RCW 85 HII region / SFO 74 in Centaurus 

Wonderful Description and Annotation By Alex Zaytsev

RCW 85 HII region / SFO (Star Forming Region) 74 [1-4] around HD 125158 star [5] consists of a prominent ionization front, complete with “wing rims” on both sides followed by a large collection of neutral gas clouds stretching across the area in the sky almost 0.6 deg across. With the distance to RCW 85 estimated at 1.5 kpc [4] that angular size corresponds to a linear size of about 16 pc. A detailed summary of structure of the shock front of SFO 74 is given in Fig. 10-12 from [4] as shown in Fig. S1. 

Fig. S1. The structure of the shock front of SFO 74 as given in Fig. 10-12 in [4].

The “exciting star” for the main ionization front of RCW 85 / SFO 74 as identified in [6, 7] is HD 124314 [8] which is located at a significant angular distance from the front, as shown in Fig. S2, yet the estimated linear distance between them is estimated to be “only” about 15 pc. 

Fig. S2. Identifying the ionizing star HD 124314 [8] for the shock front of RCW 85 / SFO 74, using the data from a wide angle image for the same area of the sky available under [9]. 

The most active area of star formation identified in this region in [4] as “Cloud B”, is rather compact: only 2 arc.min across, which is under 1pc of linear size at the estimated distance, and located behind the “tip rim” of the shock front, as illustrated in Fig. S3.

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