Mairi W. O'Brien, Pier-Emmanuel Tremblay, Boris T. Gaensicke, Mark A. Hollands, Detlev Koester, Snehalata Sahu, Antoine Bedard, Andrew M. Buchan, Tim Cunningham, John H. Debes, J. J. Hermes, Piotr M. Kowalski

We present a comprehensive multi-wavelength spectroscopic and photometric analysis of the 44 confirmed white dwarfs within 13 pc of the Sun. Combining flux-calibrated ultraviolet (UV) spectroscopy from the Hubble Space Telescope (STIS and COS) with ground-based optical spectroscopy, as well as photometry from Gaia, 2MASS, and WISE, we employ a hybrid fitting method to calculate atmospheric parameters. Each white dwarf was fitted with a bespoke model depending on its detailed atmospheric composition, aside from two strongly magnetic stars. We find a systematic discrepancy in H-atmosphere white dwarfs with Teff < 10,000 K, where fits incorporating UV spectra result in effective temperatures that are 2 - 6 per cent higher than those derived from optical and infrared photometry alone. We re-classify three He-rich white dwarfs as metal enriched following a magnesium detection in their near-UV spectra: WD 0435-088, WD 1132-325 and WD 1917+386. In total, we identify six stars in the sample for which metals were only detected in the UV. Overall we find that 30 per cent of the 13 pc white dwarfs show spectroscopic evidence of evolved planetary systems. Our analysis revealed no measurable difference between the hydrogen content of DQ and DC white dwarfs, although the upper limits of carbon in DCs are significantly below that of the DQ population. We find a multiplicity fraction of 32 per cent for the 13 pc white dwarfs.