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Astron. Astrophys. 347, L51-L54 (1999)

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4. Discussion

The absorption features present in our spectrum of the sum (a+e) indicate a spectral type for star a between mid-G and K0, reddened by [FORMULA] = 0.5 [FORMULA]0.1. Using [FORMULA] (Dean, Warren and Cousins 1978) and the same amount of reddening [FORMULA] for stars a, b, c, d and e, we obtain the estimates, listed in the two right-hand columns of Table 1, for the de-reddened color index [FORMULA] and the corresponding ranges of spectral types using the colors computed by Bessell (1990) for Vilnius spectra.

The magnitudes listed in Table 1 correspond to the flux ratios [FORMULA] or [FORMULA] in the V-band and [FORMULA] or [FORMULA] in the I-band. According to Callanan et al. (1999), star a contributes 60% of the combined flux from the pair (a+e) at K, which gives a flux ratio [FORMULA] and a magnitude difference [FORMULA]. In the V-band we find [FORMULA]. Combining both results yields the difference in [FORMULA] color between the two stars, [FORMULA].

Assuming a similar amount of reddening for stars e and a, we derive the intrinsic [FORMULA] color and spectral type for star e as a function of the intrinsic [FORMULA] colour and spectral type assumed for star a, using the main sequence calibration of Johnson (1966), and these are shown in Table 2. These determinations are compatible with the V and I photometric results of Table 1 and with a color excess [FORMULA] = 0.5 [FORMULA]0.1. Assuming absolute visual magnitudes of [FORMULA] and [FORMULA] for stars a and e (Gray 1992) and taking [FORMULA], the magnitudes of Table 1 yield distances of 2.6 and 2.5 kpc, respectively, in agreement with the assumption of equal amounts of reddening for both objects. For a galactic latitude of [FORMULA], such distances put the objects 180 pc below the galactic plane, beyond most of the absorbing layer. The low-energy X-ray absorbing column densities in the line of sight to Aql X-1 reported by Verbunt et al. (1994) and Zhang et al. (1998) correspond to color excesses [FORMULA] of 0.59 [FORMULA]0.1 and 0.49 [FORMULA]0.1, respectively (using the relation of Ryter et al. 1975). These values are compatible with those used here.


Table 2. Derived colors and spectral types for star e from assumed values for star a

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999