Comet 103P/Hartley 2 at perihelion: gas and dust activity

Abstract

Context. The comet 103P/Hartley 2, target of the EPOXI mission (NASA), was supposed to be observed for 3 days around its perihelion, from October 27 to 29, 2010, but photometric data were obtained only on October 27 and 29, 2010. On both dates, the comet visibility was not optimal due to its proximity to the Moon, as projected on the plane of the sky, whereas on October 28, the comet could not be observed at all. Aims. The goal of the campaign was to give ground support to the EPOXI mission by establishing a baseline of activity at perihelion to be compared with in situ activity observed by the space mission about 7 days later on Nov. 4, 2010. We aimed to assess gas and dust production rates, to study the gas and dust coma morphology, to investigate the behaviour of the refractory component by analysing the dust colour variations with date and with projected cometocentric distance, ρ, and to determine the slope of the surface brightness profiles, B, as a function of ρ. Methods. Long-slit spectra and optical broad-and narrowband images were acquired with the instrument ACAM mounted on the William Herschel Telescope (WHT) at La Palma Observatory. We investigated the evolution of the dust coma morphology from the images acquired with specific continuum cometary filters (in the blue and red wavelength region) with image-enhancing techniques. We studied (1) the gas and dust production rates; (2) the dust radial brightness profiles; (3) the profiles of the CN, C 2 , C 3 and NH 2 column densities, and (4) the CN and C 3 coma morphologies. The dust and gas profiles were azimuthally averaged, as well as measured in both the E-W direction (∼Sun-antisolar direction) and in a direction defined by the slit orientation at PA 70 to 250 degrees. Results. The morphological analysis of the dust coma reveals only one structure. Aside from the dust tail in the west direction, a bright jet is detected in images acquired on October 27 at 03:00-04:00 UT. This jet turns on and off and it is not clearly detected at any time on the images obtained during October 29. This structure is enhanced by making use of the radial renormalization and the LarsonSekanina method. It is also confirmed by the distortion of the isophotes at the same position angle (PA). The Afρ parameter, a proxy to the dust production rate, and the gas (CN, C 3 , C 2 , and NH 2 ) production rate, Q i , have been measured at perihelion, r h ≈ 1.058 AU. The quotient Q C 2 /Q CN ∼ 1.3 places 103P/Hartley 2 as a typical comet in terms of long-chain hydrocarbon abundance. The gas-to-dust mass ratio is ∼3−6, indicating that 103P/Hartley 2 is a relatively gas-rich comet. At perihelion, Afρ, as measured in a circular aperture of ∼4700 km ranges from ∼60 cm in the blue to ∼110 cm in the red, which indicates an overall change in the optical properties of the dust grains. On the other hand, the Afρ is rather stable in the innermost coma when it is computed from the spectroscopic measurements within several continuum spectral ranges from 482-685 nm. Both 2D dust colour maps...

A Further Investigation of Apparent Periodicities and the Rotational State of Comet 103p/Hartley 2 From Combined Coma Morphology and Light Curve Data Sets

“…Earth-based observations over multiple orbits revealed a secular decrease in activity since 1991 and a strong asymmetry in production rates around perihelion (Combi et al 2011;Knight & Schleicher 2013). Many larger-scale properties at or near the time of the flyby were constrained from Earth, including an Earth ocean-like D/H ratio (Hartogh et al 2011), activity levels and relative abundances of various gas species (e.g., Dello Mumma et al 2011;Weaver et al 2011;Crovisier et al 2013;McKay et al 2013;Boissier et al 2014), and coma morphology (e.g., Lara et al 2011;Samarasinha et al 2011;Waniak et al 2012;Mueller et al 2013). A number of measurements of the apparent period were determined and are discussed in detail below.…”

Dust environment and dynamical history of a sample of short-period comets

“…In Belton (2013) the authors described the active areas migration over the lobes of the nucleus following the Sun; thus, the strong activity shown by this comet is correlated with the rotation of the nucleus. Two faint dust jets seem to have had their origin in those active areas (see e.g., Lara et al 2011;Mueller et al 2013;Tozzi et al 2013). Thanks to this correlation between active areas and rotation of the nucleus, the comet has an isotropic pattern of time-averaged outgassing from its nuclear surface.…”

Features of a comet nucleus: The case of 103P/Hartley 2

“…In other cases the values of these parameters are specified in the current text or in the tables. It should be also emphasized that for 103P/H the CO 2 /H 2 O abundance ratio is in the range of 8 % (Lara et al 2011). Therefore, 103P/H is very carbon dioxide abundant in comparison to other comets.…”