RCW 113, also called Gum 55, is a huge region of diffuse nebulosity surrounding the Sco OB1
association and its core ionising cluster, NGC 6231
Gum notes that this region is "A large roughly semicircular loop, whose ragged appearance is due in part to overlying obscuration. At its centre is the galactic cluster NGC 6231, which is the nucleus of an O-Association".
Humphreys lists 3 O-stars and 27 ionising B-class stars in Sco OB1 and gives a distance of 1910 parsecs.
Kharchenko lists 15 ionising stars for NGC 6231 including 9 O-stars, 5 B-stars and the Wolf-Rayet binary star WR 79
. The hottest O-star listed by Kharchenko is the O6 III:(f)p binary giant HD 152233
. She gives an age of 6.4 million years and a distance of 1250 parsecs for the cluster. A 2008 study gives a distance of 1640 parsecs and finds 15 O-stars in the direction of the cluster core.
You can find a better view of the bat-like edge of this nebula here
and a stunning view of the full RCW 113 nebula and the larger Sco OB1 region here
You can see the full extent of the vast nebular clouds in the Sco OB1 region in this SuperCOSMOS explorer image
and some photographs of SFO 82
, the prominent bright-rimmed cometary globule visible inside RCW 113, here
. SFO 82 is often called "the Dark Tower of Scorpius" by amateur astronomers and is described in detail in this 1976 paper
15 O-stars in the direction of the cluster core a distance of 1640 parsecs. [1
1. ^ Sana, H., Gosset, E., Nazé, Y., et al. (2008). "The massive star binary
fraction in young open clusters - I. NGC 6231 revisited", Monthly Notices
of the Royal Astronomical Society, Vol. 386, 447-460.