The IR colour is defined as
[ν1] - [ν2] = - 2.5 log[f (ν1)/f (ν2)],
where ν1 and ν2 are any two wavebands and f (ν1)
and f (ν2) are the corresponding flux densities assuming a flat
energy spectrum for the source.
In Fig. 1, we have plotted the [25]–[60] colours of RV Tauri stars
against their corresponding [12]–[25] colours derived from the
IRAS data. Filled circles represent stars of group A and open circles
stars of group B. The two sets of near-parallel lines represent the
loci of constant inner shell temperature T0 and the quantity Q
defined above. The models correspond to the case of absorption
efficiency
Qabs(ν) varying as ν (with γ = 1 and
hence
β = - 0.4). We have omitted R Sct in Fig. 1 because it shows a
large deviation from the average relation shown by all the other
objects. R Sct has a comparatively large excess at 60 μm, but the
extent of a possible contamination by the infrared cirrus (Low et
al. 1984) is unknown. Goldsmith et al. (1987) found no evidence
of the presence of a dust envelope at near-IR wavelengths and the
spectrum was consistent with a stellar continuum. This explains why R
Sct lies well below the mean relation shown by stars of groups A and C
between the [3.6]–[11.3] colour excess and the photometrically
determined (Fe/H) (Dawson 1979). R Sct has the longest period of
140 d among the RV Tauri stars detected at far-infrared wavelengths
and does not have the 10-μm emission feature seen in other objects
(Gehrz 1972; Olnon & Raimond 1986). R Sct is probably the most
irregular RV Tauri star known (McLaughlin 1932).
Figure:
Plot of [25]–[60] colours of RV Tauri stars against their
[12]–[25] colours after normalizing as indicated in Beichman et al.
(1985b). Some of the objects are identified by their variable-star
names. Typical error bars are shown in the bottom right-hand corner.
The lines represent the loci for constant inner shell temperature and
the quantity Q. Note the separation of group A and B stars at
T0∼ 460 K. Positions occupied by a sample of carbon and oxygen
Miras are also shown. The Q = 1.0 line differs from the blackbody line
by a maximum of ∼0.05.
(Walnut Creek)(1995).ISO/macros/latex209/contrib/mnras/mnsample.tex/img8.png) |
The inner shell temperatures (T0) derived for the various objects
are also given in Table 1 and we find the majority of them to have
temperatures in the narrow range 400–600 K. If the dependences of
Qabs(ν) on ν and ρ(r) on r are similar in all
the objects considered, then in the colour–colour diagram they all
should lie along a line corresponding to different values of T0 and
in Fig. 1 we find that this is essentially the case. In view of the
quoted uncertainties in the flux measurements, we cannot attach much
significance to the scatter in Fig. 1.
At 100 μm the infrared sky is characterized by emission, called
infrared cirrus, from interstellar dust on all spatial scales (Low et
al. 1984), thereby impairing the measurements at far-infrared
wavelengths. In Fig. 2, we have plotted the [60]–[100] colours of the
six RV Tauri stars detected at 100 μm against their [25]–[60]
colours, along with the grid showing the regions of different values
for inner shell temperature T0 and the quantity Q, as in Fig. 1.
The results indicated by Fig. 2 are consistent with those derived from
Fig. 1. AR Pup shows a large excess at 100 μm but, in view of the
large values for the cirrus flags given in the catalogue, the intrinsic
flux at 100 μm is uncertain.