(3200) Phaethon Polarimetry in the Negative Branch: New Evidence for the Anhydrous Nature of the DESTINY+ Target Asteroid

TL;DR

This study presents the first polarimetric analysis of asteroid Phaethon in the negative branch, providing evidence for its anhydrous composition and accurately estimating its geometric albedo.

Contribution

It offers new polarimetric data for Phaethon, confirming its anhydrous nature and refining its albedo estimate, which informs its composition and origin.

Findings

Phaethon has a minimum polarization degree of -1.3%.

The polarimetric slope is 0.22% per degree.

The geometric albedo is estimated at 0.11.

Abstract

We report on the first polarimetric study of (3200) Phaethon, the target of JAXA's DESTINY$^+$ mission, in the negative branch to ensure its anhydrous nature and to derive an accurate geometric albedo. We conducted observations at low phase angles (Sun-target-observer angle, alpha = 8.8-32.4 deg) from 2021 October to 2022 January and found that Phaethon has a minimum polarization degree $P_{min}$ = -1.3 +- 0.1 %, a polarimetric slope h = 0.22 +- 0.02 % deg$^{-1}$, and an inversion angle alpha$_0$ = 19.9 +- 0.3 deg. The derived geometric albedo is $p_V$ = 0.11 (in the range of 0.08-0.13). These polarimetric properties are consistent with anhydrous chondrites, and contradict hydrous chondrites and typical cometary nuclei.