[Paper Review] Stellar laboratories. VIII. New Zr IV - VII, Xe IV - V, and Xe VII oscillator strengths and the Al, Zr, and Xe abundances in the hot white dwarfs G191-B2B and RE0503-289

T. Rauch, Sébastien Gamrath+6 more|arXiv (Cornell University)|Nov 21, 2016

Stellar, planetary, and galactic studies37 references16 citations

TL;DR

This study presents new non-local thermodynamic equilibrium (NLTE) oscillator strengths for Zr IV–VII, Xe IV–V, and Xe VII, enabling the first detection of zirconium in a hot white dwarf (RE0503-289) with a measured abundance of log ε(Zr) = −3.5 ± 0.2 (≈11,500× solar). It also improves xenon abundance determination to −3.9 ± 0.2 (≈7,500× solar), while establishing upper limits for Zr and Xe in G191-B2B, demonstrating the critical role of precise atomic data in NLTE stellar atmosphere modeling of hot stars.

ABSTRACT

For the spectral analysis of high-resolution and high-signal-to-noise spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. To search for Zr and Xe lines in the ultraviolet (UV) spectra of G191-B2B and RE0503-289, new Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths were calculated. This allows for the first time, determination of the Zr abundance in white dwarf (WD) stars and improvement of the Xe abundance determinations. We calculated Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths to consider radiative and collisional bound-bound transitions of Zr and Xe in our NLTE stellar-atmosphere models for the analysis of their lines exhibited in UV observations of the hot WDs G191-B2B and RE0503-289. We identified one new Zr IV, 14 new Zr V, and ten new Zr VI lines in the spectrum of RE0503-289. Zr was detected for the first time in a WD. We measured a Zr abundance of -3.5 +/- 0.2 (logarithmic mass fraction, approx. 11 500 times solar). We dentified five new Xe VI lines and determined a Xe abundance of -3.9 +/- 0.2 (approx. 7500 times solar). We determined a preliminary photospheric Al abundance of -4.3 +/- 0.2 (solar) in RE0503-289. In the spectra of G191-B2B, no Zr line was identified. The strongest Zr IV line (1598.948 A) in our model gave an upper limit of -5.6 +/- 0.3 which is about 100 times solar. No Xe line was identified in the UV spectrum of G191-B2B and we confirmed the previously determined upper limit of -6.8 +/- 0.3 (ten times solar). Precise measurements and calculations of atomic data are a prerequisite for advanced NLTE stellar-atmosphere modeling. Observed Zr IV - VI and Xe VI - VII line profiles in the UV spectrum of RE0503-289 were simultaneously well reproduced.

Motivation & Objective

• To improve the accuracy of stellar abundance determinations in hot white dwarfs by providing new, reliable oscillator strengths for Zr and Xe ions.
• To enable the first detection of zirconium in a white dwarf through high-resolution UV spectroscopy and advanced NLTE modeling.
• To refine xenon abundance measurements in hot white dwarfs by identifying new spectral lines and improving atomic data.
• To establish precise upper limits for Zr and Xe in G191-B2B using high-signal-to-noise UV spectra.
• To validate the consistency of observed line profiles with NLTE model atmospheres through simultaneous fitting of multiple ionization states.

Proposed method

• Calculated new oscillator strengths for Zr IV–VII, Xe IV–V, and Xe VIII using advanced atomic structure calculations.
• Applied non-local thermodynamic equilibrium (NLTE) stellar atmosphere models to simulate spectral line formation in hot white dwarfs.
• Analyzed high-resolution, high-signal-to-noise UV spectra of G191-B2B and RE0503-289 to identify and fit spectral lines of Zr and Xe.
• Used synthetic spectrum fitting to match observed line profiles and derive elemental abundances.
• Compared model predictions with observed line profiles to validate the reliability of the new atomic data.
• Applied error analysis to determine abundance uncertainties based on spectral fitting and data quality.

Experimental results

Research questions

• RQ1 What is the abundance of zirconium in the hot white dwarf RE0503-289, and can it be detected for the first time using improved atomic data?
• RQ2 How do new oscillator strengths for Xe IV–V and Xe VII enhance the accuracy of xenon abundance determinations in hot white dwarfs?
• RQ3 What are the upper limits for zirconium and xenon in the white dwarf G191-B2B, and how do they compare to previous estimates?
• RQ4 To what extent do the observed line profiles of Zr IV–VI and Xe VI–VII in RE0503-289 match synthetic spectra generated with the new atomic data?
• RQ5 How does the inclusion of improved atomic data in NLTE models affect the reliability of elemental abundance determinations in hot stellar atmospheres?

Key findings

• Zirconium was detected for the first time in a white dwarf, with a measured abundance of log ε(Zr) = −3.5 ± 0.2 in RE0503-289, corresponding to approximately 11,500 times the solar abundance.
• Fifteen new Zr lines were identified in RE0503-289: one Zr IV, 14 Zr V, and ten Zr VI lines, all consistent with the derived abundance.
• Xenon abundance in RE0503-289 was determined as log ε(Xe) = −3.9 ± 0.2, equivalent to about 7,500 times the solar abundance, based on five new Xe VI lines.
• In G191-B2B, no Zr lines were detected, leading to an upper limit of log ε(Zr) < −5.6 ± 0.3, or about 100 times solar abundance.
• No Xe lines were identified in G191-B2B, confirming a previously established upper limit of log ε(Xe) < −6.8 ± 0.3, or roughly ten times solar abundance.
• Observed line profiles of Zr IV–VI and Xe VI–VII in RE0503-289 were simultaneously and well reproduced by the NLTE synthetic spectra, validating the new atomic data and modeling approach.