A Cool Dust Factory in the Crab Nebula: A Herschel Study of the
  Filaments
  Whether supernovae are major sources of dust in galaxies is a long-standing
debate. We present infrared and submillimeter photometry and spectroscopy from
the Herschel Space Observatory of the Crab Nebula between 51 and 670 micron as
part of the Mass Loss from Evolved StarS program. We compare the emission
detected with Herschel with multiwavelength data including millimeter, radio,
mid-infrared and archive optical images. We carefully remove the synchrotron
component using the Herschel and Planck fluxes measured in the same epoch. The
contribution from line emission is removed using Herschel spectroscopy combined
with Infrared Space Observatory archive data. Several forbidden lines of
carbon, oxygen and nitrogen are detected where multiple velocity components are
resolved, deduced to be from the nitrogen-depleted, carbon-rich ejecta. No
spectral lines are detected in the SPIRE wavebands; in the PACS bands, the line
contribution is 5% and 10% at 70 and 100 micron and negligible at 160 micron.
After subtracting the synchrotron and line emission, the remaining far-infrared
continuum can be fit with two dust components. Assuming standard interstellar
silicates, the mass of the cooler component is 0.24(+0.32)(-0.08) Msolar for T
= 28.1(+5.5)(-3.2)K. Amorphous carbon grains require 0.11 +/- 0.01 Msolar of
dust with T = 33.8(+2.3)(-1.8) K. A single temperature modified-blackbody with
0.14Msolar and 0.08Msolar for silicate and carbon dust respectively, provides
an adequate fit to the far- infrared region of the spectral energy distribution
but is a poor fit at 24-500 micron. The Crab Nebula has condensed most of the
relevant refractory elements into dust, suggesting the formation of dust in
core-collapse supernova ejecta is efficient.
