The amount of dust estimated from infrared to sub-millimetre ( submm ) observations strongly depends on assumptions of different grain sizes , compositions and optical properties .
Here we use a simple model of thermal emission from cold silicate/carbon dust at a range of dust grain temperatures and fit the spectral energy distribution ( SED ) of the Crab Nebula as a test .
This can lower the derived dust mass for the Crab by \sim 50 % and 30-40 % for astronomical silicates and amorphous carbon grains compared to recently published values ( 0.25 M _ { \sun } \to 0.12 M _ { \sun } and 0.12 M _ { \sun } \to 0.072 M _ { \sun } , respectively ) , but the implied dust mass can also increase by as much as almost a factor of six ( 0.25 M _ { \sun } \to 1.14 M _ { \sun } and 0.12 M _ { \sun } \to 0.71 M _ { \sun } ) depending on assumptions regarding the sizes/temperatures of the coldest grains .
The latter values are clearly unrealistic due to the expected metal budget , though .
Furthermore , we show by a simple numerical experiment that if a cold-dust component does have a grain-temperature distribution , it is almost unavoidable that a two-temperature fit will yield an incorrect dust mass estimate .
But we conclude that grain temperatures is not a greater uncertainty than the often poorly constrained emissivities ( i.e. , material properties ) of cosmic dust , although there is clearly a need for improved dust emission models .
The greatest complication associated with deriving dust masses still arises in the uncertainty in the dust composition .