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Title: A close encounter of the massive kind
Author: J. Maíz Apellániz, H. Sana, R. H. Barbá, J.-B. Le Bouquin, R. C. Gamen

We have used (a) HST ACS imaging and STIS spectroscopy, (b) ground-based PIONIER/VLT long-baseline interferometry, and (c) ground-based spectroscopy from different instruments to study the orbit of the extreme multiple system HD 93 129 Aa,Ab, which is composed of (at least) two very massive stars in a long-period orbit with e>0.92 that will pass through periastron in 2017/2018. In several ways, the system is an eta Car precursor. Around the time of periastron passage the two very strong winds will collide and generate an outburst of non-thermal hard X-ray emission without precedent in an O+O binary since astronomers have been able to observe above Earth's atmosphere. A coordinated multi-wavelength monitoring in the next two years will enable a breakthrough understanding of the wind interactions in such extreme close encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a binary system itself. In that case, we could witness a three-body interaction that may yield a runaway star or a stellar collision close to or shortly after the periastron passage. Either of those outcomes would be unprecedented, as they are predicted to be low-frequency events in the Milky Way.

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Title: Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate
Authors: David H. Cohen (1), Marc Gagné (2), Maurice A. Leutenegger (3,4), James P. MacArthur (1), Emma E. Wollman (1,5), Jon O. Sundqvist (6), Alex W. Fullerton (7), Stanley P. Owocki (6) ((1) Swarthmore College (2) West Chester University (3) NASA/Goddard Space Flight Center (4) CRESST and University of Maryland, Baltimore County (5) Caltech, Department of Physics (6) University of Delaware, Bartol Research Institute (7) Space Telescope Science Institute)

We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths Rstar. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star's closest visual companion at a distance of ~100 AU. The broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} x 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} x  10^{-6} Msun/yr, respectively. This is the first consistent modelling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind.

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HD 93129A is one of the most luminous stars in the Milky Way. It is an O-type supergiant located about 10,000 light-years from Earth in the bright nebula NGC 3372, the same nebula that harbors other super luminous stars, like Eta Carinae.

HD 93129A.kmz
Google Sky file

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