Strong Variability in AzV 493, an Extreme Oe-Type Star in the SMC

TL;DR

This study analyzes 18 years of photometric and 12 years of spectroscopic data of AzV 493, revealing complex variability, possible binary interactions, and insights into OBe star disk dynamics and massive binary evolution.

Contribution

It provides detailed long-term observational evidence of variability and potential binary interactions in an extreme Oe star, advancing understanding of decretion disk formation and massive binary evolution.

Findings

14.6-year dominant variability pattern

Episodes of gas ejection and infall observed

Possible eccentric binary companion inferred

Abstract

We present 18 years of OGLE photometry together with spectra obtained over 12 years, revealing that the early Oe star AzV 493 shows strong photometric (Delta I < 1.2 mag) and spectroscopic variability with a dominant, 14.6-year pattern and ~40-day oscillations. We estimate stellar parameters T_eff = 42000 K, log L/L_sun = 5.83 +/- 0.15, M/M_sun = 50 +/- 9, and vsini = 370 +/- 40 km/s. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. AzV 493 may have an unseen companion on a highly eccentric (e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493's mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors.