RCW 161 is a large faint region of diffuse nebulosity located between the Eagle
nebulae. It appears to form a connecting link between these much brighter HII regions. RCW 161, like the Eagle and Omega nebulae, appears to be ionised by members of the Ser OB1
It is not clear that RCW 161 is a single object as the ionising stars in this direction appear to be spread across several hundred parsecs.
The RCW catalog lists this as a large 80x40 arcminute medium bright nebula and associates it with the HII region [GS55] 143
Winkler associates RCW 161 with the variable B5I supergiant HD 167838
at a distance of 1620 parsecs. [1
] SIMBAD gives a warmer B3I class for this star.
Reed lists four ionising stars in this direction: LS 4936
(B2.5 Ib), LS 4929
(O9.5/B0 IAB), LS 4930
(O6) and LS 4923
(O9.5 III). According to Humphreys, LS 4930 = BD-15 4930 is a member of the Ser OB1 association. She gives a distance estimate of 1910 parsecs for Ser OB1. A 2000 paper gives a distance estimate of 2200 parsecs for the star itself. [2
Avedisova divides this nebulosity into three separate star formation regions. These can be seen in this SuperCOSMOS hydrogen-alpha image
. The easternmost of these regions (in galactic coordinates), SFR 16.35-0.16
, contains the Lockman compact radio HII region [L89b] 16.313-0.162
and is visible in infrared as can be seen in this MSX infrared image
. The more detailed Spitzer image
suggests that this region may consist of several objects. A 2009 paper
gives a distance estimate of 4100 parsecs for [L89b] 16.313-0.162 suggesting that this region likely lies far behind RCW 161.
The Ser OB1 association can be seen in this labelled false-colour hydrogen-alpha image
1. ^ Winkler, H. (1997). "Red and infrared colours of B stars and the reddening
law in the Galaxy", Monthly Notices of the Royal Astronomical Society,
Vol. 287, 481-494. [1997MNRAS.287..481W]
2. ^ Callaway, Matthew B., Savage, Blair D., Benjamin, Robert A., et al.
(2000). "Observational Evidence of Supershell Blowout in GS 018-04+44: The
Scutum Supershell", The Astrophysical Journal, Vol. 532, 943-969.