NU: Star, not so bright

[bystander]

Star, not so bright
Northwestern University | via EurekAlert | 20 Oct 2010

Model explains evolution of unusual binary system, why large star not so luminous

[url=http://chandra.harvard.edu/photo/2007/m33x7/]M33 X-7: Heaviest Stellar Black Hole Discovered in Nearby Galaxy[/url] [i]Credit: X-ray: NASA/CXC/CfA/P.Plucinsky et al.; Optical: NASA/STScI/SDSU/J.Orosz et al.[/i]

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In a galaxy far away, an exceptionally massive black hole is traveling around a massive star in an unusually tight orbit. Also odd, the star is not as bright as it should be.

Astronomers have puzzled over this X-ray binary system, named M33 X-7, but no one could explain all of its features. Now a Northwestern University research team has.

The researchers have produced a model of the system's evolutionary history and formation that explains all of the system's observational characteristics: the tight orbit, the large masses of the star and black hole, the X-ray luminosity of the black hole and why its companion star is less luminous than one would expect, given its mass.

The evolutionary model will be published Oct. 20 by the journal Nature. The research will improve astronomers' understanding of how massive stars evolve and interact with their host environment as well as shed light on the physics behind the process of black hole formation.

ScienceShot: Black Hole vs. Massive Star
Science NOW | 20 Oct 2010

[i]Credit: Matthew McCrory, Francesca Valsechhi, Vicky Kalogera[/i]

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The binary system known as M33 X-7 is an odd duck indeed. Discovered in 2007, the two objects, a 16-solar-mass black hole and a 70-solar-mass blue star, fit no known mold. The black hole is too massive, the star too dim, and both circle each other too erratically. So a team of astronomers threw away the mold. They performed several hundred thousand simulations until they found out what process had created the black hole and its binary partner, both located in a galaxy some 3 million light-years away in the constellation Triangulum. Four million years previously, the team reports online today in Nature, the black-hole's progenitor was actually the more massive of the two, tipping the scales at 97 solar masses. But the big star fused its hydrogen and helium into heavier elements with lightning speed, expelling gigantic solar winds. The companion star lapped up gobs of that expelled material, growing quickly to its current size. When the progenitor star had no more helium to fuse, it collapsed into a black hole—which we now see is pulling matter back in from the companion star at a furious clip.

Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system - F Valsecchi et al

• Nature (advance online) (20 Oct 2010) DOI: 10.1038/nature09463

[bystander]

Massive X-ray-Belching Black Hole Finally Fits Cosmic Theory
Live Science | Space | 20 Oct 2010

A massive black hole that is spewing X-rays and locked in a tight orbital dance around a huge, dim star finally has a good origins story.

Unlike binary set-ups that result when a giant star absorbs mass from a companion star that has nearly exhausted its nuclear fuel, the huge black hole M33 X-7 could have formed because, in this case, the companion still had plenty of hydrogen to burn, according to a new study.

The enormous stellar black hole has 15.7 times the mass of the sun and is orbiting an even larger star – which is 70 times the sun's mass – once every 3.45 days. Other X-ray binaries typically have stellar black holes of 10 solar masses.

The pair is located about 2.7 million light-years from Earth in the galaxy Messier 33.

Researchers had been hard-pressed to come up with a satisfying explanation for M33 X-7 using existing models of binary X-ray systems.

[Ann]

Fascinating but frustrating!

When the progenitor star had no more helium to fuse, it collapsed into a black hole

Yeah? I thought stars collapsed into black holes when they had built up an iron core.

I also don't understand why the hefty blue 70-solar mass star would be dim for its mass. Is it because it is rapidly shrinking as the black hole is sucking off its outer atmosphere, while most of its mass is sitll safely tucked inside the star's interior? In other words, is the companion star small for its mass?

Ann

[bystander]

Massive X-ray-Belching Black Hole Finally Fits Cosmic Theory

Black hole's new origins story

In their model, the black hole-star pairing stems from a giant star – the future black hole – nearly 100 times the mass of our sun, circling a second star of about 30 solar masses every three days or so.

In such a tight orbit, the future black hole is able to start transferring mass while it is still burning hydrogen into helium. As a result, it loses most of its hydrogen envelope (becoming a so-called Wolf-Rayet star) and sheds the rest of the envelope in the form of stellar wind, exposing its helium core.

Its companion grows far more massive in the process, becoming the larger and more massive of the two stars. But it remains dim because the added mass doesn't dramatically change the rate of nuclear reactions in its core, researchers said.

Finally the progenitor star collapses under its own gravity, yielding a black hole, and begins absorbing stellar wind from its companion, leading to powerful X-ray emission. Energy released during the gravitational collapse imparts a kick to the black hole that leads to an elliptical orbit, and the black hole's spin results from the spin of the star itself.