Photometry, spectroscopy, and polarimetry of distant comet C/2014 A4 (SONEAR)

Abstract

We analyze the dust environment of the distant comet C/2014 A4 (SONEAR), with a perihelion distance near 4.1~au, using comprehensive observations obtained by different methods. We present an analysis of spectroscopy, photometry, and polarimetry of comet C/2014 A4 (SONEAR), which were performed on November 5~--~7, 2015, when its heliocentric distance was 4.2~au and phase angle was 4.7${}^{\circ }$. Long-slit spectra and photometric and linear polarimetric images were obtained using the focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA (SAO RAS, Russia). We simulated the behavior of color and polarization in the coma presenting the cometary dust as a set of polydisperse polyshapes rough spheroids. No emissions were detected in the 3800~--~7200~$\mathring{A}$ wavelength range. The continuum showed a reddening effect with the normalized gradient of reflectivity 21.6$\pm$0.2\% per 1000~$\mathring{A}$ within the 4650~--~6200~$\mathring{A}$ wavelength region. The fan-like structure in the sunward hemisphere was detected. The radial profiles of surface brightness differ for r-sdss and g-sdss filters, indicating predominance of submicron and micron-sized particles in cometary coma. The dust color (g--r) varies from 0.75$\pm$0.05${}^m$ to 0.45$\pm$0.06${}^m$ along the tail. For aperture radius near 20~000~km, the dust productions in various filters were estimated as $Af\rho$~=~680$\pm$18~cm (r-sdss) and 887$\pm$16~cm (g-sdss). The polarization map showed spatial variations of polarization over the coma from about --3\% near the nucleus to --8\% at cometocentric distance about 150~000~km. Our simulations show that the dust particles were dominated (or covered) by ice and tholin-like organics. Spatial changes in the color and polarization can be explained by particle fragmentation.

14 pages, 10 figures, 3 tables

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