R-40 - THE 1ST LUMINOUS BLUE VARIABLE IN THE SMALL MAGELLANIC CLOUD

During the past decade R 40 in the SMC has brightened in the visual ra nge by about 0.5 magnitudes. Correlated with the brightening the spect ral type has changed from B8Ia to A3Ia-0. 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 obser ved 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 sp ectrograph 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 a nd LWP mode in 1987. Apart from the secular brightening at a timescale of years R 40 exhibits photometric pulsation-like microvariations wit h 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 dur ing the outburst phase in 1991 by making use of Kurucz model atmospher es: 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. and M = 16 M.. The mass-loss rate was derived from the P Cygni type Halpha- profile to 8 10(-6)M. y r-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 b y a corresponding decrease of the satellite UV continuum; i.e. as expe cted for LBVs the outburst occurs under the condition of M(bol) = cons t. Since the metallicity is down by 0.6 dex in the SMC, R 40 demonstra tes that low metallicity does not preclude LBV outbursts.