2024A&A...692A.181Z

Aims. We present observations of the high-mass star-forming region S255IR, which harbors the ∼20 M_⊙ protostar NIRS3, where a disk-mediated accretion burst was recorded several years ago. The angular resolution of these observations, of ∼15 mas, corresponds to ∼25 au, which is almost an order of magnitude better than in the previous studies of this object.
Methods. The observations were performed with the Atacama Large Millimeter/submillimeter Array (ALMA) at a wavelength of 0.9 mm in continuum and in several molecular lines.
Results. In the continuum, we detect the central bright source (brightness temperature of ∼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 a time interval of ∼1.5 years. The orientation of the jet differs by ∼20° from that on larger scales, as also mentioned 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. Certain properties of the free-free emission are typical of 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 is observed around NIRS3 of about 400 au in diameter. The absorption features in the molecular lines toward the central bright source may indicate an infall. The molecular line emission appears highly inhomogeneous at small scales, which may indicate a small-scale clumpiness in the disk.