ISO observations of globular clusters

Madelaine Hopwood ¹, Nye Evans ¹, Stewart Eyres ¹, Alan Penny ², & Michael Odenkirchen ³

¹ Keele University, ² RAL, ³ Univerity of Bordeaux

The interstellar medium in a globular cluster (GC) might be expected to contain about 0.1 solar masses of dust. In the 1E7 years since a typical cluster last crossed the galactic plane (when all the gas and dust would be tidally and ram stripped out) enough stars in a cluster will have gone through the post-AGB stage to inject this amount of dust into the cluster.

However dust at this level has not yet been detected in any GC. As the dust should be at about 50K, observations at 50 to 100 microns are needed. Pointed IRAS observations of NGC104 found 4E-4 solar masses (Gillett et al., AJ, 96, 116) and the IRAS survey only found higher upper limits for all the other clusters. Some mechanism is removing dust from clusters, on a timescale of about 1E4 - 1E5 years. Possible mechanisms include: (i) ram-pressure stripping by halo gas, (ii) removal by a cluster wind, (iii) ejection by the integrated radiation pressure of Galactic stars, (iv) erosion of grains by the hot x-ray emitting Galactic halo gas.

We have obtained 60, 70 and 90 micron maps, covering an area of 10 arcmin x 10 arcmin, of NGC104, NGC5272, NGC5139 and NGC7078 using PHT-C. The relevant properties of these clusters are listed in the table below. We have chosen clusters with high internal escape velocities since they are more likely to retain any intra-cluster dust grains injected by the GC stars. A range of metallicities has been chosen so we can distinguish between the effectiveness of grain production in both metal-rich and metal-poor GCs.

The ISO data for NGC104 go deeper, and are at higher spatial resolution, than the IRAS data. We discuss the results in terms of the stellar content of the clusters, the intra-cluster medium and the mass-loss history of the post-main-sequence stars.

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                                      Internal
                 GC        [Fe/H]      Escape      Core     
                                      velocity    radius       
                                       (km/s)      ('')      

                 NGC104    -0.71        56.8        22.4  
                 NGC5272   -1.66        34.3        30.2                       
                 NGC5139   -1.59        51.2       154.9
                 NGC7078   -2.17        40.9         4.1

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