All Figures

Figure 1: The equivalent widths as a function of temperature for three photospheric absorption lines of early-type supergiants. Circles indicate stars with emission lines in their spectra, crosses stars with no emission. Note that the emission and non-emission stars follow the same relation. The uncertainties are of order the size of the plotting symbols and are not shown. The temperatures of the spectral types were obtained from Schmidt-Kaler (1982).

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In the text

Figure 2: HR diagram illustrating the temperature of each LBV at the epoch of observation. Filled and open circles show the temperature determined at observation for LBVs and candidate LBVs respectively, with the solid lines showing the error in each measurement. The luminosities, and minimum and maximum observed temperatures were collated from the literature by Smith et al. (2004). The Humphreys-Davidson limit is marked by a dashed line. Car, Hen 3-519 and P Cyg are marked on the plot despite the fact that their temperatures could not be determined - values are taken from Smith et al. and Cox et al. (1995).

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In the text

Figure 3: Left, centre: polarization spectra of AG Car from the two dates observed. The bottom panel of each triplot shows the Stokes I, or intensity spectrum; and the middle and top panels show the degree and PA of the polarization respectively as a function of wavelength. The data has been rebinned to the 1 error indicated above each plot. The 6/5/94 data displayed erratic behaviour around the absorption component and line-centre due to reasons explained in Sect. 2, and was edited out of this plot for clarity. Right: multiple spectropolarimetric measurements of AG Car plotted in Q-U space. Circles are continuum points from SL94; the cross is from archival data; and the square is our continuum data. Dotted lines show the excursions to the H line-centre polarization. Note that the 1994 excursion was attenuated due to fluctuating data (see section on AG Car), and that SL94 averaged their three line-centre polarizations together. The mean of the line-centre polarizations is marked on the plot, with a 1rms error of the size of the symbol. Uncertainties in the continuum are limited by the external errors, of order 0.1%.

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In the text

Figure 4: Left, centre: polarization spectra of HR Car from the two dates observed. The panels show the same as Fig. 3, and the data has been rebinned to a 1 error of 0.05%. Right: multiple spectropolarimetric measurements of HR Car, plotted in Q-U space. The cross marks the continuum polarization at H for the 1992 archival data; the circle for the data taken by Clampin et al. (1995); and the square for our data. The dotted lines show the excursions to the line-centre polarization. The mean of the line-centre polarizations is marked, with a 1 rms error of the size of the symbol. Uncertainties in the continuum are limited by the external errors, of order 0.1-0.2%.

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In the text

Figure 5: Top: the polarization spectrum of Car. The panels show the same as Fig. 3. The data has been rebinned to a 1 error of 0.1%. The dotted line shows a magnification of the Stokes I spectrum to highlight the broad wings and weaker spectral lines. Bottom: the spectrum plotted in Q-U space. The arrows join points of increasing wavelength. The location of the continuum is marked "cont.''. The distinct excursions mentioned in the text are indicated with dotted lines. The excursion associated with the wings is marked "1''; the excursion associated with the narrower region containing the bulk of the line-flux is marked "2''; and the loop associated with the narrow, unresolved nebular emission is marked "3''.

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In the text

Figure 6: Left, centre: the polarization spectra of Hen 3-519 from 6/5/94 and 10/2/03 repectively. The panels show the same as Fig. 3 and the data has been rebinned to the 1 error indicated above each plot. Right: the polarization spectrum of the 10/2/03 data plotted in Q-U space. Arrows join points of increasing wavelength. The distinctly separate continuum and line-centre regions have been marked, as well as the loop corresponding to the depolarization seen in the blue wing.

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In the text

Figure 7: Polarization spectra of the Galactic LBVs WRA 751, P Cyg and HD 160529; and the SMC LBV R 40. The panels show the same as Fig. 5, and the data has been rebinned to the 1 errors stated above each plot. Note the different wavelength range of P Cyg. The jump in polarization seen in the H absorption component of P Cyg is attributed to the steep increase in Stokes I being unresolved, as explained in Sect. 2. HD 160529 did not display a line-effect at either epoch it was observed, and only the Feb. 2003 data is shown here.

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In the text

Figure 8: Polarization spectra of the LMC LBVs S Dor, R 71, R 110, R 116, R 127 and R 143. The panels show the same as Fig. 7. Note the different wavelength scale for R 127. Where line-effects are observed, the Stokes I spectrum is magnified to show the broad change in polarization associated with the wings of the emission line. A narrow P Cygni absorption component in the H line of R 127 was found to introduce artifacts in the polarization spectrum (see Sect. 2), and has been edited out.

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In the text

	Figure 9: Strength of depolarization against the line-to-continuum contrast of the H line. Circles show line-effects, crosses show non-detections. Where no depolarization was observed, the upper limit for an undetected line-effect is shown. For the sake of clarity, Car is not shown as its depolarization and line contrast are both much greater than any other object.
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In the text

	Figure 10: Strength of depolarization against apparent variability over the last ten years. The symbols show the same as Fig. 9.
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In the text

Figure 11: The light curve of AG Car for 1989-2004. Circles show data from the Tycho mission, crosses show validated data from the AAVSO records. The data has been rebinned to 50 days, and the error bars show the 1 standard deviations. The dates of our observation, the archive observation, plus the observations of SL94 have been marked. Also marked are the brightnesses at which the star is crossing the empirical H-D limit, and the bi-stability jump (BSJ) at 21 000 K (under the assumption that the star varies at constant bolometric luminosity). Also marked on the plot are the mass-loss rates derived by Stahl et al. (2001) (triangles).

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In the text

Figure A.1: A spectral atlas of early-type supergiants in the range 6240-6700 Å. The prominent photospheric absorption lines of SiII and HeI, which appear to be good indicators of effective stellar temperature, have been identified; and the spectra have been shifted such that these lines are at their rest wavelength. Spectral types of the stars were obtained from SIMBAD. The data in the atlas is available in electronic form at the CDS. *HD 183143 is a candidate LBV.


In the text