Lessons from the high-resolution spectroscopy of AW UMa and Epsilon CrA: Is the Lucy model valid?

TL;DR

This study critically examines high-resolution spectra of contact binaries AW UMa and Epsilon CrA, questioning the validity of the Lucy model by comparing spectroscopic and photometric mass ratio determinations and identifying spectral perturbations.

Contribution

It provides new spectroscopic mass ratio measurements and highlights discrepancies with photometric results, challenging the assumptions of the Lucy contact binary model.

Findings

Spectroscopic mass ratio q(sp) = 0.092 +/- 0.007 for AW UMa.

Photometric mass ratio q(ph) tends to be systematically smaller than q(sp).

Enhanced Spectral-line Perturbations (ESP) observed, possibly linked to collision fronts or gas fountains.

Abstract

A re-examination of high-resolution spectral monitoring of the W UMa-type binaries AW UMa and Epsilon CrA casts doubt on the widely utilized Lucy (1968a, 1968b) model of contact binaries. The detection of the very faint profile of the secondary component in AW UMa leads to a new spectroscopic determination of the mass ratio, q(sp) = 0.092 +/- 0.007, which is close to the previous, medium-resolution spectroscopic result of Pribulla & Rucinski (2008), q(sp) = 0.101 +/- 0.006, and remains substantially different from a cluster of generally accepted photometric results by several authors, concentrated around q(ph) = 0.080 +/- 0.005. The two approaches are independent, with the spectroscopic technique being more direct yet more demanding on telescope/spectrograph resources, while the photometric determinations are accessible to smaller telescopes but entirely dependent on the Lucy model. A survey of binaries with the best-determined values of the mass ratio shows a common tendency for q(ph) < q(sp). The tendency for systematically smaller values of q(ph) may result from the overfilling of the primary lobe and underfilling of the secondary lobe relative to the Roche model geometry, as predicted by the Stepien (2009) model; the tendency may be variable in time. Despite the observed moderate inter-systemic velocities, the photometric Lucy model may remain useful in providing approximate, though biased, results for the mass ratio. A complicating factor in detailed spectral analysis may be the occurrence of Enhanced Spectral-line Perturbations (ESP) projected over the secondary profiles, appearing in different numbers in the two studied binaries. The ESPs are tentatively identified within the Stepien model as collision fronts or fountains of hot, primary-component gas from the circumbinary, energy-carrying flow.