[Paper Review] Astro2020 Science White Paper: Understanding the evolution of close white dwarf binaries

Odette Toloza, Elme Breed+28 more|arXiv (Cornell University)|Mar 11, 2019

Astronomy and Astrophysical Research4 citations

TL;DR

This white paper advocates for a comprehensive, unbiased survey of close white dwarf binaries using all-sky optical and high-resolution ultraviolet spectroscopy to resolve long-standing discrepancies in binary evolution models. It emphasizes that Hubble's successor must enable precise white dwarf parameter measurements via UV spectroscopy to map the full evolutionary parameter space, constrain Type Ia supernova progenitors, and calibrate future gravitational wave missions like LISA.

ABSTRACT

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain precise measurements of their physical properties, and to further develop the theory to satisfactorily reproduce the properties of the entire population. While ongoing and future all-sky high- and low-resolution optical spectroscopic surveys allow us to enlarge the sample of these systems, high-resolution ultraviolet spectroscopy is absolutely essential for the characterization of the white dwarfs in these binaries. The Hubble Space Telescope is currently the only facility that provides ultraviolet spectroscopy, and with its foreseeable demise, planning the next ultraviolet mission is of utmost urgency.

Motivation & Objective

• Address the severe selection biases and incompleteness in current samples of white dwarf binaries, which hinder accurate theoretical modeling.
• Develop a homogeneous, large-scale sample of white dwarf binaries spanning all evolutionary states to resolve discrepancies between theory and observation.
• Enable precise measurement of white dwarf masses, temperatures, and chemical abundances through high-resolution ultraviolet spectroscopy.
• Constrain the formation channels of diverse systems such as CVs, AM CVn stars, SSSs, and SN Ia progenitors by characterizing their physical properties.
• Ensure the next generation of ultraviolet space telescopes is prioritized to succeed Hubble, given its irreplaceable role in UV spectral analysis.

Proposed method

• Utilize all-sky multi-epoch optical spectroscopic surveys to identify white dwarf binaries via emission lines and radial velocity variations.
• Apply low-resolution (R ≈ 5000) spectroscopy for short-period systems and high-resolution (R ≥ 20,000) spectroscopy for longer-period systems like SSS progenitors.
• Employ high-resolution (R ≈ 20,000–40,000) ultraviolet spectroscopy to model photospheric Lyman series absorption lines and derive white dwarf effective temperatures and masses.
• Use synthetic atmosphere modeling of far-UV metal lines (C, Si, Al) to determine chemical abundances and rotation rates, resolving line blends via high spectral resolution.
• Analyze P-Cygni profiles in UV resonance lines to probe disk wind structure and variability in accretion flows.
• Extend UV coverage down to the Lyman limit (λ ≈ 912 Å) to disentangle white dwarf photospheric flux from accretion flow contamination, improving measurement robustness.

Experimental results

Research questions

• RQ1 What are the physical mechanisms driving common envelope ejection, and what is the true value of the energy efficiency parameter α in binary evolution models?
• RQ2 What fraction of white dwarf binaries evolve through stable versus unstable mass transfer, and how does this affect the formation of SSSs, CVs, and AM CVn systems?
• RQ3 Do the majority of SN Ia progenitors originate from degenerate or non-degenerate donor stars, and how can this be distinguished through UV spectral analysis?
• RQ4 How do the chemical abundances and mass accretion rates in white dwarf binaries constrain their formation channels and evolutionary histories?
• RQ5 Can ultraviolet spectroscopy uniquely identify post-supersoft X-ray sources via anomalous N V /C IV abundance ratios and other spectral signatures?

Key findings

• Current population models fail to reproduce observed white dwarf binary properties due to incomplete, biased samples and reliance on simplified assumptions like a single magnetic braking law.
• High-resolution ultraviolet spectroscopy is essential for measuring white dwarf masses and temperatures with precision, especially for systems where the white dwarf is outshone by its companion, such as in SSSs.
• The Hubble Space Telescope remains the only current facility capable of providing the necessary UV spectral resolution and sensitivity for detailed atmospheric modeling of white dwarfs.
• To achieve robust measurements, future UV instruments must achieve a signal-to-noise ratio >30 at flux levels of 1.2 × 10⁻¹⁶ erg/s/cm²/Å at 1480 Å, corresponding to white dwarfs up to 500 pc distant.
• Time-tagged photon detection in UV spectroscopy enables the construction of arbitrary wavelength-time data cubes, crucial for mitigating geocoronal airglow and intrinsic variability effects.
• A sample of several hundred systems per population type is required to fully map the parameter space of close binary evolution and test theoretical models across all evolutionary pathways.