Volume 538, February 2012
|Number of page(s)||21|
|Section||Interstellar and circumstellar matter|
|Published online||26 January 2012|
Molecule survival in magnetized protostellar disk winds
I. Chemical model and first results
1 Centro de Astrofísica, Universidade do Porto, 4150-752 Porto, Portugal
2 LERMA, Observatoire de Paris, ENS, UPMC, UCP, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France
3 IAS, UMR 8617 du CNRS, Université de Paris-Sud, 91405 Orsay, France
4 Universidade do Porto, Faculdade de Engenharia, Laboratório SIM Unidade FCT n 4006, Portugal
5 Institut de Plantologie et d’Astrophysique de Grenoble, UMR 5521 du CNRS, 38041 Grenoble Cedex, France
6 Laboratoire Astroparticule & Cosmologie, Université Paris 7, UMR 7164 du CNRS, 75205 Paris Cedex 13, France
Received: 9 July 2009
Accepted: 14 November 2011
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 H2 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 H2 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 H2 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 H2O 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 H2 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 H2, CO and H2O will allow detailed tests of the model against observations.
Key words: astrochemistry / ISM: molecules / accretion, accretion disks / stars: formation / ISM: jets and outflows / stars: mass-loss
© ESO, 2012
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