Protostellar Outflows at the EarliesT Stages (POETS)

VII. Circumstellar gas kinematics traced by water masers inside the HC HII region NGC 7538 IRS1

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Dipartimento di Fisica, Universitá degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato (CA), Italy
³ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão, 1226, São Paulo, SP 05508-090, Brazil
⁴ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁵ National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
⁶ SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^★ Corresponding author; luca.moscadelli@inaf.it

Received: 21 January 2025
Accepted: 3 March 2025

Abstract

Context. The hyper-compact (HC) HII region phase of a newly born massive star is presently poorly understood, particularly in relation to how the enhanced UV radiation impacts the kinematics of the surrounding gas and affects mass accretion.

Aims. This article focuses on NGC 7538 IRS1, one of the most luminous and studied HC HII regions in the northern hemisphere. Our aim is to identify the young stellar objects (YSOs) embedded within the ionized gas and study their nearby kinematic structures. This work expands on a recent survey called Protostellar Outflows at the EarliesT Stages (POETS), which has been devoted to studying young outflow emission on scales of 10–100 au near luminous YSOs, before they start photoionizing the surrounding medium.

Methods. We carried out multi-epoch Very Long Baseline Array (VLBA) observations of the 22 GHz water masers toward NGC 7538 IRS1 to measure the maser 3D velocities, which, following POETS’ findings, are reliable tracers of the protostellar winds. Recently, we reobserved the water masers in NGC 7538 IRS1 with sensitive global very long baseline interferometry (VLBI) observations to map weaker maser emission and also study the maser time variability.

Results. Our study confirms the presence of two embedded YSOs, IRS1a and IRS1b, at the center of the two linear distributions of 6.7 GHz methanol masers observed in the southern and northern cores of the HC HII region, which have been previously interpreted in terms of edge-on rotating disks. The water masers trace an extended (≥200 au) stationary shock front adjacent to the inner portion of the disk around IRS1a. This shock front corresponds to the edge of the southern tip of the ionized core and might be produced by the interaction of the disk wind ejected from IRS1a with the infalling envelope. The water masers closer to IRS1b follow the same local standard of rest (LSR) velocity (V_LSR) pattern of the 6.7 GHz masers rotating in the disk, but the direction and amplitude of the water maser proper motions are inconsistent with rotation. We propose that these water masers are tracing a photo-evaporated disk wind, where the maser V_LSR traces mainly the disk rotation and the proper motions the poloidal velocity of the wind. Finally, a sensitive NSF’s Karl G. Jansky Very Large Array (JVLA) 1.3 cm image of the HC HII region obtained from archival data reveals a disk-jet system, illuminated by the UV radiation from IRS1a, associated with an YSO, IRS1c, placed ≈0^′′.5 (or ≈1350 au) to the south of the ionized core.

Conclusions. This work shows that VLBI observations of the 22 GHz water masers can be used to trace disk winds near ionizing YSOs embedded within compact HII regions.