tokens: in previous studies (Kapranidis Liebert 1986 ; Dufour et al. 2007 ), atmosphere model codes used to analyze this object relied on constitutive physics that are not totally appropriate for the dense atmosphere of this cool white dwarf. In particular, the analysis of Kapranidis Liebert ( 1986 ) relied on models that assumed a Thomas–Fermi equation of state (Kapranidis 1983 ). Because of this assumption, they found that electron thermal conduction was an important energy transport mechanism, a conclusion that does not hold if we use more realistic equations of state (Bergeron et al. 1995 ; Kowalski et al. 2007 ). Also, Kapranidis Liebert ( 1986 ) computed their synthetic spectra from pure helium atmosphere structures, thus ignoring the impact of heavy elements on the pressure and temperature stratification. Moreover, a simple Lorentzian profile was assumed for the Ca i resonance line. Despite all those simplifying assumptions, they managed to obtain an excellent fit to the resonance line (with T eff = 4500 K and ) and were even able to find a good agreement to the unknown feature at 4500 Å. Unfortunately, no detail regarding this puzzling structure was given, so we cannot know how they were able to explain it. Dufour et al. ( 2007 ) revisited WD 2251−070 using more realistic atmosphere models, where heavy elements are included in the calculation of the atmosphere structures. That being said, a number of unjustified approximations remained, such as the ideal gas law, the ideal Saha ionization equilibrium, and Lorentzian profiles. Their best solution ( T eff = 4000 K and ) was in good agreement with the photometry and with the Ca i resonance line (but they were unable to explain the absorption feature near 4500 Å). Here, we revisit this star using our improved models (Blouin et al. 2018a ) and our new Ca i resonance line profiles. To do so, we use the spectroscopic observations of Dufour et al. ( 2007 ), the Gaia data release 2 (DR2) parallax measurement ( π = 117.15 ± 0.05 mas; Gaia Collaboration 2016 , 2018 ), photometry from Bergeron et al. ( 1997 ), the Panoramic Survey Telescope And Rapid Response System, Chambers et al. ( 2016 ), and the Two Micron All-Sky Survey. Our best fit to the spectroscopic and photometric observations is shown in Figure 9 , and the corresponding atmospheric parameters are given in Table 1 . Figure 9. Our best solution for WD 2251−070. The top panel shows our spectroscopic fit, and the bottom panel shows our fit to the BVRI , grizy, and JHK photometry. Table 1 WD 2251−070 Atmospheric Parameters To obtain this solution, we used the photometric technique (Bergeron et al. 1997 ) to determine T eff and . More precisely, the solid angle π ( R / D ) 2 and the effective temperature were found by adjusting the model fluxes to the photometric observations. From the solid angle, we computed the white dwarf radius R ( D is given by the Gaia parallax) and, from there, we found the mass and the surface gravity using the
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