1 Introduction

3C 400.2 (G 53.6-2.2) belongs to a poorly understood class of supernova remnants (SNRs) characterized by a non-thermal radio shell, and thermal X-ray emission filling the interior.

Based on VLA observations performed in the radiocontinuum at 327.5 and 1465 MHz, Dubner et al. (1994) have shown that 3C 400.2 has a complex morphology which resembles two synchrotron shells that overlap on the northwestern side of the remnant. The larger shell in the southeast, looks almost complete and is centered near J2000: $\rm 19^{h} 38^{m} 53^{s}$, $17^{\circ} 12'55''$, the smaller one towards the northwest, is less complete and centered near J2000: $\rm 19^{h} 38^{m} 10^{s}$, $17^{\circ} 17' 52''$ (Fig. 1).

  

Figure 1: Grayscale of the SNR 3C 400.2 at 1465 MHz as taken from Dubner et al. (1994)

In the present paper we will refer to them as the "large shell" and the "small NW shell", respectively. The interior X-ray emission is thermal in nature and roughly ellipsoidal in shape. The brightest X-ray peak is attained in the region of superposition of the two radio rings, over the edge of the larger circle and close to the center of the smaller one (Seward 1990; Saken et al. 1995). The X-ray data have been interpreted in terms of White & Long's (1991) model of evaporation of embedded cloudlets. In the optical range, Winkler et al. (1993) obtained in the H$\alpha$ and [S II] lines the only images that cover the full extent of 3C 400.2. The optical filaments appear clearly correlated with the portions of the radio rings that overlap in the NW (Dubner et al. 1994).

It is important to discern the influence of the characteristics of the surrounding interstellar medium on the morphology of this SNR in the different spectral regimes. In that sense, the investigation of the distribution and kinematics of the neutral hydrogen around the SNR is a very useful tool, because it can provide a three-dimensional picture of the environs where the remnant evolves and may help to distinguish among alternative scenarios that can result in similar morphologies.

In order to investigate the environs of 3C 400.2 we carried out HI $\lambda$21 cm line observations in a field of $2\hbox{$.\!\!^\circ$}1 \times 2\hbox{$.\!\!^\circ$}1$ centered on J2000: $\rm 19^{h}38^{m}$, $+17^{\circ}13'$, using the DRAO Synthesis Telescope. Based on these observations we find that a possible scenario which simultaneously represents the X-ray, optical, radio continuum and HI characteristics of 3C 400.2, is a supernova exploding near the border of a clumpy neutral cloud about 5 times denser than its surroundings.