Spectrophotometry of compact embedded infrared sources in the 0.6-1.0 micron wavelength region

Abstract

Spectrophotometry at 20 Å resolution in the 0.6- 1.0 μm wavelength range is presented for the compact embedded infrared sources GL 490, GL 437S, IC 1848A, GL 961, S106/IRS 3, M17/IRS 1, S235B, and M8E, for the compact H II regions GL 437W and NGC 7538/IRS 2, and for the visible objects LkHα 101, MWC 297, MWC 349, V645 Cyg, and the fan nebula associated with GL 989. The heavily obscured objects display extremely red continua, emission lines due to the Paschen series of hydrogen, and in many cases O I λ8446 and the Ca II infrared triplet lines at λλ8498, 8542, and 8662, plus other weaker lines. These and other newly identified spectral characteristics of the embedded sources show similarities to those of Be stars, T Tauri stars, and to a class of emission-line stars with infrared excesses. These facts point to similar physical conditions in the circumstellar envelopes of the compact embedded sources and those of other young stellar objects that have long histories of study. Estimates of the source extinctions have been obtained from the continuum slopes in this region. Comparison of the 1 μm continuum magnitudes with estimates for the stellar photospheric fluxes based on the source luminosities suggest that scattering and circumstellar shell emission are important in forming the 1 μm continuum in several sources. The Paschen line ratios are not in general consistent with Case B recombination; this extends a conclusion deduced from the ratios of the infrared hydrogen lines Brα, Brγ, and Pfγ. The Paschen line decrements are qualitatively consistent with high optical depth in these lines; restrictions on possible excitation mechanisms and geometries are deduced. In those cases where they are detected, the Ca II triplet lines are equal in strength. This indicates that the optical depths in these lines are substantial. The O I λ8446 line is excited by Lyβ fluorescence in most of the objects. This shows that the H I lines are excited in predominantly neutral regions and that the Hα optical depth is ~ 1000. The physical conditions in the dense, warm, and only partially ionized medium required in the embedded sources may be similar to those proposed for the broadline region of Seyfert galaxies and QSOs, but T_e is probably ~ 5000 K.