Intrinsic IR colours of normal B-type stars using the Geneva visual and ESO IR photometric systems

We present analysis of IR and visual observations of bright, normal B-type stars to derive colour-colour relationships between the visual and IR wavelength regions. The reddening-free parameters X and Y of the Geneva photometric system (Cramer & Maeder 1979) allow the calculation of precise colour excesses at visual wavelengths and de-reddened observations. We find well-defined linear relationships between the intrinsic [B - V]₀ and [V - m_λ]₀ colours, where mλ is the intrinsic near-IR magnitude at the ESO IR J, H, K, L passbands and the IRAS 12μm passband. We show that the derived visual-IR colour-colour relationships are in good agreement with those of Johnson (1966) yet are bluer than the relationships derived by Kooruneef (1983b). We attribute this to the fact that Koornneef did not account for interstellar reddening. The [V - 12μm] colour relationship is also in good agreement with that of Waters et al. (1987). Theoretical colours derived from Kurucz (1979) model atmospheres of B-type stars are also in good agreement with the near-IR passbands. However, the Kurucz models fail to reproduce the L band and 12μm magnitudes of B stars by about 8 - 12% as also found by previous authors. The results of this work will be used in a study of the properties of the near-IR excess emission in Be stars.