Photometric and Spectroscopic Observations of Comet 2P/Encke's Nucleus in the Mid-Infrared Wavelength Region

Abstract

We observed comet 2P/Encke with the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the Subaru telescope on UT 2003 November 12.3, when the comet was at heliocentric and geocentric distances of 1.09 and 0.27 au, respectively. Imaging observations of the comet with narrow-band filters centered at 8.8, 10.5, 11.7, 12.4, 18.8, and 24.5 $μ$m and low-resolution spectroscopic observations in the N-band (8-13 $μ$m, $R\sim250$) of the comet were performed. The observed spatial profiles of the comet were consistent with those of a standard star (FWHM $\sim$ 0.3'' at 10.5 $μ$m, close to the diffraction-limited image size), indicating that the observed spectrum was dominated by thermal emission from the cometary nucleus. The semi-axial ratio ($a$/$b$) of the nucleus of 2P/Encke is estimated as 2.26$^{+0.33}_{-0.21}$ based on the isothermal, prolate ellipsoid model. The photometric data with the correction for rotational phases, can be explained by a blackbody temperature of $328\pm7$ K, consistent with the nucleus surface temperature empirically estimated at 1.09 au from the Sun (327 K). This blackbody approximation for the nucleus leads to $a = 3.6^{+0.5}_{-0.3}$ km and $b = 1.6^{+0.2}_{-0.2}$ km by assuming an emissivity of unity. The observed low-resolution N-band spectrum showed an overall smooth continuum with no prominent excess emission feature at $\sim$10 $μ$m due to fine silicate grains in the cometary coma. The relative emissivity spectrum of the nucleus showed a negative feature at $\sim$9.5 $μ$m, as seen in the mid-infrared nucleus spectrum of 460P/PanSTARRS [T. Ootsubo, et al. (2021), Icarus, 363, 114425].