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Astron. Astrophys. 324, 457-460 (1997)

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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: [FORMULA] mag, [FORMULA], and [FORMULA].

[FIGURE] Fig. 3. The V vs ([FORMULA]) diagram for the measured stars.

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 [FORMULA] and [FORMULA], we derive [FORMULA]. Based on galactic HI column density Burstein & Heiles (1984) obtained a galactic extinction [FORMULA] 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 [FORMULA] 4.3 [FORMULA] 0.73 mag. With the mean of two estimates, 0.44 mag, we get the modulus 28.57 mag or [FORMULA] 5.2 Mpc.

The situation with the brightest red stars of the galaxy looks rather uncertain. For three stars (31,2,42) with colors [FORMULA] their mean apparent magnitude amounts to [FORMULA], that gives us a modulus of 28.70 mag at [FORMULA] (de Vaucouleurs, 1978). However, the standard error of the mean apparent magnitude for them is too large [FORMULA] mag), to attach any significance to this estimate.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998