The Detection of Cold Dust in Cas A: Evidence for the Formation of Metallic Needles in the Ejecta

Ejecta from core collapse supernovae contain a few solar masses of refractory elements and can therefore be the most important source of interstellar dust if these elements condense efficiently into solids. However, infrared observations of young supernova remnants, such as Cas A or Kepler, and observations of SN 1987A have detected only ∼ 10 M⊙ of hot dust in these objects. Recently, Dunne et al. (2003) obtained 450 and 850 μm SCUBA images of Cas A, and reported the detection of 2–4 M⊙ of cold, 18 K, dust in the remnant. Here we show that their interpretation of the observations faces serious difficulties. Their inferred dust mass ignores the effect of grain destruction by sputtering, and is larger than the mass of refractory material in the ejecta of a 10 to 30 M⊙ star. The cold dust model faces even more difficulties if the 170 μm observations of the remnant are included in the analysis, decreasing the cold dust temperature to ∼ 8 K, and increasing its mass to & 20 M⊙. We offer here a more plausible interpretation of their observation, in which the cold dust emission is generated by conducting needles in the ejecta. The needle properties are completely determined by the combined submillimeter and X-ray observations of the remnant. The needles are collisionally heated by the shocked gas. They are very efficient emitters at submillimeter wavelengths, and with a resistivity of a few μΩ cm can readily attain a temperature of 8 K. Taking the destruction of needles into account, a dust mass of only 10 to 10 M⊙ is needed to account for the observed SCUBA emission. The needles consist of metallic whiskers with . 1% of embedded impurities, that may have condensed out of blobs of material that were expelled at high velocities from the inner metal–rich layers of the star in an asymmetric explosion. Conductive needles may also be the source of the cold dust emission detected by Morgan et al. (2003) in Kepler. Aligned in the magnetic field, needles may give rise to observable polarized emission. The detection of submillimeter polarization will therefore offer definitive proof for a needle origin for the cold dust emission. Supernovae may still be proven to be important sources of interstellar dust, but the evidence is still inconclusive. Subject headings: ISM: supernova remnants – ISM: individual (Cassiopeia A) – ISM: interstellar dust – Infrared: general