Central Bureau for Astronomical Telegrams
IAUC 7290: S Dor; GRB 990712
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Circular No. 7290
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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S DORADUS
P. Massey, Kitt Peak National Observatory, reports a dramatic
change in the spectrum of S Dor, the archtype of luminous blue
variables (LBVs): "Spectra (resolution 0.1 nm; range 410-470 nm)
taken with the 1.5-m telescope at Cerro Tololo Interamerican
Observatory (CTIO) on Oct. 21, 23, and 24 superficially resemble
those of an extreme F-type supergiant, with the strong absorption-
line 'pseudo photospheric' spectrum characteristic of an LBV at
visual maximum. H-beta is in emission, and H-gamma and H-delta are
in absorption, superposed upon broad emission; otherwise the
spectrum contains only absorption lines. The spectrum is very
similar to that observed in eta Car during an outburst in 1893
(Walborn and Liller 1977, Ap.J. 211, 181), and that currently seen
in the Hubble-Sandage (1953, Ap.J. 118, 353) LBV star 'B' = V0330
in M33 (Massey et al. 1996, Ap.J. 469, 629, fig. 8). However, such
a spectral appearance of S Dor has not been previously reported in
past decades of study. Rather, the optical spectrum has always
been described as being dominated by emission when the star has
been in a low state, or by P-Cyg lines when the star has been in a
high state (cf. Thackeray 1964, MNRAS 129, 169; Wolf and Kaufer
1997, in Luminous Blue Variables, ASP Conf. Ser. 120, 26). For
instance, a spectrum (range 375-510 nm) obtained by me three years
ago with the CTIO 1.5-m on 1996 Oct. 28 shows a rich assortment of
emission lines comprised primarily of hydrogen and singly-ionized
metal lines, with only the upper Balmer lines and Mg II 448.1-nm
in absorption, similar to that observed in AF And (in M31), and the
Hubble-Sandage variable star 'C' (V0268) in M33, which are usually
considered examples of 'quiescent' LBVs. The strong evolution of
the spectral appearance of S Dor corresponds to only a modest
change in its photometry, with somewhat redder colors, and a small
increase in visual luminosity. I obtained UBV CCD data nearly
simultaneously with the spectroscopy using the CTIO 0.9-m (1996
Nov. 1) and Curtis Schmidt (1999 Oct. 25) telescopes; the star has
changed from V = 9.43, B-V = +0.18, U-B = -0.85 to V = 9.16, B-V =
+0.28, U-B = -0.59. S Dor clearly merits further monitoring during
the current Magellanic Cloud observing season."
GRB 990712
Corrigendum. On IAUC 7225, line 7, for (IAUC 7721) read
(IAUC 7221)
(C) Copyright 1999 CBAT
1999 October 26 (7290) Daniel W. E. Green
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