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Modelling the ArH+ emission from the Crab nebula

Priestley, F. D., Barlow, M. J. and Viti, S. 2017. Modelling the ArH+ emission from the Crab nebula. Monthly Notices of the Royal Astronomical Society 472 (4) , pp. 4444-4455. 10.1093/mnras/stx2327

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We have performed combined photoionization and photodissociation region (PDR) modelling of a Crab nebula filament subjected to the synchrotron radiation from the central pulsar wind nebula, and to a high flux of charged particles; a greatly enhanced cosmic-ray ionization rate over the standard interstellar value, ζ0, is required to account for the lack of detected [C i] emission in published Herschel SPIRE FTS observations of the Crab nebula. The observed line surface brightness ratios of the OH+ and ArH+ transitions seen in the SPIRE FTS frequency range can only be explained with both a high cosmic-ray ionization rate and a reduced ArH+ dissociative recombination rate compared to that used by previous authors, although consistent with experimental upper limits. We find that the ArH+/OH+ line strengths and the observed H2 vibration–rotation emission can be reproduced by model filaments with nH = 2 × 104 cm−3, ζ = 107ζ0 and visual extinctions within the range found for dusty globules in the Crab nebula, although far-infrared emission from [O i] and [C ii] is higher than the observational constraints. Models with nH = 1900 cm−3 underpredict the H2 surface brightness, but agree with the ArH+ and OH+ surface brightnesses and predict [O i] and [C ii] line ratios consistent with observations. These models predict HeH+ rotational emission above detection thresholds, but consideration of the formation time-scale suggests that the abundance of this molecule in the Crab nebula should be lower than the equilibrium values obtained in our analysis.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: Oxford University Press
ISSN: 0035-8711
Date of First Compliant Deposit: 6 August 2019
Date of Acceptance: 5 September 2017
Last Modified: 04 May 2023 19:16

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