Photometry of an unusual small distant object 2016 ND21

TL;DR

This study presents the first photometric and rotational analysis of the distant object 2016 ND21, revealing its elongated shape, extremely red surface, and possible outer belt origin, with no cometary activity detected.

Contribution

First detailed surface color and rotational property measurements of 2016 ND21, indicating its unusual red surface and potential outer belt origin.

Findings

Rotational period of approximately 17.53 hours.

Object has an elongated shape with axis ratio ~1.3.

Surface colors indicate an extremely red surface, similar to RR type TNOs.

Abstract

We present the first-ever measurements of the surface colors and rotational properties of a recently discovered distant object in an unusual orbit 2016 ND21. In 2017, this object was close to its perihelion, which allowed us to observe it with a 2.0-m telescope at the Peak Terskol observatory. Broadband photometric observations of 2016 ND21 were carried out in October and December of 2017 in standard BVR filters of the Johnson-Cousins photometric system. During our observations, we did not detect any cometary activity. We found the rotational period of 17.53{\pm}0.02 hr, while another a little longer value of 17.65{\pm}0.02 hr is also possible. Assuming an equatorial aspect of observations, a peak-to-peak amplitude of A = 0.31{\pm}0.05 mag (or even higher since only one maximum and one minimum were well-measured) corresponds to an elongated body with an axis ratio a/b ~1.3. The lightcurve behavior indicates a complex, possibly non-convex, shape of this object. The visible absolute magnitude is HV =12.4{\pm}0.1 mag, which was estimated by using the linear phase slope 0.04 mag/deg as the most probable value from our observations. Assuming a surface albedo in the range of 0.04-0.10, the size of 2016 ND21 should be about 15-23 km. From our multi-color observations, we determined surface colors V-R = 0.69{\pm}0.04 mag, B-R = 1.79{\pm}0.08 mag, and B-V = 1.10{\pm}0.08 mag. The measured colors indicate an extremely red surface of this object. A very red surface is unusual for comets, which is in agreement with the fact, that no cometary activity was detected for 2016 ND21. The B-R color is higher than the typical B-R colors of the red D-type asteroids, but it is consistent with colors of the red Centaurs and TNOs classified as RR type in TNOs classification. This result gives a first evidence of a possible outer belt origin of this small body.