Possible spatial asymmetry in semi-regular variable UZ Arietis

TL;DR

This study uses lunar occultation observations at 2.2 μm over three epochs to detect potential asymmetry in the brightness distribution of the semi-regular variable star UZ Arietis, linking it to observed optical polarization.

Contribution

First multi-epoch lunar occultation measurements of UZ Arietis reveal possible brightness asymmetry in a semi-regular variable star, connecting it with optical polarization data.

Findings

Detected differences in uniform disk angular diameters across epochs.

Indications of brightness asymmetry in UZ Arietis.

Correlation between brightness asymmetry and polarization axis.

Abstract

Semi-regular variables (SRVs) though closely related to Mira variables, are a less studied class of AGB stars. While asymmetry in the brightness distribution of many Mira variables is fairly well known, it is detected only in a few SRVs. Asymmetry in the brightness distribution at the level of a few milliarcsecond (mas) can be detected by high angular resolution techniques like lunar occultations (LO), long baseline interferometry, and aperture masking interferometry. Multi-epoch LO observations have the potential to detect a departure of brightness profile from spherical symmetry. Each LO event provides a uniform disk (UD) angular diameter along the position angle of the occultation. Any significant difference in the UD angular diameter values of multi-epoch LO observations signifies a brightness asymmetry. In this paper, we report for the first time three epoch UD angular diameter values of a SRV UZ Arietis using the LO technique at 2.2 $\mu m$. Optical linear polarization of the source observed by us recently is also reported. The asymmetric brightness distribution of UZ Ari suggested by a small difference in the fitted UD values for the three epochs, is discussed in the context of optical polarization exhibited by the source and the direction of polarization axis in the plane of the sky.