EDP Sciences
Free access
Volume 474, Number 2, November I 2007
Page(s) 541 - 547
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20077527

A&A 474, 541-547 (2007)
DOI: 10.1051/0004-6361:20077527

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

W. W. Tian1, 2, D. A. Leahy2, and Q. D. Wang3

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 )

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 $20\pm5$ 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 $20\pm5$ km s-1 are ~ $6 \times 10^{5}$ $M_{\odot}$ and ~ $2\times 10^{2}$ cm-3. We find an atomic hydrogen column density in front of G18.8+0.3 of $N_{\rm HI} \sim 2\times10^{22}$ 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 $2.2\times 10^{-12}$ erg cm-2 s-1, suggesting an intrinsic luminosity of $3.6\times 10^{34}$ erg s-1 for G18.8+0.3.

Key words: ISM: supernova remnants -- galaxies: ISM -- galaxies: kinematics and dynamics -- ISM: structure

© ESO 2007

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