Question

Vega is one of six stars that was used to set the UBV photometric system and as such has U-B = B-V = 0.0. Vega is a bright (V = +0.03) blue-white star with a surface temperature of 9600 K and a CU-B value of -0.87.

Estimate the colour index U-B for Vega.

Show your working and compare your answer with the value of -0.87.

Answer 1

Assuming V=+0.03 and U-B = B-V = V-R_c = R_c-I_c = 0.00, we find for the A0V star Vega the following colour index:

    g   = -0.08  (±0.03)
    u-g = +1.02  (±0.08)
    g-r = -0.25  (±0.03)
    r-i = -0.23  (±0.02)
    i-z = -0.17  (±0.02)

where we used the Bilir, Karaali & Tunçel (2005) transformation for g and the Rodgers et al. (2006) transformations for the u-g, g-r, r-i, and i-z colors.

• a star with (U-B) < 0 is bluer than Vega
• a star with (U-B) > 0 is redder than Vega
• So a star having U-B value -0.87 is bluer than vega.