The SOUL view of IRAS 20126+4104

Kinematics and variability of the H₂ jet from a massive protostar^★,★★

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: fabrizio.massi@inaf.it
² INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
e-mail: alessio.caratti@inaf.it
³ Dublin Institute for Advanced Studies, School of Cosmic Physics, Astronomy and Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, Ireland
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁷ Large Binocular Telescope Observatory, 933 N. Cherry Ave #552, Tucson, AZ 85721, USA
⁸ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Dr., Fairfax, VA 22030, USA

Received: 17 October 2022
Accepted: 21 December 2022

Abstract

Context. We exploit the increased sensitivity of the recently installed adaptive optics SOUL at the LBT to obtain new high-spatial-resolution near-infrared images of the massive young stellar object IRAS20126+4104 and its outflow.

Aims. We aim to derive the jet proper motions and kinematics, as well as to study its photometric variability by combining the novel performances of SOUL together with previous near-infrared images.

Methods. We used both broad-band (K_s, K′) and narrow-band (Brγ, H2) observations from a number of near-infrared cameras (UKIRT/UFTI, SUBARU/CIAO, TNG/NICS, LBT/PISCES, and LBT/LUCI1) to derive maps of the continuum and the H₂ emission in the 2.12 µm line. Three sets of images, obtained with adaptive optics (AO) systems (CIAO, in 2003; FLAO, in 2012; SOUL, in 2020), allowed us to derive the proper motions of a large number of H₂ knots along the jet. Photometry from all images was used to study the jet variability.

Results. We derived knot proper motions in the range of 1.7–20.3 mas yr⁻¹ (i.e. 13–158 km s⁻¹ at 1.64 kpc), implying an average outflow tangential velocity of ~80 km s⁻¹. The derived knot dynamical age spans a ~200–4000 yr interval. A ring-like H₂ feature near the protostar location exhibits peculiar kinematics and may represent the outcome of a wide-angle wind impinging on the outflow cavity. Both H₂ geometry and velocities agree with those inferred from proper motions of the H₂O masers, located at a smaller distance from the protostar. Although the total H₂ line emission from the knots does not exhibit time variations at a ⪞0.3 mag level, we have found a clear continuum flux variation (radiation scattered by the dust in the cavity opened by the jet) which is anti-correlated between the blue-shifted and red-shifted lobes and may be periodic (with a period of ~12–18 yr). We suggest that the continuum variability might be related to inner-disc oscillations which have also caused the jet precession.

Conclusions. Our analysis shows that multi-epoch high-spatial-resolution imaging in the near-infrared is a powerful tool to unveil the physical properties of highly embedded massive protostars.