Thermal properties, sizes, and size distribution of Jupiter-family cometary nuclei

Abstract

International audienceWe present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4-5 AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03 +/- 0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3 km radius, and even for comets that approach to similar to 2 AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above similar to 2 km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around -1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3-6 km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects. (C) 2013 Elsevier Inc. All rights reserved

Physical Observations of (196256) 2003 Eh1, Presumed Parent of the Quadrantid Meteoroid Stream

“…We used the absolute red magnitude, m R (1, 1, 0), to calculate the effective object radius in meters, r e , using Russell (1916) where R e = −27.1 is the apparent red magnitude of the Sun (Cox 2000). We adopt the typical value of geometric albedo, p v (≈p R ) = 0.04, from the visible and thermal (mid-infrared) measurements for JFC nuclei (Lamy et al 2004;Fernández et al 2013). For the averaged absolute red magnitude m R (1, 1, 0) = 15.82 ± 0.17 mag, Equation (3) gives r e = 1950 ± 150 m, which we approximate as r e = 2.0 ± 0.2 km.…”

“…Quantity A is the Bond albedo, defined by A = p v q = 0.012, where p v = 0.04 (Lamy et al 2004;Fernández et al 2013) and q ∼ 0.3 is the phase integral determined from cometary nuclei and Jupiter Trojan asteroids Buratti et al 2004). The latent heat of sublimation for water at temperature T (in K) is given by L(T) = (2.875 × 10 6 ) − (1.111 × 10 3 )T in J kg −1 , taking the polynomial fit to the thermodynamic data in Delsemme & Miller (1971).…”

“…The lowest value, χ = 1, corresponds to subsolar ice on a non-rotating object, while the highest value, χ = 4, corresponds to an isothermal, spherical nucleus. For comet-like objects, the nightside thermal radiation is negligible (i.e., day-side emission only) due to the low thermal diffusivity of the surface layers, suggesting the intermediate value, χ = 2, is appropriate for providing a maximum active fractional area and minimum specific mass ross rate (Fernández et al 2013;. However, since we are interested in obtaining a limit to f A , we assume the lowest possible surface temperatures (corresponding to the isothermal case, χ = 4) and find dm/ dt = 7.5 × 10 −6 kg m −2 s −1 and T = 180 K at R = 2.139 AU using Equation (6).…”

The Perihelion Emission of Comet C/2010 L5 (WISE)

“…The coma removal process involves fitting the coma in azimuthal wedges as a function of the angular distance from the central brightness peak (see Lisse et al 1999Lisse et al , 2009Fernández et al 2000Fernández et al , 2013, and has previously been successfully applied to WISE data . The extracted nucleus signal is then fit to a NEATM, described in more detail in Bauer et al (2015), yielding a 1σ upper bound on the nucleus diameter of 0.7 km, and a 3σ upper bound of 2.2km.…”

Why are Jupiter-family comets active and asteroids in cometary-like orbits inactive?

“…In comparison to comets, ACOs are inactive small bodies, but with similar orbital parameters (Fernández et al 2013;Kim et al 2014;Belton 2014). …”