A New Generation of Cool White Dwarf Atmosphere Models. II. A DZ Star with Collision-induced Absorption

Abstract

In the first paper of this series , we presented our upgraded cool white dwarf atmosphere code. In this second paper, we use our new models to analyze SDSS J080440.63+223948.6 (J0804+2239), the first DZ star to show collision-induced absorption (CIA). This object provides a crucial test for our models, since previous versions of our code were unable to simultaneously fit the metal absorption lines and the CIA. We find an excellent fit to both the spectroscopic and photometric data, which further validates the improved constitutive physics of our models. We also show that the presence of metal lines allows to lift the degeneracy between high and low hydrogen abundances that usually affects the fits of white dwarfs with CIA. Finally, we investigate the potential impact of spectroscopically undetected metals on the photometric solutions of DC stars.

Gaia white dwarfs within 40 pc – I. Spectroscopic observations of new candidates

“…Additionally, the model exhibits appreciable H 2 -He CIA, while the observations do not. H 2 -He CIA has been observed for other DZ stars, such as J0804+2239 which has a comparable T eff and hydrogen abundance (Blouin et al 2018). While Blouin et al (2018) do find two photometric solutions for J0804+2239, these are either side of the H 2 -He CIA maximum at log(H/He) = −2.5, with the higher hydrogen abundance solution also permitting an adequate spectroscopic fit.…”

“…H 2 -He CIA has been observed for other DZ stars, such as J0804+2239 which has a comparable T eff and hydrogen abundance (Blouin et al 2018). While Blouin et al (2018) do find two photometric solutions for J0804+2239, these are either side of the H 2 -He CIA maximum at log(H/He) = −2.5, with the higher hydrogen abundance solution also permitting an adequate spectroscopic fit. We advocate that the hydrogen-dominated solution is most likely to be correct, given its good agreement with all photometry and more typical surface gravity, though the reason for the overly narrow predicted Ca lines remains unexplained.…”

The 100 pc White Dwarf Sample in the SDSS Footprint

“…J0804+2239 is within 100 pc, and included in our sample. Blouin et al (2018b) find best-fit parameters of T eff = 4970 ± 100 K, log g = 7.98 ± 0.05, and log H/He = −1.6 ± 0.2. Hence, the term "IR-faint" is more appropriate for these objects.…”

“…For example, Blouin et al (2018b) performed a detailed model atmosphere analysis of J0804+2239, a DZ white dwarf with significant absorption in the nearinfrared. J0804+2239 is within 100 pc, and included in our sample.…”

“…The intensity of CIA is controlled by the photospheric density and the H 2 abundance. The former decreases and the latter increases with an increase in the H/He ratio, which implies that there is a degeneracy between high and low hydrogen abundances below and above log H/He ≈ −3 (Blouin et al 2018b). For example, in-stead of the solution shown in Figure 20, the spectral energy distribution of J2138−0056 can also be fitted with a model that has T eff = 4630 K, M = 0.429 M , and log H/He = −2.…”