A fast expanding H I shell in W44: A preexisting wind-blown shell overtaken by a supernova remnant

Astronomy and Astrophysics – Astronomy

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Gas Expansion, Hydrogen Plasma, Line Spectra, Radio Astronomy, Radio Spectra, Stellar Mass Ejection, Stellar Winds, Supernova Remnants, Astronomical Spectroscopy, Mass Distribution, Shock Waves, Spectrum Analysis, Velocity Distribution

Scientific paper

We confirm from higher resolution observations that the previously detected high-velocity H I gas in W44 is an expanding shell associated with the supernova remnant (SNR). The H I shell is expanding at 150 km/s and has a kinetic energy of 8 x 1049 ergs, which is very likely to have been imparted by the supernova (SN) explosion that produced W44. However, the radius of the H I shell RH approximately = 9 pc, which is obtained by extrapolating the high-velocity H I data, is considerably smaller than Rc approximately = 15 pc of the W44 radio continuum shell. It is possible that the uncertainty associated with our extrapolation has led us to significantly underestimate the size of the H I shell. But if RH approximately = Rc approximately 15 pc, then the coexistence of the H I shell, the centrally peaked X-ray emission, and the 2 x 104 yr old pulsar PSR 1853+01 in W44 is very difficult to understand based on available theoretical models. Furthermore, there are optical filaments mainly confined within the H I shell. We therefore propose that the H I and radio continuum shells are physically distinct shells. The double-shell structure may have been produced by an SN explosion inside a preexisting wind bubble. We interpret the inner H I structure as a preexisting wind shell that has been rejuvenated and disrupted by the SN blast wave, and the outer radio continuum structure as an SNR shell composed of newly swept-up ambient medium. The well-separated double-shell structure suggests that the SNR W44 is in 'memory-losing' phase, where the SN shell structure on the evolutionary history of W44.

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