Shocked molecular hydrogen towards the Tornado nebula

Abstract

We present near-infrared and millimetre-line observations of the Tornado nebula (G357.7-0.1). We detected 2.12-mum H-2 1-0 S(1) line emission towards the suspected site of interaction with a molecular cloud revealed by the presence of an OH(1720-MHz) maser. The distribution of the H-2 emission is well correlated with the non-thermal radio continuum emission from the Tornado, and the velocity of the H-2 emission spans over 100 km s(-1), which both imply that the H-2 emission is shock excited. We also detected millimetre lines from (CO)-C-12 and (CO)-C-13 transitions at the velocity of the maser, and mapped the distribution of the molecular cloud in a 2x2 arcmin(2) region around the maser. The peak of the molecular cloud aligns well with an indentation in the radio continuum distribution of the nebula, suggesting that the shock of the nebula is being decelerated at this location, which is consistent with the presence of the OH(1720-MHz) maser and shocked H-2 emission at that location.