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3. Identification of C-rich star candidates

 figure247
Figure 2: [12-25] vs. K-L diagram for the 199 observed stars in the K and L bands. The triangles represent the 8 new infrared stars confirmed by spectroscopic measurements, the dots, the 12 other new carbon star candidates, the open squares the stars previously known as carbon stars through their optical spectra (GCCCS) or their IRAS low resolution spectra, the filled squares, the 2 objects which might be contaminated by a nearby star, and the crosses, the other stars. The black-body line, the areas defined by Epchtein et al. (1987) and the reddening vector corresponding to tex2html_wrap_inline985 are also drawn

 figure251
Figure 3: Red spectra of 10 objects (03118+6144, 04256+5054, 04391+4904, 04469+3228, 05391+4757, 05581+2232, 05592+4627, 06175+2347, 07152+1228, 19580+2552). The spectrum of 06175+2347 is represented to illustrate the typical features of a carbon star obtained with the same instrumentation. Some of the CN band heads as well as the NaD doublet and Htex2html_wrap_inline987 are indicated. Some of the band heads of TiO characteristics of M type star are indicated on the spectrum of 05581+2232

 figure257
Figure 3: continued

 
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Table 4: The number of observed stars according to various classifications: LRS, VH, Epchtein et al. (1987), GCCCS

The colours of the observed objects have been plotted in a [12-25] vs. K-L diagram (Fig. 2), in order to discriminate between oxygen and carbon rich stars. Following our classification method (Epchtein et al. 1987), the objects can be roughly splitted into 3 main groups. One group is located along a vertical strip at low K-L and corresponds to O-rich stars surrounded by a thin dust shell, a second group follows a sequence located above the blackbody line and corresponds to C-rich stars (region c), finally, a few objects with larger colour indices correspond to oxygen stars with thick dust shells (region o2 and o3). The numbers of objects that belong to each of these classes are given in Table 4. The brightest stars in our sample have a thin CDS (K-L<0.7). It is among them that the percentage of carbon stars is the lowest. Most of them are listed in the GSC, and their tex2html_wrap_inline1003 range approximately between 3 and 5. The 8 stars that were not observed in the L band are unlikely to be IRCS according to their location in other two-colour diagrams such as [12-25] vs. J-K. They are likely to be M stars with thin CDS.

The complete list of the 20 new IRCS candidates that we propose is given in Table 5. All of them were discovered by IRAS, but were, so far, of unknown nature, i.e. not already identified as carbon stars (not in the GCCCS, not LRS "4n"). Flag "C" in Col. 3 indicates that their carbon rich nature has been confirmed by optical spectroscopic observations (see Sect. 4). The 3 objects with LRS "0n" or "1n" included in this list have LRS of too poor quality to identify their chemical type. The 17 others have no LRS. Only, 06000+3445 has a GSC counterpart. According to our previous studies, since these objects have tex2html_wrap_inline1011 (Fouqué et al. 1992) and are not located at the edge of the class c area, their probability of being IRCS is very high (Paper I). Two IRCS 18240+2326 and 21489+5301 have a K-L slightly larger than 4 in the "4n" samples. 21489+5301 has been already pointed out as a candidate of extreme carbon star (Volk et al. 1992b, and Omont et al. 1993).

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Table 5: List of the 20 new infrared carbon star candidates


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