The SMC B-type supergiant AzV322: a g-mode pulsator with a circumstellar disc

TL;DR

This study characterizes AzV322, a B-type supergiant with a circumstellar disk, revealing pulsation frequencies, spectral features, and possible binary interactions, suggesting it belongs to a new class of pulsating supergiants with unique disk features.

Contribution

First detailed photometric and spectroscopic analysis of AzV322, identifying pulsation modes, spectral properties, and disk characteristics, proposing it as a new type of pulsating supergiant with a circumstellar disk.

Findings

Identified four significant pulsation frequencies in AzV322.

Confirmed early B-type spectral classification with emission lines.

Detected long-term quasi-cyclic timing variations suggesting binary effects.

Abstract

We present a photometric and spectroscopic study of AzV322, an emission line object located in the Small Magellanic Cloud previously classified between O9 and B0. We analyze 17.5 years of $I$ and $V$ band OGLE-II, III and IV light curves and find four significant frequencies, viz. $f_1$= 0.386549 $\pm$ 0.000003, $f_2$= 0.101177 $\pm$ 0.000005, $f_3$= 0.487726 $\pm$ 0.000015 and $f_4$= 0.874302 $\pm$ 0.000020 c/d. The $f_1$ frequency (period 2.58700 $\pm$ 0.00002 days) provides the stronger periodogram peak and gives a single wave light curve of full amplitude 0.066 mag in the $I$-band. High-resolution optical spectroscopy confirms the early B-type spectral type and reveals prominent double peak Balmer, Paschen, OI 8446 and HeI 5875 emissions. The spectral energy distribution shows significant color excess towards long wavelengths possibly attributed to free-free emission in a disk-like envelope. Our analysis yields $T_{eff}$ = 23000 $\pm$ 1500 K, log g = 3.0 $\pm$ 0.5, $M$ = 16 $\pm$ 1 M$_{\odot}$, $R$ = 31.0 $\pm$ 1.1 R$_{\odot}$, and $L_{bol}$ = 10$^{4.87 \pm 0.06}$ $L_{\odot}$. AzV322 might be a member of the new class of slowly pulsating B supergiants introduced by Saio et al. (2006) and documented by Lefever, Puls & Aerts (2007), however its circumstellar disk make it an hitherto unique object. Furthermore, we notice that a O-C analysis for $f_1$ reveals quasi-cyclic changes for the times of maximum in a time scale of 20 years which might indicate a light-travel time effect in a very wide orbit binary with an undetected stellar component.