Monday, July 28, 2014

Discovery of a Stellar Behemoth in Westerhout 49

Very massive stars with masses exceeding ~100 times the Sun’s mass are incredibly rare. Nevertheless, these stars have strong influence on their environment through their powerful winds and prodigious amounts of ionizing radiation. The existence of very massive stars with reported masses of up to ~300 times the Sun’s mass indicate that there may not be a ‘real’ upper mass limit for very massive stars. These very massive stars are found in massive star-forming clusters such as NGC 3603, the Arches cluster and R136 in the Large Magellanic Cloud.

Artist’s impression of the Sun in comparison to R136a1 - a very massive star with an estimated ~300 times the Sun’s mass at birth. Image credit: ESO/M. Kornmesser.

A study by Shiwei Wu et al. (2014) presents the spectroscopic identification of a very massive star in the heart of the star-forming region Westerhout 49, hereafter, referred to as W49. The central cluster of W49 consists of dozens of massive OB-type stars. Intervening interstellar dust greatly obscures the massive stars in W49. As a result, these stars can only be observed in the near-infrared waveband. Referred to as W49 nr1, this very massive star resides in the heart of W49. It was spectroscopically identified using instruments on three telescopes - the Very Large Telescope (VLT) at Paranal in Chile, the New Technology Telescope (NTT) at La Silla in Chile, and the Large Binocular Telescope (LBT) at Mount Graham in Arizona.

W49 nr1 is the brightest star in the central cluster of W49 and it is classified as an O2-3.5If* star. Its estimated effective temperature is between 40,000 and 50,000 K, and its estimated luminosity is between 1.7 million and 3.1 million times the Sun’s luminosity. W49 nr1is more luminous than tens of billions of the coolest red dwarf stars put together. Stellar evolutionary models of W49nr1 suggest an initial mass of 100 to 180 times the Sun’s mass. If more variations are included in the models, the initial mass range is 90 to 250 times the Sun’s mass. Very massive stars live fast and die young. W49 nr1 is believed to be no more than 3 million years old.

Shiwei Wu et al. (2014), “The Discovery of a Very Massive Star in W49”, arXiv:1407.4804 [astro-ph.SR]