Investigating the Near‐Infrared Properties of Planetary Nebulae. II. Medium‐Resolution Spectra

We present medium-resolution (R~700) near-infrared (lambda = 1 - 2.5 micron)spectra of a sample of planetary nebulae (PNe). A narrow slit was used whichsampled discrete locations within the nebulae; observations were obtained atone or more positions in the 41 objects included in the survey. The PN spectrafall into one of four general categories: H I emission line-dominated PNe, H Iand H_2 emission line PNe, H_2-dominated PNe, and continuum-dominated PNe.These categories correlate with morphological type, with the elliptical PNefalling into the first group, and the bipolar PNe primarily in the H_2 andcontinuum emission groups. Other spectral features were observed in allcategories, such as continuum emission from the central star, C_2, CN, and COemission, and warm dust continuum emission. Molecular hydrogen was detected for the first time in four PNe. An excitationanalysis was performed using the H_2 line ratios for all of the PN spectra inthe survey where a sufficient number of lines were observed. One unexpectedresult from this analysis is that the H_2 is excited by absorption ofultraviolet photons in most of the PNe surveyed, although for several PNe inour survey collisional excitation in moderate velocity shocks plays animportant role. The correlation between bipolar morphology and H_2 emission hasbeen strengthened with the new detections of H_2 in this survey.Comment: 13 pages, 8 tables, 33 figure