Hydrogen Masers Reveal New Secrets of a Massive Star
Seattle, WA – Scientists have discovered something unexpected while studying masers around oddball star MWC 349A: a previously unseen jet of material launching from the star's gas disk at impossibly high speeds. The team, based at the Center for Astrophysics | Harvard & Smithsonian, believes the jet is caused by strong magnetic forces surrounding the star.
The new observations, completed with the Atacama Large Millimeter/submillimeter Array (ALMA), were presented today in a press conference at the 241st meeting of the American Astronomical Society (AAS) in Seattle, Washington. The discovery could help researchers understand the nature and evolution of massive stars and how hydrogen masers are formed in space.
Located roughly 3,900 light-years away from Earth in the constellation Cygnus, MWC 349A's unique features make it a hot spot for scientific research in optical, infrared and radio wavelengths. The massive star — roughly 30 times the mass of the Sun — is one of the brightest radio sources in the sky, and one of only a handful of objects known to have hydrogen masers. These masers amplify microwave radio emissions, making it easier to study processes that are typically too small to see. It is this unique feature that allowed scientists to map MWC 349A’s disk in detail for the first time.
"A maser is like a naturally occurring laser," says Sirina Prasad, an undergraduate research assistant at the Center for Astrophysics (CfA), and the lead author of the paper. "It's an area in outer space that emits a really bright kind of light. We can see this light and trace it back to where it came from, bringing us one step closer to figuring out what’s really going on."
Leveraging the resolving power of ALMA's Band 6, developed by the U.S. National Science Foundation's National Radio Astronomy Observatory (NRAO), the team was able to use the emitted masers to uncover previously unseen structures in the star’s immediate environment.
"We used masers generated by hydrogen to probe the physical and dynamic structures in the gas surrounding MWC 349A and revealed a flattened gas disk with a diameter of 50 au, approximately the size of the Solar System, confirming the near-horizontal disk structure of the star," says Qizhou Zhang, a senior astrophysicist at CfA, and the project's principal investigator. "We also found a fast-moving jet component hidden within the winds flowing away from the star."
The observed jet is ejecting material away from the star at a blistering 310 miles per second. That's akin to traveling the distance between San Diego, California and Phoenix, Arizona in the literal blink of an eye. It is probable that a jet moving this fast is being launched by a magnetic force, the researchers say. In the case of MWC 349A, that force could be a magnetohydrodynamic wind — a type of wind where movement is dictated by the interplay between the star’s magnetic field and gases present in its surrounding disk.
"Our previous understanding of MWC 349A was that the star was surrounded by a rotating disk and photo-evaporating wind. Strong evidence for an additional collimated jet had not yet been seen in this system. Although we don’t yet know for certain where it comes from or how it is made, it could be that a magnetohydrodynamic wind is producing the jet, in which case the magnetic field is responsible for launching rotating material from the system," says Prasad. "This could help us to better understand the disk-wind dynamics of MWC 349A, and the interplay between circumstellar disks, winds and jets in other star systems."
This project builds on previous work by Harvard undergraduates who observed MWC 349A with the CfA's Submillimeter Array (SMA) in Maunakea, Hawaii. The project was developed as part of an undergraduate course, Astro 191, where students conduct laboratory and observational astrophysics.
Zhang says, "It's great to see generations of students engaging in the forefront astronomical research using the SMA and ALMA."
Additional researchers who contributed to this project are Yue Cao of Nanjing University; James Moran of the CfA; Alejandro Báez Rubio of Mahindra United World College; and Izaskun Jiménez-Serra, Jesus Martín-Pintado and Antonio Martínez of El Centro de Astrobiología (CSIC-INTA).
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