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27 June, 2023

The Shortest-Period Binary Star System Known in the Galaxy is Resisting Inspiral

A study led by James Munday (PhD student at the University of Warwick and support astronomer in the ING Studentship Programme 2022/2023) has recently been published that exploits optical photometry of HM Cancri, spanning more than 20 years.

This long time-baseline has led to the determination of an incredibly precise orbital decay constraint for the system by timing the phasing of an optical modulation, which would not have been possible without the Isaac Newton Telescope (through the RATS survey), and the William Herschel Telescope (through the high-speed photometer ULTRACAM). Uniquely of any binary star system, not only could the orbital decay of HM Cancri be measured at high precision, but the authors also discovered that HM Cancri's inspiral is slowing down – not speeding up.

Some of the most-compact binary systems in the Galaxy comprise two white dwarf stars, stellar fossils resulting from the evolution of stars like our Sun. As they orbit each other, they continuously emit gravitational wave radiation; the closer the two stars, the stronger this radiation. The radiation carries away orbital energy, causing the binary to inspiral. With time, the stars can get close enough to merge, which can lead to a type Ia supernova, or other exotic stellar products.

A graphical representation of a compact and mass transferring binary, such as HM Cancri. Credit: Pablo Carlos Budassi, CC BY-SA 4.0, via Wikimedia Commons. Large format: PNG.

To be able to understand type Ia supernovae, which enable us to measure the expansion of the universe, and the exotic merger population, which can constrain the physics of stars, we first need to find the shortest period binaries.

There are different ways to achieve this. If a system is compact enough that mass is transferred between the stars, it might have an X-ray signature detectable from space. Complementarily, eclipses or other causes of light variability can be detected at optical wavelengths with ground-based instruments on telescopes such as the Isaac Newton Telescope or the William Herschel Telescope.

Efforts in the hunt for the shortest orbital period systems over the last decades have revealed one particular system with the shortest period of them all – HM Cancri. Every 5.4 minutes the two white dwarfs complete one orbital cycle. In other words, HM Cancri completes nearly 100,000 orbits in the time it takes for the Earth to orbit the Sun once. The two white dwarfs are separated by just 1/10th of the radius of the Sun, causing one star to donate mass to the other.

HM Cancri was discovered over 20 years ago from a bright X-ray detection which exhibits a pulsed signal every 5.4 minutes, but discovering the system was only the start. In the years that followed, the orbital decay of the binary was measured with the Nordic Optical Telescope on La Palma and found to indeed show a decreasing orbital period, such that the two stars are drawing closer together with a trajectory dominated by gravitational wave radiation. In recent years, Munday and collaborators have continued to monitor the orbital decay and have found that not only are the stars drawing closer, but the decay is slowing, such that the system is resisting inspiral.

The residual fit to the timing measurements expected from a constant orbital period, clearly displaying the long-term trend of HM Cancri's orbital decay (plotted in red). On the left y-axis, the observed decay minus a calculated linear (constant period) fit. The right y-axis is the same as the left, except this decay is in terms of a number of seconds. Black crosses are individual measurements of the orbital timing, with small errorbars. Figure adapted from Munday et al., 2023, MNRAS, 518, 5123. Large format: PNG.

This revelation is somewhat surprising. The measurement of a slowing inspiral implies that the binary will reach an orbital period minimum in roughly 2000 years and may not in fact merge. After 2000 years if the trend continues, the binary will 'turn around' and evolve to longer orbital periods – hence surviving inspiral – with the only consequence being the donor star becoming stripped of its outer layers. Alternatively, HM Cancri may undergo a nova or supernova eruption, depending on the exact details of the surface composition and masses of the two stars, which seem to be close to the Chandrasekhar mass limit.

Even after 20 years HM Cancri is still providing insight into the last moments of binary stellar evolution. Looking towards the upcoming decades, HM Cancri will continue to be observed and further observations can hopefully be exploited to predict the future of the system conclusively. But, as we probe deeper into the Galaxy with optical/X-ray telescopes and unlock a new window into the Galaxy, will we find more-compact binaries on the cusp of explosion?

About the William Herschel and Isaac Newton Telescopes

Based on observations made with the William Herschel Telescope (WHT) and the Isaac Newton Telescope (INT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

Journal article

James Munday, T R Marsh, Mark Hollands, Ingrid Pelisoli, Danny Steeghs, Pasi Hakala, Elmé Breedt, Alex Brown, V S Dhillon, Martin J Dyer, Matthew Green, Paul Kerry, S P Littlefair, Steven G Parsons, Dave Sahman, Sorawit Somjit, Boonchoo Sukaum, James Wild, 2023, "Two decades of optical timing of the shortest-period binary star system HM Cancri", MNRAS, 518, 5123. Paper: ADS.


James Munday
University of Warwick and Isaac Newton Group of Telescopes.

Javier Méndez (ING PR Officer)

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Last modified: 27 June 2023