Colours of Transneptunian Binaries from WFC3 Archive Data.

Binary Transneptunian Objects (TNOs) constitute one of the most primordial and least altered planetesimal populations in the solar system, giving valuable insights into outer solar system planetesimal formation (Morbidelli et al., 2008; Brown, 2012). Conditions within the protoplanetary disk where these objects formed can be constrained by examining the compositions of these objects, as well as effects of planetary migration being inferred from their dynamics. Benecchi et al. (2009) measured optical colours of 23 TNO binaries using Hubble Space Telescope (HST) observations, finding a strong correlation between binary component colours. Marsset et al. (2020) extended this analysis with three additional binary objects observed with the Gemini-North Telescope, further strengthening the optical correlation and indicating a similar correlation may exist for component near-infrared (NIR) colours.

We re-examine archival HST Wide Field Camera 3 (WFC3) data of TNO binaries through the application of a purpose-built Markov Chain Monte Carlo (MCMC) based point spread function (PSF) fitting code. The code utilises emcee (Foreman-Mackey et al., 2013) to draw component position and brightness values which are used to produce Tiny Tim (Krist et al., 2011) PSFs for each component. Both component PSFs are injected into a model image and the model is compared to the observed data to obtain best-fit component brightness values and positions. Simultaneously fitting both binary components allows for any dependencies between component parameters to be taken into account, including in the statistical uncertainties for each fitted parameter drawn from the MCMC chains. Final data processing and analysis is ongoing at the time of abstract submission. We will present the results of applying our PSF fitting algorithm to optical and NIR WFC3 archival data of 28 previously reported TNO binaries, covering λ ~ 430-1530 nm. Our preliminary results are shown with the results of Benecchi et al. (2009) in Figure 1. We find the optical component colours show a weaker correlation than that found by Benecchi et al. (2009), with excited objects displaying a larger range of component colours compared to the Cold Classicals. While outliers are present in both dynamical groups, the data remains consistent with similar component colours within 3σ. We will also explore our codes’ sensitivity to binary properties, such as component separation and absolute magnitude.