8.2 Methodology

It can be shown from the interstellar extinction law of Rieke and Lebofsky (1985) that

(8.1)

where B and V are the wider and slightly offset, equivalent Johnson filters to Strömgren b and y filters. It should be noted that several different extinction laws have been derived (see e.g.,  He et al., 1995; Martin and Whittet, 1990; Mathis, 1990), each produces similar results, however to be consistent with Chapter 7 the value ( = 0.194) from Rieke and Lebofsky (1985) is used.

Whilst relates the extinction to B and V , the filters used were b and y, however Crawford and Mandwewala (1976) relate the two

(8.2)

Therefore it can be said that

(8.3)

The value of E(b - y)_is may now be calculated from the E(H - K)_is derived in Chapter 7. The intrinsic values (b - y)₀ are extracted from Crawford (1978) and rely on the spectral types derived in Steele et al. (1999). In Chapter 7 it was possible from previous observations at the same wavelength (see Dougherty et al., 1994) to derive a relationship between the circumstellar excess at one wavelength to that of another. This has not been possible in this chapter as no previous separation of circumstellar excess at the Strömgren wavelengths has been found, with listed excesses, in the literature, except that of Fabregat and Torrejon (1998) - whose method it is preferable to remain independent from. However it is still possible to derive the circumstellar excess from the standard three parameter relationship

(8.4)

as two of the parameters are known. Using this method it as possible to derive results of 23 members of the representative sample of Be stars. It is these results which are analysed below.