**title** Asp Conference Series, Vol. **volume**, **publication Year** **editors** G28.17+0.05, a Giant Atomic/molecular Cloud

HI 21 cm observations with the NRAO 140 Foot telescope have revealed a giant HI cloud (G28.17+0.05) in the Galactic plane that has unusual properties. The cloud is 150 pc in diameter, is at a distance of 5 kpc, and contains as much as 105M⊙ of atomic hydrogen. The cloud consists of a cold core, T ∼ 10 K, and a hotter outer envelope, T ≥ 200 K. There is no observable difference in the HI line widths, ∼ 7 km s, between the core and the envelope. Anomalously-excited 1720 MHz OH emission, with a similar line width, is associated with the core of the cloud. The cloud core also exhibits CO and CO self-absorption which indicates that most of the cloud mass is in molecules. The total mass of the cloud is > 2×105M⊙. The cloud has only a few sites of current star formation. If similar clouds are associated with other observed sites of anomalously-excited 1720 MHz OH emission, there may be as many as 100 more of these objects in the inner galaxy. 1. G28.17+0.05 In HI G28.17+0.05 was discovered in 21cm HI observations made with the NRAO 140 Foot Telescope (Minter et al. 2001). At the time of these observations these data comprised the most detailed look at this part of the Galactic Plane in HI. G28.17+0.05 is shown in Figure 1. It appears as a shell of bright emission surrounding an area of reduced emission centered on l,b = 28. ◦17 + 0. ◦05. G28.17+0.05 is estimated to have a distance of ∼ 5 kpc at which its diameter would be 150 pc. The reduced emission in the central area of G28.17+0.05 (hereafter referred to as the core) is the result of HI self-absorption (see Figure 2). Models of the 21cm HI emission from G28.17+0.05 indicate the temperature in the core is T ≤ 40 K while the shell has T ≥ 200 K. The core contains 1/3 of the total HI mass. The column density of atomic hydrogen through the center is NHI ≥ 5− 12× 10 20 cm, depending on the exact value of the temperature in the core. The total atomic hydrogen mass within G28.17+0.0 is ∼ 105M⊙. The line widths (FWHM) of HI in the core and shell are both ∼ 7 km s. This results in G28.17+0.05 being gravitationally bound from just its atomic mass alone!