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Astron. Astrophys. 339, 201-207 (1998)

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1. Introduction

The bright galactic radio source Cassiopeia A (Cas A) is believed to be the supernova remnant (SNR) of a massive star with a zero age main sequence mass in excess of [FORMULA]. The progenitor probably evolved into a Wolf-Rayet star type WN8 (Fesen et al. 1987). Since Cas A is also the youngest known galactic SNR, studying this object potentially reveals important facts about the evolution, circumstellar medium and core collapse of its progenitor and, consequently, about massive stars in general. Spectroscopic data (Chevalier & Kirshner 1978, 1979) show that the remnant's blast-wave is ploughing through the helium and nitrogen enriched wind of the progenitor. A recent estimate of the swept-up and ejected mass based on the X-ray emissivity amounted to [FORMULA] and [FORMULA], respectively (Vink et al. 1996). The ratio of the two indicates that Cas A is presumably not anymore in its free expansion phase, but rather in a transition phase between the reverse-shock dominated phase and the adiabatic or Sedov phase of its evolution.

The best way to verify the dynamical status of Cas A is to measure the current expansion rate of Cas A. This has been done in the optical (Van den Bergh & Kamper 1983) and in the radio (Tuffs 1986, and Anderson & Rudnick 1995- hereafter AR95 -) with different results. AR95 reported an expansion timescale of [FORMULA] yr for the radio knots and [FORMULA] yr for the diffuse component, whereas the optical expansion time indicates an explosion date of AD [FORMULA]. These results seem contradictory, but the optical knots consist of high density gas ([FORMULA] cm-3) and suffer little deceleration. Furthermore, the optical knots comprise only a small [FORMULA] fraction of the total mass of the remnant. So the optical expansion of Cas A gives us important information on the age of Cas A, but little on the dynamical status. The radio emission, on the other hand, is associated with the relativistic electrons and magnetic fields, rather than the bulk mass of Cas A. The most direct handle on the dynamical status of Cas A comes from the emission closely associated with the bulk mass of Cas A, i.e. the X-ray emission from the shock heated gas.

Some kinematical information based on the X-ray emission has already been provided by Doppler shift measurements of X-ray line complexes implying an expansion asymmetry between the Northwest and Southeast parts of the remnant with a velocity difference of [FORMULA] 1500 km/s (Markert et al. 1983, Holt et al. 1994, Vink et al. 1996). The interpretation of the data is not clear: are we dealing here with bipolar mass ejection, a ring-like morphology (Markert et al.) or small but measurable deviations from spherical symmetry? Vink et al. (1996) reported a velocity line broadening of [FORMULA]4000 km/s, but it was later found that the ASCA SIS0, the instrument used to measure the broadening, suffered from a somewhat degraded spectral resolution. So [FORMULA] km/s should be regarded as an upper limit to the actual line broadening (Vink et al. 1997).

Here we present a direct measurement of the angular expansion of Cas A in X-rays over an interval of almost 17 years.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998