6 Absorption lines

We have listed in Table 3 all absorption features appearing in our spectra. Half of them belong to well known interstellar bands, whereas the remainder is composed by lines belonging to a hot star. Among the interstellar lines we find those corresponding to NaI (M.1) and K I (M.1) which cannot be resolved into components due to lack of resolving power. We can thus not decide if they come from the interstellar or from the circumstellar medium. If the observed equivalent widths of the interstellar lines are taken at face value, we can use Herbig's (1975) relation between strength and B-V color excess to derive an estimate of the latter. From four features, we derive 0.41 $\pm$ 0.48 which speaks in favor of a rather low value of E(B-V). It should be added that the large uncertainty attached to our result is quite natural when dealing with equivalent width-color excess relations. Brosch et al. (1978) had found a value of 0.5, whereas Bergner et al. (1990) found 2.1. However all values have a rather large uncertainty attached.

Among the lines attributable to the hot component, we find lines belonging to CIII and O II. Such lines are observable in stars of spectral types between O 7-B 0.5 (C III) and O9-B3 (O II). As a compromise we adopt a spectral type of about B 0. Nothing can be said on the luminosity of the star, since both elements show a positive luminosity effect. The spectral type found agrees well with the 28 000 K star found by Bergner et al. (1990).

  

Table 3: Absorption lines present in MWC 342

Notes: The first column provides the observed wavelength, the second the equivalent width, the third the theoretical wavelength and the last the identification. IB = interstellar band; Herbig = Herbig (1975); Sanner et al. = Sanner et al. (1978).