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Title:
A fast expanding H I shell in W44: A preexisting wind-blown shell overtaken by a supernova remnant
Authors:
Koo, Bon-Chul; Heiles, Carl
Affiliation:
AA(Seoul National Univ., Seoul, Korea), AB(Seoul National Univ., Seoul, Korea)
Publication:
Astrophysical Journal (ISSN 0004-637X), vol. 442, no. 2, Part 1, p. 679-684 (ApJ Homepage)
Publication Date:
04/1995
Category:
Astronomy
Origin:
STI
NASA/STI Keywords:
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
DOI:
10.1086/175473
Bibliographic Code:
1995ApJ...442..679K

Abstract

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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