Time-dependent asymmetries in the atmosphere of the Mira variable R Trianguli through infrared interferometry

We report high-resolution ( < 0.05 mas) angular size measurements of the Mira variable star R Tri using the Palomar Testbed Interferometer. Observations were conducted in the K band (2.0-2.4 μm) between the visual phases of 0.77 and 0.88, and one period later at phase 0.91. The spatial coverage of the measurements spans 40° in position angle. Three simple geometries were modeled: a uniform ellipse model and a uniform sphere with a brighter disk model have lower χ(2)(μ) than the uniform spherical disk model by factors of 5-10. For the axially symmetric models, the axis of symmetry is clustered between 20degrees and 35degrees. The position angles are roughly perpendicular to visual polarization position angles, which supports an axially symmetric source of light scattering. For the elliptical geometry, averaging the semimajor and semiminor axes throughout the data set yields an ensemble average angular diameter of 5.22 +/- 0.30 mas; the ensemble average axial ratio of 2a/2b = 0.75 is similar to that previously determined for other Mira and semiregular variable stars.