Rare emission nebula driven by O star

Image credit: Don Goldman

The halo of NGC 6164 imaged here is likely a result of a previous episode of mass loss in the lifetime of the central O star, HD 148937. The nebula itself is bioplar and, as seen in bipolar planetary nebulae, its shape may arise because of interaction between the central star and a companion. In this case, the star is thought to be a triple system.

NGC 6164-5 is a bipolar nebula surrounding the peculiar O 7f star HD 148937. Although the nebula was believed initially to represent a planetary nebula, it is now understood to represent a shell-like ejected nebula, formed by the winds of its young central star. Wind blown nebulae are usually considered the outcome of Wolf-Rayet type stars (WR), evolved O-type stars that have left the main sequence, molding their surroundings by way of fierce stellar winds. In the case of the S-shaped bipolar nebula NGC 6164-5, the central star is not a WR star, but a young O-type supergiant of 40 solar masses. Relatively few examples of such objects are known to exist, making NGC 6164-5 particularly unique. The true age of the star is not known although estimates of its age vary from a pre-main sequence age of 350,000 years to a more evolved age of one to two million years. HD 148937 is a peculiar type of Of star with exotic emission lines (carbon emission lines (CIII) equal in strength to Nitrogen (NIII)) and other peculiar spectra indicative of significant mass loss. Only five such stars are known to exist, three in our galaxy and two in Small Magellanic Cloud.

(Source)

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