Title: The Sydney Radio Star Catalogue: Properties of radio stars at megahertz to gigahertz frequencies
Authors: Laura Nicole Driessen, Joshua Pritchard, Tara Murphy, George Heald, Jan Robrade, Barnali Das, Stefan William Duchesne, David L. Kaplan, Emil Lenc, Christene R. Lynch, Jackson Mitchell-Bolton, Benjamin J.S. Pope, Kovi Rose, Beate Stelzer, Yuanming Wang and Andrew Zic
First Author’s Institution: Sydney Institute for Astronomy, School of Physics, The University of Sydney, Sydney, NSW, Australia
Status: Published in PASA [open access]
Our Milky Way galaxy is home to many billions of stars. By now, we’ve gotten pretty good at finding and cataloguing these stars (for example with the Gaia mission) because they’re bright; stars are also very hot and so via blackbody emission they emit most of their light in the optical or near-infrared spectrum. As we look towards lower energy emission (radio light with a long wavelength), the blackbody brightness of these stars tends to fall off so much that it’s difficult to detect them. Despite this, a subset of stars broadcast their location — and interesting physics — to us through various thermal and non-thermal radio emission mechanisms.
In today’s paper the authors introduce a brand new collection of 839 of these radio-bright stars: the Sydney Radio Star Catalogue (SRSC). The catalogue consists of a broad range of star types, from the low-mass M dwarfs all the way up to the mighty Wolf-Rayet stars, and in a distribution that covers much of the sky (see Figure 1). To detect these stars in the radio, the authors used exclusively Square-Kilometre Array (SKA) pathfinder (i.e. proof of concept) telescopes, mainly the Australian Square Kilometre Array Pathfinder (ASKAP) located in the Western Australian desert. This telescope is smaller and not as sensitive compared to the eventual SKA, but is doing important scientific work to validate SKA’s science case. The majority of the catalogued stars are in the southern hemisphere sky because of the telescope’s location, but this is where the Milky Way is most easily visible and with the least radio interference (hence why it was built there!).
Figure 1: Each radio star in the SRSC is plotted on top of an all-sky radio image with Galactic coordinates (the white horizontal strip shows the Galactic plane). The colour of each superimposed star shows its peak radio luminosity, with purple stars being more bright in radio. The black dashed line traces the celestial equator, and the dotted line shows Declination = +40 deg (i.e. in the north); most of the stars are in the southern hemisphere sky due to ASKAP’s location in Australia. Source: Figure 2 in the paper.
ASKAP was designed as a wide-field, sensitive radio observatory and allows the authors to detect stars as radio point-sources through polarised light. After detecting the point-like radio sources, the authors cross-match the source positions with known star locations using four optical/x-ray star catalogues. The SRSC is not discovering new stars entirely, but is revealing known stars in a brand new light – radio! To ensure that their cross-matches are reliable, the authors performed a Monte Carlo analysis — where they randomly offset the position of the stars and re-ran the cross-matching — and only accept radio-star matches that have a 98% reliability or higher.
What’s interesting about radio?
We probe different physics when we look at different portions of the electromagnetic spectrum, and radio is no different. Radio emission arises from two main processes: thermal and non-thermal. The former is mostly in the form of bremsstrahlung where free particles emit photons when ‘colliding’ with other particles; the latter comes mainly from synchrotron radiation where relativistic particles accelerate in a magnetic field, emitting photons as they do so. The SRSC is made up from radio observations up to frequencies of 3 gigahertz (GHz), where non-thermal radio processes dominate the brightness, and so the stars in the catalogue are correlated with particularly strong magnetic fields in the environment of the stars.
In Figure 2 we see the distribution of the SRSC stars on the Gaia colour-magnitude diagram. This shows that stars across the main sequence — the prominent diagonal band on the diagram where stars spend most of their lives — are capable of being radio bright. This shows that some main sequence stars probably have strong magnetic fields! In particular we can see that stars brighter in optical wavelengths (those in the upper regions of the diagram) in the SRSC tend to also be more radio luminous. In fact, the brightest radio star in the catalogue is the Wolf-Rayet colliding wind binary Apep (the top left of Figure 2), and the dimmest is the M dwarf EXO 040830-7134.7 (the bottom right of Figure 2).
Figure 2: Similar to Figure 1, the authors plot the SRSC stars on top of the Gaia DR3 colour-magnitude diagram. This shows that the SRSC encompasses a diverse population of star types. Hot, blue stars are on the left while cooler, redder stars are on the right; dim stars are at the bottom and bright stars at the top. As before, the SRSC stars are coloured by their radio luminosity. Source: Figure 4 in the paper.
Where to next?
As ASKAP, and eventually the SKA in the late 2020s, continue observing the Galaxy, the SRSC will be updated to reflect the newly observed stars. Many stars are transient in the radio, for example eccentric colliding wind binaries who are brighter in the radio at large separations or M dwarfs that stochastically flare, and so more observations will reveal these long-period or fleeting phenomena. Further, there are plans to extend the catalogue up to 20 GHz radio emission which may help us observe some thermal radio-emitting stars. In the meantime, the SRSC is available in your nearest Vizier or browser of choice at https://radiostars.org/ where you can search for light curves of your favourite radio star!
Astrobite edited by Veronika Dornan and Storm Colloms
Featured image credit: CSIRO, Driessen et al 2024
