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Date: Mon, 12 Apr 93 19:09:16 GMT From: gav@astro.as.utexas.edu (Gerard de Vaucouleurs) Subject: Foreground extinction in M81 field Dear Roberta: I thought this could interest you and a few others. Best regards. Gerard >From G. de Vaucouleurs (gav@astro.as.utexas.edu) Foreground (Galactic) color excess in the M81 field. To assist in the interpretation of the photometry/spectrophotometry of SN1993J, I have estimated - by comparison with the standard main sequence of Johnson and Morgan (ApJ 117, 313, 1953) - the color excess of 10 field stars for which UBV magnitudes are available. Sources: J. Neff, McDonald (ApJS 24, 421, 1972), 3 stars (N); A. Sandage, Palomar (AJ 89, 621, 1984), 4 stars (S); H. Corwin, McDonald (IAUC 5742), 3 stars (C). The range of individual values of E(B-V) is from 0.00 to +0.26, with means 0.065 (S), 0.077 (C), and 0.167 (N). For the total sample the unweighted mean is <E(B-V)> = 0.099 +/- 0.025 (n = 10, s.d. = 0.080) or, after rejection of two largest values, 0.069 +/- 0.017. With the standard Av/E = 3.3, this implies Av = 0.30 or 0.23, and A(B) = 0.43 or 0.30. For comparison, published values for the reddening in the M81 field range from A(B) = 0.43 (Jacoby et al. ApJ 344, 704, 1988) to 0.30 (RC2, 1976), 0.17 (Tonry, ApJ 373, L1, 1991), 0.16 (RC3, 1991 after Burstein and Heiles), and 0.07 (Sandage, loc.cit), for an unweighted average of <A(B)> = 0.23 +/- 0.06. These figures may be compared with the much higher total extinction quoted by R. Humphreys for the suspected precursor star.
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