A FAST EXPANDING H-I SHELL IN W44 - A PREEXISTING WIND-BLOWN SHELL OVERTAKEN BY A SUPERNOVA REMNANT

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(-1) and has a kinetic energy of 8 x 10(49) 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 R(H) similar or equal to 9 pc, which is obtained by extrapolating the high-velocity H I data, is considerably smaller than R(C) similar or equal to 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 R(H) similar or equal to R(C) similar or equal to 15 pc, then the coexistence of the H I shell, the centrally peaked X-ray emission, and the 2 x 10(4) 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 a 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 a SNR shell composed of newly swept-up ambient medium. The well-separated double-shell structure suggests that the SNR W44 is in the ''memory-losing'' phase, where the SN blast wave has now obliterated the presence of the wind bubble. We discuss the implications of the double-shell structure on the evolutionary history of W44.