4 Summary

We performed HI 21 cm line observations in a $2\hbox{$.\!\!^\circ$}1 \times 2\hbox{$.\!\!^\circ$}1$field around the SNR 3C 400.2 with the DRAO Synthesis Telescope. These observations revealed the existence of neutral gas features associated with the remnant. Namely: i) a clumpy HI cloud delineating the north, west and southern border of 3C 400.2 in the velocity range +14 to +42 km s^-1  and ii) gas probably accelerated by the shock front at v=-49.5 and +69 km s^-1.

A simple picture that can explain the observed morphology of 3C 400.2 in the different spectral regimes is as follows: the remnant originated from a supernova explosion occurring near the border of a neutral gas cloud approximately 21 cm^-3 in density. The position of the explosion coincides with the center of the HI void detected at v=+27 km s^-1  and with the geometrical center of the bright optical filaments. The "small NW shell" evolved within the dense cloud until it broke out originating the "large radio shell" where it is evolving into a medium about five times lighter. We argue that the optical filaments originated in regions where the front shock encountered and overtook density enhancements in the surrounding interstellar medium. The excellent correspondence between regions of H$\alpha$ emission with regions of enhanced HI emission detected between +26 and +28 km s^-1 confirms this scenario. By comparing the HI observations with the X-ray image, we found a hole in the HI distribution at +28 km s^-1  coinciding exactly with the maximum in the X-ray emission, this suggests that the neutral gas could have been partially evaporated thus producing centrally condensed X-ray morphology.

From the present observations a distance of 2.8 $\pm$ 0.8 kpc to 3C 400.2 was calculated.

Acknowledgements

We would like to thank Dr. Tom Landecker for his valuable contribution in the first stages of this work. E.B.G. thanks DRAO for support and the kind hospitality during her visit to Penticton. The Dominion Radio Astrophysical Observatory Synthesis Telescope is operated as a national facility by the National Research Council of Canada. This research was supported by the CONICET grant PMT-PICT 0107.