Trajectories within the quadruple stellar system HD 74438: the two close pairs, having orbital periods of 21 and 4 days, orbit around each other in 6 years. At some point, the two pairs could merge into white dwarfs giving rise to a thermonuclear supernova, as illustrated in the bottom right. (Image Credit: Merle, A)

Stars like to be in company, and unlike our Sun, most of the stars in the Galaxy have one or more stellar companions. Binary stars are now recognized to play a major role in a large range of astrophysical events, such as the 2017 gravitational wave emission detection. In addition, binary stars allow us to derive fundamental stellar parameters like masses, radii and luminosities with better accuracy compared to single stars. They represent the gems on which various astrophysics studies rely.

While binaries have received much attention so far, higher-order stellar systems have remained aside until recently, despite the fact that they show a wide variety of interactions, especially in tight systems. Stellar quadruples only represent a marginal fraction (a few percent) of all multiple systems. The complex evolution of such high-order multiples involves mass transfer and collisions, leading to mergers that are also possible progenitors of thermonuclear supernovae. These supernovae represent standard candles for fixing the Universe distance scale, despite the fact that the evolutionary channel(s) leading to the progenitors of such supernova explosions are still highly debated.

A spectroscopic quadruple (HD 74438) was discovered in 2017 in the Gaia-ESO Survey. The Gaia-ESO Survey is a public spectroscopic survey providing a detailed overview of the stellar content of the Milky Way by characterizing more than 100 000 stars. Subsequent follow-up spectroscopic observations of HD 74438 were obtained with high-resolution spectrographs at the University of Canterbury Mount John Observatory in New Zealand and at the Southern African Large Telescope in South Africa. These observations allowed us to determine that this stellar quadruple is made of 4 gravitationally-bound stars: a short-period binary orbiting another short-period binary on a longer orbital period (2+2 configuration). Its membership in the open cluster IC 2391 makes it the youngest (43 million years) spectroscopic quadruple discovered so far and among the quadruple systems with the shortest outer orbital period (6 years).

Thanks to the spectroscopic analysis it was possible to show that this quadruple system is undergoing dynamic effects on long time scales compared to the orbital periods. Indeed, one of the inner binaries should have evolved into a circular orbit whereas it has an eccentric one. This is explained by the gravitational effect of the distant binary companion which can pump up the eccentricity. State-of-the-art simulations of this system’s future evolution show that such gravitational dynamics can lead to one or multiple collisions and merger events producing white dwarfs with masses just below the Chandrasekhar limit.

A star like our Sun will end its life as a white dwarf, and the mass of white dwarfs cannot go above the so-called Chandrasekhar limit. If it does, as a result of mass transfer or merger events, it does collapse and produces a thermonuclear supernova. Interestingly, 70 to 85% of all thermonuclear supernovae are now suspected to result from the explosion of white dwarfs with sub-Chandrasekhar masses. The evolution of stellar quadruples such as HD 74438 thus represents a new promising channel to form them.

Reference

1. Merle, A. S. Hamers, S. Van Eck, A. Jorissen, M. Van der Swaelmen, K. Pollard, R. Smiljanic, D. Pourbaix, Tomaž Zwitter, G. Traven, G. Gilmore, S. Randich, A. Gonneau, A. Hourihane , G. Sacco and C. C. Worley

A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae 

Nature Astronomy, Letters https://doi.org/10.1038/s41550-022-01664-5

The scientists observed an ‘accreting’ neutron star as it entered an outburst phase in an international collaborative effort involving five groups of researchers, seven telescopes (five on the ground, two in space), and 15 collaborators. 

The telescopes involved include two space observatories: the Neil Gehrels Swift X-ray Observatory, and the Neutron Star Interior Composition Explorer (NICER) on the International Space Station; as well as the ground-based Las Cumbres Observatory network of telescopes and the Southern African Large Telescope (SALT). 

“Optical spectra obtained with SALT were crucial in demonstrating the powering-up,” said the SALT Transient programme lead, Dr David Buckley. “We observed on 6 occasions during August 2019. The first two observations showed it was very faint, while by the time of the third observation, on 6 August, is was clearly in outburst. This demonstrates the importance of flexible telescope scheduling that can quickly react to changing circumstances,” he concluded. SALT has made many advances in the study of compact objects, like neutron stars and black holes, including studies of the most energetic events in the Universe, like gamma-ray bursts and gravitational wave events.

It is the first time such an event has been observed in this detail – in multiple frequencies, including high-sensitivity measurements in both optical and X-ray. 

This illustration shows a neutron star — the core of a star that exploded in a massive supernova. This particular neutron star is known as a pulsar because it sends out rotating beams of X-rays that sweep past Earth like lighthouse beacons.

The physics behind this ‘switching on’ process has eluded physicists for decades, partly because there are very few comprehensive observations of the phenomenon. 

The researchers caught one of these accreting neutron star systems in the act of entering outburst. They witnessed the onset of the outburst, from the first sign of optical activity to the beginning of X-ray emission, all the way to the end of the outburst. 

The observations revealed that it took 12 days for material to swirl inwards and collide with the neutron star, substantially longer than previously thought. 

“These observations allow us to study the structure of the accretion disk, and determine how quickly and easily material can move inwards to the neutron star,” said study lead PhD candidate Adelle Goodwin from the Monash School of Physics and Astronomy in Melbourne, Australia. 

“Using multiple telescopes that are sensitive to light in different energies we were able to trace that the initial activity happened near the companion star, in the outer edges of the accretion disk, and it took 12 days for the disk to be brought into the hot state and for material to spiral inward to the neutron star, and X-rays to be produced,” she said.

In an ‘accreting’ neutron star system, a pulsar which is a dense remnant of an old star, strips material away from a nearby star, forming an accretion disk of material spiralling in towards the pulsar, where it releases extraordinary amounts of energy – about the total energy output of the sun in 10 years, over the period of a few short weeks. 

This is so energetic that most of the radiation is released in the highest energy portion of the electromagnetic spectrum: in X-rays. 

Some accreting neutron stars are not always active and can spend years in a quiet state, known as quiescence, where they emit barely any light at all and accrete at a very low rate. They can suddenly go into outburst and become extremely bright in X-rays for around a month. 

The pulsar in a binary system is named SAX J1808.4−3658, and spins at a staggering rate of 400 times per second. 

What the researchers saw was unexpected: it took 12 days from the first sign of increased optical activity before any high energy X-ray emission was observed. 

This is longer than anyone thought it would take, with most theories suggesting there should be only a two- to three-day delay. 

“This work enables us to shed some light on the physics of accreting neutron star systems, and to understand how these explosive outbursts are triggered in the first place, which has puzzled astronomers for a long time,” said New York University researcher, Dr David Russell, one of the study’s co-authors. 

Accretion disks are usually made of hydrogen, but this particular object has a disk that is made up of 50% helium, more helium than most disks. The scientists think that this excess helium may be slowing down the heating of the disk because helium “burns” at a higher temperature, causing the “powering up” to take 12 days.

The research, led by PhD candidate Adelle Goodwin from the Monash School of Physics and Astronomy in Melbourne, Australia, was featured at the recent 236th virtual meeting of the American Astronomical Society meeting (1-3 June 2020), before it is published in Monthly Notices of the Royal Astronomical Society. Adelle leads a team of international researchers, including her supervisor, Monash University Associate Professor Duncan Galloway, and Dr David Russell from New York University Abu Dhabi and Dr David Buckley from the South African Astronomical Observatory.  

The preprint of the submitted paper is available on the Astro-PH arXiv here: https://arxiv.org/abs/2006.02872

The SALT Annual Report 2019 (.pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.