Optical spectroscopy of the blue supergiant Sk-69 279 and its circumstellar shell with SALT

TL;DR

This study uses optical spectroscopy from SALT to analyze the blue supergiant Sk-69 279 and its circumstellar shell, revealing its stellar properties, chemical abundances, and the shell's composition, suggesting recent stellar evolution or binary interaction.

Contribution

First detailed spectroscopic analysis of Sk-69 279 and its shell, linking stellar surface abundances with circumstellar material and implications for massive star evolution.

Findings

Sk-69 279 classified as O9.2 Iaf star.

Shell composed mainly of CNO processed material.

Star likely recently evolved off the main sequence or from binary evolution.

Abstract

We report the results of optical spectroscopy of the blue supergiant Sk-69 279 and its circular shell in the Large Magellanic Cloud (LMC) with the Southern African Large Telescope (SALT). We classify Sk-69 279 as an O9.2 Iaf star and analyse its spectrum by using the stellar atmosphere code CMFGEN, obtaining a stellar temperature of \approx 30 kK, a luminosity of log(L_*/Lsun)=5.54, a mass-loss rate of log(\dot{M}/(Msun/yr))=-5.26, and a wind velocity of 800 km/s. We found also that Sk-69 279 possesses an extended atmosphere with an effective temperature of \approx 24 kK and that its surface helium and nitrogen abundances are enhanced, respectively, by factors of \approx 2 and 20--30. This suggests that either Sk-69 279 was initially a (single) fast-rotating (\ga 400 km/s) star, which only recently evolved off the main sequence, or that it is a product of close binary evolution. The long-slit spectroscopy of the shell around Sk-69 279 revealed that its nitrogen abundance is enhanced by the same factor as the stellar atmosphere, which implies that the shell is composed mostly of the CNO processed material lost by the star. Our findings support previous propositions that some massive stars can produce compact circumstellar shells and, presumably, appear as luminous blue variables while they are still on the main sequence or have only recently left it.