Saturated hydrocarbons in comet 153P/Ikeya-Zhang: Ethane, methane, and monodeuterio-methane

Emissions of the methane ν₃, band and the ethane ν ₇ band were detected in the high-dispersion infrared spectra of comet 153P/Ikeya-Zhang taken by the Subaru Telescope with the Infrared Camera and Spectrograph (IRCS) on 2002 May 28 and 29 UT. The rotational temperature is estimated to be about 40-50 K from R₀ and R₁ lines of the methane ν₃ band. The production rates of methane and ethane were estimated by applying the rotational temperature of 50 K. Resultant production rates of methane and ethane are (2.13 ± 0.40) × 10²⁶ and (1.75 ± 0.48) × 10²⁶ molecules s^-1, respectively. The abundance ratio between ethane and methane is 0.82 ± 0.27, which is comparable with other comets within error bars. Although the emissions of monodeuterio-methane (CH₃D) were searched for carefully, there is no evidence of them. The 2 σ (95% confidence) upper limit of the CH₃D/CH₄ ratio is 0.3 in comet 153P/Ikeya-Zhang. Comparing the relative abundance of methane and ethane in the interstellar ices, we discussed the formation conditions of cometary nuclei.