The Tarantula Nebula is a gift that keeps on giving – if you’re into really massive stars, that is. It makes for a pretty picture, sure, but Tarantula, in our little galactic sibling the Large Magellanic Cloud at a distance of 170,000 lightyears (50 kpc), is the largest region of ionised hydrogen (HII) in the entire Local Group of galaxies. That includes our own Milky Way, the Andromeda galaxy, and a smattering of dwarfs. Tarantula’s huge mass of HII gas is testament to a rich star formation history: the region, also known as 30 Doradus, has given birth to at least 5 distinct populations of stars, hosting a complex of clusters. This nebula gets stellar astrophysicists really excited.
Some of the region’s clusters contain the most massive stars we know today. Stars so massive that our current theories have trouble explaining how they even are able to form. Their evolution too is very different from our billions of Average-Joe stars, and their impact on their surroundings far more profound, as their powerful winds heat and pummel the surrounding medium. And now, recent observations of these stars have thrown another curveball: a supermassive star that appears to be all on its own.
The most massive stars form almost exclusively in clusters, as the clouds they form out of have to be so massive that many smaller stars are inevitably born in the process. At least, that ‘s how we understand it, and it’s what we generally observe. In our own Galaxy, we know several such clusters packed with massive stars, particularly near the Galactic Centre. Tarantula contains the spectacular cluster Radcliffe 136 (R136), and just last year British researchers discovered not one, but several stars with masses over 150 times our Sun.
Some scientists even propose that the combined winds from supermassive clusters like R136 could even be responsible for blasting heavy elements out of their galaxies entirely, into the intergalactic medium.
The newly discovered behemoth star, named VFTS 682, is around a million times more luminous than the Sun, and around 150 times as massive. The extreme mass of the newly discovered VFTS 682 in Tarantula is nothing new – we already knew that the region hosted many of the most massive known stars. But what makes this one unique is that it’s not located in or near any of the known clusters. The obvious question then is: how did it end up there? Was it born in isolation, or was it slung out of the nearest cluster, R136? Both scenarios pose problems.
Our current modes of how massive stars form have no way of explaining the formation of a 150 solar star all on its own. Stars do get ejected from clusters, as a result of the dynamical interactions in the densely packed cluster centres. Just last year the same survey of the Tarantula nebula yielded some stunning images of a runaway star from a cluster in the very same nebula. The velocity VFTS 682 appears to be travelling suggest this is the more likely scenario.
The paper describing the newly discovered star, authored by Joachim Bestenlehner of Armagh Observatory, is the just one of a series of neat results to come out of a large-scale spectroscopic survey of the nebula with VLT’s optical spectrograph FLAMES. I hope there’s more to come.
Joachim M. Bestenlehner, Jorick S. Vink, G. Gräfener, F. Najarro, C. J. Evans, N. Bastian, A. Z. Bonanos, E. Bressert, P. A. Crowther, E. Doran, K. Friedrich, V. Hénault-Brunet, A. Herrero, A. de Koter, N. Langer, D. J. Lennon, J. Maíz Apellániz, H. Sana, I. Soszynski, & W. D. Taylor (2011). The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136 A&A Letters arXiv: 1105.1775v1
Paul A Crowther, Olivier Schnurr, Raphael Hirschi, Norhasliza Yusof, Richard J Parker, Simon P Goodwin, & Hasan Abu Kassim (2010). The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit MNRAS 308 , 731-751 (2010) arXiv: 1007.3284v1
Evans, C., Walborn, N., Crowther, P., Hénault-Brunet, V., Massa, D., Taylor, W., Howarth, I., Sana, H., Lennon, D., & van Loon, J. (2010). A massive runaway star from 30 Doradus The Astrophysical Journal, 715 (2) DOI: 10.1088/2041-8205/715/2/L74
Norman Murray, Brice Ménard, & Todd A. Thompson (2010). Radiation pressure from massive star clusters as a launching mechanism for super-galactic winds Arxiv arXiv: 1005.4419v1