  Molecular counterparts to atomic jets have been detected within 1000 AU of
young stars. Reproducing them is a challenge for proposed ejection models. We
explore whether molecules may survive in an MHD disk wind invoked to reproduce
the kinematics and tentative rotation signatures of atomic jets in T Tauri
stars. The coupled ionization, chemical and thermal evolution along dusty flow
streamlines is computed for a prescribed MHD disk wind solution, using a method
developed for magnetized shocks in the interstellar medium. Irradiation by
wind-attenuated coronal X-rays and FUV photons from accretion hot spots is
included, with self-shielding of H2 and CO. Disk accretion rates of 5e-6, 1e-6
and 1e-7 solar masses per year are considered, representative of low-mass young
protostars (Class 0), evolved protostars (Class I) and very active T Tauri
stars (Class II). The disk wind has an onion-like thermo-chemical structure,
with streamlines launched from larger radii having lower temperature and
ionisation, and higher H2 abundance. The coupling between charged and neutral
fluids is sufficient to eject molecules from the disk out to 9 AU. The launch
radius beyond which most H2 survives moves outward with evolutionary stage. CO
survives in the Class 0 but is significantly photodissociated in the Class
I/II. Balance between ambipolar heating and molecular cooling establishes an
asymptotic temperature 700-3000 K, with cooler jets at earlier protostellar
stages. Endothermic formation of H2O is efficient with abundances up to 1e-4,
while CH+ and SH+ can exceed 1e-6 in the Class I/II winds. A centrifugal MHD
disk wind launched from beyond 0.2-1 AU can produce molecular jets/winds up to
speeds 100 km/s in young low-mass stars. The model predicts a high ratio H2/CO
and an increase of molecular launch radius, temperature, and flow width as the
source evolves, in agreement with current observed trends.
