No abstract

No 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

No 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.