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Astronomers from the University of Sheffield and Pennsylvania State University have performed X-ray and optical observations of a massive star known as Melnick 39. As a result, they found that Melnick 39 is a colliding-wind binary system composed of two O-type supergiant stars. The finding was on March 21 on the pre-print server arXiv.

Observations show that hot, massive stars are so luminous that their radiation can drive fast stellar winds with terminal velocities of 1,000–3,000 km/s. Wolf-Rayet (WR) stars, which are evolved massive stars with their hydrogen-rich envelopes stripped off, showcase the most powerful fast winds.

In general, it is assumed that such powerful stellar winds are associated with three types of shocks that can produce X-ray emission: shocks in the wind itself, colliding winds in a binary system, and shocked wind in a circumstellar bubble.

Melnick 39, also known as Brey 78, was first identified as a WR star in the Tarantula Nebula—a large region of ionized atomic hydrogen (H II) in the Large Magellanic cloud (LMC). The star's mass was inferred to be about 145 solar masses, which makes it one of the most massive stars detected so far. Melnick 39 is also one of the brightest X-ray sources in the Tarantula Nebula.

Previous observations of Melnick 39 have suggested that it may be a binary system with an estimated preliminary orbital period of 92.6 days and now a team of astronomers led by University of Sheffield's Andrew Pollock has provided evidence confirming these assumptions.

"The inclusion of optical time-series spectroscopy from the VLT FLAMES Tarantula Survey and archival HST [Hubble Space Telescope] spectroscopy confirms Melnick 39 as a double-lined O2.5 If/WN6+O3 V–III spectroscopic binary with orbital period near 648 days," the researchers wrote in the paper.

The observations found that the primary and secondary stars of Melnick 39 have minimum masses of 105 and 80 solar masses, respectively. The mass ratio for the system was determined to be 0.76.

According to the paper, the primary and secondary stars have effective temperatures at a level of 44,000 and 48,000 K, respectively. The age of the primary star was estimated to be about 1.5 million years, while its companion is assumed to be some 400,000 years younger. The wind velocity for both components was calculated to be approximately 2,600 km/s.

The astronomers added that the X-ray luminosity of Melnick 39 scales with the inverse of the binary separation, as expected for colliding-wind binaries in the adiabatic regime. Therefore, they concluded that Melnick 39 is a colliding-wind binary system.

The study also found that Melnick 39 reaches a maximum count rate near 10 cts/ks at the minimum periastron orbital separation of 3.5 AU, according to the new orbital solution. Thus, it may be compared with Melnick 34, the brightest colliding-wind binary from the Chandra X-ray Visional Project 'The Tarantula—Resolved by X-rays'—at its minimum near 35 cts/ks.

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