H2 temperatures in the Crab Nebula

Abstract

We used K-band spectra to measure the H 2 excitation temperatures in six molecular knots associated with the filaments in the Crab Nebula. The temperatures are quite high -in the range T ∼ 2000-3000 K, just below the H 2 dissociation temperature. This is the temperature range over which the H 2 1-0 S(1) line at λ2.121 μm has its maximum emissivity per unit mass, so there may be many additional H 2 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 H 2 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 H 2 . 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.

Modelling the ArH+ emission from the Crab nebula

“…Our final models all predict H2 surface brightnesses below the observed range, given in Table 5. Loh et al (2012) used H2 vibration-rotation emission to derive the excitation temperature and density in Crab Nebula H2 knots, finding a temperature range of 2000 − 3000 K, and a lower limit of nH = 2 × 10 4 cm −3 for the density. While the limit on the density is consistent with two of our final models, D4Z7 and D4Z8, the gas temperatures for these models are higher than the derived excitation temperatures.…”

“…While the limit on the density is consistent with two of our final models, D4Z7 and D4Z8, the gas temperatures for these models are higher than the derived excitation temperatures. Table 6 lists the surface brightnesses of the six H2 vibrational transitions detected by Loh et al (2012), relative to the 1-0 S(1) line, for a representative Crab Nebula knot from that paper and for our final PDR models. With the exception of D4Z8, all our models predict line ratios in agreement with 2 × 10 6 cm −3 Figure 18.…”

“…Loh et al (2012) also calculated the electron densities of the H2 knots based on [S II] emission. Their typical electron abundances of n(e − )/nH ≈ 0.1 are very close to the value of 0.11 from our D4Z7 model, which also has the gas temperature closest to the measured H2 exctiation temperature of all our final models.…”

“…Their models required an additional heating source to reproduce the observed H2 vibrational spectrum, which they assumed to be either energy input from shocks or turbu- Table 6. H 2 vibrational line surface brightness relative to the 1-0 S(1) line, for Knot 51 from Loh et al (2012) and for our final models. lence, or from the ionising particle flux from the PWN.…”

“…The Crab Nebula is a young supernova remnant (SNR), consisting of a central pulsar wind nebula (PWN) and a surrounding network of filaments ionised by the PWN synchrotron emission (Hester 2008). Molecular hydrogen (H2) (Graham et al 1990;Loh et al 2010Loh et al , 2011Loh et al , 2012 emission had previously been detected in the Crab Nebula, with modelling by Richardson et al (2013) suggesting it originates from a trace molecular component in mostly atomic gas, shielded from ionising radiation at the centre of the filaments. Barlow et al (2013) noted that ArH + is principally formed by the reaction Ar + + H2 → ArH + + H (Roach & Kuntz 1970), while the main destruction mechanism is ArH + + H2 → Ar + H + 3 (Schilke et al 2014).…”