[Paper Review] Stellar laboratories II. New Zn IV and Zn V oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE0503-289

T. Rauch, K. Werner+2 more|arXiv (Cornell University)|Mar 10, 2014

Stellar, planetary, and galactic studies6 references19 citations

TL;DR

This paper presents newly calculated oscillator strengths for Zn iv and Zn v ions using the Hartree-Fock method with configuration interaction, enabling precise non-LTE modeling of ultraviolet spectra. The improved atomic data successfully reproduce 31 Zn iv and 16 Zn v lines in the hot white dwarf G191-B2B (log Zn = -5.52 ± 0.2) and 128 Zn v lines in RE 0503-289 (log Zn = -3.57 ± 0.2), confirming highly supersolar zinc abundances and validating the reliability of the new atomic data for stellar abundance analysis.

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. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191-B2B, 21 Zn IV lines were newly identified. Because of the lack of Zn IV data, transition probabilities of the isoelectronic Ge VI were adapted for a first, coarse determination of the photospheric Zn abundance. We performed new calculations of Zn IV and Zn V oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn IV - V spectrum exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. In the UV spectrum of G191-B2B, we identify 31 Zn IV and 16 Zn V lines. Most of these are identified for the first time in any star. We can reproduce well almost all of them at log Zn = -5.52 +/- 0.2 (mass fraction, about 1.7 times solar). In particular, the Zn IV / Zn V ionization equilibrium, which is a very sensitive indicator for the effective temperature, is well reproduced with the previously determined Teff = 60000 +/- 2000 and log g = 7.60 +/- 0.05. In the spectrum of RE0503-289, we identified 128 Zn V lines for the first time and determined log Zn = -3.57 +/- 0.2 (155 times solar). Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Zn IV and Zn V line profiles in two white dwarf (G191-B2B and RE0503-289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed us to determine the photospheric Zn abundance of these two stars precisely.

Motivation & Objective

• To address the lack of reliable atomic data for Zn iv and Zn v, which previously limited accurate photospheric abundance determinations in hot stars.
• To improve the precision of zinc abundance measurements in the DA-type white dwarf G191-B2B and the hotter DO-type white dwarf RE 0503-289.
• To validate the newly calculated oscillator strengths by reproducing observed Zn iv and Zn v line profiles in high-resolution ultraviolet spectra.
• To identify new Zn lines in the spectra of these stars, particularly in the case of RE 0503-289, where trans-iron elements are highly enriched.
• To demonstrate that accurate atomic data are essential for reliable non-LTE stellar atmosphere modeling and ionization equilibrium analysis.

Proposed method

• Calculated Zn iv and Zn v oscillator strengths using the pseudo-relativistic Hartree-Fock (HFR) method with configuration interaction for relevant configurations.
• Optimized radial integrals and effective interaction parameters via a least-squares fitting process to experimental energy levels from Sugar & Musgrove (1995).
• Applied the new oscillator strengths in non-LTE stellar atmosphere models for G191-B2B and RE 0503-289, using established model parameters (Teff = 60,000 K, log g = 7.60 for G191-B2B; Teff = 70,000 K, log g = 7.50 for RE 0503-289).
• Constrained the Zn abundance by matching synthetic line profiles to observed high-signal-to-noise UV spectra from the Hubble Space Telescope and FUSE.
• Used ionization equilibrium between Zn iv and Zn v as a sensitive diagnostic for effective temperature, validating the model atmosphere parameters.
• Evaluated line profile reproduction and identified blends or cancellation effects via the cancellation factor (CF) in the HFR calculations.

Experimental results

Research questions

• RQ1 Can newly calculated oscillator strengths for Zn iv and Zn v improve the accuracy of photospheric zinc abundance determination in hot white dwarfs?
• RQ2 To what extent can the new atomic data reproduce observed Zn iv and Zn v line profiles in high-resolution UV spectra of G191-B2B and RE 0503-289?
• RQ3 Does the ionization equilibrium between Zn iv and Zn v provide a consistent and reliable constraint on the effective temperature of these stars?
• RQ4 How many previously unidentified Zn lines can be detected and confirmed in the UV spectra of these white dwarfs using the improved atomic data?
• RQ5 To what extent does the high Zn abundance in RE 0503-289 align with the abundances of other trans-iron elements in this metal-rich white dwarf?

Key findings

• The newly calculated Zn iv and Zn v oscillator strengths successfully reproduce 31 Zn iv and 16 Zn v lines in the UV spectrum of G191-B2B, with a photospheric Zn abundance of log Zn = -5.52 ± 0.2 (1.7 times solar).
• The Zn iv / Zn v ionization equilibrium is well reproduced with the previously determined effective temperature of 60,000 ± 2,000 K and log g = 7.60 ± 0.05, validating the model atmosphere parameters.
• In RE 0503-289, 128 Zn v lines were identified for the first time, and the Zn abundance was determined as log Zn = -3.57 ± 0.2, corresponding to about 155 times the solar abundance.
• The highly supersolar Zn abundance in RE 0503-289 is consistent with the abundances of other trans-iron elements such as Ge (650× solar), Kr (450× solar), and Xe (3,800× solar).
• The cancellation factor (CF) analysis indicates that very few transitions are affected by strong cancellation effects, suggesting high reliability of the majority of the calculated oscillator strengths.
• The study confirms that accurate atomic data are essential for precise non-LTE modeling and abundance determination in hot, metal-rich white dwarfs.