This small nebula is part of the Sh 2-15 to Sh 2-20 complex of HII regions located in the Sgr OB5
association in the Sagittarius arm. It lies next to RCW 140 (Sh 2-19).
No ionising stars are cited in the scientific literature but the nebula is believed to contain the infrared cluster [DB2000] 1
which may perhaps contain ionising stars.
Avedisova includes Sh 2-18 in the star formation region SFR 0.01-0.55
, along with three masers and the massive young stellar object IRAS 17441-2910
More details on the massive young stellar object IRAS 17441-2910.[1
Dutra and Bica conclude that the infrared cluster [DB2000] 1
is embedded in RCW 139 (Sh 2-18).[2
The RCW catalog explicitly identifies RCW 139 with Sh 1-14 = Sh 2-18 but gives the coordinates for RCW 139 as (0.1°, -0.3°), half a degree away from Sh 2-18. SIMBAD currently identifies RCW 139 with the emission line star 2MASS J17465744-2900419 which does indeed lie closer to the coordinates given in the RCW catalog. I've chosen to maintain the original RCW identification and use the coordinates for Sh 2-18. However, it may be that the RCW compilers were confused and were observing a star which they misidentified with the Sharpless nebula.
1. ^ Kendall, T. R., de Wit, W. J., & Yun, J. L. (2003). "VLT/ISAAC H-band
spectroscopy of embedded massive YSOs", Astronomy and Astrophysics, Vol.
408, 313-322. [2003A&A...408..313K]
2. ^ Dutra, C. M. & Bica, E. (2000). "New star clusters projected close to the
Galactic Centre", Astronomy and Astrophysics, Vol. 359, L9-L12.