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Orion, the Hunter, is one of the most well-known constellations in the sky. The "belt," which consists of three bright stars in a line and can be seen without a telescope, is one of Orion's most well-known features.
Delta Orionis is the official name for the westernmost star in Orion's belt. (Because it has been observed for centuries by sky-watchers all over the world, it is also known by many other names in various cultures, such as "Mintaka.") Modern astronomers know that Delta Orionis is a complex multiple star system, rather than a single star.
Delta Orionis is a small stellar group consisting of three components and five stars in total: Delta Ori A, Delta Ori B, and Delta Ori C. Delta Ori B and Delta Ori C are single stars that may emit X-rays. Delta Ori A, on the other hand, has been discovered as a strong X-ray source and is a triple star system, as depicted in the artist's illustration.
Two closely spaced stars orbit each other every 5.7 days in Delta Ori A, while a third star orbits this pair with a period of over 400 years. The more massive, or primary, star in the closely separated stellar pair weighs approximately 25 times the mass of the Sun, while the less massive, or secondary star, weighs approximately ten times the mass of the Sun.
From Earth's perspective, the chance alignment of these two stars causes one to pass in front of the other during each orbit. This type of star system is known as a "eclipsing binary," and it allows astronomers to measure the mass and size of the stars directly.
Massive stars, despite their rarity, can have profound effects on the galaxies they inhabit. The radiation from these massive stars is so powerful that it blows powerful winds of stellar material away, affecting the chemical and physical properties of the gas in their host galaxies. These stellar winds also influence the fate of the stars, which will eventually explode as supernovas, leaving behind a neutron star or black hole.
A team of researchers gleaned important information about massive stars and how their winds play a role in their evolution and affect their surroundings by observing this eclipsing binary component of Delta Orionis A (dubbed Delta Ori Aa) with NASA's Chandra X-ray Observatory for the equivalent of nearly six days. The Chandra image is seen in the inset box in context with an optical view of the Orion constellation obtained from a ground-based telescope.
Because Delta Ori Aa is the nearest massive eclipsing binary, it can be used as a decoder key to better understand the relationship between stellar properties derived from optical observations and wind properties revealed by X-ray emission.
Delta Ori Aa's lower-mass companion star has a very weak wind and is very faint in X-rays. Astronomers can use Chandra to observe how the companion star blocks different parts of the more massive star's wind. This enables scientists to better understand what happens to the X-ray emitting gas surrounding the primary star, assisting in answering the long-standing question of where the X-ray emitting gas is formed in the stellar wind. The data show that the majority of the X-ray emission comes from the giant star's wind and is most likely caused by shocks caused by collisions between rapidly moving clumps of gas embedded within the wind. The researchers also discovered that the X-ray emission from specific atoms in Delta Ori Aa's wind changes as the stars in the binary move around. This could be caused by collisions between the winds of the two stars, or by a collision of the primary star's wind with the surface of the secondary star. As a result of this interaction, some of the wind from the brighter star is obstructed.
Parallel optical data from the Canadian Space Agency's Microvariability and Oscillation of Stars Telescope (MOST) revealed evidence for primary star oscillations caused by tidal interactions between the primary and companion stars as the stars traveled in their orbits. The parameters of the two stars were refined using measurements of changes in brightness in optical light as well as detailed analysis of optical and ultraviolet spectra. The researchers were also able to resolve some previously reported discrepancies between stellar parameters and models of how stars are expected to evolve over time.
These findings were recently published in The Astrophysical Journal in four coordinated papers led by Michael Corcoran (NASA's Goddard Space Flight Center & Universities Space Research Association), Joy Nichols (Harvard-Smithsonian Center for Astrophysics), Herbert Pablo (University of Montreal), and Tomer Shenar (University of Potsdam). The Chandra program is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for NASA's Science Mission Directorate in Washington. Chandra's science and flight operations are overseen by the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.