Near-IR [Fe II] emission diagnotics applied to cold disk winds in young stars

N. Pesenti, C. Dougados, S. Cabrit, D. O'Brien, P. J. V. Garcia, J. Ferreira

Abstract
We investigate the emissivity properties of the main near-IR transitions of the Fe⁺ ion in the conditions prevailing in the inner regions of jets from young stars, based on a simplified 16-level atom model. We present new diagnostic diagrams involving prominent near-IR line ratios that allow us to constrain the electronic density, temperature, and Fe gas phase abundance ratio, independently of the heating process. Comparison with recent near-IR observations of a sample of HH objects indicates gas phase Fe abundances ranging from 15-50% up to 100% of the solar value (depending on the assumed temperature and on the HH object), in agreement with the moderate depletions previously derived from optical line ratios or shock models. Hence, it appears that Fe-bearing dust is efficiently destroyed in stellar jets. We then use our Fe⁺ emissivity model to predict near-IR [Fe II] emission maps for self-similar, cold MHD disk wind models. We show that near-IR [Fe II] lines are stronger than [S II] lambda 6731 and [O I] lambda 6300 in the cool regions (T 〈=7000 K) near the wind base, and that observations in [Fe II] with AMBER on the VLTI could severely constrain the MHD solution and the inner launch radius of the jet. We also compare theoretical predictions with recent observations in the near-IR [Fe II] lines of the L1551-IRS5 and DG Tau jets. The cold disk wind model reproduces quite well the two velocity components observed at -100 and -300 km s^-1, although the high velocity component appears overestimated by a factor of 1.5 in the DG Tau jet. However, the model predicts too little emission at intermediate velocities and insufficient densities. Similar problems were encountered in previous model comparisons in the optical range with jets from T Tauri stars. Denser disk winds with stronger heating at the jet base, which have been invoked for optical jets, also appear needed in younger, embedded Class I jet sources.

Astronomy and Astrophysics
Volume 410, Page 155
October 2003

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