3 Discussion

3.1 Slowly spinning M asteroids

A list of M-type asteroids consisting of 60 objects was given by Belskaya & Lagerkvist (1996). It included all asteroids which were classified as M at least in one of the available taxonomic systems. At present rotation periods are known for 49 asteroids in this list. All classified asteroids with diameters larger than 50 km have their rotation rates determined. However, there are still some unclassified objects in this size range. The statistics is practically complete for M-asteroids with diameters larger than 70 km (Belskaya & Lagerkvist 1996).

The rotation periods versus diameters are shown in Fig. 12 for M-type asteroids. There are six asteroids having a rotation period longer than 15 hours (black dots). The other three black dots in the figure represents asteroids with colour indices or albedos differing from typical M-type values.

How typical are the classification parameters for these asteroids? To answer this question we have plotted in Fig. 13 U-B and B-V colour indices versus albedo for the known M-type asteroids. These parameters are critical for distinguishing M-type objects from others. One can see that many slow rotators have albedos and (or) colour indices different from the typical values for the M-type asteroids. Also the asteroids 359, 785 and 798 have classification parameters that are non-typical for asteroids of type M. Below we give more detailed comments on the six M-type asteroids with long rotation periods.

  

Figure 12: Rotation period plotted versus diameter for M-type asteroids

  

Figure 13: U-B and B-V versus albedo for M type asteroids

92 Undina

was first classified as C by Zellner & Bowell (1979) because of a small value of the minimum polarization (0.77$\%$) and a large U-B colour compared to other M-type objects. Tholen (1989) classified it as X while the u-v colour index of the asteroid differed too much from typical values. Barucci et al. (1987) and Tedesco et al. (1989) classified it as M. Jones et al. (1990) and later Rivkin et al. (1995) found the 3 $\mu$m absorption band in spectra which indicate the presence of hydrated minerals on the asteroids surface and is therefore inconsistent with a metallic composition.

97 Klotho

has quite typical classification parameters compared with the whole population of M-type asteroids. Radar observations of 97 Klotho failed to show any evidence of a metallic surface composition (Ostro et al. 1985). Thus, 97 Klotho should not be considered as a metal-rich body.

224 Oceana

was classified as M based on its colours U-B=0.20 mag and B-V=0.75 mag (Bowell et al. 1979). Both colours are at the border for the M-type population (see Fig. 13). Additional data are needed to check the classification.

322 Phaeo

was classified as X by Tholen (1989) and as M by Barucci et al. (1987) and Tedesco et al. (1989) based on its IRAS albedo of 0.088 (Tedesco et al. 1992). The albedo is one of the lowest among the M-type population. On the other hand the value of the v-z colour (Zellner et al. 1985) is the highest among the EMP classes.

498 Tokio

was classified as U because of the large U-B=0.42 mag (Bowell et al. 1979). Later it was classified as M by Tholen (1989) and as D3 by Barucci et al. (1987). Fitzsimmons et al. (1994) determined a spectral slope $S^\prime= 8.8$% which is quite typical for asteroids of taxonomic type D (Dahlgren & Lagerkvist 1995). Since 498 Tokyo has an albedo of only 0.07 (Tedesco et al. 1992) we conclude that the correct taxonomic type for this asteroid must be D.

1210 Morosovia

was classified as SM by Bowell et al. (1979) because of the large B-V=0.83 mag which is closer to the mean value for the S-type population (0.86 mag) than to the mean value of B-V (0.70 mag) for the M-type asteroids (Belskaya & Lagerkvist 1996). Tholen (1989) classified the asteroid as MU since the ECAS data were noisy. The asteroid 1210 Morosovia is a member of the Eos family. Most probably it is of taxonomic type S since all other classified asteroids in the Eos family are of this taxonomic type (Tholen 1984).

The main conclusion of the discussion above is that these six asteroids cannot be considered to be members of the M-type population.

3.2 Spin rates of asteroids of different taxonomic types

The increase of the data set of M-type asteroids with known rotation periods, and the exclusion of six asteroids previously considered to be belonging to the M-type population, justifies a new comparison of the spin rates between the M, S and C asteroiods.

For comparison we chose asteroids with diameters larger than 70 km for which the statistics is almost complete for M-type asteroids. Considered asteroids of C- and S-type have semi-major axes between 2.3 and 3.2 AU which is the same range as for M-asteroids. However, asteroids of taxonomic types C and S are not completely sampled down to this diameter regarding rotation periods. In practice this means that available rotational data for C and S asteroids down to this diameter is overrepresented by asteroids with short rotation periods.

  

Figure 14: Spin rate distributions for asteroids of taxonomic types M, S and C

In Fig. 14 we present the histograms for the rotation periods of the M, S and C type asteroids. The individual M asteroids shown above being untypical M asteroids have been excluded. It is clearly evident that M type asteroids in general spin faster than asteroids of type C and S. Considering the bias in the C and S population, favouring short periods, this difference is even greater than seen in Fig. 14. The distribution of the spin rates of M type asteroids is also much flatter than for the other types. This is also verified by a Kolmogorov-Smirnov test (e.g. Press et al. 1989). The result of the test shows that the spin rate distribution for M-type asteroids is completely different from those of S- and C-type asteroids, while the later two types have large similarities of the observed spin rate distributions.