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

TL;DR

This paper reports the discovery of a new isolated Galactic O2If* star, MTT 68, which is a binary system with a potential additional massive companion, providing insights into massive star formation outside clusters.

Contribution

First identification of an isolated O2If* star in the Milky Way, revealing its binary nature and possible additional companions, challenging existing star formation theories.

Findings

MTT 68 is an isolated O2If* star 3 pc from NGC 3603.

MTT 68 is a visual binary with a 0.38 arcsec separation.

X-ray luminosity suggests a possible unresolved massive companion.

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 star-burst 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 x 10^{6} L_Sun. It was found an interesting similarity between MTT 68 and the well known multiple system HD 93129. From Hubble Space Telescope 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 x 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 x 10^{-2} pc. On the other hand, HD93129A 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 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 not resolved by the HST archive images.