1999AJ....118..930D

We have carried out a study of the interaction of the supernova remnant (SNR) G18.8+0.3 with the surrounding interstellar medium. Observations of the H I, ¹²CO, ¹³CO, and OH (1720 MHz) lines were performed toward a large field around G18.8+0.3 using the Parkes (Australia) 64 m single-dish telescope, the 4 m NANTEN millimetric telescope (Las Campanas Observatory, Chile), and the Very Large Array (NRAO). The present survey has revealed the existence of an elongated molecular cloud (about 21'x6' in size) adjacent to the more flattened borders of the SNR and to the far side of the remnant. The overall CO and H I morphology and kinematics allow us to conclude that the explosion occurred near the border of a preexisting molecular cloud, driving a slow shock into the cloud. The presence of diffuse shock-heated dust with a color temperature of about 30 K was shown in coincidence with the molecular feature using IRAS data. The shocked CO and H I gas was detected between +10 and +27 km.s^–1 (LSR). The systemic velocity of this complex, about +19 km.s^–1, yields a kinematic distance of about 1.9 kpc for G18.8+0.3. The shock is presently expanding into the cloud at ∼10 km.s^–1. Masses of the order of 7300, 1100, and 55 M_☉ are estimated for the associated molecular hydrogen, atomic hydrogen, and heated dust, respectively. The total kinetic energy transferred by the supernova shock to the surrounding interstellar medium is of the order of 10⁴⁹ ergs. An age of ∼16,000 yr is calculated for G18.8+0.3. The extended molecular feature with a density of ∼600 cm^–3 has denser clumps immersed in it, with densities ranging from ∼2500 to ∼6000 cm^–3. Three out of five of these clumps were found to contain luminous IRAS pointlike sources compatible with protostellar candidates, suggesting a causal relationship with the supernova explosion that deserves further investigation. The interferometric search for OH (1720 MHz) masers gave negative results. These OH masers can be short-lived occurrences and dissipate quickly in the cooling postshock gas because of the very restrictive conditions under which masers form. Therefore, their absence does not preclude the hypothesis of shock-cloud interaction.