Grain properties of Oort cloud comets: Modeling the mineralogical composition of cometary dust from mid-infrared emission features

We carried out spectroscopic observations (R∼250R∼250) of Oort cloud comet C/2001 Q4 (NEAT) at the post-perihelion (rh=1.02AU, Δ=0.93AU, 2004 June 4.2 UT) in mid-infrared 8–13μm region using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on 8.2 m Subaru Telescope. The silicate feature of comet Q4 shows the crystalline olivine peak at 11.2μm along with the broad amorphous silicate features. We present the detailed results of our fitting analysis by using a thermal emission model of cometary coma grains, and compare with other results of C/2001 Q4 (NEAT) at the different heliocentric distance pre-perihelion (rh=0.97AU), comet C/2002 V1 (NEAT), and comet C/1995 O1 (Hale–Bopp). The mass ratios of crystalline to amorphous silicate in sub-μmμm grain size are between 1.9 and 2.5 for these comets. The similarity of coma dust mineralogy among these Oort cloud comets implies that the dust grains in these comets represent the similar physical conditions in the solar nebula and potentially similar regions of icy planetesimal formation.