Delta Orionis
Credit
X-ray: NASA/CXC/GSFC/M.Corcoran et al.;
Optical: Eckhard Slawik
One of the most recognizable constellations in the sky is Orion,
the Hunter. Among Orion's best-known features is the "belt," consisting
of three bright stars in a line, each of which can be seen without a
telescope.
The westernmost star in Orion's belt is known officially as Delta
Orionis. (Since it has been observed for centuries by sky-watchers
around the world, it also goes by many other names in various cultures,
like "Mintaka".) Modern astronomers know that Delta Orionis is not
simply one single star, but rather it is a complex multiple star system.
Delta Orionis is a small stellar group with three components and five
stars in total: Delta Ori A, Delta Ori B, and Delta Ori C. Both Delta
Ori B and Delta Ori C are single stars and may give off small amounts of
X-rays. Delta Ori A, on the other hand, has been detected as a strong X-ray source and is itself a triple star system as shown in the artist's illustration (below).
In Delta Ori A, two closely separated stars orbit around each other every 5.7 days, 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 about 25 times the mass of the Sun, whereas the less massive, or secondary star, weighs about ten times the mass of the Sun.
Illustration
Credit: NASA/CXC/M.Weiss
The chance alignment of this pair of stars allows one star to pass in front of the other during every orbit from the vantage point of Earth. This special class of star system is known as an "eclipsing binary," and it gives astronomers a direct way to measure the mass and size of the stars.
Massive stars, although relatively rare, can have profound impacts on the galaxies
they inhabit. These giant stars are so bright that their radiation
blows powerful winds of stellar material away, affecting the chemical
and physical properties of the gas in their host galaxies. These stellar
winds also help determine the fate of the stars themselves, which will
eventually explode as supernovas and leave behind a neutron star or black hole.
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, a team of researchers gleaned
important information about massive stars and how their winds play a
role in their evolution and affect their surroundings. 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.
Since Delta Ori Aa is the nearest massive eclipsing binary, it can be
used as a decoder key for understanding the relation between the
stellar properties derived from optical observations, and the properties
of the wind, which are revealed by X-ray emission.
The lower-mass companion star in Delta Ori Aa has a very weak wind
and is very faint in X-rays.
Astronomers can use Chandra to watch as
the companion star blocks out various parts of the wind of the more
massive star. This allows scientists to better see what happens to the
X-ray emitting gas surrounding the primary star, helping to answer the
long-standing question of where in the stellar wind the X-ray emitting
gas is formed. The data show that most of the X-ray emission comes from
the wind of the giant star, and is likely produced by shocks resulting
from collisions between rapidly-moving clumps of gas embedded within the
wind.
The researchers also found that the X-ray emission from certain atoms
in the wind of Delta Ori Aa changes as the stars in the binary move
around. This may be caused by collisions between winds from the two
stars, or from a collision of the wind from the primary star with the
surface of the secondary star. This interaction, in turn, obstructs some
of the wind from the brighter star.
Parallel optical data from the Canadian Space Agency's
Microvariability and Oscillation of Stars Telescope (MOST) revealed
evidence for oscillations of the primary star produced by tidal
interactions between the primary and companion star as the stars travel
in their orbits. Measurements of the changes of brightness in optical
light plus detailed analysis of optical and ultraviolet spectra were
used to refine the parameters of the two stars. The researchers were
also able to resolve some previously claimed inconsistencies between the
stellar parameters and models of how the stars are expected to evolve
with time.
These results were published in four coordinated papers that were
recently published in The Astrophysical Journal 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). NASA's Marshall Space Flight Center in Huntsville, Alabama,
manages the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for Delta Orionis:
Scale: Main image is 21 x 25 degrees (462 x 540 light years); X-ray inset image is 44 arcsec across (0.25 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 05h 32m 00.40s | Dec -00° 17' 56.70"
Constellation: Orion
Observation Date: 4 pointings between 19 and 27 Dec 2012
Observation Time: 138 hours 63 min. (5 days 18 hours 63 min)
Obs. ID: 14567-14570
Instrument: ACIS/HETG
Color Code X-ray: Pink; Optical: Red, Green, Blue
Distance Estimate: About 1,240 light years
References:
- Corcoran, M et al, 2015, ApJ, 809, 132; arXiv:1507.05101
- Nichols, J et al, 2015, ApJ, 809, 133, arXiv:1507.04972
- Pablo, H et al, 2015, ApJ, 809, 134, arXiv:1504.08002
- Shenar, T et al, 2015, ApJ, 809, 135, arXiv:1503.03476
Source: NASA’s Chandra X-ray Observatory
