A MILLIMETER SEARCH FOR DUST IN THE GLOBULAR-CLUSTERS M-3 AND M-22

We have conducted the first millimetre wavelength search for interstel lar dust within globular clusters, by carrying out observations of M 3 and M 22. We have found no dust in the central 14'' and 18'' of the c lusters, respectively, to a limit of 10(-3) ... 10(-1) M. (M 3) and si milar to 10(-5) ... 10(-3) M. (M 22), depending on the absorption effi ciency assumed for the dust. If, as we argue, the appropriate absorpti on efficiency is for circumstellar rather than interstellar dust, the smaller upper limits are appropriate. These limits are comparable with , or somewhat tighter than, those obtained in previous searches at oth er wavelengths, although tighter upper limits on mass per unit area or on mass per unit beam are generally provided by IRAS data. However, o bservations at millimetre wavelengths are sensitive to dust at signifi cantly lower temperatures. These results are consistent with previous searches for dust in clusters, which have given upper limits below 10( -2) M., except for a possible detection in 47 Tuc at the 3 10(-4) M. l evel. M 3 and M 22 are expected to have produced some 0.05 M. and 8 10 (-4) M. of dust respectively since their last Galactic plane crossing, as calculated from the rate of injection of dust into the interstella r medium by evolving member stars and from the orbits of the clusters. The discrepancy between observation and theory is discussed.