3. The distance estimate
Fig. 3 reproduces a "color-magnitude" diagram derived from the data of Table 1. As the brightest blue supergiant candidates we selected the stars: 13, 15, and 10, excluding the star 52 as a likely foreground one. Their apparent magnitudes and colors are: mag, , and .
It is known that the luminosity of the brightest blue stars in a galaxy correlates with its integral luminosity (Sandage & Tammann, 1974, de Vaucouleurs, 1978). To estimate the distance modulus we use the relation
whose parameters have been calibrated by Karachentsev & Tihkonov (1994) from the galaxies with Cepheid distance estimates and members of the nearby groups. It should be noted that our photometry of stars in the NW-area, as well as the blinking of B,V plates did not reveal brighter blue stars throughout the galaxy body. Therefore, adopting the above mentioned values of and , we derive . Based on galactic HI column density Burstein & Heiles (1984) obtained a galactic extinction 0.14 mag. However, the total colour of NGC 6503 presented in RC3 as well as the mean colour of the brightest blue stars suggest a larger extinction 4.3 0.73 mag. With the mean of two estimates, 0.44 mag, we get the modulus 28.57 mag or 5.2 Mpc.
The situation with the brightest red stars of the galaxy looks rather uncertain. For three stars (31,2,42) with colors their mean apparent magnitude amounts to , that gives us a modulus of 28.70 mag at (de Vaucouleurs, 1978). However, the standard error of the mean apparent magnitude for them is too large mag), to attach any significance to this estimate.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998