Navigating the cosmos with Georgia State’s CHARA Array

New instruments and plans for a seventh telescope at Georgia State’s CHARA Array will allow scientists to see the stars in greater detail than ever before.

Reports and Proceedings

GEORGIA STATE UNIVERSITY

 

IMAGE: GEORGIA STATE'S CHARA ARRAY IS AN OPTICAL INTERFEROMETER LOCATED ON MOUNT WILSON, CALIFORNIA view more 

CREDIT: GEORGIA STATE UNIVERSITY

ATLANTA—Plans are underway to add a seventh movable telescope to Georgia State University’s Center for High Angular Resolution Astronomy— known as the CHARA Array—that would increase the resolution, or the ability to see small objects, by a factor of three.

Located at Mount Wilson Observatory in Southern California and operated by Georgia State, the new telescope will be connected using fiber optics to transport the starlight, a technique that will serve as a pathfinder for future expansion of the Array. The update comes after a group of international scientists gathered in Atlanta to take part in the 2023 CHARA Science Meeting to share the latest developments in high-resolution astronomical imaging using the CHARA Array.

“Adding a seventh moveable telescope to the Array represents a great leap forward in stellar astronomy,” says Doug Gies, Regents’ Professor of Physics and Astronomy and director of the center. “Collaboration is truly fundamental for an undertaking like the CHARA Array. With scientists all over the world using our telescopes, this annual gathering is an important forum for us to share our latest discoveries.”

The CHARA Array combines the light from six optical telescopes spread across the mountaintop to image stars with a spatial resolution equivalent to a single telescope 331 meters (over 1000 ft) in diameter. The visible and infrared observatory offers astronomers the opportunity to capture images of space with better resolution than any other telescope in the world.

More than 40 members of the CHARA Consortium, which represents 10 institutions around the world, took part in the annual review of the latest scientific and technical progress.

Scientists gathered at Georgia State University in March 2023 for the CHARA Science Meeting and Imaging Workshop.

CHARA features a new suite of instruments built by partner institutions at the University of Michigan, University of Exeter, and Observatoire de la Côte d’Azur in France. This next generation of instrumentation provides unprecedented capabilities to image the surfaces of stars and their circumstellar environments at a variety of different wavelengths from the near-infrared to the visible part of the spectrum. Georgia State University is also building a new instrument that will increase the sensitivity of the CHARA Array to measure light 30 times fainter than possible now. This improvement will help astronomers probe the gas clouds swirling around supermassive black holes in very distant active galaxies.

With funding from the National Science Foundation (NSF), CHARA has expanded its user base over the last six years by offering open access time to the global community of astronomers through a competitive proposal process offered through the National Optical-Infrared Astronomy Research Laboratory. In addition to over 60 active observers at Georgia State University and partner institutions, the open-access program has received applications from over 350 visiting astronomers around the world.

“Expanding the user community brings new opportunities for innovative science projects that broaden the impact and productivity of the CHARA Array,’’ says Gail Schaefer, Director of the CHARA Array.

At the recent meeting, members presented some science highlights and findings from the CHARA Array.

• Georgia State graduate student Katherine Shepard presented results on a sample of evolved massive binary star systems surrounded by outflowing disks. The disks in these fascinating systems form as one star in the system grows in size as it evolves and material from that star is transferred to the companion. Some of the mass escapes into a disk that surrounds the system. Katherine is using the CHARA Array to resolve the structure of these disks and search for interactions between the disk and the inner binary system.
• Noura Ibrahim, a graduate student from the University of Michigan, imaged the ring-like structure of a circumstellar disk around the young star V1295 Aquila. Two images taken one month apart show a bright spot in the ring that rotates between the two epochs. This variation could be caused by a stellar companion, an exoplanet in formation, or asymmetries in the density distribution.
• Visiting astronomer Willie Torres at the Harvard-Smithsonian Center for Astrophysics mapped the orbits in the Castor multiple star system. The system consists of Castor A and B that revolve around each other every 450 years, and each component in turn are short-period binary systems with periods of a few days. They are joined by a more distant component Castor C, which is also a binary. Torres used the CHARA Array to resolve the close, faint companions in Castor A and B for the first time. He combined these observations with historical observations spanning the past three centuries to map the orbits of the stars in the Castor system and measure their stellar masses with a precision better than 1%. The CHARA observations were also used to measure the radii of the two brightest stars to infer an age for the system of 290 million years.
• Rachael Roettenbacher, a Postdoctoral Associate from the University of Michigan, presented recent work on mapping starspots over a rotation cycle for the sun-like star Epsilon Eridani, which is orbited by an exoplanet. The starspot images, in combination with data from other telescopes, were used to develop a technique to distinguish between small changes in the stellar spectrum caused by starspots and those caused the orbiting planet. These techniques will improve the detection of planets around other stars.

The annual meeting was followed by a workshop on imaging and modeling of interferometric observations. Participants were given an overview of modeling and imaging software packages available to analyze data from stellar interferometers (arrays of telescopes that combine light together), and the workshop included interactive hands-on sessions where participants used the software tools to analyze data. Participants also brought their own data for review in order to get the most from observations made with the CHARA Array.

Scientists map gusty winds in a far-off neutron star system

The 2D map of this “disk wind” may reveal clues to galaxy formation.

Peer-Reviewed Publication

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

 

IMAGE: MIT ASTRONOMERS MAPPED THE “DISK WINDS” ASSOCIATED WITH THE ACCRETION DISK AROUND HERCULES X-1, A SYSTEM IN WHICH A NEUTRON STAR IS DRAWING MATERIAL AWAY FROM A SUN-LIKE STAR, REPRESENTED AS THE TEAL SPHERE. THE FINDINGS MAY OFFER CLUES TO HOW SUPERMASSIVE BLACK HOLES SHAPE ENTIRE GALAXIES. view more 

CREDIT: CREDIT: JOSE-LUIS OLIVARES, MIT. BASED ON AN IMAGE OF HERCULES X-1 BY D. KLOCHKOV, EUROPEAN SPACE AGENCY.

An accretion disk is a colossal whirlpool of gas and dust that gathers around a black hole or a neutron star like cotton candy as it pulls in material from a nearby star. As the disk spins, it whips up powerful winds that push and pull on the sprawling, rotating plasma. These massive outflows can affect the surroundings of black holes by heating and blowing away the gas and dust around them.

At immense scales, “disk winds” can offer clues to how supermassive black holes shape entire galaxies. Astronomers have observed signs of disk winds in many systems, including accreting black holes and neutron stars. But to date, they’ve only ever glimpsed a very narrow view of this phenomenon.

Now, MIT astronomers have observed a wider swath of winds, in Hercules X-1, a system in which a neutron star is drawing material away from a sun-like star. This neutron star’s accretion disk is unique in that it wobbles, or “precesses,” as it rotates. By taking advantage of this wobble, the astronomers have captured varying perspectives of the rotating disk and created a two-dimensional map of its winds, for the first time.

The new map reveals the wind’s vertical shape and structure, as well as its velocity — around hundreds of kilometers per second, or about a million miles per hour, which is on the milder end of what accretion disks can spin up. 

If astronomers can spot more wobbling systems in the future, the team’s mapping technique could help determine how disk winds influence the formation and evolution of stellar systems, and even entire galaxies. 

“In the future, we could map disk winds in a range of objects and determine how wind properties change, for instance, with the mass of a black hole, or with how much material it is accreting,” says Peter Kosec, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “That will help determine how black holes and neutron stars influence our universe.”

Kosec is the lead author of a study appearing in Nature Astronomy. His MIT co-authors include Erin Kara, Daniele Rogantini, and Claude Canizares, along with collaborators from multiple institutions, including the Institute of Astronomy in Cambridge, U.K.

Fixed sight

Disk winds have most often been observed in X-ray binaries — systems in which a black hole or a neutron star is pulling material from a less dense object and generating a white-hot disk of inspiraling matter, along with outflowing wind. Exactly how winds are launched from these systems is unclear. Some theories propose that magnetic fields could shred the disk and expel some of the material outward as wind. Others posit that the neutron star’s radiation could heat and evaporate the disk’s surface in white-hot gusts.  

Clues to a wind’s origins may be deduced from its structure, but the shape and extent of disk winds has been difficult to resolve. Most binaries produce accretion disks that are relatively even in shape, like thin donuts of gas that spins in a single plane. Astronomers who study these disks from far-off satellites or telescopes can only observe the effects of disk winds within a fixed and narrow range, relative to their rotating disk. Any wind that astronomers manage to detect is therefore a small sliver of its larger structure. 

“We can only probe the wind properties at a single point, and we’re completely blind to everything around that point,” Kosec notes.

In 2020, he and his colleagues realized that one binary system could offer a wider view of disk winds. Hercules X-1 has stood out from most known X-ray binaries for its warped accretion disk, which wobbles as it rotates around the system’s central neutron star. 

“The disk is really wobbling over time every 35 days, and the winds are originating somewhere in the disk and crossing our line of sight at different heights above the disk with time,” Kosec explains. “That’s a very unique property of this system which allows us to better understand its vertical wind properties.”

A warped wobble

In the new study, the researchers observed Hercules X-1 using two X-ray telescopes — the European Space Agency’s XMM Newton and NASA’s Chandra Observatory. 

“What we measure is an X-ray spectrum, which means the amount of X-ray photons that arrive at our detectors, versus their energy. We measure the absorption lines, or the lack of X-ray light at very specific energies,” Kosec says. “From the ratio of how strong the different lines are, we can determine the temperature, velocity, and the amount of plasma within the disk wind.”

With Hercules X-1’s warped disk, astronomers were able to see the line of the disk moving up and down as it wobbled and rotated, similar to the way a warped record appears to oscillate when seen from edge-on. The effect was such that the researchers could observe signs of disk winds at changing heights with respect to the disk, rather than at a single, fixed height above a uniformly rotating disk. 

By measuring X-ray emissions and the absorption lines as the disk wobbled and rotated over time, the researchers could scan properties such as the temperature and density of winds at various heights with respect to its disk and construct a two-dimensional map of the wind’s vertical structure. 

“What we see is that the wind rises from the disk, at an angle of about 12 degrees with respect to the disk as it expands in space,” Kosec says. “It’s also getting colder and more clumpy, and weaker at greater heights above the disk.” 

The team plans to compare their observations with theoretical simulations of various wind-launching mechanisms, to see which could best explain the wind’s origins. Further out, they hope to discover more warped and wobbling systems, and map their disk wind structures. Then, scientists could have a broader view of disk winds, and how such outflows influence their surroundings — particularly at much larger scales. 

“How do supermassive black holes affect the shape and structure of galaxies?” poses Erin Kara, the Class of 1958 Career Development Assistant Professor of Physics at MIT. “One of the leading hypotheses is that disk winds, launched from a black hole, can affect how galaxies look. Now we can get a more detailed picture of how these winds are launched, and what they look like.”

This research was supported in part by NASA.

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Written by Jennifer Chu, MIT News Office

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JOURNAL

Nature Astronomy

ARTICLE TITLE

“Vertical wind structure in an X-ray binary revealed by a precessing accretion disk”