Molecule survival in magnetized protostellar disk winds

Abstract

Context. Molecular counterparts to atomic jets have recently been detected within 1000 AU of young stars at early evolutionary stages. Reproducing these counterparts is an important new challenge for proposed ejection models. Aims. We explore whether molecules may survive in the magneto-hydrodynamic (MHD) disk wind solution currently invoked to reproduce the kinematics and tentative rotation signatures of atomic jets in T Tauri stars. Methods. The coupled ionization, chemical, and thermal evolution along dusty flow streamlines is computed for the prescribed MHD disk wind solution, using a method developed for magnetized shocks in the interstellar medium. Irradiation by (wind-attenuated) coronal X-rays and far-ultraviolet photons from accretion hot spots is included, with an approximate self-shielding of H 2 and CO. Disk accretion rates of 5 × 10 −6 , 10 −6 and 10 −7 M yr −1 are considered, representative of low-mass young protostars (so-called "Class 0"), evolved protostars ("Class I") and very active T Tauri stars ("Class II") respectively. Results. The disk wind has an "onion-like" thermo-chemical structure, with streamlines launched from larger radii having lower temperature and ionization, and higher H 2 abundance. The coupling between charged and neutral fluids is sufficient to eject molecules from the disk out to at least 9 AU. The launch radius beyond which most H 2 survives moves outward with evolutionary stage, from 0.2 AU (sublimation radius) in the Class 0 disk wind, to 1 AU in the Class I, and >1 AU in the Class II. In this molecular wind region, CO survives in the Class 0 but is significantly photodissociated in the Class I/II. Balance between ambipolar heating and molecular cooling establishes a moderate asymptotic temperature 700−3000 K, with cooler jets at earlier protostellar stages. As a result, endothermic formation of H 2 O is efficient, with abundances up to 10 −4 , while CH + and SH + can reach ≥10 −6 in the hotter and more ionised Class I/II winds. Conclusions. A centrifugal MHD disk wind launched from beyond 0.2−1 AU can produce molecular jets/winds up to speeds 100 km s −1 in young low-mass stars ranging from Class 0 to active Class II. The model predicts a high abundance ratio of H 2 to CO and an increase of molecular launch radius, temperature, and flow width as the source evolves, in promising agreement with current observed trends. Calculations of synthetic maps and line profiles in H 2 , CO and H 2 O will allow detailed tests of the model against observations.

Origin of the wide-angle hot H2in DG Tauri

“…For a launch radius r 0 10 AU, compatible with the rim radius at the base of the blue lobe, this particular MHD disc wind solution predicts that the streamline will reach a radius 50 AU at an altitude of 50 AU above the disc (see Fig. 1 of Panoglou et al 2012), similar to the measured maximum FWHM of the H 2 cavity at the corresponding projected distance (see Fig. 2).…”

“…10. Panoglou et al (2012) recently carried out a thorough study of the thermal and chemical structure of a steady, self-similar MHD disc wind solution from Casse & Ferreira (2000), with a lever arm parameter λ = 13 selected to provide a good fit to the tentative rotation signatures in the atomic jet of DG Tau (Pesenti et al 2004). For an accretion rateṀ acc ∼ 10 −6 −10 −7 M yr −1 , Panoglou et al find that H 2 molecules can survive against both collisional dissociation and photodissociation by the stellar FUV and X-ray photons when launched from disc radii greater than about 1 AU.…”

“…All C-shock models in Table 1 assume a preshock ionization from cosmic rays alone, as appropriate for ambient gas. But a molecular wind launched within 10 AU is irradiated by stellar FUV photons and X-rays and will reach a higher ionization 10 −4 that remains "frozen-in" as the wind expands (e.g., Glassgold et al 1991;Panoglou et al 2012). A J-shock will then result even if b > 0.1, due to the strong ion-neutral coupling.…”

“…The v = 1-0 S(1) transition of H 2 at 2.12 μm is often detected in embedded protostars, with the emission formed primarily in shock-excited collimated outflows (e.g., Davis et al 2011Davis et al , 2002. The presence of H 2 in jets might be an indication that MHD ejection operates out to disc radii ≥0.2-1 AU where molecules can survive against dissociation (Safier 1993;Panoglou et al 2012). If such extended MHD disc winds persist until the later planet building phase, they could maintain fast accretion across the disc "dead-zone" where ionization in the midplane is too low to activate MHD turbulence (Bai 2013).…”

Spatially resolved H2emission from a very low-mass star

“…Therefore, the jet collimation must take place within the first 50 AU from the source, in agreement with MHD wind models (Dougados et al 2004). Panoglou et al (2012) show that molecular hydrogen can survive along MHD disk-wind streamlines. The presence of H 2 emission detected at only ∼48 AU from the source, together with the broad line profiles shown in Fig.…”