Dissecting the disk and the jet of a massive (proto)star

An ALMA view of IRAS 20126+4104

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
³ Institut de Ciències de l’Espai (ICE, CSIS), Carrer de Can Magrans s/n, 08193 Bellaterra, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Barcelona, Spain

^★ Corresponding author; riccardo.cesaroni@inaf.it

Received: 21 October 2024
Accepted: 2 December 2024

Abstract

Context. The study of disks around early-type (proto)stars has recently been boosted by a new generation of instruments, and additional evidence has been found of disk+jet systems around stars of up to ~20 M_⊙. These results appear to confirm theoretical predictions that even the most massive stars may form though disk-mediated accretion.

Aims. We want to investigate one of the best examples of disk+jet systems around an early B-type (proto)star, IRAS 20126+4104. The relatively simple structure of this object and its relative proximity to Earth (1.64 kpc) make it an ideal target for resolution of its disk and the determination of its physical and kinematical structure.

Methods. Despite the high declination of IRAS 20126+4104, it has been possible to perform successful observations with the Atacama Large Millimeter and submillimeter Array at 1.4 mm in the continuum emission and a number of molecular tracers of high-density gas (for the disk) and shocked gas (for the jet).

Results. The new data allow us to improve on previous similar observations of IRAS 20126+4104 and confirm the existence of a Keplerian accretion disk around a ~12 M_⊙ (proto)star. From methyl cyanide, we derived the rotation temperature and column density as a function of disk radius. We also obtained a map of the same quantities for the jet using the ratio between two lines of formaldehyde. We also use two simple models of the jet and the disk to estimate the basic geometrical and kinematical parameters of the two. From the temperature and column density profiles, we conclude that the disk is stable at all radii. We also estimate an accretion rate of ~10⁻³ M_⊙ yr⁻¹.

Conclusions. Our analysis confirms that the jet from IRAS 20126+4104 is highly collimated, lies close to the plane of the sky, and expands with velocity increasing with distance. As expected, the gas temperature and column density peak in the bow shock. The disk is undergoing Keplerian rotation but a non-negligible radial velocity component is also present that is equal to ~40% of the rotational component. The disk is slightly inclined with respect to the line of sight and has a dusty envelope that absorbs the emission from the disk surface. This causes a slight distortion of the disk structure observed in high-density tracers such as methyl cyanide. We also reveal a significant deviation from axial symmetry in the SW part of the disk, which might be caused by either tidal interaction with a nearby, lower-mass companion or interaction with the outflowing gas of the jet.