Friday, October 18, 2013

An Isolated Giant

Very massive stars with ~100 times the Sun’s mass are exceedingly rare. It is still not known if such massive stars can form in isolation or only in star clusters. The centre region of the Milky Way galaxy hosts a unique environment where the process of star formation may differ from elsewhere in the galaxy. This makes the Galactic Centre an important test-bed for determining if very massive stars can indeed form in isolation. In the Galactic Centre are 3 very massive star clusters. The central Cluster surrounds the galaxy’s supermassive black hole, while 2 other massive star clusters, the Arches cluster and the Quintuplet cluster, are located no more than ~100 light years away.

Figure 1: Artist’s impression of the Arches cluster.

Images of the Galactic Centre acquired by the Hubble Space Telescope (HST) reveal a number of isolated massive stars located outside of the 3 known massive star clusters. Four scenarios that may explain the origin of these isolated massive stars are that these stars (1) were formed in isolation; (2) were formed within clusters that have already dispersed; (3) were ejected from one of the 3 known massive star clusters; (4) belong to clusters that have yet to be discovered.

WR 102ka is one of the most massive and most luminous stars known in the galaxy. It also happens to be one of those isolated massive stars in the Galactic Centre. The current mass of WR 102ka is estimated to be ~110 times the Sun’s mass. Since a massive star like WR 102ka has a very high mass loss rate, the initial mass of WR 102ka is believed to be ~150 times the Sun’s mass. WR 102ka blazes with ~3 million times the Sun’s luminosity. Its estimated age is ~2 million years. Very massive stars like WR 102ka live fast and die young.

Figure 2: Artist impression of a massive star.

 Figure 3: A massive star with ~20 times the Sun’s mass is shown next to a stack of lighter, Sun-like stars. For every one such massive star, there could be 500 to as many as 2000 smaller stars. Such a proportion of small to big stars is also expected for WR 102ka if it formed in a massive star cluster where it would be accompanied by a large number of less massive counterparts. Credit: NASA/JPL-Caltech.

In 2009, a team of astronomers used the European Southern Observatory’s (ESO) Very large Telescope (VLT) in Chile to study WR 102ka and its surroundings. If WR 102ka belongs in a star cluster that has yet to be discovered, simulations predict that in such a massive star cluster initially containing a single star with the mass of WR 102ka, there should be ~300 stars exceeding 20 times the Sun’s mass and ~10 of these stars may even exceed 100 times the Sun’s mass. Nevertheless, observations reveal no massive star cluster is associated with WR 102ka, even though such a cluster would have been clearly detectable in the observations.

It is also unlikely that WR 102ka formed within such a massive star cluster ~2 million years ago and that the cluster has since dispersed. This is because ~2 million years is too short a time for such a massive star cluster to disperse. For example, the Quintuplet cluster is an older massive star cluster with an estimated age of 3 to 5 million years and it is still very much intact.

It may be possible that WR 102ka was ejected from one of the 3 massive star clusters. There are a number of mechanisms that can eject a star from it natal cluster. Measurements show that WR 102ka has a radial velocity of 60 km/s. Even with a slow runaway velocity, say equal to its radial velocity of 60 km/s, WR 102ka could easily have arrived at its current location, far from any of the 3 massive star clusters in the Galactic Centre within its ~2 million year lifetime.

However, a number of observational evidences do not support an ejection scenario for WR 102ka. Firstly, it is unlikely that a very massive star like WR 102ka (possible the most massive star were it part of a cluster) can be ejected from a cluster while many less massive stars remain bound. Secondly, infrared observations using the Spitzer space telescope show that a bow shock in the direction of motion is absent around WR 102ka, while the same observations detected bow shocks around two other isolated massive stars in the Galactic Centre. For these two stars, one was ejected from the Central Cluster while the other was ejected from the Quintuplet cluster. Each star has a bow shock in its direction of motion as it ploughs through the interstellar medium.

Thirdly, images from the Spitzer space telescope and from the Wide-field Infrared Survey Explorer (WISE) show the presence of a dusty circumstellar nebula around WR 102ka. The nebula is probably stellar material thrown out by WR 102ka during previous evolutionary stages. The central position of WR 102ka within its nebula suggests that the star has remained at the same location.

All these observations show that WR 102ka, one of the most massive and most luminous stars in the galaxy, may have formed in isolation. In fact, the majority of isolated massive stars in the Galactic Centre do not display obvious bow shocks, suggesting that the massive star population in the Galactic Centre consists of stars formed in clusters, stars ejected from clusters and stars that formed in relative isolation.

L. M. Oskinova et al. (2013), “One of the most massive stars in the Galaxy may have formed in isolation”, arXiv:1309.7651 [astro-ph.SR]