Insights on Ultra-red Main Belt Asteroid (203) Pompeja from TESS Photometry

During its 2021 apparition, large Main Belt asteroid (203) Pompeja was observed to have an extremely steeply red sloped spectrum in the visible and near-infrared [1]. The resemblance of this ultra-red spectrum to that of some classes of trans-Neptunian Objects (TNOs) led to the hypothesis that Pompeja originated from the primordial TNO parent population and was transported to its current location in the Main Belt during the era of giant planet migration [1]. However, subsequent observations of Pompeja [2, 3] showed that the asteroid’s VNIR spectrum was not consistently ultra-red, suggesting that the spectral appearance of Pompeja varies across its surface and thus varies with time, related to rotational phase and viewing geometry at the time of observation [2].  Understanding the viewing geometry for any given spectral observation requires knowledge of Pompeja’s rotational state (shape and spin pole), which can be derived from light curves using convex inversion [4,5]. Obtaining ground-based, densely sampled light curves of Pompeja is complicated by its Earth commensurate (~24 hour) rotational period [6], necessitating observations via either a global network of telescopes or a space-based observing platform with an observation cadence unaffected by Earth’s rotational period. To this end, we obtained photometric measurements of Pompeja from TESS Full Frame Images using existing open-source software tools for accessing TESS data [7, 8] to obtain a dense, continuous light curve of Pompeja. We compare the results of shape modeling incorporating TESS data to previous models of Pompeja’s rotational state and shape and discuss the implications for the hypothesis of spectral variability on Pompeja’s surface in light of the TESS observations, including recommendations for future observations of Pompeja that target the ultra-red material.   Figure 1: Light curve of Pompeja derived via photometric measurements of TESS FFIs, folded to the best fit synodic period of 24.0921 hours. A rotational phase of 0 corresponds to JD = 2457000. References:  [1] Hasegawa, S., Marsset, M., DeMeo, F. E., et al. 2021 ApJL, 916, L6, doi: 10.3847/2041-8213/ac0f05 [2] Hasegawa, S., DeMeo, F. E., Marsset, M., et al. 2022 ApJL, 939, L9, doi: 10.3847/2041-8213/ac92e4 [3] Humes, O. A., Thomas, C. A., & McGraw, L. E. 2024, PSJ, 5, 80, doi: 10.3847/PSJ/ad2e99 [4] Kaasalainen, M., & Torppa, J. 2001, Icarus, 153, 24,doi: 10.1006/icar.2001.6673 [5] Kaasalainen, M., Torppa, J., & Muinonen, K. 2001, Icarus,153, 37, doi: 10.1006/icar.2001.6674 [6] Pilcher, F., Ferrero, A., Hamanowa, H., & Hamanowa, H. 2012, Minor Planet Bulletin, 39, 99 [7] Brasseur, C. E., Phillip, C., Fleming, S. W., Mullally, S. E., & White, R. L. 2019, Astrocut: Tools for creating cutouts of TESS images, Astrophysics Source Code Library,121 record ascl:1905.007 [8] Lightkurve Collaboration, Cardoso, J. V. d. M., Hedges, C., et al. 2018, Lightkurve: Kepler and TESS time series analysis in Python, Astrophysics Source Code Library. http://ascl.net/1812.013