R 40 : the first luminous blue variable in the Small Magellanic Cloud.

Abstract

During the past decade R 40 in the SMC has brightened in the visual range by about 0.5 magnitudes. Correlated with the brightening the spectral type has changed from B8Ia to A3Ia-O. This behaviour shows R 40 to be an LBV, the first one discovered in the SMC. During the past eight years we obtained extensive Stromgren photometry. R 40 was also observed in the IR range in the JHKL filter system. High resolution spectra were observed in Dec. 1991 and Jan. 1993 in the optical range at ESO with CASPEC at the 3.6m telescope and with our fiber-linked echelle spectrograph at the 2.2m telescope. In addition one high resolution LWP- IUE spectrum and one SWP and LWP low resolution spectrum were secured during the present outburst phase of R 40. For comparison IUE archive data were consulted; R 40 had been exposed in the low resolution SWP and LWP mode in 1987. Apart from the secular brightening at a timescale of years R 40 exhibits photometric pulsation-like microvariations with a quasi-period of 120 days which is also typical for LBVs. From the continuum energy distribution (from the satellite UV to the IR range) and from the CASPEC spectrum we derived stellar parameters of R 40 during the outburst phase in 1991 by making use of Kurucz model atmospheres: T_eff_ = 8700 K, log g = 0.75 and with a distance module of 18.85 for the SMC M_bol_ = -9.4 and R = 280 R_sun_ and M = 16 M_sun_. The mass-loss rate was derived from the P Cygni type Hα- profile to 8 10^- 6^M_sun_yr^-1^ which is small but not exceptional for an LBV (cf. e.g. R110 of the LMC). Comparison of the present spectral energy distribution with the one of 1987 shows that the brightening in the visual is accompanied by a corresponding decrease of the satellite UV continuum; i.e. as expected for LBVs the outburst occurs under the condition of M_bol_ = const. Since the metallicity is down by 0.6 dex in the SMC, R 40 demonstrates that low metallicity does not preclude LBV outbursts.