We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emissions from high ionization lines that will not be emitted by the Crab's filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The near-NIR [Ne VI] λ7.652 μm line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV λ1549 doublet is predicted to be the strongest absorption line from the shell, which is in agreement with Hubble Space Telescope observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually "remember" its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion.

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Published in The Astrophysical Journal, v. 774, no. 2, 112, p. 1-13.

© 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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G.J.F. acknowledges support by NSF (1108928; and 1109061), NASA (10-ATP10–0053, 10-ADAP10–0073, and NNX12AH73G), and STScI (HST-AR-12125.01, GO-12560, and HST-GO-12309). J.A.B. and C.T.R. acknowledge support by NSF (1006593). C.T.R., J.A.B., and E.D.L. are grateful to NASA for support through ADAP grant NNX10AC93G.