On the effect of helium enhancement on bolometric corrections and Teff-colour relations

We evaluate the effects that variations in He content have on bolometric corrections and T-eff-colour relations. To do this, we computed ATLAS9 model atmospheres and spectral energy distributions for effective temperatures ranging from 3500 K to 40 000 K for dwarfs and from 3500 K to 8000 K for giants, considering both "He-non-enhanced" and "He-enhanced" compositions. The variations in He content that were considered are of Delta Y = +0.1 and +0.2 for the metallicity [M/H] = +0.5 and Delta Y = +0.1 for [M/H] = -0.5 and -1.5. Synthetic photometry was then carried out for the UBVRIJHK system. We conclude that the changes in bolometric corrections, caused by the adopted He-enhancements are in general too small (less than 0.01 mag), for both dwarfs and giants, to be affecting present-day tables of bolometric corrections at a significant level. The only possible exceptions are found for the U-band at T-eff between 4000 K and 8000 K, where \Delta BCU\ amounts to similar to 0.02 mag, and for T-eff equal to 3500 K, where \Delta BCS lambda\ values clearly become much higher (up to 0.06 mag for passbands from U to V). However, even in the latter case the overall uncertainty caused by variations in the He content may be not so significant, because the ATLAS9 results are still approximative at their lowest temperature limit.