2007 Outburst of 17p/Holmes: The Albedo and the Temperature of the Dust Grains

Abstract

Based on optical and infrared observations, we study the albedo and the temperature of the dust grains associated with the spectacular 2007 outburst of Jupiter-family comet 17P/Holmes. We found that the albedo at the solar phase angle ∼16 • was 0.03-0.12. While the color temperature around 3-4 μm was 360 ± 40 K, the color temperature at 12.4 μm and 24.5 μm was ∼200 K, which is consistent with that of a blackbody. We studied the equilibrium temperature of the dust grains at 2.44 AU and found that the big discrepancy in the temperature was caused by the heterogeneity in particle size, that is, hotter components consist of submicron absorbing grains whereas colder components consist of large ( 1 μm) grains. The contemporaneous optical and mid-infrared observations suggest that the albedo and the temperature could decrease within ∼ 3 days after the outburst and stabilized at typical values of the other comets. We estimated the total mass injected into the coma by the outburst on the basis of the derived albedo and the optical magnitude for the entire dust cloud, and found that at least 4 × 10 10 kg (equivalent to a few meter surface layer) was removed by the initial outburst event. The derived mass suggests that the outburst is explainable by neither the exogenetic asteroidal impact nor water ice sublimation driven by solar irradiation, but by an endogenic energy source. We conclude that the outburst was triggered by the energy sources several meters or more below the nuclear surface.

Comet 17p/Holmes: Contrast in Activity Between Before and After the 2007 Outburst

“…It is found that the infrared spectrum of the diffuse emission can be explained by a mixture of amorphous and crystalline silicate materials as observed in most comets (Watanabe et al 2009;Reach et al 2010). Total ejecta mass was estimated to be 10 10 -10 13 kg (Montalto et al 2008;Altenhoff et al 2009;Reach et al 2010;Ishiguro et al 2010;Boissier et al 2012). More than a 2-m-thick surface layer could be blown off by the initial event although this is strongly dependent on the size distribution and maximum size of the dust par-ticles .…”

“…In the range, we obtained i n = (5.3-6.0)×10 −10 Jy/m 2 . In contrast, the flux from the dust coma, i c that has an equivalent total cross-sectional area was derived in the manner described in Ishiguro et al (2010). We calculated the equilibrium temperature of a 1-µm-sized particle r h =2.23 AU using the optical constants of astronomical silicate (217 K), magnetite (229 K), and graphite (235 K) and derived a thermal flux at 18-µm AKARI wavelength of i c = 2.0×10 −9…”

“…There is a big uncertainty in the total mass in the range of 10 10 -10 13 kg (Montalto et al 2008;Altenhoff et al 2009;Reach et al 2010;Ishiguro et al 2010;Boissier et al 2012). In addition, little is known about the excavated depth by the initial outburst.…”

Comet 17P/Holmes: Possibility of a CO driven explosion

“…Reach et al (2010) distinguished three coma components: a shell, a 'blob' of ejecta containing separate filaments and a 'core' centered on the nucleus where the shell is due to small particles while the core and blob can be explained by larger particles. Ishiguro et al (2010) estimated the lower limit of the ejected mass removed by the initial outburst for 4 · 10 10 kg (equivalent to a few meters thick surface layer of the nucleus) -presumably not only the dust mantle but also the pristine layer beneath the surface. Schleicher (2009) estimated a total amount of water ice and dust that was vaporized and emitted during four months of subsequent activity.…”

“…Generally, the huge outburst of Comet Holmes is attributed to an internal explosive source of energy in the nucleus. In the literature are considered various exothermic chemical reactions involving free radicals or pockets of volatile gas stored beneath the surface (Altenhoff et al, 2009;Ishiguro et al, 2010;Reach et al, 2010;Schleicher, 2009;Sekanina, 2008). It should be also noted, that evolution of gas pressure within cometary nuclei and mechanisms for outbursts were investigated also in relations to other comets (e.g Prialnik and Bar-Nun, 1990;Huebner et al, 2006).…”