We calculate dust spectral energy distributions ( SEDs ) for a range of grain sizes and compositions , using physical properties appropriate for five pulsar wind nebulae ( PWNe ) from which dust emission associated with the ejecta has been detected .
By fitting the observed dust SED with our models , with the number of grains of different sizes as the free parameters , we are able to determine the grain size distribution and total dust mass in each PWN .
We find that all five PWNe require large ( \geq 0.1 { \mu m } ) grains to make up the majority of the dust mass , with strong evidence for the presence of micron-sized or larger grains .
Only two PWNe contain non-negligible quantities of small ( < 0.01 { \mu m } ) grains .
The size distributions are generally well-represented by broken power laws , although our uncertainties are too large to rule out alternative shapes .
We find a total dust mass of 0.02 - 0.28 { M } _ { \odot } for the Crab Nebula , depending on the composition and distance from the synchrotron source , in agreement with recent estimates .
For three objects in our sample , the PWN synchrotron luminosity is insufficient to power the observed dust emission , and additional collisional heating is required , either from warm , dense gas as found in the Crab Nebula , or higher temperature shocked material .
For G 54.1 + 0.3 , the dust is heated by nearby OB stars rather than the PWN .
Inferred dust masses vary significantly depending on the details of the assumed heating mechanism , but in all cases large mass fractions of micron-sized grains are required .