Star Quality

First-generation stars, referred to as Population III (or Pop III) stars, are believed to be the first objects that formed out of the pristine gas a few hundred million years after the Big Bang. Direct detection of Pop III stars has never been possible but a team from HKU have discovered a novel method for hunting them which focusses on tidal disruption events.

It was the recent discovery of several galaxies and supermassive black holes in the very early phase of our universe that prompted the Department of Physics researchers – led by Professor Jane Dai Lixin, her postdoctoral fellow Dr Rudrani Kar Chowdhury and PhD student Ms Janet Chang – to explore the properties of the first stars.

“The discovery led us to think about the population of stars in those early galaxies and their fate in the event of close interaction with the associated black holes,” said Dr Kar Chowdhury. “Theoretical studies suggest that the first stars in these galaxies had very different properties from the sun-like stars that we find around us in the local universe.”

Knowing more about Pop III stars is important to the understanding of the complex distribution of gas, stars and galaxies in the later universe. “In the very early phase of the universe when Pop III stars were formed, only hydrogen and helium were present all over the universe. Hence, the main composition of these stars was also hydrogen and helium,” explained Dr Kar Chowdhury.

“All other heavier elements that we find around us today started to form in the cores of the Pop III stars by means of nuclear fusion. Then when they ended their lives with explosions, called supernovae, these elements were distributed in the surrounding medium, thereby enriching our universe with all different types of elements. Afterwards, next-generation stars like our own sun started to form from the gas enriched with heavy elements. So, without Pop III stars, it would not have been possible to form our own sun and our existence on Earth.”

The researchers’ theory proposes that if a Pop III star enters into the vicinity of a massive black hole, it can be torn apart into pieces by tidal force – an event known as a tidal disruption event.

Luminous flare

“After a tidal disruption event, some fraction of the disrupted stellar debris cannot escape the huge gravitational pull of the black hole and gets eaten by it,” said Professor Dai. “Then by Einstein’s general relativity theory, mass is energy, and part of the energy is used to form a very luminous flare. This flare can shine across billions of light years – it even outshines all the stars in the galaxy hosting the massive black hole. So, we can observe this very bright flare, and decode the star’s information by analysing the properties of the flare.

“We further demonstrate that a large fraction of the flare emissions is redshifted to infrared wavelengths, and we believe that with our proposed technique and the identifying properties of Pop III stars, observers will be able to detect them using the current and upcoming powerful telescopes.”

These are the existing James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope (Roman), both of which are highly sophisticated infrared instruments which can peer into the very early epoch of the universe. Roman is scheduled to launch in 2027, and scientists are already planning different survey strategies for it. “We expect that a few dozen of these events will be detected by Roman with proper observation strategy,” said Ms Chang.

Transient exploration

One such mission with Roman is Transient Exploration in the high latitude Wide Area Survey (TEWAS). Professor Dai and Dr Kar Chowdhury are both part of the TEWAS collaboration and will have immediate access to the data when the survey starts.

The researchers’ proposal for understanding Pop III stars was published in The Astrophysical Journal Letters, and has received wide acknowledgement from the international astronomy community.

Dr Kar Chowdhury said: “Understanding Pop III stars is important to understanding the distribution of gas, stars, galaxies and black holes in the local universe. Moreover, we can be confident about the existence of massive black holes at the very early stages of the universe that are otherwise too dim to detect directly.

“Additionally, Pop III stars are much heavier than the next-generation stars. Hence, astronomers believe that a fraction of these first stars ends up being heavy black holes in the early universe. These seed black holes eventually grow in mass to become supermassive black holes which are present at the centres of most of the galaxies in the local universe, including our very own Milky Way.

“Without a proper understanding of the Pop III stars, our knowledge about the local universe will be incomplete. This work has opened a new window to look for Pop III stars and lots can be done in this field in future,” she concluded.