The optical and IR properties of southern galactic unclB[e] stars: The case of CD-42 11721

(abridged) In this work we present the results of an optical and IR study for the curious B[e] star CD-42 11721. This object has a doubtful evolutionary stage, being either a pre-main sequence star (HAeB[e]) or an evolved star (sgB[e]). Our optical investigation could be splitted in two parts, a qualitative study based on the identification of the numerous emission lines present in the spectra and the classification of their line profiles, which indicate a non-spherically symmetric circumstellar environment, and a quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [NII] and [SII]. Assuming a typical circumstellar scenario for a sgB[e], i.e. a fast, low-density polar wind and a slow, high-density disc forming equatorial wind, we can reproduce very well the line luminosities of the forbidden lines. From this analysis, we can determine the mass loss rate of the star lying in the range from 4.4 x 10^(-6) to 2.2 x 10^(-5) M_sun/yr, depending on the considered redenning. Our IR study could also be splitted in two parts: the identification of several features in the SWS ISO spectrum, and the modeling of the SED of CD-42 11721. The first part shows the presence of a mixed chemistry, i.e. C- and O-rich dust in the same circumstellar medium. We have tried to model the SED, by using a numerical code written by us which considers a spherical circumstellar scenario. Although the answer concerning the evolutionary stage of CD-42 11721 is still not very clear, we believe that our analysis will improve the discussion about the nature of this curious star.