Origin of the wide-angle hot H₂ in DG Tauri

New insight from SINFONI spectro-imaging

¹ Universidade do Porto, Faculdade de EngenhariaDepartamento Engenharia fisica, SIM Unidade FCT 4006, rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
e-mail: vaa@fe.up.pt
² LERMA, UMR 8112 du CNRS & Observatoire de Paris, ENS, UPMC, UCP, 61 avenue de l’Observatoire, 75014 Paris, France
³ Laboratoire Franco-Chilien d’Astronomie, UMI 3386 du CNRS, 1515 Camino el observatorio, Casilla 36-D correo central, Santiago, Chile
⁴ IPAG, UMR 5274 du CNRS & Univ. Joseph Fourier, 414 rue de la Piscine, 38041 Grenoble, France
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
⁶ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 1 October 2012
Accepted: 30 December 2013

Abstract

Context. The origin of protostellar jets remains a major open question in star formation. Magnetohydrodynamical (MHD) disc winds are an important mechanism to consider, because they would have a significant impact on planet formation and migration.

Aims. We wish to test the origins proposed for the extended hot H₂ at 2000 K around the atomic jet from the T Tauri star DG Tau, in order to constrain the wide-angle wind structure and the possible presence of an MHD disc wind in this prototypical source.

Methods. We present spectro-imaging observations of the DG Tau jet in H₂ 1–0 S(1) with 0.̋ 12 angular resolution, obtained with SINFONI/VLT. Thanks to spatial deconvolution by the point spread function and to careful correction for wavelength calibration and for uneven slit illumination (to within a few km s^-1), we performed a thorough analysis and modeled the morphology and kinematics. We also compared our results with studies in [Fe II], [O I], and FUV-pumped H₂. Absolute flux calibration yields the H₂ column/volume density and emission surface, and narrows down possible shock conditions.

Results. The limb-brightened H₂ 1–0 S(1) emission in the blue lobe is strikingly similar to FUV-pumped H₂ imaged 6 yr later, confirming that they trace the same hot gas and setting an upper limit <12 km s^-1 on any expansion proper motion. The wide-angle rims are at lower blueshifts (between –5 and 0 km s^-1) than probed by narrow long-slit spectra. We confirm that they extend to larger angle and to lower speed the onion-like velocity structure observed in optical atomic lines. The latter is shown to be steady over ≥4 yr but undetected in [Fe II] by SINFONI, probably due to strong iron depletion. The rim thickness ≤14 AU rules out excitation by C-type shocks, and J-type shock speeds are constrained to ≃10 km s^-1.

Conclusions. We find that explaining the H₂ 1–0 S(1) wide-angle emission with a shocked layer requires either a recent outburst (15 yr) into a pre-existing ambient outflow or an excessive wind mass flux. A slow photoevaporative wind from the dense irradiated disc surface and an MHD disc wind heated by ambipolar diffusion seem to be more promising and need to be modeled in more detail. Better observational constraints on proper motion and rim thickness would also be crucial for clarifying the origin of this structure.