Westerlund 1 is the most massive starburst cluster known in the Milky Way galaxy. It contains more than 50 known massive O-type stars, Wolf-Rayet stars and red supergiants (RSGs) at various stages of post-main sequence evolution. Wright et al. (2013) acquired images that show the existence of an ionized circumstellar nebula surrounding the RSG W26 in Westerlund 1. W26 is one of the most luminous and largest RSGs known in the galaxy. It is estimated to have ~320,000 times the Sun’s luminosity and ~1500 times the Sun’s diameter. If placed in the centre of the Solar System, the visible surface of W26 would reach to the orbit of Jupiter and another 260 million kilometres further. The presence of a nebula around W26 suggests it is a highly evolved RSG with extensive mass-loss in its recent history.
An artist’s impression of a RSG from the surface of hypothetical planet. Even though the planet is over a billion kilometres away, the RSG still spans a large breadth of the sky and the planet’s surface scorches under the intense heat. Credit: Scott Cornett.
The ionized circumstellar nebula surrounding W26 consists of a detached circumstellar shell or ring surrounding the star and a triangular nebula located ~0.2 pc from the star. Images taken by the Hubble Space Telescope (HST) reveal filamentary structures in the triangular nebula. The filaments appear to be oriented towards the nearby blue supergiant (BSG) W25 and may be associated with this star instead of W26. However, a thin nebulous strip is shown to connect the triangular nebula with the circumstellar nebula around W26. Such a connection probably suggests that the triangular nebula is associated with W26, although it may be somewhat shaped or photoionized by W25.
The triangular nebula may be some sort of outflow from W26, similar to bipolar outflows observed around other RSGs. Then again, if the triangular nebula originates from a bipolar outflow from W26, the absence of a similar nebula on the other side of W26 is puzzling. Nevertheless, observations of bipolar outflows around a number of RSGs, such as the RSG Sher 25, show one side of the outflow to be significantly brighter than the other. This suggests that a second outflow exists on the other side of W26, but is undetected. Alternatively, outflows exist on both sides of W26, but only one side is being photoionized by the nearby BSG W25. Yet another possibility is the triangular nebular could be a flow of material being blown off the circumstellar nebula by the cumulative cluster wind and radiation field of Westerlund 1.
W26 is unusual for having an ionized nebula surrounding it because RSGs are too cool to produce ionizing photons. As a result, the ionized nebular is probably due to photoionization by the nearby BSG W25 or by the presence of a hot companion star around W26, although the latter possibility is unlikely. Other potential sources of photoionization include the radiation field of Westerlund 1 and shock-induced ionization from the collision of the nebula with the intra-cluster medium. W26 provides a unique opportunity to observe extensive mass-loss from a highly evolved RSG. Further observations with higher resolution spectra can allow the expansion velocity of the nebula around W26 to be measured.
Wright et al. (2013), “The Ionized Nebula surrounding the Red Supergiant W26 in Westerlund 1”, arXiv:1309.4086 [astro-ph.SR]