Up: Analysis of correlations between stars

5 Location of other type stars with IR excesses and polarization of radiation at the $\log p/E(V-L)$ diagram

For statistical purposes we have compared the location of young stars with the position of the stars which are definitely not young but also show some polarization of radiation.

5.1 Classical Be stars

It was previously noted by Yudin ([1988]) that the classical Be stars (40 objects) did not follow the relation derived for young stars but were distributed in a separate group. To make statistically significant conclusions we collect the data of polarimetry and V, L photometry for a sample of 228 classical Be stars (for detail discussion see Yudin [2000]). With few exceptions most of the selected stars are dwarfs that allow us to compare their behaviour with that of HAEBE stars. The relation between $\log p$ and E(V-L) for classical Be and HAEBE stars is shown in Fig. 17. It is safe to say that most classical Be stars are largely concentrated in the region of small IR excesses $0^{\rm m}<E(V-L)<1^{\rm m}$ while their polarization varies from very low values up to $\approx2$ %. The correlation between $\log p$ and E(V-L) for Be stars is weak or even absent and most of them are concentrated well away from the above derived dependence for young HAEBE stars. It is well known that the polarization of Be stars originates from scattering of free electrons in a CS gaseous shell, as well as the excess in near IR, and even at 12 $\mu$ m is mainly caused by free-free emission (see Waters & Marlborough [1992]) and is not due to the thermal emission from CS dust. Therefore, the nature of IR excesses and polarization in classical Be stars and HAEBE stars is quite different. We are currently collecting data on optical polarization, near-IR excesses and projected rotational velocities for a sample of $\approx$ 650 classical Be stars and we are planning to publish a detailed study in the nearest future (Yudin [2000]).

5.2 Red giants and supergiants

Another group of stars with IR excesses and polarization is red giants and supergiants (hereafter RSG). The importance of the comparison of their polarimetric behaviour with that observed for young TT stars is that the objects in both classes are surrounded by dust shells, have approximately the same spectral classes but are on a different stage of evolution. RSG are post-MS objects and they are located on the diagram near $E(V-L)\approx 0$ with a considerable scatter in the polarization values (up to 4%, see Fig. 3). It is well known that for most RSG the dust envelopes are optically thin $\tau\leq 0.1-0.2$ (Dyck et al. [1971]). There is a consensus now that the intrinsic polarization of RSG originates due to scattering by nonradially oriented dust grains in the stellar envelope. In this case linear polarization appears even if the stellar envelope is spherical and even for small optical depth (resulting in small near IR excesses). This suggestion is strongly supported by the detection of high value circular polarization in some RSG (up to $p_{\rm c}\approx 0.5$ %). Comparison of the location of TT stars and RSG on the diagram leads to some interesting conclusions:
a) Most TT stars are located outside the box occupied by RSG which indicates different physical conditions in their CS shells (for instance, significantly larger optical depth in CS shells of TT stars);
b) A small fraction of TT stars ( $\approx$ 20%) are located close to the position derived for RSG and for some of them we may suppose the important contribution of the scattering by nonspherical dust grains to their intrinsic polarization (see also Lucas & Roche [1998]). This fact has been corroborated by the detection of circular polarization in some TT stars (Bastien et al. [1989]). Note however that the level of circular polarization in TT stars is generally lower ( $p_{\rm c}<0.1$ %) than that for RSG. At present circular polarization has been detected for only a few HAEBE stars and on the level lower than that for TT stars. Thus a slight difference in position of HAEBE and TT stars and respectively larger scattering of the data points for TT stars on the diagram may also occur due to the presence of nonspherical oriented dust grains in CS shells of some objects.

5.3 Main sequence stars in solar neighborhood

The next group is MS stars in solar neighborhood and 68 stars from Leroy's ([1993]) catalogue within 50 pc from the Sun with available L-band photometry and $p/\sigma p>2$ were selected. It is believed that for most of these stars the interstellar component of polarization is negligible. As follows from Fig. 3, all selected stars are largely concentrated in a small region of the diagram around $E(V-L)\approx0^{\rm m}$ and in the range of polarization $0.01\%<p<0.1\%$ , i.e. in the same region as young solar-type stars and some Vega-type stars. This is a side benefit on the small polarization of MS stars.

5.4 Early-type supergiants

Finally, we consider the Serkowski et al. ([1975]) catalogue. About 120 stars for which the data on L-band photometry were found in CIO5 (Gezari et al. [1999]) have been selected. Most of them have MK-classification and are supergiants of early spectral type which allows us to exclude the suggestion of their possible youth. As follows from Fig. 2 most of these normal stars are clustered close to the value of $E(V-L)\approx0^{\rm m}$ in the range of polarization 0.3% $\leq p\leq$ 8%. This fact suggests that these stars have no near IR excesses (and no hot dust in their CS environment) and all observed polarization has an interstellar origin. If the component of interstellar polarization is removed from the observed values, these MS stars will be located in the lower left side of the diagram discussed here. In any case their position on the diagram is very different from the dependence obtained for young stars which gives additional support to the use of this diagram as a selection criteria for young stars.