The effect of 53 μm IR radiation on 18 cm OH megamaser emission

OH megamasers ( OHMs) emit primarily in the main lines at 1667 and 1665 MHz and differ from their Galactic counterparts due to their immense luminosities large line widths and 1667/ 1665 MHz flux ratios which are always greater than 1. We find that these maser properties result from strong 53 mu m radiative pumping combined with line overlap effects caused by turbulent line widths similar to 20 km s(-1) pumping calculations that do not include line overlap are unreliable. A minimum dust temperature of similar to 45 K is needed for inversion and maximum maser efficiency occurs for dust temperatures similar to 80-140 K. We find that warmer dust can support inversion at lower IR luminosities in agreement with observations. Our results are in good agreement with a clumpy model of OHMs with clouds sizes less than or similar to 1 pc and OH column densities similar to 5 x 10(16) cm(-2) that is able to explain both the diffuse and compact emission observed for OHMs. We suggest that all OH main line masers may be pumped by far IR radiation with the major differences between OHMs and Galactic OH masers caused by differences in line width produced by line overlap. Small Galactic maser line widths tend to produce stronger 1665 MHz emission. The large OHM line widths lead to inverted ground state transitions having approximately the same excitation temperature producing 1667/ 1665 MHz flux ratios greater than 1 and weak satellite line emission. Finally the small observed ratio of pumping radiation to dense molecular gas as traced by HCN and HCO+ is a possible reason for the lack of OH megamaser emission in NGC 6240.