Description
The study of binary objects becomes a significant challenge when extended to the faintest extremes. Ultracool dwarfs (UCDs, M < 0.1M⊙) are difficult targets to resolve into their binary components due to their inherent faintness, especially at closer separations. The distribution of UCD binaries seemingly peaks between 1 and 10AU, dropping off at smaller separations. Whether observations at these close separations are rare due to observational limitations or intrinsic formation processes is a long-standing question. Observing and understanding the limits at which these systems form is vital for understanding star and planet formation across the deuterium burning limit of 13MJ. Detection of UCDs at the smallest separations requires particularly deep, sharp images. A wealth of such UCD data is available from the Hubble Space Telescope’s now retired Wide Field Planetary Camera 2 (WFPC2). With modern post-processing techniques, this data can be analysed and further information, spanning the whole lifetime of WFPC2, can be extracted. We use a tested and proven MultiNest based double-PSF fitting method built on extensive ePSF libraries, run over 1145 archived WFPC2 images, covering ∼150 of the best-studied UCDs. Firstly, we apply the method to known UCD binaries and recover the previously resolved companions and measure their parameters to a significantly higher accuracy, effectively halving the previous error measurements. We then repeat the process with individual UCDs, probing closer separations than previously achievable, and analyse the potential binarity of each. We uncover previously unrecognized binaries and place the tightest constraints yet on companions down to ∼1 AU.
