Line Profiles of the Calcium I Resonance Line in Cool Metal-polluted White Dwarfs

Abstract

Metal-polluted white dwarfs (DZ stars) are characterized by a helium-rich atmosphere contaminated by heavy elements traces originating from accreted rocky planetesimals. As a detailed spectroscopic analysis of those objects can reveal the composition of the accreted debris, there is a great interest in developing accurate DZ atmosphere models. However, the coolest DZ white dwarfs are challenging to model due to the fluidlike density of their atmospheres. Under such extreme conditions, spectral absorption lines are heavily broadened by interactions with neutral helium and it is no longer justified to use the conventional Lorentzian profiles. In this work, we determine the theoretical profiles of the Ca I resonance line (the most prominent spectral line for the coolest DZ white dwarfs) in the dense atmospheres of cool DZ white dwarfs. To do so, we use a unified theory of collisional line profiles and accurate ab initio potential energies and transition dipole moments for the CaHe molecule. We present the resulting profiles for the full range of temperatures and helium densities relevant for the modeling of cool, metal-polluted white dwarfs (from 3000 to 6000 K and from 10 21 to 10 23 cm −3 ). We also implement these new profiles in our atmosphere models and show that they lead to improved fits to the Ca I resonance line of the coolest DZ white dwarfs.

A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs

“…These fits were obtained using their photometric data and trigonometric parallaxes (as they did not have any parallax measurement for WD 0423+120, we assumed log g = 8 for this object). in Blouin et al (2019a). As in Papers I, II and III, our models are in excellent agreement with the observations.…”

“…In particular, continuum opacities are corrected for collective interactions (Iglesias et al 2002;Rohrmann 2018), an ab initio equation of state for hydrogen and helium is assumed (Becker et al 2014), the pressure ionization of helium is modeled using the ab initio calculations of Kowalski et al (2007), an accurate description of the pressure broadening of Lyα is included (Kowalski & Saumon 2006), the H 2 −He CIA profiles are corrected for many-body collisions (Blouin et al 2017) and CIA from He−He−He interactions is included (Kowalski 2014). Moreover, our code includes significant improvements relevant for the accurate modeling of cool metal-polluted atmospheres, such as accurate line profiles for important heavy element lines (Paper III; Allard & Alekseev 2014;Allard et al , 2016aAllard et al ,b, 2018Blouin et al 2019a), a state-of-the-art treatment of the nonideal ionization equilibrium of C, Ca, Fe, Mg and Na (Paper I), and the distortion of the C 2 Swan bands in DQpec stars (Kowalski 2010). Note also that we now include the C 2 Swan bands opacity using a line-by-line approach with the linelist of Hornkohl et al (2005).…”

Magnesium abundances in cool metal-polluted white dwarfs

“…While it is certainly the case, this formalism remains the most accurate theory we have to describe pressure broadening in cool DZ atmospheres and has already led to improved fits to Ca, Na and Mg lines. Moreover, we note that even for very cool objects the unified theory leads to consistent abundance determinations when different lines of the same element are fitted (e.g., Blouin et al 2019b, Figure 7). This suggests that the agreement with the observations is not just superficially good, but also that the determined abundances are accurate.…”

“…The model atmosphere code used in this paper is described at length in Blouin et al (2018a,b). Of particular importance for this work are the unified line profiles that are used to retrieve the photospheric abundances of Ca and Mg (Ca ii H & K, Allard & Alekseev 2014; Ca i λ4226, Blouin et al 2019b;Mg b triplet, Allard et al 2016a). We computed a six-dimensional grid of model atmospheres with T eff varying from 4000 to 9000 K in steps of 500 K, log g from 7.0 to 9.0 in steps of 0.5 dex, log H/He 2 from −5 to −2 in steps of 1 dex, log Ca/He from −11 to −7 in steps of 0.5 dex, log Mg/Ca from 0.24 to 2.24 in steps of 0.5 dex, and log Fe/Ca from 0.16 to 2.16 in steps of 0.5 dex.…”

Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts

“…Additionally, a unified profile for the Ca I 4,227 Å resonance line broadened by neutral helium is now used. Our calculations used the potentials and dipole moments provided by Thierry Leininger and recently presented with similar profile calculations 37 .…”