5 Conclusions

Our observations indicate that the extinction law in the HM1 region is anomalous and takes place in the intracluster material. It is also possible that part of the interstellar material just in front of HM1 contributes to this fact. We also noticed that the properties of the interstellar material in this zone strongly change in a very reduced portion of the sky. At only $2{\hbox{$^\prime$ }}$ southwest of the cluster centre where the polarisation of the stars is similar to the foreground polarisation, we found stars with the highest polarisation values that, in turn, show high R-values. However, at the same time, no extra absorption across the cluster surface is evident from our diagrams. So, to find stars with high polarisation and anomalous extinction law sharing a same location does not imply a physical relationship at all. This assumption is favoured by the polarisation maps of Klare & Neckel (1977) that reveal that nearby small dust clouds produce high polarisation values across the area that surrounds HM1.

It still has to be explained why highly reddened stars ( $A_V \geq 6$), as seen in the cluster centre, have, however, a polarisation comparable to foreground stars. It is probable that, when looking at the cluster centre, the star light passes through clouds with different orientations of their magnetic fields undergoing a depolarisation process. We have already found a situation of this type in other young cluster in this zone, Trumpler 27 (Feinstein et al. 2000) as well. There are examples of this: the average $P_{\max}/ E_{B-V}$ ratio is in the order of 5 (Serkowski et al. 1975) but there are zones in our galaxy with smaller values e.g. Cygnus OB2 ( $P_{\max} = 1.7 \times E_{B-V}$, McMillan & Tapia 1977) or R Coronae Australis ( $P_{\max} = 2 \times E_{B-V}$, Vrba et al. 1981), where the polarisation and reddening can be produced by more than one source. Unfortunately, there is no available spectroscopy of foreground stars close to the cluster front to study the material between the sun and the cluster and subtract its contribution to the total polarisation shown by cluster stars.

We obtained a better distance estimation as we were able to reach stars 2 mag fainter than in any previous work: the cluster is at a distance d=3.3kpc, beyond the Sagittarius spiral arm. HM1 is 2-4 Myr old, an age that agrees with the presence of two WN7-8 stars and several O and Of stars, but excludes the possibility that star 35 is a red super-giant member. Our data show that it is a foreground star. Further, both WR stars have the same absolute magnitude, -7.3, and only LSS 4064 shows hints of infrared excess that can be attributed to the intracluster material.

Acknowledgements

The authors acknowledge the financial suport from La Plata Observatory and the CONICET. Special thanks are given to Bob Garrison for the kind allocation of telescope time at UTSO and to the CASLEO staff for the technical support. We also acknowledge useful comments from Dr. H. G. Marraco. We are grateful to our referee, Dr. M. Geffert, for his valuable suggestions which improved the final version of this article