3 Results

The principal morphological features of the light curve of V705 Cas can be described well on the basis of long-term visual brightness estimates displayed in Fig. 3. These data represent a quasi-regular coverage of an observational interval of more than 1000 days following the outburst. The light curve is characterized by a moderately rapid decline from the maximum brightness which is interrupted after approximately 50 days by a very abrupt decline to a minimum at the level of about 16.1 mag which occurred on day 103 after the maximum. Since then the brightness started to increase again and it can be assumed that by day 165 after the outburst the phase of the formation and development of the dust envelope had already been completed. The brightness of the nova on the shorter time scales appeared to be quite stable afterwards, exhibiting only relatively minor fluctuations of 0.5 mag at most. Nevertheless, the overall brightness of V705 Cas started to decline again soon. The nova has become fainter by 1.8 mag (it declined slowly from 12.2 mag to 14.0 mag) during about 1000 days which have elapsed since the formation and development of the dust shell were completed.

In order to study the phase of formation and development of the dust shell in more detail, we have analyzed the temporal change of the value of the (U-B) index (Fig. 4). The variation of the index since the brightness maximum is characterized by a sharp decrease which can be associated with the heating of the nova's expanding shell. On day 33 after the maximum, the value of the index has become erratic. Then, on day 47 after the maximum, it started to exhibit an unambiguous sharp increase. Such a behaviour can be attributed to the cooling of the expanding shell and especially to the formation of the dust component of the shell. This interpretation is consistent with the behaviour of the UBV photoelectric light curves of the nova which commonly exhibited an abrupt decline in all three colours at about 50 days after the maximum.

We have used the photoelectric data to determine the basic physical parameters of the nova. At first, we eliminated the data spanning the formation and development of the dust shell from the photoelectric light curves during the data processing. Then, we fitted the remaining data with polynomials of the fourth degree, in order to be able to determine the overall trend of the brightness decline more accurately. Thereby we obtained the following values: t_2,B = 40 d, t_2,V = 33 d, t_3,B = 63 d, t_3,V = 61 d, with the error of $\pm 1$ day. We have calculated the absolute magnitude of the nova at maximum in the B colour according to the empirical relations derived by Pfau & Livio (1992). This has provided us with the relevant values of $M_{B}= (-7.43 \pm{0.20})$ mag and $M_{B}= (-7.27\pm{0.20})$ mag, respectively. As for the V colour, we have used the relation published by Schmidt (1957) and obtained the value $M_{V}= (-7.29 \pm{0.22})$ mag, as well as that by Della Valle & Livio (1995) and obtained the value $M_{V}= (-7.34 \pm{0.20})$ mag. In order to be able to advance further and select the best value of the M_B, one needs to perform a short discussion of the methods used. Livio (1996) has compared the individual statistical calibrations for the determination of the absolute brightness of a nova at maximum and showed that the linear relation proposed by Pfau (1976) is fitting the data file worse than more modern Livio's relation. On this basis, we have decided to use in further calculations the value $M_{B}= (-7.27\pm{0.20})$ mag as the mean one and the value determined according to Pfau (1976) as the lower limit for the distance determination of the nova. Nevertheless, we realize that in order to be as exact as possible one should limit the application of these methods only to the use of fits by Della Valle & Livio (1995) and Livio (1996) in the future.

For our further calculations, we have used $M_{V}= (-7.32\pm{0.22})$ mag, representing the mean of two values of this parameter presented above. The relation by Livio (1992) has enabled us to calculate the mass of the white dwarf component of the nova system responsible for the outburst of V705 Cas. We have obtained $M_{\rm WD}= (0.79\pm{0.06})$$M_{\odot}$. This mass of the white dwarf component then yielded the corresponding Eddington luminosity $M_{\rm Edd}= (-6.49\pm{0.06})$ mag. Using the bolometric correction B.C. = -0.28, (Livio 1994), we derived the Eddington luminosity in the V colour, $M_{V, {\rm Edd}}= (-6.21 \pm{0.06})$ mag. Comparing this value with the empirically determined value of $M_{V}= (-7.32\pm{0.22})$ mag. In fact, Starrfield (1988) published the value of Eddington limit for slow novae as $M_{\rm bol}= -7$ mag, on the other hands the models by Kato & Hachisu (1994) and by Kato (1994) reduce the Eddington luminosity at corresponding value of $M_{V,{\rm Edd}}= -6$ mag. Thus we can state that the outburst of nova V705 Cas was probably a super-Eddington one.

Using $V_{\rm max}= 5.53$ mag and $B_{\rm max}= 6.14$ mag (Munari et al. 1994b) and the intrinsic (B-V) index (0.05 mag), the colour excess at brightness maximum for intermediate values E_B-V= 0.56 mag and the corresponding visual extinction A_v= 1.74 mag. Using then the well-known distance modulus relation, the distance to V705 Cas $r= 1.67 \pm{0.34}$ kpc. We have deduced the error of the distance determination as follows: the extinction determined on the basis of the value of $M_{B}= (-7.43 \pm{0.20})$ mag according to Pfau (1976) is A_v= 2.23 mag and the corresponding distance is r= 1.33 kpc. We consider this value to be the lower limit of the distance to the nova and the difference between it and the mean distance to be the value of the error. Within the upper limit of the distance values interval created in such a way, there is the value determined through the method according to Miroshnichenko (1988). We consider this value to represent the upper limit of the distance determination (see Discussion).