The optical characteristics of the dust of sungrazing comet C/2012 S1 (ISON) observed at large heliocentric distances

Abstract

We present an analysis of the photometric and spectroscopic data of the comet C/2012 S1 (ISON) observed at the heliocentric distances of 6.21÷4.81 AU. The photometric observations were made with the 60-cm Zeiss-600 telescope (ICAMER, peak Terskol, Russia) and the spectroscopic observations were performed using the SCORPIO-2 focal reducer mounted in the prime focus of the 6-m BTA telescope (SAO RAS, Russia). We analyse the B, V and R-band images to describe the dusty cometary coma and to investigate its brightness, colours and dust production rate. The spectra cover the wavelength range of 3600÷7070 Å. No emissions which are expected in this wavelength region were detected above the 3$σ$ level. The continuum shows a reddening effect with the normalized gradient of reflectivity along dispersion of (9.3$\pm$1.1)% per 1000 Å. A dust-loss rate was derived using the obtained values and under the different model assumptions. Our simulations clearly indicate that to retrieve dust production from the observational Af$ρ$ parameter is an ambiguous task. The result of such a procedure is strongly dependent on dynamical (e.g. effective density and cross-section) as well as optical (e.g. scattering coefficient and phase function) characteristics of dust grains. A variation of the mentioned parameters can lead to dramatic changes in the evaluation of mass production. We demonstrate that the dynamic and optical properties are interconnected via the microscopic properties of dust grains (effective size and porosity).