S-154 in the Large Magellanic Cloud: Spectral Evolution from a Luminous Fe II Variable to a Symbiotic-like Star

The emission-line star, S-154, is a luminous variable in the Large Magellanic Cloud. Between 1988 February and December, S-154 changed its spectral appearance from a low-excitation "Fe II star" into a high- excitation state that resembles symbiotic stars. Before the transition (1984-1988), the spectral features had been primarily confined to emission lines of H and Fe II; afterward, we observed emission from He I, He II, O III, and the λ6830 band. This transition is reminiscent of spectral variations observed in RX Puppis, but the spectrum of S-154 out to 1 micron shows no evidence of the Mira variable contained in that symbiotic star. The simplest interpretation for the spectral evolution of S-154 is the optical thinning of the nebula. This study implies that spectral type of the central star(s) does not always dominate the physical conditions in the circumstellar material and thereby determine the nebular classification. The membership of S-154 in the LMC was confirmed with a radial velocity measurement of +274 km s^-1^, enabling the measurement of absolute luminosity. The historical light curve (1880- 1990) obtained from 346 photographic plates of the Harvard Plate Library exhibits ~4 mag of variations, with a range of - 6 < M_B_ < -2. There is no evidence of coherent modulations that would represent the orbital period of a symbiotic binary. We originally investigated S-154 as an optical candidate of a HEAO 1 X-ray source on grounds of positional coincidence. However, efforts to confirm the X-ray emission with EXOSAT were negative, and the status of S-154 as an X-ray source remains unclear.