Zeta Puppis, also known as Naos, is a blue supergiant star, one of the most luminous stars in our Milky Way Galaxy. It is about 60 times more massive than our Sun, 7 times hotter and over 10,000 times brighter. Its enormous brightness helps produce a stellar wind that blows erratically at a fierce 5.14 million mph (2,300 km per second). Professor Dany Vanbeveren of the Vrije Universiteit Brussel and co-authors have now discovered that spots on the star’s surface are driving huge spiral structures in this stellar wind.
Artist’s impression of Zeta Puppis, an O-type star approximately 1,400 light-years away in the constellation Puppis. Image credit: Tahina Ramiaramanantsoa.
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.
Massive stars are rare, and usually found in pairs called ‘binary systems’ or small groups known as ‘multiple systems’.
Zeta Puppis is special however, because it is a single massive star, moving through space alone, at a velocity of 134,216 mph (60 km per second).
“Imagine an object about 60 times the mass of the Sun, travelling about 60 times faster than a speeding bullet,” Professor Vanbeveren said.
“One theory is that Zeta Puppis interacted with a binary or a multiple system in the past, and was thrown out into space at an incredible velocity”.
Using a network of ‘nanosatellites’ from the BRIght Target Explorer (BRITE) space mission, Professor Vanbeveren and colleagues monitored the brightness 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.
“The observations revealed a repeated pattern every 1.78 days, both at the surface of the star and in the stellar wind,” said lead author Tahina Ramiaramanantsoa, a Ph.D. student at the Université de Montréal.
“The periodic signal turns out to reflect the rotation of the star through giant ‘bright spots’ tied to its surface, which are driving large-scale spiral-like structures in the wind, dubbed co-rotating interaction regions (CIRs).”
“By studying the light emitted at a specific wavelength by ionized helium from the star’s wind, 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 astronomers 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.
“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 Université de Montréal Professor Anthony Moffat, co-author of the study.
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 space observatories, large ground-based facilities, and small telescopes alike.
The findings are published in the Monthly Notices of the Royal Astronomical Society.
_____
Tahina Ramiaramanantsoa et al. BRITE-Constellation high-precision time-dependent photometry of the early-O-type supergiant ζ Puppis unveils the photospheric drivers of its small- and large-scale wind structures. MNRAS, published online October 13, 2017; doi: 10.1093/mnras/stx2671
