Insights on the Rotational State and Shape of Asteroid (203) Pompeja from TESS Photometry

Abstract

The Main Belt asteroid (203) Pompeja shows evidence of extreme variability in visible and near-infrared spectral slope with time. The observed spectral variability has been hypothesized to be attributed to spatial variations across Pompeja's surface. In this scenario, the observed spectrum of Pompeja is dependent on the geometry of the Sun and the observer relative to the asteroid's spin pole and surface features. Knowledge of the rotational spin pole and shape can be gleaned from light curves and photometric measurements. However, dense light curves of Pompeja are only available from two apparitions. Further, previous estimates of Pompeja's sidereal period are close to being Earth-commensurate, making ground-based light curves difficult to obtain. To overcome these difficulties, we implement a pipeline to extract a dense light curve of Pompeja from cutouts of TESS Full Frame Images. We succeeded in obtaining a dense light curve of Pompeja covering $\sim$22 complete rotations. We measure a synodic period of $P_{syn} =24.092 \pm 0.005$ hours and amplitude of 0.073 $\pm$ 0.002 magnitudes during Pompeja's 2021 apparition in the TESS field of view. We use this light curve to refine models of Pompeja's shape and spin pole orientation, yielding two spin pole solutions with sidereal periods and spin pole ecliptic coordinates of $P_{\mathrm{sid}, 1} = 24.0485 \pm 0.0001$ hours, $\lambda_1 = 132^{\circ}$, $\beta_1 = +41^{\circ}$ and $P_{\mathrm{sid}, 2} = 24.0484 \pm 0.0001$ hours, $\lambda_2 =307^{\circ}$, $\beta_2 =+34^{\circ}$. Finally, we discuss the implications of the derived shape and spin models for spectral variability on Pompeja.