2018A&A...618A.120L

Context. The disk-outflow connection is thought to play a key role in extracting excess angular momentum from a forming protostar. HH30 is a rare and beautiful example of a pre-main sequence star exhibiting a flared edge-on disk, an optical jet, and a CO molecular outflow, making this object a case study for the disk-jet-outflow paradigm.
Aims. We aim to clarify the origin of the small-scale molecular outflow of HH30 and its link and impact on the accretion disk.
Methods. We present ALMA 0.25" angular resolution observations of the circumstellar disk and outflow around the T Tauri star HH30 in the dust continuum at 1.33 mm and of the molecular line transitions of ¹²CO(2-1) and ¹³CO(2-1). We performed a disk subtraction from the ¹²CO emission, from which we analysed the outflow properties in detail in the altitudes z≤250au. We fit the transverse position-velocity diagrams across the ¹²CO outflow to derive the ring positions and projected velocity components (including rotation). We use the results of these fits to discuss the origin of the CO outflow.
Results. The 1.3 mm continuum emission shows a remarkable elongated morphology along PA=31.2±0.1deg that has a constant brightness out to a radius of r=75au. The emission is marginally resolved in the transverse direction, implying an intrinsic vertical width ≤24 au and an inclination to the line-of-sight i≥84.8°. The ¹³CO emission is compatible with emission from a disk in Keplerian rotation, in agreement with the previous findings. The monopolar outflow, detected in ¹²CO, arises from the north-eastern face of the disk from a disk radius r≤22 au and extends up to 5" (or 700au) above the disk plane. We derive a lower limit to the total mass of the CO cavity/outflow of 1.7x10^–5M_☉. The CO cavity morphology is that of a hollow cone with semi-opening angle ∼35°. The derived kinematics are consistent with gas flowing along the conical surface with constant velocity of 9.3±0.7km/s. We detect small rotation signatures (V_phsini∈[0.1; 0.5]km/s) in the same sense as the underlying circumstellar disk. From these rotation signatures we infer an average specific angular momentum of the outflow of 38±15au.km/s at altitudes z≤250au. We also report the detection of small amplitude wiggling (1.2°) of the CO axis around an average inclination to the line of sight of i=91°.
Conclusions. The derived morphology and kinematics of the CO cavity are compatible with expectations from a slow disk wind, originating either through photo-evaporation or magneto-centrifugal processes. Under the steady assumption, we derive launching radii in the range 0.5-7au. In that scenario, we confirm the large minimum mass flux of 9x10^–8M_☉/yr for the CO wind. The wind would therefore extract a significant amount of the accreted mass flux through the disk and would likely play a crucial role in the disk evolution. If the CO flow originates from a steady-state disk wind, our ALMA observations rule out the 18 au binary orbital scenario previously proposed to account for the wiggling of the optical jet and favour instead a precession scenario in which the CO flow originates from a circumbinary disk around a close (separation≤3.5au) binary. Alternatively, the CO outflow could also trace the walls of a stationary cavity created by the propagation of multiple bow shocks. Detailed numerical simulations are under way to fully test the entrainment hypothesis.