Astron. Astrophys. 333, 613-618 (1998)

7. Summary and conclusion

Photometric observations in the near infrared for 24 Tc-rich and 20 Tc-deficient S stars are presented in this paper. Several color-color diagrams and energy distributions including these new data together with the IRAS fluxes and some magnitudes in V or B are presented, and the IRAS Low-Resolution Spectra are also discussed. From analyses above it is concluded that Tc-rich S stars at the AGB phase have more infrared excess, hence more mass loss than Tc-deficient ones in binary systems so that one can distinguish them properly by using their different properties in the infrared. In fact, these two categories of S stars have different color especially in the 2 µm - 25 µm range, thus by using the () - () color-color diagram, the IRAS LRS classification and the pattern of energy distribution fitting they could be well separated as follows:

1. In the () - () diagram the criteria lines of segregations are and . Almost all Tc-rich S stars can be found in the upper-right area, whereas all Tc-deficient S stars are located in the lower-left area. As the () - () diagram seems to be more efficient to split off the two categories of S stars, the forthcoming two-micron data that will come out of 2 MASS and DENIS, in combination with the corresponding IRAS data, will soon offer new opportunities to identify new S stars of both types.
2. A S star in LRS class E is likely to be Tc-rich, whereas candidates of Tc-deficient S stars can be selected from S and F groups in the LRS classification.
3. A S star whose energy distribution can be fitted by a double blackbody curve is likely to be Tc-rich, whereas candidates of Tc-deficient S stars can be found from sources whose energy distribution can be fitted by a single blackbody curve.

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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