Publication Date

2007

Journal or Book Title

Astronomy and Astrophysics

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.

Comments

This is the pre-published version harvested from ArXiv. The published version is located at http://www.aanda.org/index.php?option=com_article&access=doi&doi=10.1051/0004-6361:20077527&Itemid=129

Volume

474

Pages

541-547

Issue

2

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