

Contribution

It presents the first detailed observation and modeling of a photoevaporative outflow with associated maser emission in a massive protostar, highlighting ongoing accretion despite strong feedback.

Findings

Ionized outflow is a photoevaporation flow with T_e=10,000 K and n_e~1.5x10^7 cm^-3.

Detection of millimeter recombination line masers in G45.47+0.05.

Estimated mass loss rate of the outflow is (2-3.5)x10^-5 Msun/yr.

Abstract

Massive protostars generate strong radiation feedback, which may help set the mass they achieve by the end of the accretion process. Studying such feedback is therefore crucial for understanding the formation of massive stars. We report the discovery of a photoionized bipolar outflow towards the massive protostar G45.47+0.05 using high-resolution observations at 1.3 mm with the Atacama Large Millimeter/Submillimeter Array (ALMA) and at 7 mm with the Karl G. Jansky Very Large Array (VLA). By modeling the free-free continuum, the ionized outflow is found to be a photoevaporation flow with an electron temperature of 10,000 K and an electron number density of ~1.5x10^7 cm^-3 at the center, launched from a disk of radius of 110 au. H30alpha hydrogen recombination line emission shows strong maser amplification, with G45 being one of very few sources to show such millimeter recombination line masers. The mass of the driving source is estimated to be 30-50 Msun based on the derived ionizing photon rate, or 30-40 Msun based on the H30alpha kinematics. The kinematics of the photoevaporated material is dominated by rotation close to the disk plane, while accelerated to outflowing motion above the disk plane. The mass loss rate of the photoevaporation outflow is estimated to be ~(2-3.5)x10^-5 Msun/yr. We also found hints of a possible jet embedded inside the wide-angle ionized outflow with non-thermal emissions. The possible co-existence of a jet and a massive photoevaporation outflow suggests that, in spite of the strong photoionization feedback, accretion is still on-going.