Abstract
Context. Line shapes of the magnesium resonance lines in white dwarf spectra are determined by the properties of magnesium atoms and the structure of the white dwarf atmosphere. Through their blanketing effect, these lines have a dominant influence on the model structure and thus on the determination from the spectra of other physical parameters that describe the stellar atmosphere and elemental abundances. Aims. In continuation of previous work on Mg+He lines in the UV, we present theoretical profiles of the resonance line of neutral Mg perturbed by He at the extreme density conditions found in the cool largely transparent atmosphere of DZ white dwarfs. Methods. We accurately determined the broadening of Mg by He in a unified theory of collisional line profiles using ab initio calculations of MgHe potential energies and transition matrix elements among the singlet electronic states that are involved for the observable spectral lines. Results. We computed the shapes and line parameters of the Mg lines and studied their dependence on helium densities and temperatures. We present results over the full range of temperatures from 4000 to 12 000 K needed for input to stellar spectra models. Atmosphere models were constructed for a range of effective temperatures and surface gravities typical for cool DZ white dwarfs. We present synthetic spectra tracing the behavior of the Mg resonance line profiles under the low temperatures and high gas pressures prevalent in these atmospheres. Conclusions. The determination of accurate opacity data of magnesium resonance lines together with an improved atmosphere model code lead to a good fit of cool DZ white dwarf stars. The broadening of spectral lines by helium needs to be understood to accurately determine the H/He and Mg/He abundance ratio in DZ white dwarf atmospheres. We emphasize that no free potential parameters or ad hoc adjustments were used to calculate the line profiles.
Highlights
The UV spectra of cool DZ white dwarfs, which are rich in helium, show the resonance lines of Mg at 2852 Å and Mg+ at 2796/2803 Å
With nHe densities above 1021 cm−3 for Ross 640 (Blouin et al 2018), reaching 2 × 1022 cm−3 for vMa2 (Dufour, priv. comm.), multiple perturber effects have to be taken into account. Each one of these white dwarfs constitutes a very interesting case study. As in these very cool DZ white dwarfs, a reliable determination of the line profiles that is applicable in all parts of the line at all densities is the Anderson semi-classical theory (Anderson 1952), which uses the Fourier transform of an autocorrelation function
The semiempirical Hartree Fock Damped dispersion ansatz (HFD) methods have been used in Reho et al (2000) and accurate ab initio calculations for the Mg–He and Ca–He van der Waals potential energy curves of the ground state have been presented in Hinde (2003)
Summary
The UV spectra of cool DZ white dwarfs, which are rich in helium, show the resonance lines of Mg at 2852 Å and Mg+ at 2796/2803 Å. Since our review paper 36 years ago (Allard & Kielkopf 1982), considerable progress in unified line broadening theory and in computational technology enables us to calculate neutral atom spectra given the potential energies and radiative transition moments for relevant states of the radiating atom interacting with other atoms in its environment. As in these very cool DZ white dwarfs, a reliable determination of the line profiles that is applicable in all parts of the line at all densities is the Anderson semi-classical theory (Anderson 1952), which uses the Fourier transform of an autocorrelation function. A unified theory of spectral line broadening (Allard et al 1999) has been developed to calculate neutral atom spectra given the interaction and the radiative transition moments of relevant states of the radiating atom with other atoms in its environment. The spectrum I(∆ω) can be written as the Fourier transform (FT) of the dipole autocorrelation function Φ(s), I(∆ω)
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