Laycock and Balchunas locate stars in IC10 galaxy

Physics doctoral student Andrew Balchunas presented new work, Optical Spectroscopy of Candidate X-ray Binaries in the Starburst IC 10 at the American Astronomical Society’s High Energy Astrophysics Division (HEAD) meeting in Newport, RI on September 7, 2011. Balchunas worked with Dr. Silas Laycock of the Physics and Applied Physics Department, and undergraduates Rigel Cappallo and Kate Oram, who are co-authors on the study.

The poster reported on the optical spectra of 25 stars that are associated with X-ray emission seen by the Chandra X-ray Observatory (which is an orbiting observatory operated by NASA). Although ranking among the most luminous stars in existence, the stars lie in a galaxy 2 million light years away in the constellation Cassiopeia. Consequently the stars studied are about 15 million times fainter than the limit of human vision. Collecting enough of their distant light to disperse into a spectrum was a job for Chandra’s giant 8-meter Gemini telescope.

By measuring the doppler shift of spectral lines in each star, Laycock and Balchunas were able to prove that most of the X-ray stars lie in the galaxy IC10, which is rushing towards our own Milky Way at 340 km/s. (No cause for alarm, 2 million lightyears is a very very long way away!) At this huge distance, the only objects luminous enough to be visible in X-rays are X-ray Binaries (XRBs).

When one member of binary star system goes supernova, the survivor can suddenly find itself with a black hole or neutron star for a partner. The companion continues shedding material in a stellar wind (as all stars including our Sun, do to varying extent), and some of this matter gets dragged onto the compact object by gravity. As the gas spirals into the strong gravitational field, it is heated to millions of degrees by friction, releasing huge amounts of energy across the electromagnetic spectrum, but mostly in X-rays.

The X-ray binary phenomenon is dramatic, but very short-lived, which presents an intriguing way to probe the evolution of stars and galaxies. Laycock’s team is studying this particular galaxy because it contains the youngest and most massive stars. The basic idea is to discover how its XRBs differ from those in other older galaxies. For example, is there a higher rate of black hole binaries, or, are more massive stars involved, are their orbits tighter?

In addition to presenting the UMass Lowell astronomy group’s poster, Balchunas listened to talks by leading astronomers, and had the pleasure of meeting the author of the textbook he is studying in Dr. Laycock's Astronomy & Astrophysics I course this semester. “The highlight was probably hearing the latest cosmology findings on the dark-energy driven expansion of the universe, from supernova studies,” said Balchunas.

Gemini optical spectrum of one suspected X-ray binary. It is a hot, luminous supergiant shedding gas that likely fuels the neutron star or black hole companion. The emission lines are evidence of a powerful outflow. The inset shows the line is doppler shifted by about 340 km/s, placing it in the starburst galaxy.

Gemini image of Starburst IC10 galaxy with positions of X-ray stars marked.

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This page contains a single entry by Martin, Fred published on September 26, 2011 2:56 AM.

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