Radio Observations of the Star Formation Activities in the NGC 2024 FIR 4 Region

TL;DR

This study used archival VLA data to investigate star formation in NGC 2024 FIR 4, revealing a low-mass protostar with associated radio jets and masers, challenging assumptions about high-mass star formation indicators.

Contribution

First detailed radio imaging of FIR 4 showing a low-mass protostar with associated masers and jets, indicating methanol masers are not exclusive to high-mass YSOs.

Findings

VLA 9 aligns with the bipolar outflow axis.

FIR 4 contains a low-mass protostar, not a high-mass YSO.

Methanol masers can be excited by low-mass protostars.

Abstract

Star formation activities in the NGC 2024 FIR 4 region were studied by imaging centimeter continuum sources and water maser sources using several archival data sets from the Very Large Array. The continuum source VLA 9 is elongated in the northwest-southeast direction, consistent with the FIR 4 bipolar outflow axis, and has a flat spectrum in the 6.2-3.6 cm interval. The three water maser spots associated with FIR 4 are also distributed along the outflow axis. One of the spots is located close to VLA 9, and another one is close to an X-ray source. Examinations of the positions of compact objects in this region suggest that the FIR 4 cloud core contains a single low-mass protostar. VLA 9 is the best indicator of the protostellar position. VLA 9 may be a radio thermal jet driven by this protostar, and it is unlikely that FIR 4 contains a high-mass young stellar object (YSO). A methanol 6.7 GHz maser source is located close to VLA 9, at a distance of about 100 AU. The FIR 4 protostar must be responsible for the methanol maser action, which suggests that methanol class II masers are not necessarily excited by high-mass YSOs. Also discussed are properties of other centimeter continuum sources in the field of view and the water masers associated with FIR 6n. Some of the continuum sources are radio thermal jets, and some are magnetically active young stars.