Physical parameters of erupting luminous blue variables: NGC 2363-V1 caught in the act

A quantitative study of the luminous blue variable NGC 2363-V1 in the Magel lanic galaxy NGC 2366 (D = 3.44 Mpc) is presented, based on ultraviolet and optical Hubble Space Telescope STIS spectroscopy. Contemporary WFPC2 and W illiam Herschel Telescope imaging reveals a modest V-band brightness increa se of similar to0.2 mag per year between 1996 January-1997 November, reachi ng V = 17.4 mag, corresponding to MV = -10.4 mag. Subsequently, V1 underwen t a similar decrease in V-band brightness, together with a UV brightening o f 0.35 mag from 1997 November to 1999 November. The optical spectrum of V1 is dominated by H emission lines, with Fe II, He I and Na I also detected. In the ultraviolet, a forest of Fe absorption features and numerous absorpt ion lines typical of mid-B supergiants (such as Si II, Si III, Si IV, C III , C IV) are observed. From a spectral analysis with the non-LTE, line-blank eted code of Hillier & Miller, we derive stellar parameters of T-* = 11 kK, R-* = 420 R., log (L/L.) = 6.35 during 1997 November, and T-* = 13 kK, R-* = 315 R., log (L/L.) = 6.4 for 1999 July. The wind properties of V1 are al so exceptional, with (M) over dot similar or equal to 4.4 x 10(-4) M. yr(-1 ) and upsilon (infinity) similar or equal to 300 km s(-1), allowing for a c lumped wind (filling factor = 0.3) and assuming H/He similar to 4 by number . The presence of Fe lines in the UV and optical spectrum of V1 permits an estimate of the heavy elemental abundance of NGC 2363 from our spectral syn thesis. Although some deficiencies remain, allowance for charge exchange re actions in our calculations supports a SMC-like metallicity, that has previ ously been determined for NGC 2363 from nebular oxygen diagnostics. Conside ring a variety of possible progenitor stars, V1 has definitely undergone a giant eruption, with a substantial increase in stellar luminosity, radius, and almost certainly mass-loss rate, such that its stellar radius increased at an average rate of similar to4 km s(-1) during 1992 October-1995 Februa ry. The stellar properties of V1 are compared to other LBVs, including eta Car and HD 5980 during its brief eruption in 1994 September, the latter new ly analyzed here. The mass-loss rate of the HD 5980 eruptor compares closel y with V1, but its bolometric luminosity was a factor similar to6 times lar ger.