Near infrared spectroscopy of highly reddened stars in the galactic plane

Near Infrared spectroscopic observations of highly reddened stars in the Galactic Plane compiled by Stephenson [AJ, 103,263 (1992)] are presented. We observed nine highly reddened stars and detected the 3.4μm absorption feature in the spectra of five stars. The 3.4μm absorption depth and the visual extinction show a positive correlation. The observed strength of this feature is τ₃/A_v∼ 0.003, similar to previous results by Pendleton et al. [ApJ, 437, 683 (1994)] for lines of sight other than the Galactic Center. One star shows the 3. 1μm absorption feature which is probably of molecular cloud origin, but no 3.4μm absorption feature. This is consistent with Brooke et al. [ApJ, 459, 209 (1996)] who find that the spectra of objects embedded in or located behind molecular cloud material consistently show a 3.1μm ice band, but never show a 3.4μm band which resembles the diffuse interstellar medium band. These results support the hypothesis that the 3.4μm absorption feature is caused by organic materials ubiquitously distributed in the diffuse interstellar medium. Although these stars were originally selected from an I band objective prism survey for the absence of molecular features in their spectra, we detected the CO absorption feature near 2.3 μm in all nine stars we observed. Consequently, these stars are likely to be moderately to highly reddened stars of late spectral type.