Fine structure and kinematics of the ionized and molecular gas in the jet and disk around S255IR NIRS3 from high resolution ALMA observations

TL;DR

This study uses high-resolution ALMA observations to analyze the detailed structure and motion of ionized and molecular gas around the high-mass protostar NIRS3 in S255IR, revealing a complex jet-disk system with evidence of episodic ejections and inhomogeneous disk features.

Contribution

First high-resolution ALMA imaging of S255IR NIRS3 revealing detailed jet and disk structures, including double ejections and small-scale clumpiness in the disk.

Findings

Detection of a double ejection event separated by ~1.5 years.

Observation of a sub-Keplerian, inhomogeneous disk around NIRS3.

Identification of free-free emission dominating jet knots and hypercompact H II region characteristics.

Abstract

We present observations of the high-mass star-forming region S255IR, which harbors the $\sim$20 M$_\odot$ protostar NIRS3, where a disk-mediated accretion burst was recorded several years ago, with the angular resolution of $\sim$15 mas, which corresponds to $\sim$25 au and is almost an order of magnitude better than in the previous studies of this object. The observations were performed with ALMA at a wavelength of 0.9 mm in continuum and in several molecular lines. In the continuum we detected the central bright source (brightness temperature $\sim$850 K) elongated along the jet direction and two pairs of bright knots in the jet lobes. These pairs of knots imply a double ejection from NIRS3 with the time interval of $\sim$1.5 years. The orientation of the jet differs by $\sim$20$^\circ$ from that on larger scales, as mentioned also in some other recent works. The 0.9 mm continuum emission of the central source represents a mixture of the dust thermal emission and free-free emission of the ionized gas. Properties of the free-free emission are typical for hypercompact H II regions. In the continuum emission of the knots in the jet the free-free component apparently dominates. In the molecular lines a sub-Keplerian disk around NIRS3 about 400 au in diameter is observed. The absorption features in the molecular lines towards the central bright source may indicate an infall. The molecular line emission appears very inhomogeneous at small scales, which may indicate a small-scale clumpiness in the disk.