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Astron. Astrophys. 317, 203-211 (1997)

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Neutral hydrogen compact absorption features in Cassiopeia A

E.M. Reynoso * 1, W.M. Goss 2, G.M. Dubner ** 1, P.F. Winkler *** 3 and U.J. Schwarz 4

1 Instituto de Astronomía y Física del Espacio, C.C.67, Suc. 28, 1428 Buenos Aires, Argentina (ereynoso@iafe.uba.ar)
2 National Radio Astronomy Observatory, Very Large Array, P.O. Box 0, Socorro, New Mexico 87801, USA
3 Department of Physics, Middlebury College, Middlebury, VT 05753, USA
4 Kapteyn Astronomical Institute, Postbus 800, NL-9700 AV Groningen, The Netherlands

Received 2 April 1996 / Accepted 2 May 1996


Very Large Array (VLA) observations of the [FORMULA] 21 cm H I line towards Cassiopeia A (Cas A) have been performed. The velocity interval observed was from -38 to -362 (LSR) km s-1, with a resolution of 2.6 km s-1 and a beam size of [FORMULA]. Opacity images show the presence of a number of small absorption H I features in the velocity interval -62 to -69 km s-1 (LSR) which, on the basis of several arguments, are postulated to be physically associated with Cas A. These cold H I knots have sizes of <0.1 pc and show spatial substructure. Their velocities lie well outside the range of the intervening H I in the Perseus spiral arm, i.e. 0 to -55 km s-1. We demonstrate that the H I knots cannot be described as shocked interstellar cloudlets engulfed by the blast wave of the supernova remnant (SNR). Instead, we propose that these low velocity H I features are recombined wind driven clumps moving ahead of the supernova (SN) shock front. This hypothesis is supported by the ages derived for the knots, varying from [FORMULA] to [FORMULA] years, which implies that these features were originally accelerated during the Wolf-Rayet (WR) phase of the precursor star.

Key words: ISM: supernova remnants — ISM: individual: Cas A — stars: mass loss

* Fellowship of CONICET, Argentina
** Member of the Carrera del Investigador Científico of CONICET, Argentina.
*** Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy under contract to the National Science Foundation.

Send offprint requests to: Estela M. Reynoso

© European Southern Observatory (ESO) 1997