Radial distribution of dust

Figure: Plot of the [60]–[100] colours of RV Tauri stars against their [25]–[60] colours after normalizing as indicated in Beichman et al. (1985b). The solid lines represent the loci for constant inner shell temperature and the quantity Q. The dashed line shows the locus for a blackbody distribution.
\begin{figure*} \vspace*{174pt} \end{figure*}

From Fig. 1, it is evident that all RV Tauri stars lie between the lines corresponding to Q = 1.5 and 0.5. With

α = - (1 + Q)β - 3,

these values suggest limits of r-2.0 and r-2.4 for the dust density variation, indicating a near-constant mass-loss rate. Jura (1986) has suggested that the density in the circumstellar envelope around RV Tauri stars varies as r-1, implying a mass-loss rate that was greater in the past than it is currently. By fitting a power law to the observed fluxes, such that f (ν) varies as νq, values of q determined by him for the various objects given in Table 1 lie in the range 0.6–1.2, with a mean $\skew$5$\bar{q}$ = 0.98. The assumption of a power law corresponds to the case of X0 = 0 in equation (3) and hence we get

q = 2 + γ - Q.

Since we assume that Qabs(ν) varies as ν, the resulting value for Q=2.0. None of the objects is found to lie in the corresponding region in the colour–colour diagram. Even this extreme value for Q implies a density which varies as r-1.8.

Goldsmith et al. (1987) have reported that the simultaneous optical and near-IR data of AC Her can be fitted by a combination of two blackbodies at 5680 and 1800 K, representing, respectively, the stellar and dust shell temperatures, and suggested that in RV Tauri stars the grain formation is a sporadic phenomenon and not a continuous process. Apparently, they have been influenced by the remark by Gehrz & Woolf (1970) that their data in the 3.5–11 μm region of AC Her indicated a dust temperature of ∼300 K. We find that the K–L colours given by Gehrz (1972), Lloyd Evans (1985) and Goldsmith et al. (1987) are all consistent with each other. Surely, hot dust ( ∼1800 K), if present at the time of observations by Goldsmith et al. (1987), would have affected the K–L colour significantly. AC Her, like other members of its class, is found to execute elongated loops in the (U–B), (B–V) plane (Preston et al. 1963), indicating that significant departure of the stellar continuum from the blackbody is to be expected. Further, their data show only a marginal excess at the near-IR wavelengths. We feel that the case for the existence of hot dust around AC Her and hence for the sporadic grain formation around RV Tauri stars is not strong. In Fig. 3 we find that AC Her and RU Cen lie very close to R Sct which, according to Goldsmith et al. (1987), shows no evidence for the presence of a hot dust envelope.


Subsections