J. Pittichova

A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)

We present infrared images and spectra of comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger program to observe comets inside of 5 AU from the Sun with the Spitzer Space Telescope. The nucleus of comet 2P/Encke was observed at two vastly different phase angles (20° and 63°). Model fits to the spectral energy distributions of the nucleus suggest that comet Enckes infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence. The observed emission from comet Enckes dust coma is best modeled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 μm, and the silicate contribution by mass to the submicron dust coma is constrained to <31%. Comet 67P/Churyumov-Gerasimenko was observed with distinct coma emission in excess of a model nucleus at a heliocentric distance of 5.0 AU. The coma detection suggests that sublimation processes are still active or grains from recent activity remain near the nucleus. Comet C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the spectrum obtained at 3.2 AU, and we derive a silicate-to-carbon dust ratio of 0.6. The ratio is an order of magnitude lower than that derived for comets 9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).

WISE/NEOWISE Observations of Comet 103P/Hartley 2

We report results based on mid-infrared photometry of comet 103P/Hartley 2 taken during 2010 May 4-13 (when the comet was at a heliocentric distance of 2.3 AU, and an observer distance of 2.0 AU) by the Wide-field Infrared Survey Explorer. Photometry of the coma at 22 μm and data from the University of Hawaii 2.2 m telescope obtained on 2010 May 22 provide constraints on the dust particle size distribution, d log n/d log m, yielding power-law slope values of alpha = –0.97 ± 0.10, steeper than that found for the inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. The extracted nucleus signal at 12 μm is consistent with a body of average spherical radius of 0.6 ± 0.2 km (one standard deviation), assuming a beaming parameter of 1.2. The 4.6 μm band signal in excess of dust and nucleus reflected and thermal contributions may be attributed to carbon monoxide or carbon dioxide emission lines and provides limits and estimates of species production. Derived carbon dioxide coma production rates are 3.5(± 0.9) × 10^(24) molecules per second. Analyses of the trail signal present in the stacked image with an effective exposure time of 158.4 s yields optical-depth values near 9 × 10^(–10) at a delta mean anomaly of 0.2 deg trailing the comet nucleus, in both 12 and 22 μm bands. A minimum chi-squared analysis of the dust trail position yields a beta-parameter value of 1.0 × 10^(–4), consistent with a derived mean trail-grain diameter of 1.1/ρ cm for grains of ρ g cm^(–3) density. This leads to a total detected trail mass of at least 4 × 10^(10) ρ kg.

Ground-based optical and Spitzer infrared imaging observations of comet 21p/Giacobini-Zinner

We present ground-based optical and Spitzer Space Telescope infrared (IR) imaging observations of the ecliptic (Jupiter-family) comet 21P/Giacobini-Zinner, the parent body of the Draconid meteor stream, during its 2005 apparition. The onset of nucleus activity occurred at a pre-perihelion heliocentric distance, r(h) ≃ 3.80 AU, while post-perihelion 21P was dusty (peak Afρ = 131 cm^(-1)) and active out to heliocentric distances ≳ 3.3 AU following a logarithmic slope with rh of -2.04. Coma colors, V-R = 0.524 ± 0.003, R-I = 0.487 ± 0.004, are redder than solar, yet comparable to colors derived for other Jupiter-family comets. A nucleus radius of 1.82 ± 0.05 km is derived from photometry at quiescence. Spitzer images post-perihelion exhibit an extensive coma with a prominent dust tail, where excess emission (over the dust continuum) in the 4.5 µm Infrared Array Camera ( IRAC) image arises from volatile gaseous CO and/or CO2. No dust trail was detected (3σ surface brightness upper limit of 0.3 MJy sr^(-1) pixel^(-1)) along the projected velocity vector of comet 21P in the MIPS 24 µm image suggesting that the number density of trail particles is less than or similar to 7 x 10^(-11) m^(-3). The bolometric albedo of 21P derived from the contemporaneous optical and Spitzer observations is A(θ = 22°) = 0.11, slightly lower than values derived for other comets at the same phase angle.

Thermal evolution and activity of Comet 9P/Tempel 1 and simulation of a deep impact

ABSTRACT We use a quasi–three‐dimensional thermal evolution model for a spherical comet nucleus that takes into account the diurnal and latitudinal variation of the solar flux but neglects lateral heat conduction. We model the thermal evolution and activity of comet 9P/Tempel 1, in anticipation of the Deep Impact mission encounter with the comet. We also investigate the possible outcome of a projectile impact, assuming that all the energy is absorbed as thermal energy. An interesting result of this investigation is that the estimated amount of dust ejected due to the impact is equivalent to 2–2.6 days of activity during “quiet” conditions at perihelion. We show that production rates of volatiles that are released in the interior of the nucleus depend strongly on the porous structure, in particular on the surface‐to‐volume ratio of the pores. We develop a more accurate model for calculating this parameter, based on a distribution of pore sizes, rather than on a single, average pore size.

P/2010 A2 LINEAR - I. An impact in the asteroid main belt

Comet P/2010 A2 LINEAR is an object on an asteroidal orbit within the inner Main Belt, therefore a good candidate for membership with the Main Belt Comet family. It was observed with several telescopes (ESO New Technology Telescope, La Silla, Chile; Gemini North, Mauna Kea, Hawai‘i; University of Hawai‘i 2.2 m, Mauna Kea, Hawai‘i) from 14 Jan. until 19 Feb. 2010 in order to characterize and monitor it and its very unusual dust tail, which appears almost fully detached from the nucleus; the head of the tail includes two narrow arcs forming a cross. No evolution was observed during the span of the observations. Observations obtained during the Earth orbital plane crossing allowed an examination of the out-of-plane 3D structure of the tail. The immediate surroundings of the nucleus were found dust-free, which allowed an estimate of the nucleus radius of 80‐90 m, assuming an albedo p = 0.11 and a phase correction with G = 0.15 (values typical for S-type asteroids). A model of the thermal evolution indicates that such a small nucleus could not maintain any ice content for more than a few million years on its current orbit, ruling out ice sublimati on dust ejection mechanism. Rotational spin-up and electrostatic dust levitations were also rejected, leaving an impact with a smaller body as the favoured hypothesis. This is further supported by the analysis of the tail structure. Finston-Probstein dynamic al dust modelling indicates the tail was produced by a single burst of dust emission. More advanced models (described in detail in a companion paper), independently indicate that this burst populated a hollow cone with a half-opening angleα ∼ 40 ◦ and with an ejection velocity vmax ∼ 0.2 m s −1 , where the small dust grains fill the observed tail, while the arcs are foreshortened sections of the burst cone. The dust grains in the tail are measured to have radii between a = 1‐20 mm, with a differential size distribution proportional to a −3.44±0.08 . The dust contained in the tail is estimated to at least 8× 10 8 kg, which would form a sphere of 40 m radius (with a density ρ = 3 000 kg m −3 and an albedo p = 0.11 typical of S-type asteroids). Analysing these results in the framework of crater physics, we conclude that a gravity-controlled crater would have grown up to∼ 100 m radius, i.e. comparable to the size of the body. The non-disruption of the body suggest this was an oblique impact.

Spitzer observations of the asteroid-comet transition object and potential spacecraft target 107P (4015) Wilson-Harrington

Context. Near-Earth asteroid-comet transition object 107P/ (4015) Wilson-Harrington is a possible target of the joint European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) Marco Polo sample return mission. Physical studies of this object are relevant to this mission, and also to understanding its asteroidal or cometary nature. Aims. Our aim is to obtain significant new constraints on the surface thermal properties of this object. Methods. We present mid-infrared photometry in two filters (16 and 22 μm) obtained with NASAs Spitzer Space Telescope on February 12, 2007, and results from the application of the Near Earth Asteroid Thermal Model (NEATM). We obtained high S/N in two mid-IR bands allowing accurate measurements of its thermal emission. Results. We obtain a well constrained beaming parameter (η = 1.390.26) and obtain a diameter and geometric albedo of D = 3.46 ± 0.32 km, and p_V = 0.059 ± 0.011. We also obtain similar results when we apply this best-fitting thermal model to single-band mid-IR photometry reported by Campins et al. (1995, P&SS, 43, 733), Kraemer et al. (2005, AJ, 130, 2363) and Reach et al. (2007, Icarus, 191, 298). Conclusions. The albedo of 4015 Wilson-Harrington is low, consistent with those of comet nuclei and primitive C-, P-, D-type asteorids. We establish a rough lower limit for the thermal inertia of W-H of 60 Jm^(-2)s^(-0.5) K^(-1) when it is at r = 1 AU, which is slightly over the limit of 30 Jm^(-2) s^(-0.5) K^(-1) derived by Groussin et al. (2009, Icarus, 199, 568) for the thermal inertia of the nucleus of comet 22P/Kopff.

Dust Evolution of Comet 9P/Tempel 1

We present wide-field g, r, and i-band images of comet 9P/Tempel 1, target of the Deep Impact mission, taken with the MegaCam CCD (FOV 1° ×1°) on the {3.6-m} Canada-France-Hawaii Telescope on Mauna Kea. Our observations on UT July 3–6, 2005, covered the night prior to impact to ensure that we have data for the pre-impact dust environment. The two nights following impact allowed us to monitor the inner coma for changes in grain size properties as a result of the impact. We created a deep composite image (from three hours of observing time) for each night. Our morphological studies can establish the pattern of dust emission from the nucleus, which is determined by the surface distribution of discrete sources of dust on the rotating nucleus and their temporal evolution. The scientific questions we address are: How does post-impact dust differ from pre-impact dust? Is there any compositional difference between surface and subsurface grains? Does surface mantling change this? From the relative photometry we observed that some of the impact ejecta leaves our smallest aperture within the first hour after the impact. The light curves through different apertures implies that the ejecta was traveling roughly 200 ms

Physical survey of 24 Centaurs with visible photometry

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Activity of comets at large heliocentric distances pre-perihelion

projected on the sky. Fifty hours after the impact, the core of the comet was within a few tenths of a magnitude back to its pre-impact brightness. Also by our last night the morphology of the coma had begun to return to its pre-impact shape.