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An international team of astronomers have discovered, for the first time, observational evidence in how some features at the surface of the hot massive supergiant star ‘Zeta Puppis’ induce the formation of fundamental structures in its wind.

In contrast to cool low-mass stars like the Sun, hot massive stars are scarce, possess extremely strong winds, and catastrophically end their lives as supernovae that stir up and enrich the interstellar medium with chemical elements involved in the creation of new stars and even planets like Earth. Thus, the research team’s breakthrough results on the hot massive supergiant star Zeta Puppis are a significant step towards a better understanding of the true nature of hot massive stars which play a crucial role in the evolution of the Universe.

The team used the network of nanosatellites of the BRIght Target Explorer (BRITE) space mission to monitor the visible brightness changes coming from the surface of Zeta Puppis over about six months, and simultaneously monitored the behavior of the wind of the star from several ground-based professional and amateur observatories.

The observations, published in Monthly Notices of the Royal Astronomical Society, revealed a 1.78-day periodicity both at the surface and in the wind of Zeta Puppis. The behaviour of this periodic signal turns out to reflect the spinning of the star through the presence of slowly evolving bright spots tied to its surface, which are driving large-scale spiral-like structures dubbed corotating interaction regions (CIRs) in its wind.

Tahina Ramiaramanantsoa, PhD student at the Université de Montréal and member of the Centre de Recherche en Astrophysique du Québec (CRAQ), who led the investigation, said, “Once we found that the variations in the brightness of Zeta Pup arise because bright spots on its surface are carried into and out of our view by the star’s rotation every 1.78 days, we employed an algorithm that used those brightness variations to make maps showing where the bright spots are on the star’s surface and how they change over time. Then by studying the light emitted at a specific wavelength by ionized helium from the star’s wind, we clearly saw some “S” patterns that are caused by arms of CIRs induced in the wind by the bright surface spots.”

The 1.78 day periodicity was initially discovered in 2014 using an instrument called SMEI (Solar Mass Ejection Imager), designed and built at the University of Birmingham. However, at that point the origin on the period was unclear. The new data from the BRITE nanosatellites has enabled this mystery to be solved.

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 correlated with the behavior of small regions of higher density in the wind known as “clumps” that travel outward from the star.

Anthony Moffat, professor emeritus at Université de Montréal, and Principal Investigator for the Canadian contribution to the BRITE mission, explained, “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.”

The southern naked-eye bright star Zeta Puppis is an evolved massive star currently at the stage of supergiant. It is often considered as the archetype of hot massive stars with strong stellar winds. Indeed, about sixty times more massive and seven times hotter than the Sun, Zeta Puppis has a stellar wind about a billion times stronger than that of the Sun. In that sense, the solar wind that drives aurorae and shapes the tails of comets appears like a light breeze when compared to the gale-force wind from Zeta Puppis.

Also, most massive stars occur in binary or multiple systems. However, Zeta Puppis is particular because not only is it amongst the few massive stars known to be single, but also it is moving through space at a particularly fast velocity of about 60 km/s.

Dany Vanbeveren, professor at Vrije Universiteit Brussels , said, “The existing theoretical scenarios that explain this high peculiar space velocity for Zeta Pup involve past interactions within a binary or a multiple system, and predicted a relatively short rotation period for the star. That prediction is now supported by these new observational results.”

Dr Ian Stevens , from the School of Physics and Astronomy at the University of Birmingham, said, “Zeta Pup will become either a black hole or a neutron star at the end of its life. In that regard, it is interesting to realise that, if Zeta Pup had survived its past interactions within a massive binary system, it would have been a perfect candidate for the generation of a gravitational-wave signals from the coalescence of a pair involving any combination of stellar-mass black holes or neutron stars in the future!”

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.

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 nanosatellites 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 at their backyard observatories can play a significant role at the scientific front!”, said investigating team member Paul Luckas from the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia. Paul is one of the six amateur astronomers who intensively observed Zeta Puppis from their backyard small observatories during the observing campaign, as part of the Southern Amateur Spectroscopy initiative.