The Ara OB 1 a Association

The Ara OB1a association is one of the closest sites where triggered star formation is visible for multiple generations of massive stars. At about 1.3 kpc distance, it contains complex environments including cleared young clusters, embedded infrared clusters, CO clouds with no evidence of star formation, and clouds with evidence of ongoing star formation. In this review we discuss the research on this region spanning the last half-century. It has been proposed that the current configuration is the result of an expanding wave of neutral gas set in motion between 10–40 million years ago in combination with photoionization from the current epoch. 1. Historical Summary (1960–1985) Ara OB1a, while somewhat enigmatic, may be one of the best examples of triggered star formation in the local Galaxy. The triggering in the most active portion is easily imagined from images of the < 3 Myr NGC 6193/RCW 108-IR complex such as Figure 1. However the full association may be as much as 50 Myr in age and cover several degrees on the sky. Ara OB1 was first studied in detail by Whiteoak (1963). He used photographic photometry and objective prism spectroscopy to identify about 35 O and B star members. Whiteoak’s work was a follow up to an Hα survey by Rodgers, Campbell & Whiteoak (RCW; 1960), which cataloged 181 Hα emission regions in the southern sky, including RCW 108. NGC 6193 is an open cluster, discovered by James Dunlop in 1826, that is dominated by a pair of O stars, HD 150135 and HD 150136. These are the brightest optically revealed O stars in the association and are thought to be responsible for ionizing the bright rim of emission to the west (NGC 6188, discovered by John Herschel in 1836) which separates NGC 6193 from RCW 108–IR. The youth of the region is clear in sky survey plates and Figure 1 which show concurrent regions of ionized gas and dust lanes. Whiteoak noted two additional clusters in the region, NGC 6204 – about 2 degrees to the northeast – and NGC 6167 – about 1 degree