The intrinsic colours of stars in the ultraviolet

Abstract

Twenty-five intrinsic ultraviolet colours for normal stars, covering most spectral types and luminosity classes, between O6.5 V and K3 III have been derived from observations by the S2/68 sky survey telescope. The 25 colours, in the form (mλ–V)0, are based on absolute fluxes that cover the wavelength range 1400 to 2740 Å: (1) four 330 Å broad-band fluxes centred on 1565, 1965, 2365, 2740 Å; (2) nine 110 Ä narrow-band fluxes on 1455, 1565, 1675, 1855, 1965, 2075, 2255, 2365, 2475 Ä; (3) twelve 100 Å narrow-band fluxes centred every 100 Å between 1400 and 2500 Å. The fluxes have been corrected from the absolute calibration of S2/68 to the standard calibration of Bohlin et al. (1980) for the star η UMa. Complementary tables giving the number of stars, the mean error, (B–V)0, (U–B)0, absolute visual magnitude, and some reddening-free colours, are also given. The printed paper contains just the four broad-band colours and mean error. The accompanying microfiche contains both the broad-band and the narrow-band colours and the complementary tables. The complete set of tables is also available on magnetic tape, arranged by MK spectral type. The local interstellar extinction curve has been re-examined, using 1200 stars of spectral type B5 and earlier, and the mean values of A(λ)/E(B–V) calculated for all 25 passbands. These values are in good agreement with previous determinations and no significant variations from the mean curve, either with galactic direction or with wavelength, are found.