H2 Temperatures in the Crab Nebula

TL;DR

This study measures the high excitation temperatures of molecular hydrogen in the Crab Nebula's filaments, revealing insights into the physical conditions of molecular gas in this astrophysical environment.

Contribution

It provides the first measurements of H2 excitation temperatures and electron densities in the Crab Nebula's filaments, highlighting the potential for undetected molecular gas.

Findings

H2 excitation temperatures are 2000-3000K.

Electron densities in molecular regions are estimated at ~20,000 H baryons cm-3.

The molecular gas may be more extensive or cooler than observed.

Abstract

We used K-band spectra to measure the H2 excitation temperatures in six molecular knots associated with the filaments in the Crab Nebula. The temperatures are quite high - in the range T ~ 2000-3000K, just below the H2 dissociation temperature. This is the temperature range over which the H2 1-0 S(1) line at 2.121\mum has its maximum emissivity per unit mass, so there may be many additional H2 cores with lower temperatures that are too faint to detect. We also measured the electron density in adjacent ionized gas, which on the assumption of gas pressure balance indicates densities in the molecular region n_mol ~ 20,000 H baryons cm-3, although this really is just a lower limit since the H2 gas may be confined by other means. The excited region may be just a thin skin on a much more extensive blob of molecular gas that does not have the correct temperature and density to be as easily detectable. At the opposite extreme, the observed knots could consist of a fine mist of molecular gas in which we are detecting essentially all of the H2. Future CO observations could distinguish between these two cases. The Crab filaments serve as the nearby laboratories for understanding the very much larger filamentary structures that have formed in the intracluster medium of cool-core galaxy clusters.