10 mins read 10 Apr 2021

Radio Stars Emerge in ASKAP’s Survey of the Southern Skies

Using one of the most powerful radio telescopes in the world, Australian scientists have surveyed and analysed the southern skies in search of elusive stars that emit radio waves.

Artist rendition of auroral activity on a cool red star interacting with an exoplanet. Credit: Danielle Futselaar.

When we look up into the night sky, the light from the stars that we see is due to the visible bands of frequencies that our eyes are tuned into. This electromagnetic (EM) radiation, which has travelled across light-years, was generated in the cores of the stars through nuclear fusion processes. But not all light emitted from stars arrives in the visible range of wavelengths – with lots of light emitted from stars spread across the entire spectrum.

However, radio waves emitted from most regular stars (main sequence) are fairly undetectable, especially given the large distances that the emissions would need to travel across the galaxy to reach us.

Now, a new paper that has been recently published in the Monthly Notices of the Royal Astronomical Society journal by a collaboration of astronomers, led by Australian scientists, has utilised an efficient analysis of the southern sky in radio frequencies to find radio-wavelength detections for 10 known radio stars, and 23 which have never been detected in these frequencies prior.

The sample of radio stars found in the analysis includes late-type dwarf stars, interacting binary pairs, young stellar objects and peculiar magnetic stars that exhibit interesting chemistry, with all detections exhibiting polarization above a certain threshold to stand out against the background noise. And each tells a different story about the in-situ processes occurring at each of the stars.

“Radio emission from stars reveals a lot of information about the magnetic properties of their atmospheres, which is otherwise difficult to measure,” said Sydney University Ph.D. candidate and lead author, Joshua Pritchard. 

“Stars also produce powerful flares and coronal mass ejections that may impact whether the planets they host are able to support life—the radio emission produced by these events is potentially one of the few ways to trace that impact.” 

“There are a wide range of processes that lead to radio emission from stars—with these detections we are mostly seeing emission produced by plasma that is being shaped by strong magnetic fields,” he said. 

“This can be driven by aurorae where the plasma is channelled to the magnetic poles of the star, or by sudden realignment of magnetic fields in a flare.”

Several Australian institutions were involved in the study, including the University of Sydney, Australia’s national science agency, CSIRO, OzGrav, the International Centre for Radio Astronomy Research (ICRAR), and ASTRO3D. International collaborators include the University of Wisconsin and the National Radio Astronomy Observatory (NRAO) in New Mexico, USA.  

Credit: Emil Lenc/CSIRO ATNF.

To complete the survey, CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) telescope was utilised, being made up of 36 x 12m dish antennas which are located in central Western Australia, at the Murchison Radio-astronomy Observatory – a government dedicated radio quiet zone.

“With ASKAP we mostly find M-dwarfs and closely separated binary systems, both of which tend to have rapid rotation speeds and strong magnetic fields. A few of the stars we detected are persistent radio sources, but most show short-duration bursts that may be associated with flares, aurorae, or interaction with a nearby planet,” said Joshua. 

Each of ASKAP’s 36 dishes is equipped with a phased array feed (PAF) allowing 36 beams per telescope to capture data of the night sky, across 288 MHz of bandwidth, surrounding a central frequency of 887.5 MHz (which translating to a wavelength of roughly 33cm).

“Each ASKAP field contains thousands of astronomical objects, most of which are distant
galaxies. We used a special type of signal (circularly polarised emission) to find the radio stars: needles in the haystack,” said Sydney University’s Professor Tara Murphy, also a co-author on the paper. 

This new analysis is the first of its kind – an all-sky circular polarization survey in centimetres of the southern sky, and forms part of the Rapid ASKAP Continuum Survey (RACS) which late last year mapped the entire southern sky below 41-degrees north in very high resolution, completing the massive task in under two weeks – an unrivalled achievement across radio astronomy. The baselines used between the ASKAP dishes as part of the survey ranged from as small as 22m through to over 6km.

Where are all the radio stars?

An artist’s depiction of a coronal mass ejection event on Proxima Centauri. Red dwarfs like Proxima Centauri give off enormous flares, causing them to double in brightness in just minutes. This also generates a lot of radio signals that can be detected from Earth. Credit: Mark Myers / OZGrav.

The EM spectrum that Earth-based telescopes can see ranges across a number of available ‘windows’ in which the atmosphere allows photons from astrophysical sources to arrive at the surface. It is for this reason that a number of different methods of observation and telescope locations are required to detect different frequencies.

For example, infrared telescopes often work best atop high mountains, away from the moisture that is trapped in the lower layers of Earth’s atmosphere. And higher energy radiation – like UV, X-rays and Gamma-rays, are often blocked out all together by Earth’s atmosphere, so these telescopes must get even higher, some even residing in space.

Radio-frequency bands have long wavelengths, often in the millimetre to km range and as such can mostly penetrate the opacity of our atmosphere, arriving at telescopes on the surface. Due to the longer wavelengths, telescopes that are attuned to these frequencies are often larger in size, which allows the collection of more radio light.

The electromagnetic spectrum and Earth’s ‘atmospheric windows’. Not all of the EM spectrum comes through to the surface. Visible light only occupies a small portion of the greater spectrum. Credit: Uni. of Delaware.

What type of EM emissions we detect is dependent on the processes occurring at the origin of the source, in this case, astrophysical sources like stars. This can be broken down into two major categories known as thermal and non-thermal radiation.

Thermal emissions are dependent on the temperature of the emitting source, and broadly speaking, can include all objects with temperatures above absolute zero (because the internal motion of atoms within each body emits in thermal ranges above absolute zero). In this context, we can consider stars as massive thermal emitters because of the energy they create through processes like nuclear fusion.

Non-thermal emissions are not generated by heat, but rather by the way atoms are excited or move in magnetic fields. For example, when an electron (which usually travels at near the speed of light) passes through a magnetic field, its pathway becomes curved as a result of this field. This change in direction produces acceleration and thus, generates non-thermal emissions, known as synchrotron radiation. As this radiation is emitted it takes the shape of a narrow cone, pointing in the direction of the particle's motion.

This type of non-thermal radiation has been observed in other bodies in deep space, such as pulsars (which are not considered main sequence stars), AGN, radio galaxies and supernovae remnants, and is also known to occur at other wavelengths like UV and X-rays.

Given that main sequence stars are fusing hydrogen in their cores, the majority of emissions detected from these bodies are thermal emissions, so we tend not to see many ‘radio stars’, unless we are considering bodies like pulsars.

Science Check: Polarization

Unpolarized view of the sky, with crosshairs showing the location of the Pulsar that was discovered with ASKAP in 2019. Credit: CSIRO.

Polarized view of the same region, showing the variance in observation. Credit: CSIRO.

All light (i.e., the entire EM spectrum) is represented by electric and magnetic fields that are positioned at right angles to each other and perpendicular to the direction they travel in, so we consider light as a transverse wave.

As light waves travel towards us, they can vibrate in a number of different directions, including up and down, side by side, and so on.  A lot of the light that we experience around us, from sunlight through LED lights, produce what is known as unpolarized light – that is the oscillations of the fields that are propagating in our direction are vibrating in all directions, not one preferred direction.

The reason why light is unpolarized to begin with is that it is made up of interactions with particles like atoms (or electrons), which are randomly distributed in space. As such, the quanta of light are all travelling in different directions. These same atoms can however be aligned to some degree, thereby making their electrical fields also aligned.

When this occurs, the electric field aspect of light then has a preferred orientation (imagine in this case it only being up and down) it is now said to be polarized light – that is, it only vibrates/oscillates in the up and down direction.

The plane in which light oscillates - from right to left: unpolarized, linearly polarized and then circular polarized (in this case left-handed). Credit: Dave3457/Wiki Commons.

When the electric field is limited to this single plane as it travels along the path of propagation, the light is then said to experience linear polarization. If this plane is rotating in a circle around the direction of travel, then this is classed as circular polarization.

As light travels through different mediums it can be reflected, refracted, or scattered causing some degree of polarization. Circular polarization indicates the presence of a magnetic field in the region, as electrons then start to spiral along heliacal paths, causing the linear field to rotate as it propagates forward.

Studying polarization of light from astronomical sources can reveal a lot of information about the source of light producing it, the magnetic field present or along the way, the way the light is reflected off another object and many more applications.

In particular, the focus on circular polarization across this analysis provided an advantage of finding radio stars across the galaxy, as the background of the type of radiation being captured is usually noisy, through the generation of synchrotron emissions generated by Active Galactic Nuclei (AGN).

However, as AGN do not mostly account for this type of polarization, the number of radio stars vs. false positives generated by the background were better confidence, revealing the radio stellar objects more clearly.

“Out of the millions of radio sources detected in RACS only a few dozen are associated with stars while most are completely unrelated background galaxies, so a search for radio stars is usually dominated by false positives. To overcome this we made use of the fact that the emission from flaring radio stars is often highly circularly polarised while most other radio sources are not,” said Joshua.

The Radio Stars of RACS

Figure one (left) showcasing visually inspected candidates. Radio stars are indicated with red stars. Figure two (right) shows the position of radio stars (red) on the Hertzsprung-Russell diagram. Credit: Pritchard et al.

As part of the analysis, 33 radio stars were detected within the sample of this study, from which 10 were previously known radio stars, but 23 had no prior radio emissions detected.

The findings included 18 K-class and M-class cool dwarf stars, which are expected to produce their non-thermal radio emissions in the star’s magnetically contained corona (the stellar atmosphere) and included YZ CMi – a known eruptive M-class star which produced bright optical flares, and as such is a well-known radio flare star.

As well as the cool dwarf stars, six pairs of interacting binary systems, which are known to possess strong magnetic fields generated by rapid, tidally induced rotation periods, were detected with radio emissions. One such example is the HR 1099/V711 Tau system which is known to exhibit strong radio flaring and electron cyclotron maser-driven auroral emissions.

Two young stellar objects (early-stage stellar objects, which are observed prior to the star commencing its nuclear fusion within its core) were also identified in the study. These objects are known to be magnetically active, producing polarized emissions.

All of the stars detected as part of this survey are within 500 light-years of Earth, so relatively nearby in terms of galactic distances. What makes these non-common radio stars so interesting is that we still have so much more to learn about the emission mechanisms that drive these non-thermal emissions, as well as being presented with an opportunity to further increase our understanding of the properties and behaviours of magnetic fields around other stars.

Of the roughly 4,500 exoplanets already discovered, the majority of ideally habitable locations are based around K-class and M-class stars – so by understanding these stars through their radio emissions, and in particular how their behaviour and outburst impacts their surrounding environments, we might get a better idea about which would likely be the ones that could harbour life as we know it.