In the seventies Cassatella et al. (1973, 1975) developed in Roma a processing technique for the analysis of OP spectra based on the use of PDS-type microphotometers. The technique was applied to investigate the OP spectra of the symbiotic nova V1016 Cyg (Baratta et al. 1974) and of other peculiar objects. Modern techniques of image processing may give a fundamental contribution to the matter, although to exploit fully the relatively high resolving power of the photographic emulsions (down to 10 m), and, simultaneously, their large collecting surface (up to a few thousand cm) one needs a very expensive hardware.
Within a National Project aiming the scientific use of old astronomical plates, we have analysed a set of OP plates obtained during 1969-1975 to follow the spectral evolution of the symbiotic nova HBV 475 (also named V1329 Cyg). Our aim is to make a photometric calibration of the plates by using a number of stars in the field with known broad-band magnitudes. Here we present the preliminary results of our work and discuss some technical aspects.
The nova-like outburst of HBV 475 was discovered by Lubos Kohoutek in an OP plate taken in August 1969 at the 80/120/240 cm Schmidt telescope of the Hamburg-Bergedorf Observatory equipped with a 4 OP (Kohoutek 1969). With the aim of investigating the spectral evolution of HBV 475 following its nova-like explosion, Kohoutek collected during 1969-1975 a set of OP plates centred on the position of the symbiotic star using 2323 cm 103aO, 103aD, and 103aE plates. Casually, in late 1970 a nova (Nova Cyg 1970) exploded in the same sky region, so that also the spectrum of the nova is recorded on several plates of the set.
The mid and low resolution 1969 spectrum of HBV 475 has been analysed by Crampton et al. (1970) and Baratta & Viotti (1989).
Then, the wavelength calibration was derived using a code purposely written for OP spectrograms running outside the IRAF environment. The resulting OP dispersion curve is: ( - 1647 Å)(s + 567 m) = 1.26 10 m Å. In the absence of a zero point in the individual spectrograms, we set s(H) = 2000 m. The reciprocal dispersion d/ds at H is 575 Å/mm. Fig. 1 (left) shows some examples of wavelength-calibrated spectrograms of A-type stars. The OP plate was obtained on 1969 September 7 on a 103aE emulsion. Fig. 1 (right) shows the spectrum of HBV 475 in two epochs and of Nova Cygni 1970 just exploded.
For the photometric calibration of the plates, we have computed the average Baker density in a wavelength interval centred near the maximum response of the Johnson B-filter, and compared with the Johnson's B-magnitudes from the Tycho-2 Catalogue (Høg et al. 2000). The results for the 1969 plate are shown in Fig. 2 (left). For 26 calibration stars we have compared their B-magnitudes with the average plate density in the wavelength interval 4150-4310 Å. The derived regression line is shown in the figure (correlation coefficient r=0.96). The response appears linear within at least three magnitudes. The blue plate contrast (d/dm) is with a r.m.s. of 0.05. This procedure can be used to estimate the blue magnitude of all the early-type stars in the field. A blue-continuum magnitude of 12.7 was derived for the symbiotic star HBV 475 (see Fig. 2). We have also used a larger wavelength interval that includes the H and H lines, and found the same result with a slightly larger dispersion. The corresponding blue magnitude of HBV 475 turns out to be 12.2, due to the large contribution of the emission lines.
The situation is more complicate for the Johnson V-band, since its response curve includes the green-yellow region where the plate sensitivity changes rapidly with wavelength. In Fig. 2 (right) we plot against V the average plate density in the wavelength interval 5400-6000 Å. It is clear in the figure that the dynamical range is smaller, with a large dispersion for the faintest stars. The visual plate contrast is about 0.33. We are going to extend the analysis to other unblended stars in the field, and derive their B and V magnitude and spectral type, as described in Nesci et al. (2003).
The next steps of our work will be the spectrophotometric calibration of the OP plates using field stars with known photometry and spectral type, to which we attribute the energy distribution of standard stars scaled to their B, V magnitudes. This will allow us to study the spectral evolution of HBV 475 in a wide energy range.
This work has made use of the SIMBAD database at the CDS (Strasbourg).
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