Polarimetric analysis of STEREO observations of sungrazing Kreutz comet C/2010 E6 (STEREO).

Twin STEREO spacecraft pre-perihelion photometric and polarimetric observations of the sungrazing Kreutz comet C/2010 E6 (STEREO) in March 2010 at heliocentric distances $3-28 R_{\odot}$ were investigated using a newly-created set of analysis routines. The comet fully disintegrated during its perihelion passage. Prior to that, a broadening and an increase of the intensity peak with decreasing heliocentric distance was accompanied by a drop to zero polarisation at high phase angles (~105-135°, STEREO-B) and the emergence of negative polarisation at low phase angles (~25-35°, STEREO-A). Outside the near-comet region, the tail exhibited a steep slope of increasing polarisation with increasing cometocentric distance, with the slope showing a marked decrease with decreasing heliocentric distance. The steep slope is attributed to sublimation of refractory organic matrix and the accompanied processing of the fluffy aggregate dust grains, exposing silicates. The decrease in slope is likely caused by the gradual sublimation of all refractory material closer to the Sun, with resulting gases suppressing polarisation signal of the dust. Near-zero polarisation closer to the comet head may be explained by the same mechanism, stronger there due to the large amounts of material being ejected and sublimated from the presumably disintegrating core, which correlates with intensity data. Negative polarisation at small phase angles may be explained by the presence of freshly ejected large silicate-rich aggregates. Despite both zero and negative polarisation being observed simultaneously, the two hypotheses on their causes are not easily reconciled. The need for further studies of such comets, both observational and theoretical, is highlighted.