X-ray halos and large grains in the diffuse interstellar medium

Recent observations with dust detectors on board the interplanetary spacecr afts Ulysses and Galileo have recorded a substantial flux of large interste llar grains with radii between 0.25 and 2.0 mum entering the solar system f rom the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by an a(-3.5) power law in grain radii a and extends to a maximum grain radius of 0.25 mum. The extension of the i nterstellar grain size distribution to such large radii will have a major e ffect on the median grain size and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to i nvestigate whether this population of larger dust particles prevails in the general interstellar medium or whether it is merely a local phenomenon. Th e presence of large interstellar grains can be inferred mainly from their e ffect on the intensity and radial profile of scattering halos around X-ray sources. In this Letter, we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study con firm the need to extend the interstellar grain size distribution in the dir ection of this source to and possibly beyond 2 mum. The model that gives th e best fit to the halo data is characterized by (1) a grain size distributi on that follows an power law up to a(-3.)5 a 0.50 mum, followed by an a(-4. 0) extension from 0.50 to 2.0 mum, and (2) silicate and graphite (carbon) d ust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundance constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.