The Near-Infrared Spectrum of the Planetary Nebula IC 5117

Abstract

Infrared spectroscopy from 0.8 to 2.5 μm is presented for the planetary nebula IC 5117. The emission lines of IC 5117 span a wide range of ionization that includes He II, [S III], [S II], [N I], and H2. The reddening measured from the hydrogen lines is E(B-V) = 0.79, most of which is probably interstellar in origin. The He/H abundance ratio is 0.113 ± 0.015, with approximately 10% of the helium being doubly ionized. Using our measurements of [S II] and [S III] lines and published observations of [S IV], we find a sulfur abundance, relative to hydrogen, of N(S)/N(H) = 7.8 × 10-6, or approximately half the solar value. Fluxes and flux limits for several lines of molecular hydrogen are presented. Measurements of 1–0 transitions, together with the limits on 2–1 transitions, indicate Tvib ~ Trot = 1900 K, suggesting a purely collisional excitation mechanism. The ortho-to-para ratio is ~3, a value that is also indicative of collisional excitation. The presence of [C I] λ9850 is consistent with previous studies of IC 5117 that indicated carbon is more abundant than oxygen. IC 5117 follows the trend of planetary nebulae that display bipolar outflows and H2 emission to be carbon-rich. We confirm the results of Zhang & Kwok, who reported infrared continuum emission substantially in excess of that produced by the ionized gas. This emission is most likely due to hot dust (T ~ 1300 K) and accounts for roughly half of the continuum between 1.5 and 2 μm.