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4 The interstellar reddening

As has been noted firstly by Liller (2000), CI Aql suffers from a significant amount of interstellar reddening. Since the most interesting parameters of a nova system depends criticially on the inferred luminosity, and consequently, the distance, an accurate determination of the colour excess and visual extinction is highly desirable. Several spectroscopic methods using absorption features originating from the interstellar matter in the line of sight exist (see, e.g. Jenniskens & Désert 1994; Munari & Zwitter 1997; Oudmaijer et al. 1997), though these methods suffer from significant limitations. For instance, the Na I D doublet provides reliable excesses only in the moderately reddened region (up to E(B-V)=0 $.\!\!^{\rm m}$4, Munari & Zwitter 1997). Some of the diffuse interstellar bands (DIBs) provide good reddenings (e.g. DIB 5849, Oudmaijer et al. 1997), while other give only rough estimates. The internal consistency is in the order of 0 $.\!\!^{\rm m}$1-0 $.\!\!^{\rm m}$2 even in the best cases, therefore, a certain amount of uncertainty cannot be exceeded.

We have surveyed all of the medium resolution spectra to identify possible DIBs taken from the list of Jenniskens & Désert (1994). We have unambiguously found DIB 5849 and 6613. Their equivalent widths (W) was measured using the IRAF task splot. The resulting values are: $W_{\rm 5849}=0.04\pm0.01$ Å and $W_{\rm 6613}=0.25\pm0.03$ Å. Unfortunately, the latter is slightly affected by a telluric line at 6612 Å, that is why its width has a larger uncertainty. Jenniskens & Désert (1994) gave the following ratios for the W/E(B-V): 0.048 (err. 0.008) for 5849 and 0.231 (err. 0.037) for 6613. The corresponding reddenings are E(B-V) $_{\rm 5849}=0\hbox{$.\!\!^{\rm m}$ }83\pm0\hbox{$.\!\!^{\rm m}$ }20$and E(B-V) $_{\rm 6613}=1\hbox{$.\!\!^{\rm m}$ }08\pm0\hbox{$.\!\!^{\rm m}$ }20$. The interstellar line Ca II 3933.66 can be also used through the empirical relationships between the width of DIB 5780 and interstellar lines (see Table 2 in Jenniskens & Désert 1994). Therefore, although we have not detected DIB 5780, we could convert the measured Ca II equivalent width to $W_{\rm 5780}$ which resulted in an E(B-V)=0 $.\!\!^{\rm m}$$66\pm0$ $.\!\!^{\rm m}$30.

In the case of CI Aql the Na I D doublet is of lower significance due to the saturation effects. Interestingly, as has been noted by the referee, there is an apparent difference between the strength of Na I D lines obtained by us and that of Greiner et al. (1996), i.e. the latter data suggest a weaker, unresolved doublet. A real difference would query the interstellar origin of this resonance line and the whole reddening estimation should be reconsidered. We attribute this phenomenon to the lower resolution of that spectrum by Greiner et al. (1996), because a close inspection of their Fig. 1 reveals a broad ($\sim$15 Å) single Na I D line. We could reproduce this kind of appearance with a resampling and Gaussian convolution of our spectrum mimicking the same resolution as quoted by Greiner et al. (about 1 Å FWHM). However, we cannot solidly exclude the possibility of other origin, e.g. some kind of circumstellar absorption. The measured equivalent widths are $W_{\rm Na D1}=0.84\pm0.02$ Å and $W_{\rm Na D2}=0.76\pm0.02$ Å. Their ratio is 1.10, far from the theoretically expected 2.0 at the lowest optical depths, but exactly what is found for the asymptotic behaviour at high reddenings (Munari & Zwitter 1997). If one checks the relation between equivalent width and reddening presented in Fig. 2 in Munari & Zwitter (1997), only a weak and approximative conclusion can be drawn as 0 $.\!\!^{\rm m}$8 <E(B-V)<1 $.\!\!^{\rm m}$5.

Further constraints on the reddening are provided by the published colour measurements. Hanzl (2000) gave B-V=0 $\hbox{$.\!\!^{\rm m}$ }69\pm0\hbox{$.\!\!^{\rm m}$ }02$ on May 7.03 UT ($\Delta t$ = +2 d), while Jesacher et al. (in Wilson et al. 2000) presented B-V=0 $.\!\!^{\rm m}$82 on May 10.97 UT ($\Delta t$ = +6 d). The B-V colour of novae around maximum tends to be about B-V=0 $\hbox{$.\!\!^{\rm m}$ }23\pm0\hbox{$.\!\!^{\rm m}$ }06$ with a significant dispersion of $\sigma=0\hbox{$.\!\!^{\rm m}$ }16$ (Warner 1995). The resulting reddening lies between 0 $.\!\!^{\rm m}$46-0 $.\!\!^{\rm m}$59 mag (with 0 $.\!\!^{\rm m}$06 formal error). Two magnitudes down from maximum the dispersion decreases, therefore the relation $(B{-}V)_{\rm0}^{V({\rm max})+2}\approx0\hbox{$.\!\!^{\rm m}$ }0$ can be also used. There are a few BV CCD photometric measurements in the VSNET database obtained in early June, resulting in again a reddening about 0 $.\!\!^{\rm m}$5. However, three prominent emission complexes are covered by the standard V passband (N II 5679, He I 5876/Na I D, H$\alpha $), therefore we consider these colour measurements to be heavily affected by the presence of such strong emission. It is difficult to say which reddening is more reliable. In the following discussion we adopt the unweighted mean of spectroscopic values which is 0 $.\!\!^{\rm m}$$85\pm0$ $.\!\!^{\rm m}$3 (formal error). Despite the limitations of using interstellar lines, their observational data are much less affected by the nova itself as in the case of multicolour photometry.


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