The nature of the H2-emitting gas in the Crab nebula★

Abstract

Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers we have mapped the Crab Nebula in a roto-vibrational H 2 emission line, and then measured the molecular excitation temperature for a few of the brighter H 2 -emitting knots that we have found to be scattered throughout the Crab Nebula's filaments. We found that H 2 emission is often quite strong, correlates with optical low-ionization emission lines, and has a surprisingly high excitation temperature. Here we study Knot 51, a representative, bright example. It is a spatially isolated structure for which we have available long slit optical and NIR spectra covering emission lines from ionized, neutral, and molecular gas, as well as HST visible and SOAR Telescope NIR narrow-band images. We present a series of CLOUDY simulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H 2 emission. There is still considerable ambiguity about the geometry of Knot 51, so we do not try for an exact match between model and observations. Rather, we aim to explain how the bright H 2 emission lines can be formed from within a cloud the size of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by

Modelling the ArH+ emission from the Crab nebula

“…These enhanced values cover the limits estimated by Richardson et al (2013), based on the observed Crab synchrotron radiation and the degree of magnetic trapping. The grid was run using the UV and X-ray fluxes from the attenuated PWN spectrum described above.…”

“…The rates for reactions 17 and 18 were taken from Shull & van Steenberg (1982) and from Kingdon & Ferland (1996), respectively. We also included reactions involving the H − anion from the UMIST database (McElroy et al 2013), to account for its role in the formation of H2 in the Crab Nebula (Richardson et al 2013). Electron impact excitation rates for the first six rotational levels of ArH + were taken from Hamilton et al (2016), and we used energy levels and Einstein A-coefficients from the Cologne Database for Molecular Spectroscopy (CDMS) (Müller et al 2001(Müller et al , 2005, in order to calculate the emissivity of ArH + rotational transitions.…”

“…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. Richardson et al (2013) modelled a Crab Nebula H2-emitting knot using cloudy (Ferland et al 1998), in order to explain the combination of near-infrared H2 vibrational lines with optical emission from atomic and ionised gas. 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.…”

“…This overestimation may be due to the fact that our models do not treat any optical or UV line emission from higher ionization stages that would be produced by the high charged particle flux, and so require stronger emission from other species in order to balance the heating rate. [Fe II], which Richardson et al (2013) found to be the dominant coolant in the PDR region of their ionizing particles model, is also not included in our models.…”

“…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).…”