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R. J. Tuffs, C. C. Popescu (Max-Planck-Institut fuer Kernphysik), D. A. Green (Mullard Radio Astronomy Observatory)
We present FIR and sub-mm images of the Crab nebula, taken with the ISOPHOT instrument on board ISO at 60, 100, and 170 micron, and with the JCMT at 850 micron. The 60 and 100 micron images show a clear excess of emission above the extrapolation of the synchrotron spectrum from lower frequencies, as previously seen using IRAS. The ISOPHOT images reveal that about half this excess arises from two peaks not seen on synchrotron images, separated by ca. 80 arcsec. The 170 micron image does not show any excess emission. Spectroscopic measurements show that this FIR excess is not due to line emission. We interpret the FIR excess as emission from a small amount of warm dust (in the ranges 0.01-0.07 and 0.003-0.02 Msolar for silicate and graphite, respectively), superimposed on a synchrotron spectrum which gradually steepens towards shorter wavelengths throughout the FIR and MIR. The dust geometry is consistent with a torus of diameter ca. 0.8pc created by the supernova progenitor prior to its explosion, superimposed upon a broadly distributed component which may be supernova condensates in the filaments. Our upper limit on the total mass of graphite is consistent with the inference from gas-phase Carbon abundances that there has been no significant enrichment of the filaments in Carbon nucleosynthesis products from the progenitor. Comparison of the 850 micron image with a 20-cm VLA image shows that the only variation in spectral index of the synchrotron emission across the remnant is a small change near the centre. Given the good agreement between the integrated flux density at 850 micron and the extrapolated synchrotron spectrum, together with the different epochs of the 850 micron and 20cm images, we do not see the need for the second radio synchrotron component which has previously been proposed.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.