Radio and X-ray images of the SNR G18.8+0.3 interacting with molecular clouds

¹ National Astronomical Observatories, CAS, Beijing 100012, PR China e-mail: tww@iras.ucalgary.ca
² Department of Physics & Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada
³ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA

Received: 22 March 2007
Accepted: 20 August 2007

Abstract

New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the supernova remnant (SNR) G18.8+0.3. High-resolution ¹³CO images reveal that molecular clouds overlap the radio filaments of G18.8+0.3. The ¹³CO emission spectrum over the full velocity range in the direction of G18.8+0.3 shows two molecular components with high brightness-temperature and three molecular components with low brightness-temperature, all with respective HI absorption. This implies that these clouds are in front of G18.8+0.3. In the HI images, the highest velocity absorption feature seen against the continuum image of G18.8+0.3 is at 129 km s^-1, which corresponds to the tangent point in this direction. This yields a lower distance limit of 6.9 kpc for G18.8+0.3. Absence of absorption at negative velocities gives an upper distance limit of 15 kpc. The broadened profile at  km s^-1 in the ¹³CO emission spectra is a strong indicator of a possible SNR/CO cloud interaction. Thus, G18.8+0.3 is likely to be at the distance of about 12 kpc. The upper mass limit and mean density of the giant CO cloud at  km s^-1 are ~  and ~ cm^-3. We find an atomic hydrogen column density in front of G18.8+0.3 of  cm^-2. The ROSAT PSPC observations show a diffuse X-ray enhancement apparently associated with part of the radio shell of G18.8+0.3. Assuming an association, the unabsorbed flux is  erg cm^-2 s^-1, suggesting an intrinsic luminosity of  erg s^-1 for G18.8+0.3.