A Bright Molecular Core in a Crab Nebula Filament

TL;DR

This study identifies a bright molecular knot in the Crab Nebula with high H_2 emission, suggesting significant molecular mass and specific physical conditions that enhance infrared emission, providing insights into nebular composition.

Contribution

First near-infrared survey revealing a molecular knot with high H_2 emission in the Crab Nebula, indicating the presence of substantial molecular gas in filaments.

Findings

Detected a knot with high H_2 to Br-gamma ratio of 18±9.

Estimated a lower limit on molecular to recombination mass ratio >/- 0.9.

Inferred a minimum molecular mass of 5×10^-5 solar masses in the knot.

Abstract

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 H_2 2.12 micron line and a very large H_2 2.12 micron to Br-gamma ratio. The brightest of these knots has an intensity ratio I(H_2 2.12 micron)/I(Br-gamma) = 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 >/- 5E-5 M_sun. We argue that the knot discussed here probably is able to emit so strongly in the 2.12 micron 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.