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G28.17+0.05: An Unusual Giant H I Cloud in the Inner Galaxy

New 21 cm H I observations have revealed a giant H I cloud in the Galactic plane that has unusual properties. It is quite well defined, about 150 pc in diameter at a distance of 5 kpc, and contains as much as 105 M☉ of atomic hydrogen. The outer parts of the cloud appear in H I emission above the H I background, while the central regions show H I self-absorption. Models that reproduce the observations have a core with a temperature ≲40 K and an outer envelope as much as an order of magnitude hotter. The cold core is elongated along the Galactic plane, whereas the overall outline of the cloud is approximately spherical. The warm and cold parts of the H I cloud have similar and relatively large line widths, ~7 km s-1. The cloud core is a source of weak, anomalously excited 1720 MHz OH emission, also with a relatively large line width, which delineates the region of H I self-absorption but is slightly blueshifted in velocity. The intensity of the 1720 MHz OH emission is correlated with NH derived from models of the cold core. There is 12CO emission associated with the cloud core. Most of the cloud mass is in molecules, and the total mass is greater than 2 × 105 M☉. In the cold core the H I mass fraction may be ~10%. The cloud has only a few sites of current star formation. There may be ~100 more objects like this in the inner Galaxy; every line of sight through the Galactic plane within 50° of the Galactic center probably intersects at least one. We suggest that G28.17+0.05 is a cloud being observed as it enters a spiral arm and that it is in the transition from the atomic to the molecular state.