An O2If* star found in isolation in the backyard of NGC 3603.

Abstract

In this Letter, we communicate the identification of a new Galactic O2If* star (MTT 68) isolated at a projected linear distance of 3 pc from the centre of the starburst cluster NGC 3603. From its optical photometry, I computed a bolometric luminosity M_Bol = -10.7, which corresponds to a total stellar luminosity of 1.5 × 10⁶ L_⊙. It was found to be an interesting similarity between MTT 68 and the well-known multiple system HD 93129. From Hubble Space Telescope (HST) F656N images of the NGC 3603 field, it was found that MTT 68 is actually a visual binary system with an angular separation of 0.38 arcsec, which corresponds to a projected (minimum) linear distance of r_A-B = 1.4 × 10^-2 pc. This value is similar to that for the HD 93129A (O2If*) and HD 93129B (O3.5) pair, r_A-B = 3.0 × 10^-2 pc. On the other hand, HD 93129A has a third closer companion named HD 93129Ab (O3.5) at only 0.053 arcsec, and taking into account that the X-ray to total stellar luminosity ratio for the MTT 68 system (L_X/L_Bol ∼1 × 10^-5) is about two orders of magnitude above the canonical value expected for single stars, I suspect that the MTT 68 system probably hosts another massive companion possibly to close to be properly resolved by the HST archive images.