Spectrophotometry of the Broad Continuum Features in Magnetic AP Stars

Extract

The strongest broad absorption feature in the peculiar energy distributions of the Ap stars is that centered at about 5200 Å, thus the Stromgren y band and the Geneva VI band are most affected in stars in which this feature is strong. Fig. 1 shows bandpasses (full width at ½ intensity maximum) of three widely used photometric systems superimposed on two of our scans of Ap stars and two solar abundance line blanketed model atmospheres (Kurucz 1979). It is seen that both the y and VI bands fall entirely within the λ5200 feature. The plot (Fig. 2a) of b-y vs. Tpc (the color temperature of the red end of the Paschen continuum), shows that the b-y colors for most of our sample of Ap stars are displaced to the blue of the b-y, Teff relationship of Relyea and Kurucz (1978). In Fig. 2b, Δ(b−y) = (model b−y) − (observed b−y), for a given temperature is plotted vs. ΔWS₂(5200), a spectrophotometric index measuring the equivalent width of the λ5200 feature. There is a strong correlation between Δ(b-y) and ΔWS₂(5200), indicating quantitatively the large effect of the λ5200 feature on the y band, previously discussed by Adelman (1979). The deviations in Ap star B2-G values from the normal star B2-G vs. T curve are much less than for b-y, as the Geneva G band is largely outside the λ5200 feature (Fig. 2c). Thus B2-G is a better temperature indicator for Ap stars than is b-y (also see Hauck and North 1982).