Search for short-term color variations of five distant comets

Abstract

We present our preliminary results of searching for short-term variations of four long-period comets C/2016 M1 (PANSTARRS), C/2015 O1 (PANSTARRS), C/2014 W2 (PANSTARRS), C/2016 N4 (MASTER), and a hyperbolic comet C/2021 A1 (Leonard). All the comets reveal red color of their dust coma with slight variations, i.e., without changes of the color to blue or neutral. The observed dust productivity of the comets is quite high. Dust production respects to measurements of other long-period comets at the similar heliocentric distances. Comet C/2021 A1 (Leonard) was significantly redder comparing with the other comets and possessed weaker activity.

Introduction

In our research, we are looking for rapid variations of photometric color of comets at heliocentric distances beyond 3 au. The color reflects mainly the size distribution and also depends on the composition of dust particles (1; 2).

Variations of photometric color can be observed when we change the aperture size. It can be explained by differences in sizes and/or compositions of particles on different cometocentric distances, which can be caused by sublimation and/or fragmentation of dust particles (3; 4). On the second hand, except variations with the aperture size, the color was believed to be a constant characteristic of a cometary coma, but further studies have demonstrated changes of color during a long time (5; 6; 7). Furthermore, recent studies reveal much shorter variations of photometric color, i.e. days or months, which can be induced by changes of dust properties (3; 8; 9).

We analyze monitoring observations of 4 comets with highly eccentric elliptical orbits and a hyperbolic comet in order to find rapid color variations.

Observations

Observations of the comets were taken at the Skalnaté Pleso observatory (IAU Code – 056) using the 61-cm telescope. As CCD detector the FLI PL-230 camera have been taken. The pixel size of the camera is 15 µm, the field of view of 40.3 arcmin×40.6 arcmin, and the pixel scale was 1.18 arcsec/px. the Bessel B, V, R broadband filters were used. Images were taken without binning. During observations of comet C/2021 A1 (Leonard) in 2021 the telescope was equipped with the SBIG ST-10XME Dual CCD camera, having a pixel size of 6.8 µm, the field of view of 19.5 arcmin×13.1 arcmin and the pixel scale is 1.07 arcsec/px. We used a 2×2 binning mode. R and V filters of the Johnson-Cousins photometric system were used.

The reduction procedure (bias subtraction, dark and flat field corrections, and cleaning cosmic-ray tracks) was performed in in the standard manner, using the IDL routines (10; 8; 11). We used only photometric nights for analysis.

Results

At heliocentric distances bigger than 3 au, where the comets were observed, the coma is mostly formed by the sublimation of more volatile species, e.g., CO/CO₂. Furthermore, at such distances gaseous emissions of daughter molecules are rarely observed in the optical region (12). Therefore, we assume that the observations obtained with the broadband filters allow us to study features of a dust coma.

We have not found significant variations of color from red to blue or neutral (or vice versa) in the selected comets. All the comets possessed red color of the dust coma with slight variations. Dust production of most long-period comets is similar to other comets of these family and is significantly bigger than for the Jupiter family comets. Only comet C/2016 N4 (PANSTARRS) has possessed lower activity.

Color of hyperbolic comet C/2021 A1 (Leonard) was usually bigger than for the other sampled comets, but dust production was on contrary smaller.

Acknowledgments

Our work was supported by the Slovak Grant Agency for Science VEGA No. 2/0059/22. Also, we were supported by the Slovak Research and Development Agency under the Contract no. APVV-19-0072.

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