LABORATORY MEASUREMENTS OF WHITE DWARF PHOTOSPHERIC SPECTRAL LINES: Hβ

Abstract

We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating white dwarf photospheric conditions. Here we present timeresolved measurements of Hβ and fit this line using different theoretical line profiles to diagnose electron density, n e , and n = 2 level population, n 2 . Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n e ∼ 4 to ∼ 30 × 10 16 cm −3 throughout a 120-ns evolution of our plasma. Also, we observe n 2 to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within ∼ 55 ns to become consistent with LTE. This supports our electron-temperature determination of T e ∼ 1.3 eV (∼ 15, 000 K) after this time.At n e 10 17 cm −3 , we find that computer-simulation-based line-profile calculations provide better fits (lower reduced χ 2 ) than the line profiles currently used in the white dwarf astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.

Unified line profiles for hydrogen perturbed by collisions with protons: satellites and asymmetries

“…The latter is evident for high order lines (n f > 5), in which the red wing falls quickly for the quasi-molecular profile, while the red wing in the electronic Stark behaves similarly to the blue wing and decreases slowly. These differences may be the cause of the inconsistent densities reported by Falcon et al (2014) and Genest-Beaulieu & Bergeron (2014), which are obtained when different lines are fitted.…”

“…Hence the inconsistencies between the spectroscopic method and other methods are reduced when corrections to 3D convection are applied, but are still without a complete solution at present (Genest-Beaulieu & Bergeron 2014). Falcon et al (2014) noted that the inferred electronic density of a plasma when fitting H γ was ∼ 40 % smaller than the density obtained when fitting H β, regardless of the set of models used. They suggested that this inconsistency would lead to lower determinations of surface gravities and, by consequence, a lower mean mass would be obtained.…”

“…The input data for potentials and dipole moments were calculated by Santos & Kepler (2012) and are available at their web page 1 , except for the potential energy data used for the evaluation of the line centres, which were obtained with an improvement of the code, as will be described in Section 6. Calculations via unified theory existed in the literature only for Lyman α ( atmosphere models but especially to allow comparison with new experimental data like those published by Falcon et al (2010aFalcon et al ( , 2012Falcon et al ( , 2014. That is the scope of the present work.…”

“…Kielkopf, Allard & Decrette (2002) studied Balmer α and identified six satellites in the theoretical wing, of which only a stronger absorption at 8650Å is observed in the plasma spectrum. For Balmer β and Balmer γ, on the other hand, no satellites were identified, because only the line centre experimental profiles were published so far (Falcon et al 2012(Falcon et al , 2014 and usually there are no satellites in this region, which corresponds to radiation when the particles are far apart. In these papers and in Falcon et al (2010a), they use x-rays generated from a z-pinch dynamic hohlraum to drive plasma formation in a macroscopic gas cell and generate absorption spectra for the Balmer lines, with the intention of benchmarking white dwarf stars atmosphere models, though the published data are scarce to date.…”

Matrix Methods for Solving Hartree-Fock Equations in Atomic Structure Calculations and Line Broadening

“…Calculations of spectral line broadening are important for a variety of applications, including diagnosing laboratory plasma conditions [1,2], determining temperatures and gravities of white dwarf stars [3], and calculations of stellar opacities [4]. The accuracy of line-broadening calculations is determined by several factors: the accuracy of the atomic structure, the accuracy of the radiator-perturber interaction, and the accuracy of the perturber correlations.…”