5 Conclusions

Based on 1200 $\mu$m imaging observations of the circumstellar dust of $\beta ~ {\rm Pic}$ we conclude the following:

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At 25 $^{\prime \prime }$ resolution, the 1200 $\mu$m image of the $\beta ~ {\rm Pic}$ disk is slightly resolved in the NE-SW direction, but remains unresolved perpendicularly (NW-SE). In addition, the emission appears asymmetric and extends further SW-S to more than 1000 AU away from the star. Maximum emission (=24 mJy) is observed toward the position of the star.

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Combining our 1200 $\mu$m map with that by Holland et al. (1998) at 850 $\mu$m we infer that the dust size distribution in the $\beta ~ {\rm Pic}$ disk is significantly different from that in the general interstellar medium and appears more reminiscent of that found in protostellar disks. We argue that the thermal emission is dominated by dark big particles and that these grains constitute a population different from that dominating the scattering in the visible/NIR part of the spectrum.

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From the examination and numerical simulations of the available data (optical and submm/mm) for the southwestern blobs/extensions seen in the SIMBA maps we conclude that their reality can at present neither be excluded nor can, on the basis of the available evidence, their existence be fully confirmed.

Acknowledgements

We are grateful to Dr. José Afonso, who made available to us the Uranus calibration data, and to the SEST staff for providing additional observations. The critical comments by the anonymous referee are highly appreciated.