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