A Survey of Molecular Hydrogen in the Crab Nebula

Abstract

We have carried out a near-infrared, narrow-band imaging survey of the Crab Nebula, in the H 2 2.12 µm and Br 2.17 µm lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 2.8'  5.1' area that encompasses about 2/3 of the full visible extent of the Crab, we detect 55 knots that emit strongly in the H 2 line. We catalog the observed properties of these knots. We show that they are in or next to the filaments that are seen in optical-passband emission lines. Comparison to HST [S II] and [O III] images shows that the H 2 knots are strongly associated with compact regions of low-ionization gas. We also find evidence of many additional, fainter H 2 features, both discrete knots and long streamers following gas that emits strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the Crab's projected surface area has significant H 2 emission that correlates with [S II] emission. We measured radial velocities of the [S II] 6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Br emission. It is right at the limit of our survey, and our Br filter cuts off part of the expected velocity range. But clearly the Br emission has a quite different morphology than the H 2 knots, following the long linear filaments that are seen in Hα and in [O III] optical emission lines.

Modelling the ArH+ emission from the Crab nebula

“…Grenman et al (2017) used Hubble Space Telescope optical images of the Crab Nebula to determine the extinctions of several dusty globules observed as dark spots against the synchrotron background, obtaining values of AV = 0.20−0.34. The globule sizes range from 6×10 15 cm to 3 × 10 16 cm, similar to the sizes of the H2 emitting knots observed by Loh et al (2011), although Grenman et al (2017) note that only ∼ 10% of their dusty globules are coincident with H2 knots. While the observed globule sizes are similar to our nH = 1900 cm −3 model cloud thicknesses, none of our preferred models match the extinctions found by Grenman et al (2017).…”

“…We list the surface brightnesses of emission lines observed in the Crab Nebula for each model in Table 5, along with observational upper and lower limits. Loh et al (2010Loh et al ( , 2011 observed emission from the H2 2.12 µm line from 55 knots in the Crab Nebula, with average surface brightnesses of ∼ 1 − 5 × 10 −5 erg cm −2 s −1 sr −1 . Figure 17 shows the surface brightness of this line versus ζ for our models, integrated up to AV = 3.…”

“…Absolute surface brightness integrated up to A V = 0.05 (as for models D3Z7 and D3Z8) of the H 2 2.12 µm transition versus cosmic ray ionisation rate ζ for models with n H = 1900 cm −3 (black), 2×10 4 cm −3 (red), 2×10 5 cm −3 (blue) and 2×10 6 cm −3 (green). The dashed lines show upper and lower observational bounds from Loh et al (2011).…”

“…The ratio of globule angular area to PACS spaxel area is ∼ 3 × 10 −3 , which is low enough to explain the discrepancy between our models and the observations if each spaxel contains only a few globules, although unlike the moleuclar emission, there is no reason to assume that the far-infrared lines only originate in dense, shielded clumps. The Hershel SPIRE FTS beam size is also much larger than the typical angular sizes of the H2 knots from Loh et al (2011), so assuming that the ArH + and OH + emission also originates in these knots, a higher model surface brightness would be necessary to reproduce observed values. If this was obtained by increasing the filament AV value, it would also increase the H2 2.12 µm surface brightness, potentially removing the issue of our models predicting lower values than those from Loh et al (2011).…”

“…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).…”