Polarimetry of M-type asteroids in the context of their surface composition

¹ V. N. Karazin Kharkiv National University, 4 Svobody Sq., Kharkiv 61022, Ukraine
e-mail: irina@astron.kharkov.ua
² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, Sofia, Bulgaria
⁴ Planetary Science Group, Universidad Nacional de San Juan and CONICET, Av. José I. de la Roza 590 (O), J5402DCS Rivadavia, San Juan, Argentina
⁵ Leibniz-Insititut für Sonnenphysic, 79104 Freiburg, Germany
⁶ Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980–8578, Japan

Received: 30 November 2021
Accepted: 15 March 2022

Abstract

Aims. We aim to investigate how polarimetric observations can improve our understanding of the nature and diversity of M/X-type asteroids.

Methods. Polarimetric observations of the selected M/X-type asteroids were carried out at the Tohoku 0.6-m telescope at Haleakala Observatory, Hawaii (simultaneously in BVR filters), the 2-m telescope of the Bulgarian National Astronomical Observatory in Rozhen (in R filter), and the 2.15-m telescope of the Complejo Astronómico El Leoncito (CASLEO), Argentina (in V filter). We analysed the polarimetric characteristics of M/X-type asteroids along with the available data obtained by other techniques.

Results. New polarimetric observations of 22 M/X-type asteroids combined with published observations provide a data set of 41 asteroids for which the depth of a negative polarisation branch and/or inversion angle were determined. We found that the depth of the negative polarisation branch tends to increase with decreasing steepness of the near-infrared spectra. Asteroids with a deeper negative polarisation branch tend to have a higher radar circular polarisation ratio. We show that, based on the relationship of the depth of the negative polarisation branch and inversion angle, two main sub-types can be distinguished among M-type asteroids. We suggest that these groups may be related to different surface compositions similar to (1) irons and stony-irons and (2) enstatite and iron-rich carbonaceous chondrites.