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 = 0.5 0.1. Using (Dean, Warren and Cousins 1978) and the same amount of reddening 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 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 or in the V-band and or 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 and a magnitude difference . In the V-band we find . Combining both results yields the difference in color between the two stars, .
Assuming a similar amount of reddening for stars e and a, we derive the intrinsic color and spectral type for star e as a function of the intrinsic 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 = 0.5 0.1. Assuming absolute visual magnitudes of and for stars a and e (Gray 1992) and taking , 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 , 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 of 0.59 0.1 and 0.49 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
© European Southern Observatory (ESO) 1999
Online publication: June 6, 1999