6. Individual samples
A very peculiar star is DY Gem that has no Tc in its spectrum (Smith & Lambert 1990), however it is located in the Tc-rich area in () - (), () - () and () - () diagrams as showing in Figs. 2, 3 and 4, respectively. Its LRS classification is F that indicates little infrared excess in about 7-23 µm. This is consistent with its energy distribution in Fig. 5a showing a single blackbody feature. Groenewegen (1993) pointed out that it also populates the Tc-rich region in the color , and considered that one should not put too much weight on the "non detection" of Tc in DY Gem, since its spectrum is very complicated and difficult to measure. Therefore Groenewegen proposed that DY Gem has all characteristics of a true AGB star and is probably in the TP-AGB phase.
Another peculiar Tc-deficient star is V Cnc that populates the Tc-rich regions in the () - () and () - () diagrams as shown in Figs. 3 and 4, respectively. Groenewegen (1993) considered that V Cnc is a Mira with V magnitude variation from 7.5 to 13.9 in the GCVS and in the SAO, and if has been taken then its is very close to the value of Tc-deficient sample. However as shown above, it still presents Tc-rich property in the infrared colors, therefore future investigations should be done for V Cnc.
Among the Tc-rich sample, Ori (with the LRS in group S) and NQ Pup (with the LRS in group I) populate the same regions as Tc-deficient stars in the () - () and () - () diagrams, and HR Peg (with the LRS in group S) is also in the same region as Tc-deficient stars in the () - () diagram. These three stars are examples of Tc-rich S stars with photospheric feature. In addition, Ori is known to have a WD companion (Ake & Johnson 1988), and this star may be an example of an extrinsic S star that just entered the TP-AGB phase as suggested by Jorissen & Mayor (1992).
It is surprising that a Tc-rich star, Cyg is located in the Tc-deficient area in the () - () diagram as also pointed out by Jorissen et al. (1993), because Cyg has the LRS classification of E, and is well fitted with a typical double blackbody mode for its energy distribution in Fig. 6a. Further investigation is also needed for this star.
Finally, two Tc-rich stars, HD170970 and V679 Oph are also distributed in the Tc-deficient area in the () - () diagram. However none of the stars have a LRS spectrum nor good fluxes at 60 µm and 100 µm. Jorissen et al. (1993) considered HD170970 as a possible spectroscopic binary system. More information for these two stars is needed.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998