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5. Conclusions

Although, in most cases, based on near infrared data alone, it is not possible to determine the nature of a given source, the combination of our near infrared photometry with the properties observed in the far infrared and a few other additional criteria provides essential information which has been used to determine the nature and evolutionary stage of a sample of unidentified IRAS sources with far infrared colours similar to those observed in well known PNe.

Single near infrared counterparts have been detected in 80% of the IRAS fields searched. For these positive detections, we have determined the origin of the near infrared emission observed according to whether this is mainly stellar, nebular or due to hot dust (or a combination of them). As shown in Fig. 4 (click here), their distribution in the near infrared two-colour diagram is quite different to that shown by the group of well identified objects in our sample plotted in Fig. 1 (click here). However, in Fig. 5 (click here) we see that there is a very good agreement between the colours observed in the far infrared for the new objects found and those expected for each class, as determined from previous surveys in the literature, which confirms the consistency of the criteria used.

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Figure 4: J-H vs. H-K two colour diagram where we show the position occupied by the observed IRAS sources once they have been classified according to the criteria given in the text

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Figure 5: IRAS two-colour diagram where we show the position of the observed IRAS sources, once they have been classified according to the criteria given in the text

The percentage of young stars found (25%) is only slightly larger than that previously observed in the sample of well identified IRAS sources, and the new objects found show a very similar distribution in the near infrared two-colour diagram. T-Tauri and Herbig Ae/Be stars usually show stellar-like emission with a moderate near infrared excess which is attributed to the presence of circumstellar disks. Other young stellar objects, such as deeply embedded compact H II regions and Herbig-Haro objects, are strongly obscured and their near infrared colours are consistent with a black-body emission at temperatures betwen 800 and 1500 K.

Only two possible galaxies, both too faint in the near infrared to be detected, were found among the unidentified IRAS sources in our sample. This is explained by the fact that only a small number of bright active galaxies satisfy our selection criteria and most of them were already included in the sample of very well known objects in the literature.

Among the new IRAS sources, we found a very low percentage of PNe (13%), compared to the 49% observed in our sample of well identified objects. In addition, most of them show peculiar near infrared colours, which are only observed in very young and dusty PNe. The main contribution to the near infrared emission observed in these PNe comes from their central stars, sometimes affected by a strong circumstellar reddening, and/or from hot dust present in the envelope.

In contrast, we find a very large number of transition objects in the late-AGB or in the post-AGB stage (61%). They show a wide variety of near infrared colours. Late-AGB stars are always heavily obscured by their thick circumstellar shells, as expected. Among the stars already in the post-AGB stage we find both, heavily obscured objects (with or without OH maser emission), and optically bright stars. Some of them show a near infrared excess which is interpreted as the consequence of recent post-AGB mass loss.

Unfortunately, it is not possible just from the relative numbers of post-AGB stars and PNe found to estimate the lifetime of the post-AGB evolutionary phase. First, because it is not clear whether all post-AGB stars become observable PNe and second, because it is clear that the use of IRAS data produces a strong selection effect which favours the detection of post-AGB stars, which sometimes can only be recognized through the analysis of their far infrared emission. OH/IR stars, well evolved PNe and galaxies are relatively easy to discover in other spectral ranges, because of their strong molecular maser emission, rich emission line optical spectra or the presence of radio continuum emission.

One of the most interesting results obtained is the detection of a relatively large sample of post-AGB stars in a very early stage, still heavily obscured in the optical. Previous surveys have always been biased towards the search for optically bright post-AGB stars with intermediate spectral types, which are probably the result of the evolution of low mass stars which might never become PNe. Among these heavily obscured post-AGB stars we expect to find rapidly evolving massive post-AGB stars which may be the true progenitors of PNe. This possibility is supported by the discovery of a few heavily obscured transition objects already showing the presence of emission lines in their optical spectra and/or radio continuum emission. The detection of their near infrared counterparts is the first step needed for further studies, which are already in progress, including ISO observations in a few cases. This may be crucial to understand the short transition phase which precedes the formation of a PN.

Acknowledgements

This research has made use of the Simbad database, operated at CDS, Strasbourg (France) and it was partially funded through grant PB94-1274 from the Spanish Dirección General de Investigación Científica y Técnica (DGICYT). PGL is the recipient of a Grant from the Spanish Ministerio de Educación y Ciencia.


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