An analysis of the BVRI colors of 22 active comets

Abstract

Our aim was to analyze the variation of Johnson-Kron-Cousins BVRI color indexes of a sample with 22 active comets of various dynamic groups with the time, geometrical, observational and dynamical parameters. We performed photometric observations of 16 comets between 2010 and 2014, using robotic telescopes in three continents. In addition to the sample, we used data of six comets available in the literature. A statistical comparison between the distributions of color indexes was performed using the Kruskal-Wallis H-test. The color indexes of active comets can vary a few tenths up to a magnitude on time scales that range from hours to weeks. Using the B–V colors of the observed comets, we generated a relationship that correlates the cometary visual and CCD magnitudes. We did not identify any relationship between B–V and V–R colors with heliocentric distance and phase angle. The color B–V is correlated with the photometric aperture that can be described by a logarithmic function. We did not identify any differences in the distribution of B–V color among the comets analyzed at a confidence level equal to or greater than 95%. The mean color of active comets are B–R=1.20±0.24, B–V=0.76±0.16 and V–R=0.42±0.16. Active comets with V–R colors outside the three standard deviation interval can be considered objects with unusual physical characteristics.