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Spots on supergiant star drive spirals in stellar wind

Artist’s impression of the hot massive supergiant Zeta Puppis. The rotation period of the star indicated by the new BRITE observations is 1.78 d, and its spin axis is inclined by (24 ± 9)° with respect to the line of sight.

A Canadian-led international team of astronomers recently discovered that spots on the surface of a supergiant star are driving huge spiral structures in its stellar wind.

East Tennessee State University’s Dr. Richard Ignace was a member of this team.  Ignace is a professor in the College of Arts and Sciences’ Department of Physics and Astronomy and director of Undergraduate Research and Creative Activities in the Honors College.

Massive stars are responsible for producing the heavy elements that make up all life on Earth. At the end of their lives they scatter the material into interstellar space in catastrophic explosions called supernovae. Without these dramatic events, our solar system would never have formed.

Zeta Puppis is an evolved massive star known as a “supergiant.” It is about 60 times more massive than our sun, and seven times hotter at the surface.  Massive stars are rare, and usually found in pairs of stars (called “binary systems”) or small groups of more than two stars (called “multiple systems”). Zeta Puppis is special, however, because it is a single massive star, moving through space alone, at a velocity of about 60 kilometers per second. Imagine an object about 60 times the mass of the Sun, travelling about 60 times faster than a speeding bullet!

“The existing theories that explain this high peculiar space velocity for Zeta Pup still involve past interactions within a binary or a multiple system,” said investigating team member Dany Vanbeveren, professor at Vrije Universiteit Brussel.

Using a network of “nanosatellites” from the “BRIght Target Explorer” (BRITE) space mission, the group of astronomers monitored the brightness of the surface of Zeta Puppis over a six-month period, and simultaneously monitored the behavior of its stellar wind from several ground-based professional and amateur observatories.

In a paper recently published in the Monthly Notices of the Royal Astronomical Society (MNRAS), Tahina Ramiaramanantsoa (Ph.D. student at the Université de Montréal and member of the Centre de Recherche en Astrophysique du Québec; CRAQ) explains, “The observations revealed a repeated pattern every 1.78 days, both at the surface of the star and in the stellar wind. The periodic signal turns out to reflect the rotation of the star through bright spots tied to its surface, which are driving large-scale spiral-like structures in the wind, dubbed ‘co-rotating interaction regions’ or ‘CIRs.’

“By studying the light emitted at a specific wavelength by ionized helium from the star’s wind,” Ramiaramanantsoa continued, “we clearly saw some ‘S’ patterns caused by arms of CIRs induced in the wind by the bright surface spots.”

In addition to the 1.78-day periodicity, the research team also detected random changes on timescales of hours at the surface of Zeta Puppis, strongly correlating with the behavior of small regions of higher density in the wind known as “clumps” that travel outward from the star.

“These results are very exciting because we also find evidence, for the first time, of a direct link between surface variations and wind clumping, both random in nature,” said investigating team member Anthony Moffat, emeritus professor at Université de Montréal, and principal investigator for the Canadian contribution to the BRITE mission.

After several decades of puzzling over the potential link between the surface variability of very hot massive stars and their wind variability, these results are a significant breakthrough in massive star research, essentially owing to the BRITE nanosats and the large contribution by amateur astronomers. 

“It is really exciting to know that, even in the era of giant professional telescopes, dedicated amateur astronomers using off-the-shelf equipment in their backyard observatories can play a significant role at the forefront of science,” says investigating team member Paul Luckas from the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia. Luckas is one of the six amateur astronomers who intensively observed Zeta Puppis from their homes during the observing campaign, as part of the “Southern Amateur Spectroscopy initiative.”

“I have always been fascinated by how astronomers can learn so much about the cosmos without actually being able to visit any of the marvelous objects scattered throughout space,” Ignace said.  “Progress in our knowledge about how the cosmos works comes about through many different strategies.  One strategy is long-term staring.  This is what the BRITE instrument has accomplished, and I am very honored to be part of such a prestigious team of astrophysicists for this particular study. 

“Evidence has been mounting for years in relation to complex interactions between stellar atmospheres and stellar wind outflows in massive stars – the type that explode as supernova,” Ignace continued.  “The BRITE observations of Zeta Puppis have provided incredible new insights regarding the nature of these interactions.  Professionally, it is incredibly satisfying to be part of this effort that involves both professionals and amateurs to expand our knowledge of the universe.”

The physical origins of the bright surface spots and the random brightness variations discovered in Zeta Puppis remain unknown at this point, and will be the subject of further investigations, probably requiring many more observations using ground-based telescopes, space observatories and small telescopes alike.
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