Observational data and reduction

To make the atlas, two spectra were selected that were taken with the CCD echelle spectrometer ``LYNX" placed at the Nasmyth focus of the 6 m telescope (Panchuk et al., 1993). The main spectrum was obtained on December 12, 1995 near the maximum light with a CCD of ${\rm 1040\times 1160}$ pixels. It was complemented with one closest in time (taken on December 24, 1994 with a CCD of ${\rm 520 \times 580}$ pixels) from those we had available. The V magnitudes of the object which corresponded to those moments were 10.1^m (from Doroshenko, 1996) and 10.4^m (from Tomov et al., 1996). From 35 orders of the main spectrum 28 of the most informative were taken for the atlas, which represented without gaps a wavelength region of 4780 Å to 6580 Å. From the complementary spectrum 11 orders, 40-45 Å each, between 5660 and 6470 Å were entered in the atlas. The quality of the resultant spectrum was not uniform. The summation of the overlapping portions brought the signal-to-noise ratio in them to 80-100, however, in the portions represented by one spectrum, especially at the red boundary of the atlas, it is markedly lower. The spectral resolution limit is close to 0.3 Å.

Following the standard procedures of taking account of the scattered light, removal of cosmic particle traces and extraction of the spectra from the images, the spectra were normalized to the continuum and reduced to the scale of laboratory wavelengths. The dispersion curves were plotted separately for each order immediately from emission lines of the object. In so doing lines were selected which yielded radial velocities close to the symbiotic velocity of MWC 560 (+35 km/s). The averaging of the overlapping spectrum orders was performed on the wavelength scale.

The visible spectrum of MWC 560 is poor in absorption lines especially near the brightness maximum. In our atlas are visible only the blue-shifted components of $\rm H_{\beta}$, $\rm H_{\alpha}$, NaI and FeII(42), interstellar lines NaI, a few diffuse interstellar bands (DIB) and telluric lines of $\rm H_2O$ and $\rm O_2$ and, at length, traces of the spectra of the M giant. To reveal these traces, the spectrum of $\beta$ Peg M2.5 II-III obtained with the same resolution as the spectrum of MWC 560 and the list of absorption lines identified in it (Davis, 1947) were taken.

Principal attention was given to the identification of the emission spectrum. The following elements and ionization stages (species): HI, CI, NI, [NI], [OI], NaI, MgI, SiII, CaI, CaII, ScII, TiI, TiII, VII, CrI, CrII, MnII, FeI, FeII, [FeII], YII, ZrII, BaII, LaII, CeII, PrII, NdII have been revealed, which are mostly present in the blue part of the spectrum of MWC 560 (Kolev and Tomov, 1993) too.

 

Figure: Residual intensities of FeI emission in the spectrum of MWC560 vrs oscillator strengths. Filled squares denote the lines with excitation potentials of the upper level 3.3, 5.5 and 6.6

The identification was performed in two steps. At first the whole set of lines within the boundaries of the atlas with the wavelengths and oscillator strength accessible to us was compared alternatively for each of the species listed. For instance, for FeI the table of Nave et al. (1994) was used, for FeII the data of Johansson (1978) and Boyarchuk and Savanov (1986), while for [FeII] the data of Quinet et al. (1996). The closeness of the tabulated and observed values not only for the wavelength but also for the relative intensity of a line was considered a condition for reliable identification. About one-third of all emission lines were used in measuring radial velocities and plotting dispersion curves. For these the departure of the measured wavelengths from the tabulated ones does not exceed $\pm0.04$ Å. In the rest of the cases the wavelengths were estimated from the atlas, and discrepancies as large as $\pm0.1-0.2$ Å were permissible. The relationships between the tabulated gf values and the observed residual intensities, r, were presented in a diagram form. As is seen in Fig. 1, where an example of r(lggf) dependence for FeI is given, allowance for the excitation potential makes the relationships clear enough, which makes it possible to correctly refer a line to a given species. Subsequently our spectra were compared with the like and well described spectra of real objects to check that the identification was correct. As a whole, the spectrum of MWC 560 is close to the spectra of the symbiotic stars XX Oph (Merrill, 1951) and PU Vul (Belyakina et al., 1985; Iijima and Ortolani, 1984) as well as to the spectrum of the solar chromosphere (Pierce, 1968; Kastner, 1995). Its individual species are well represented in the spectra of T Tauri stars (Appenzeller et al., 1986) and $\eta$ Car (Thackeray, 1967).