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

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.Comment: To be published in MNRAS. 9 pages, 13 figures and 8 table