Ground-based Optical and SPITZER Infrared Imaging Observations of Comet 21P/Giacobini-Zinner

TL;DR

This study combines optical and infrared observations of comet 21P/Giacobini-Zinner during its 2005 apparition, revealing details about its activity, composition, and physical properties, including nucleus size and coma characteristics.

Contribution

It provides new measurements of the comet's nucleus size, activity onset, coma colors, and gas emissions, enhancing understanding of Jupiter-family comet properties.

Findings

Nucleus radius of 1.82 km was determined.

Comet activity started before perihelion at < 3.80 AU.

Excess emission at 4.5 microns indicates CO and/or CO2 gases.

Abstract

We present ground-based optical and Spitzer Space Telescope infrared imaging observations of the ecliptic (Jupiter-family) comet 21P/Giacobini-Zinner, the parent body of the Draconid meteor stream, during its 2005 apparition. Onset of nucleus activity occurred at a pre-perihelion heliocentric distance, < 3.80 AU, while post-perihelion 21P was dusty (peak Afrho = 131 cm^{-1}) and active out to heliocentric distances > 3.3 AU following a logarithmic slope with heliocentric distance -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 micron IRAC image arises from volatile gaseous CO and/or CO2. No dust trail was detected (3-sigma surface brightness upper-limit of 0.3 MJy/sr/pixel) along the projected velocity vector of comet 21P in the MIPS 24 micron image suggesting that the number density of trail particles is < 7 x 10^{-11} m^{-3}. The bolometric albedo of 21P derived from the contemporaneous optical and Spitzer observations is A(theta = 22 degr.) = 0.11, slightly lower than values derived for other comets at the same phase angle.