Featured Research

from universities, journals, and other organizations

Star, not so bright: Model explains evolution of unusual black hole binary system

Date:
October 21, 2010
Source:
Northwestern University
Summary:
Astronomers have puzzled over the oddities of the X-ray binary system M33 X-7, but no one could explain all of its features. Now researchers have. They have produced a model of the system's evolutionary history 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.

The main component of this graphic is an artist's representation of M33 X-7, a binary system in the nearby galaxy M33. In the illustration, an orange disk surrounds the black hole. This depicts material, fed by a wind from the blue companion star, which has been swept into orbit around the black hole. The inset shows a composite of data from NASA's Chandra X-ray Observatory (blue) and the Hubble Space Telescope. The bright objects in the inset image are young, massive stars around M33 X-7, and the bright, blue Chandra source is M33 X-7 itself.
Credit: Illustration: NASA/CXC/M.Weiss; X-ray: NASA/CXC/CfA/P.Plucinsky et al.; Optical: NASA/STScI/SDSU/J.Orosz et al.

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.

"We were attracted to this system because it has one of the most massive black holes to have formed from a star, and yet the rest of its characteristics, especially the mass of its companion star and its orbit, did not make any sense from an evolutionary point of view," said Vicky Kalogera, professor of physics and astronomy in the Weinberg College of Arts and Sciences.

M33 X-7 is one of the few known X-ray binary systems containing a black hole outside our galaxy, and its star is the most massive star ever discovered in such a system.

The researchers' evolutionary model of M33 X-7 starts with two stars in a binary system (or in orbit one around the other). One star is 100 solar masses (100 times the sun's mass), and the other is 30 solar masses. The stars are in a close orbit, with the larger star growing faster until it nearly envelops the other. The initially smaller star then gains material from its companion, while the initially larger and more massive star collapses into a black hole at the end of its nuclear-burning lifetime. The orbit becomes even tighter.

The star, which is now 70 solar masses, is not as luminous as stars of similar mass partially because of the way it gained its mass and partially because of the inclination of the system with respect to us. On one hand, the star accreted matter so quickly from its interaction with the other star (now a black hole) that it could not adjust fast enough to its new, greater mass. Therefore, the star does not burn as bright as an undisturbed star of this greater mass would. On the other hand, the star is deformed due to the close presence of the massive black hole, and the star's temperature and luminosity are not uniform across the surface. This effect, combined with the inclination of the system with respect to our line of sight, means we are looking at the star's dimmest equatorial regions.

And now the massive black hole is growing even larger. The companion star is feeding matter, via a stellar wind, to the black hole. In the process X-rays are emitted, allowing astronomers to observe the black hole.

"Solitary black holes are very difficult to observe, but X-ray binary systems, such as M33 X-7, make black holes visible to us," said Francesca Valsecchi, a doctoral student in Kalogera's research group and lead author of the paper. "These systems provide a unique physical laboratory for the study of massive compact objects."

Valsecchi, Kalogera and colleagues performed detailed binary system evolution calculations to explore possible evolutionary tracks. They used information known about the physics of binary stellar interactions and black hole formation processes.

In their initial work, they ran more than 200,000 sequences on a high-performance computing cluster, which took a couple of months. The researchers then examined a number of these sequences in further detail and were able to identify the final model, consistent with all observational characteristics of M33 X-7.

M33 X-7 is an X-ray binary system discovered in 2007 in the Messier 33 galaxy, known as M33. (An X-ray binary system is a class of binary stars luminous in X-rays.) The Messier 33 galaxy, slightly farther away from us than the Andromeda galaxy, is among the farthest permanent objects that can be viewed with the naked eye.

In addition to Kalogera and Valsecchi, other authors of the paper are Evert Glebbeek, Will M. Farr, Tassos Fragos, Bart Willems, Jerome A. Orosz and Jifeng Liu.


Story Source:

The above story is based on materials provided by Northwestern University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Francesca Valsecchi, Evert Glebbeek, Will M. Farr, Tassos Fragos, Bart Willems, Jerome A. Orosz, Jifeng Liu, Vassiliki Kalogera. Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system. Nature, 2010; DOI: 10.1038/nature09463

Cite This Page:

Northwestern University. "Star, not so bright: Model explains evolution of unusual black hole binary system." ScienceDaily. ScienceDaily, 21 October 2010. <www.sciencedaily.com/releases/2010/10/101020131704.htm>.
Northwestern University. (2010, October 21). Star, not so bright: Model explains evolution of unusual black hole binary system. ScienceDaily. Retrieved July 23, 2014 from www.sciencedaily.com/releases/2010/10/101020131704.htm
Northwestern University. "Star, not so bright: Model explains evolution of unusual black hole binary system." ScienceDaily. www.sciencedaily.com/releases/2010/10/101020131704.htm (accessed July 23, 2014).

Share This




More Space & Time News

Wednesday, July 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Raw: Cargo Craft Undocks from Space Station

Raw: Cargo Craft Undocks from Space Station

AP (July 22, 2014) A Russian Soyuz cargo-carrying spacecraft undocked from the International Space Station on Monday. The craft is due to undergo about ten days of engineering tests before it burns up in the Earth's atmosphere. (July 22) Video provided by AP
Powered by NewsLook.com
NASA Ceremony Honors Moon Walker Neil Armstrong

NASA Ceremony Honors Moon Walker Neil Armstrong

AP (July 21, 2014) NASA honored one of its most famous astronauts Monday by renaming a historic building at the Kennedy Space Center in Florida. It now bears the name of Neil Armstrong, the first man to walk on the moon. (July 21) Video provided by AP
Powered by NewsLook.com
Neil Armstrong's Post-Apollo 11 Life

Neil Armstrong's Post-Apollo 11 Life

Newsy (July 19, 2014) Neil Armstrong gained international fame after becoming the first man to walk on the moon in 1969. But what was his life like after the historic trip? Video provided by Newsy
Powered by NewsLook.com
This Week @ NASA, July 18, 2014

This Week @ NASA, July 18, 2014

NASA (July 18, 2014) Apollo 11 yesterday, Next Giant Leap tomorrow, Science instruments for Europa mission, and more... Video provided by NASA
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins