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The physical origin of the near IR hydrogen emission lines in low-luminosity young stellar objects (YSOs) is of long-standing interest since these lines are among the few prominent spectral features available for the study of some of the youngest (deeply embedded) pre-main sequence objects. As in the case of the hydrogen Balmer lines in the spectra of T Tauri stars, these lines have generally been assumed to form in a wind; and the spectroscopic study of these lines has been viewed as one of the most promising ways to determine the properties of winds in young, extremely active YSOs. Since hydrogen line emission may arise from other components of the YSO system, (e.g. chromospheres, boundary layers, and magnetic accretion flows), high resolution line profiles are required to determine whether these lines are indeed suitable wind diagnostics.
We have obtained high resolution (R = 20,000) spectra of the Br$\gamma$ emission line in a small sample of embedded low-luminosity YSOs and active T Tauri stars using the cryogenic echelle spectrograph (CSHELL) at the IRTF. We find line profiles which possess broad wings (400-700 km/s FWZI) but which conspicuously lack\/ the blueshifted absorption components generally expected for emission from a wind. Even T Tauri stars that show strong wind absorption features in the Balmer and Na D lines show no obvious evidence of a wind origin for the Br$\gamma$ line. These results imply that the winds in these YSOs are likely much cooler than previously estimated and that mass loss rates derived from Br$\gamma$ line strengths require revision. Thus, these results bear on the larger issue of whether embedded YSOs possess neutral winds with momenta sufficient to drive their associated molecular outflows. We also find a tendency for the centroid of the emission to be blueshifted with respect to the stellar velocity, as is often observed in the higher Balmer lines of T Tauri stars. This tendency is generally consistent with emission from infalling rather than outflowing gas.
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