By Prof. Simon Jeffery

Enter the name ‘PHL 417’ in the public database which tells you nearly everything known about nearly every star in the sky, and you will find an almost blank entry.  It is enigmatically identified as a ‘Blue object’ of the 17th magnitude, or about 200,000 times fainter than the faintest stars visible to the naked eye. It has no other names, and no scientific paper refers to it, so far. The only clue is that it is object number 417 in a catalogue of 8746 faint blue stars near the South Galactic Pole made by astronomers Haro and Luyten at the Palomar Observatory in 1962. In all likelihood, it would have remained so had not the Kepler spacecraft, re-purposed and re-badged as K2 after a partial breakdown, pointed to a small patch of sky in the south-western corner of constellation Pisces. With precise coordinates from modern surveys, the same very blue star can now be identified as the even less memorable SDSS J231105.09-013706.0 

A colour composite image of the sky around the faint blue star PHL 417 obtained from the Strasbourg Astronomical Data Center. Credit: Sloan Digital Sky Survey and the CDS.   

Although PHL 417 is very faint, K2 observations obtained every 2 minutes for 90 days were more than sufficient to show that it varied with several periods ranging from 40 minutes to 2 hours. Lead astronomer Roy Østensen quickly recognised the signature of a pulsating hot subdwarf. These are low-mass stars in an advanced stage of evolution; they usually have helium interiors, with a very thin layer of almost pure hydrogen on the surface.  

Artists impression of the Kepler spacecraft against the Milky Way. Some of the K2 pointings are shown in light grey. Credits: NASA/Ames/JPL-Caltech/T Pyle 

Follow-up observations were quickly made to confirm the discovery. To the team’s surprise, the star was much hotter than hot subdwarfs showing similar pulsations and showed a surprising amount of helium on its surface. Armagh astronomer Simon Jeffery was tasked to make sense of the spectrum and, to his surprise, detected evidence of the element zirconium. To be visible, this element must be at least 10,000 times more abundant than it is on the Sun. 

PHL 417 turns out to be the third member of an extremely rare class of stars – the V366 Aquariids. The first – V366 Aquarii or LS IV-14 116 — was identified by Armagh astronomers Amir Ahmad, Naslim Neelamkodan and Simon Jeffery in a series of discoveries between 2005 and 2012. A second pulsating zirconium subdwarf, Feige 46, was announced by Marylin Latour and colleagues in 2018 in Armagh at the 4th conference on hydrogen-deficient stars.  

The V366 Aquariids are identified on this Hertzsprung-Russell diagram of pulsating variable stars.  The bottom axis shows the surface temperature, increasing from 2000 K to 150 000 K from right to left. The top axis shows approximate spectral types corresponding to those surface temperatures. The vertical axis shows the brightness, going from 1/10 000 to 1 000 000 times the brightness of the Sun from bottom to top. Different types of pulsations are represented by left-diagonal, right-diagonal, horizontal and vertical shading. Credit: Simon Jeffery, AOP.
An artist’s impression of the pulsating zirconium subdwarf: V366 Aqr. Clouds of zirconium, strontium, germanium and yttrium lie in layers above the blue surface of the star. Image: Natalie Behara 

Because it was observed from space for more than 90 days in early 2017 and in 2018, the pulsations in PHL 417 are the most clearly measured of all V366 Aquariids, despite being so faint. The pulsations are difficult to explain if, like other hot subdwarfs, the stellar interior is mostly helium. However, like other V366 Aquariids, coauthor Hideyuki Saio showed the pulsations can be explained if most of the interior is a roughly equal mix of helium, carbon and oxygen. Explaining that remains a puzzle.  

A paper describing the discovery of PHL 417 will appear in the Monthly Notices of the Royal Astronomical Society (https://arxiv.org/abs/2010.02978).  


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