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Astron. Astrophys. 333, 613-618 (1998)
3. Color - color diagram
From data in Tables 3 and 4, after the IRAS fluxes are
transformed into magnitudes without color corrections according to
Cheeseman et al. (1989), different infrared color-color diagrams can
be plotted: ( ) - ( ) in
Fig. 1; ( ) - ( ) in
Fig. 2; ( ) - ( ) in
Fig. 3 and ( ) - ( ) in
Fig. 4. In those figures, the open and filled circles indicate S
stars with and without Tc, respectively, and the straight line
corresponds to the blackbody distribution.
![[FIGURE]](img18.gif) |
Fig. 1. - diagram for observed S stars
|
![[FIGURE]](img22.gif) |
Fig. 2. - diagram for observed S stars
|
![[FIGURE]](img25.gif) |
Fig. 3. - diagram for observed S stars
|
![[FIGURE]](img28.gif) |
Fig. 4. - diagram for observed S stars
|
It is obvious from Fig. 1 that although samples with Tc
distribute in a rather wider area than the ones without Tc, both
species are basically located in almost the same region. These
distributions imply that, statistically, both categories of S stars
have almost the same colors and temperatures in the near infrared, and
they can not be clearly distinguished on the basis of their near
infrared color.
It is shown from ( ) -
( ) diagram in Fig. 2 that a rather clear
segregation between Tc-rich and Tc-deficient S stars is visible by
different but not by
except for a peculiar Tc-deficient star DY Gem (it will be discussed
in more detail later). This implies that the color
is a good probe for distinguishing these two
kinds of S stars, and Tc-rich S stars really exhibit on infrared
excess in the 12 µm and 25 µm bands. They
have cooler temperature and hence are probably more evolved. On the
other hand, Tc-deficient stars have no or little excess in the 12
µm and 25 µm bands which indicates a
photospheric origin of the mid-infrared flux. It is also noted that a
few Tc-rich S stars including NQ Pup, Ori,
Cyg and HR Peg populate the same region as
Tc-deficient S stars as Jorissen et al. (1993) already pointed out. In
addition, it is seen from Table 4 or Fig. 2 that
Tc-deficient S stars with good quality 60 µm flux are not
numerous (only 7, out of 20 in Table 4). Therefore, other colors
not involving 60 µm flux must be used to show their
infrared property.
In order to include more samples than that in Fig. 2, the
( ) - ( ) diagram is plotted
in Fig. 3 from which it is obviously seen that except for DY Gem
and V Cnc (they will be discussed later), Tc-deficient S stars are
concentrated on a very small region of and
, and very close to the blackbody line, whereas
almost all Tc-rich S stars are out of this region, far away from the
blackbody line, and spread over a much wider area that corresponds to
a much larger infrared excess either in or in
colors, hence with much lower temperature and
much higher mass loss. Only four stars: Ori, NQ
Pup, HD 170970 and V679 Oph are located in the Tc-deficient sample
region. This suggests that both and
are good probes to distinguish Tc-deficient S
stars from Tc-rich ones.
The ( ) - ( ) diagram is
also plotted for the two kinds of S stars in Fig. 4. Again except
for DY Gem and V Cnc, most Tc-deficient S stars can be separated from
Tc-rich ones with color, but not with
color. It should be noted that the
( ) color has been used in Figs. 3 and 4,
although the observational results in K and 25 µm were
not obtained at the same epoch. From the previous observations in K
(Gezari et al. 1993) it is found that, on the average, the difference
between the previous data and the new ones is 0.22. This has no
serious influence on the result shown in Fig. 3. However, the
similar comparison is not possible for samples in Fig. 4 for lack
of earlier data.
The conclusion that can be drawn out from the analysis of infrared
color-color diagrams mentioned above is that the more sensitive colors
for segregating Tc-rich and Tc-deficient S stars are
and , hence the more
appropriate color-color diagram for distinguishing them is the
( ) - ( ) diagram. Chen et
al. (1995) gave more than 700 IRAS associations of S stars in their
Table 1, but more than 2/3 of the samples have no good 60
µm flux. If one wants to extract candidates of
Tc-deficient S stars according to their infrared colors, only color
is not enough. Fortunately, if K
magnitudes have been measured, by using the ( ) -
( ) diagram the candidates of Tc-deficient S
stars can be well selected despite the absence of 60 µm
flux for almost all samples.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998
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