From B[e] to A[e]

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Abstract
Optical observations from 1989 of the SMC B[e] supergiant star S23 revealed the presence of photospheric HeI absorption lines. In our high-resolution optical spectra from 2000, however, we could not identify any HeI line. The observed changes in spectral behaviour of S23 lead to different spectral classifications at different observing epochs. The aim of this research is, therefore, to find and discuss possible scenarios that might cause a disappearance of the photospheric HeI absorption lines within a period of only 11 years. From our high-resolution optical spectra, we perform a detailed investigation of the different spectral appearances of S23. We further determine the projected rotational velocities of S23 in the two epochs of observations. Based on its spectral appearance in 2000, we classify S23 as A1Ib star with an effective temperature of about 9000 K. Further, an interstellar extinction value of E(B-V) = 0.03 is derived. This is considerably lower than the previously published value, which means that, if the circumstellar extinction due to the stellar wind is neglected, the interstellar extinction, and hence the luminosity of the star, are overestimated. We derive a rotation velocity of v sin i = 150km/s, which means that S23 is rotating with about 75% of its critical speed. The object S23 is thus the fourth B[e] supergiant with confirmed high projected rotational velocity. The most striking result is the apparent cooling of S23 by more than 1500 K with a simultaneous increase of its rotation speed by about 35% within only 11 years. Since such a behaviour is excluded by stellar evolution theories, we discuss possible scenarios for the observed peculiar variations in S23.
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