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Astron. Astrophys. 338, 1006-1014 (1998)

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6. Distance to the nova and absolute magnitude at maximum

Using the doublet-ratio method (Münch 1968) for the Na I D1 and D2 lines (Fig. 5 of Münch), we have a column density of Na I atoms to the nova:


from the equivalent widths of the interstellar components of Na I D1 and D2 lines, where N(Na I) is the space density of Na I atoms and `d' is the distance. If we assume the space density of Na I atoms as 2[FORMULA] cm-3 (Münch 1968), the distance to the nova should be 2.95 kpc. It is difficult to evaluate the error in the distance, because there is large uncertainty in the space density of Na I atoms (see, e.g. Binnendijk 1952). The data presented in Table 5 of Binnendijk (1952) show that the mean error in the distances derived from the equivalent widths of interstellar Na I D1 and D2 lines of the objects in the Perseus region may have been about [FORMULA]%. Therefore, here we adopt a probable error [FORMULA] kpc in the distance to the nova.

A well studied open cluster NGC 457 locates within an angular distance of [FORMULA] from the present nova. Pesch (1959) estimated the distance to the cluster as 2.88[FORMULA]0.58 kpc and the interstellar reddening as [FORMULA]. Some members of the cluster have had higher reddenings such as [FORMULA] (Pesch 1959), but the higher ones may have been due to the interstellar matter in the own cluster.

An approximate formula for the distribution of interstellar extinction in our galaxy was proposed by Parenago (1948) :


where `a' is an amount of extinction in magnitude per kpc, `b' is the galactic latitude, c[FORMULA] kpc is a scale height, and `d' is distance in kpc. Since the galactic latitude of NGC 457 is [FORMULA], the amount of the extinction in this direction may be about 0.94 mag kpc-1. Using the same amount of extinction, we have an interstellar extinction to the nova ([FORMULA], d=2.95 kpc) as [FORMULA] mag. The interstellar extinction in the area No. 17 ([FORMULA]) of Neckel and Klare (1980) is about [FORMULA] mag. at 3 kpc. After the same process we have a=0.97 [FORMULA] in this area and [FORMULA] mag. for the nova, which agree with those derived from the data of NGC 457. Here we adopt [FORMULA] mag. for the nova. The absolute magnitude at the light maximum may be [FORMULA], where [FORMULA] (Fig. 1). This absolute magnitude is not rare among slow classical novae at maxima (see, e.g. Payne-Gaposchkin 1957; van den Bergh & Younger 1987).

The UBV photometric data supplied by V. Goranskij (Fig. 1), show that the light maximum in the B band occurred on 1995 December 17 by [FORMULA]. The absolute B magnitude at the maximum may have been about [FORMULA]. Using the Eq. (6) of Livio (1992), we have a mass of the white dwarf as 0.58[FORMULA]. This value is close to the lower limit of the mass of a white dwarf which performs nova's explosion (Kovetz & Prialnik 1985).

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998