A bright molecular core in a crab nebula filament

In a sub-arcsec near-infrared survey of the Crab Nebula using the new Spartan Infrared Camera, we have found several knots with high surface brightness in the H₂ 2.12 μm line and a very large H₂ 2.12 μm to Brγ ratio. The brightest of these knots has an intensity ratio I(H₂ 2.12 μm)/I(Brγ) = 18 ± 9, which we show sets a lower limit on the ratio of masses in the molecular and recombination (i.e., ionized) zones M _mol/M _rec ≥ 0.9, and a total molecular mass within this single knot M _mol ≥ 5 × 10 ^-5 M _⊙. We argue that the knot discussed here probably is able to emit so strongly in the 2.12 μm line because its physical conditions are better tuned for such emission than is the case in other filaments. It is unclear whether this knot has an unusually large M _mol/M _rec ratio, or if many other Crab filaments also have similar amounts of molecular gas which is not emitting because the physical conditions are not so well tuned.