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Astron. Astrophys. 326, 143-154 (1997)

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5. Extinction towards the core of LH 39

The blue population of the color-magnitude diagram (Fig. 6) is centered on a color indice of V - R = 0.05, B - V = -0.07 (see also Table 2). Assuming an intrinsic color of (B - V) 0 = -0.30 for O or early-type B stars gives E(B - V) = 0.23 mag. This corresponds to [FORMULA] = 0.71 mag, or a reddening coefficient of c (H [FORMULA]) = 0.34, based on the interstellar extinction law derived by Savage & Mathis (1979 ) and R = 3.1. This result is in good agreement with Lucke's (1972 ) photometry who finds a mean E(B - V) = 0.18 [FORMULA] 0.09 (s.d.) for a sample of 14 stars belonging to the OB association LH 39 (Schild 1987 ).

[FIGURE] Fig. 6. Color-magnitude diagram of the components towards the core of the OB association LH 39 resolved by deconvolution.

The extinctions that we derive for stars #35 and #36 also fully agree with this mean value. As to #35, using an intrinsic color of V - R = -0.07 for a type B1 Ib (Johnson, 1968 ), gives E(V - R) = 0.14 mag, which corresponds to an interstellar reddening of E(B - V) = 0.18 or [FORMULA] = 0.54. For star #36 we find E(B - V) = 0.24, or [FORMULA] = 0.74 mag, using an intrinsic color indice [FORMULA].

Regarding R 84, its B - V color varied between 0.11 and 0.16 mag over a lapse of 15 nights from December 28, 1981 to January 11, 1982 (Stahl et al. 1984 ). Different values ranging from 0.04 to 0.15 were suggested for the B - V color excess of R 84 according to observations conducted at different periods between 1960 and 1990 (Feast et al. 1960 , Stahl et al. 1984 , Vacca & Torres-Dodgen 1990 , Crowther et al. 1995 ). Using Stahl et al.'s (1984 ) Aug. 1983 observations, we derive B - V = 0.16 for R 84. Comparing UV, visible, and near IR fluxes, Stahl et al. (1984 ) concluded that the late-type star contributes significantly to the flux in the V band. Therefore, one cannot directly compare the observed colors with the intrinsic (B - V) 0 of -0.30 derived by Crowther et al. (1995 ) for the W-R star in R 84. Instead, we use the measured U - B = -0.93 (Table 2) to derive the reddening of R 84. We estimated the intrinsic color (U - B) 0 from the photometric data for a sample of 7 LMC Ofpe/WN9 stars (Nota et al. 1996 ). Using Savage & Mathis's (1979 ) interstellar extinction curve, the measured B - V colors, and (B - V) 0 = -0.30 for these stars, we work out the color excess E(U - B). Then from the listed U - B data we get a mean intrinsic color (U - B) 0 = -1.09 [FORMULA] 0.07 for the sample. This parameter is then used to derive E(B - V) = 0.24 for R 84, which corresponds to [FORMULA] = 0.75 mag, in good agreement with the extinction derived above for the members of LH 39.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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