Observations of Comet P/2003 T12 = 2012 A3 (SOHO) at large phase angle in STEREO-B

Comet P/2003 T12 = 2012 A3 (SOHO) was observed by the satellite STEREO-B during the period 2012 January 13-27. During its apparition, it ventured into the highest phase angle ever observed for a comet, and the forward-scattering enhancement in brightness was marked, as large as ∼8.5 mag. Therefore,...

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Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2013-12, Vol.436 (2), p.1564-1575
Main Author: Hui, M.-T.
Format: Article
Language:English
Subjects:
Astronomy
Comets
Geometry
Scattering
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Summary:Comet P/2003 T12 = 2012 A3 (SOHO) was observed by the satellite STEREO-B during the period 2012 January 13-27. During its apparition, it ventured into the highest phase angle ever observed for a comet, and the forward-scattering enhancement in brightness was marked, as large as ∼8.5 mag. Therefore, it provided a precious opportunity to examine the compound Henyey-Greenstein (HG) comet-dust light-scattering model and it also offered valuable polarization data under an unprecedented observing geometry. Our analysis reveals that the compound HG model fits the observations very well until the phase angle exceeds ∼173°, where the brightness surge of the comet was obviously steeper than the prediction by the model. We have found that the reason for the greater steepness cannot be explained by contaminations from the proximal tail. Instead, the model of Mie spheres with radii greater than 1 μm, having a power-law distribution of power index ∼3, matches the observation very well, providing a best-fitting complex refractive index μ = 1.38 + i 0.006. The dust size was found to be consistent with the analysis of the comet's syndyne lines. The debiased polarization of the coma was ∼0 per cent in the phase angle range from 172 9 to 177 6. No convincing evidence of temporal variation of the polarization was detected.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/stt1683