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A Circular Polarisation Survey for Radio Stars with the Australian SKA Pathfinder

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A Circular Polarisation Survey for Radio Stars with the Australian SKA Pathfinder MNRAS 000,1–18 (2021) Preprint 4 February 2021 Compiled using MNRAS LATEX style file v3.0 A circular polarisation survey for radio stars with the Australian SKA Pathfinder Joshua Pritchard,1,2,3¢ Tara Murphy,1,3y Andrew Zic,1,2 Christene Lynch,4,5 George Heald,6 David L. Kaplan,7 Craig Anderson,8,2 Julie Banfield,2 Catherine Hale,6 Aidan Hotan,6 Emil Lenc,2 James K. Leung,1,2,3 David McConnell,2 Vanessa A. Moss,2,1 Wasim Raja,2 Adam J. Stewart,1 and Matthew Whiting2 1Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia 2CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia 3ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, Victoria, Australia 4International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA, Australia 5ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO3D), Bentley, WA, Australia 6CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia 7Department of Physics, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53201, USA. 8National Radio Astronomy Observatory, 1003 Lopezville Rd, Socorro, NM 87801, USA Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We present results from a circular polarisation survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is a survey of the entire sky south of X = ¸41◦ being conducted with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) over a 288 MHz wide band centred on 887.5 MHz. The data we analyse includes Stokes I and V polarisation products to an RMS sensitivity of 250 µJy PSF−1. We searched RACS for sources with fractional circular polarisation above 6 per cent, and after excluding imaging artefacts, polarisation leakage, and known pulsars we identified radio emission coincident with 33 known stars. These range from M-dwarfs through to magnetic, chemically peculiar A- and B-type stars. Some of these are well known radio stars such as YZ CMi and CU Vir, but 23 have no previous radio detections. We report the flux density and derived brightness temperature of these detections and discuss the nature of the radio emission. We also discuss the implications of our results for the population statistics of radio stars in the context of future ASKAP and Square Kilometre Array surveys. Key words: radio continuum: stars – stars: low mass – stars: chemically peculiar p 1 INTRODUCTION ond harmonic of the local plasma frequency l? = 4c=442/<4, or electron cyclotron maser (ECM) emission at the local relativistic While most stars generally have weak radio emission and are un- cyclotron frequency l2 = 4퐵/W<42 (see Dulk 1985 for a detailed arXiv:2102.01801v1 [astro-ph.SR] 2 Feb 2021 detectable over Galactic distance scales, magnetically active stars review of these processes). Here = , 4, and < are the electron den- are known to exhibit radio bursts with intensities orders of mag- 4 4 sity, charge, and mass respectively, 퐵 is the magnetic field strength, nitude greater than those produced by the Sun. Radio bursts have W is the Lorentz factor, and 2 is the speed of light. been detected from a wide range of stellar types, including chromo- spherically active M-dwarfs, ultracool dwarfs, close and interacting If the emission mechanism can be determined, these processes binaries, and magnetic chemically peculiar (MCP) stars (see Güdel are an excellent measure of either the electron density or magnetic 2002 for a review). Stellar radio bursts are often highly circularly- 12 field of the stellar magnetosphere, and provide a means to probe polarised with brightness temperatures in excess of 10 K, requir- extreme events such as particle acceleration driven by magnetic ing the operation of a non-thermal, coherent emission process. reconnection (Crosley et al. 2016) and auroral current systems (Leto Coherent radio bursts are generally attributed to one of two pro- et al. 2017). Periodic auroral bursts are often highly beamed and cesses: plasma emission which operates at the fundamental and sec- modulated by stellar rotation, providing an independent constraint on the stellar rotation period (Zic et al. 2019). Constraints on the ¢ Email: [email protected] energetics and rates of stellar radio bursts also inform models of y Email: [email protected] magnetospheric topology (André et al. 1991), cool star magnetic © 2021 The Authors 2 Joshua Pritchard et al. dynamos (Kao et al. 2016), the origin of strong magnetic fields in detected radio stars in§4 and discuss the implications of our survey hot stars (Schneider et al. 2016), and the habitability of exoplanets on stellar radio burst statistics and future ASKAP surveys in§5. (Crosley & Osten 2018). Despite the wide variety of stars demonstrating non-thermal radio bursts, studies have typically been limited to targeted observa- 2 OBSERVATIONS AND DATA REDUCTION tions of a small number of candidates, whose selection is motivated by activity indicators such as prior optical or X-ray flaring (White RACS is the first all-sky survey performed with the full 36 antenna X = ¸ ◦ et al. 1989; Güdel 1992), presence of chromospheric emission lines ASKAP telescope, and covers the entire sky south of 41 . (Slee et al. 1987), or previously known radio activity (Villadsen & Each antenna in the array is equipped with a Phased Array Feed Hallinan 2019). Surveys have also been conducted specifically tar- (PAF; Hotan et al. 2014; McConnell et al. 2016) which allows 36 geting known classes of radio star such as RS Canum Venaticorum dual linear polarisation beams to be formed on the sky. All four (RS CVn) and Algol binaries (Collier et al. 1982; Morris & Mutel cross-correlations were recorded, allowing full Stokes I, Q, U and 1988; Umana et al. 1998), hot OB-type stars (Bieging et al. 1989), V images to be reconstructed. The antenna roll-axis was adjusted young stellar objects (YSOs) (André et al. 1987; Osten & Wolk throughout the observations to maintain orientation of the linear 2009), and ultracool dwarfs (Antonova et al. 2013). The selection feeds with respect to the celestial coordinate frame, so the beam biases inherent in these targeted searches impacts the inference of footprint remained fixed on the sky and no correction for parallactic angle was required. RACS observations used a 6×6-beam square population statistics, and does not allow for the discovery of new 2 classes of radio stars. footprint giving a 32 deg field of view, and were acquired in 15 minute integrations at a central frequency of 887.5 MHz with 288 A few volume-limited surveys for radio stars have been con- channels each 1 MHz wide. RACS observations were conducted ducted, though their success has been limited by the high surface from April through May 2019, with fields originally located near density of background radio sources, resulting in a large number of the Sun observed in August 2019. false-positive matches. Helfand et al.(1999) searched 5000 deg2 of high Galactic latitude sky to a sensitivity of 0.7 mJy PSF−1 in the VLA Faint Images of the Radio Sky at Twenty-cm (FIRST; Becker 2.1 Data Reduction et al. 1995) survey identifying 26 radio stars. Kimball et al.(2009) further explored FIRST in a comparison with the Sloan Digital ASKAP data is calibrated and imaged with the ASKAPsoft pack- Sky Survey (SDSS; Adelman-McCarthy et al. 2008) selecting 112 age (Cornwell et al. 2011; Guzman et al. 2019) using the Galaxy point radio sources coincident with spectrally confirmed SDSS stars, computer cluster that is maintained at the Pawsey Supercomputing though a similar number of matches are estimated due to chance Centre. The specific methods used for RACS data reduction are alignment with background galaxies. Umana et al.(2015) surveyed described in detail by McConnell et al.(2020), but we summarise a 4 deg2 field located within the Galactic plane to a sensitivity of them here. 30 µJy PSF−1 in a search for stellar radio emission, and identified Data were prepared for imaging by flagging bad samples 10 hot stars producing thermal radio emission out of 614 detected and applying a calibration derived from the calibration source − sources. Vedantham et al.(2020) crossmatched radio sources in the PKS B1934 638, which included factors for adjustment of inter- LOFAR Two-Metre Sky Survey (LOTSS; Shimwell et al. 2017) with ferometric phase and for setting the absolute flux density scale. nearby stars in Gaia Data Release 2 (Andrae et al. 2018), discov- Images were formed for each beam as the first two terms (0, 1) ering an M-dwarf producing coherent, circularly polarised auroral of a Taylor series in frequency using multi-frequency synthesis. emission at metre wavelengths. ASKAPsoft applies weights to the visibility data using precon- ditioning (Rau 2010); RACS data were weighted to achieve the Circular polarisation surveys are a promising method for wide- equivalent of a Briggs(1995) ‘robust weight’ of A = 0.0, where A field detection of stellar radio bursts, as the synchrotron emission takes values between −2 and 2 corresponding to uniform and natural from Active Galactic Nuclei (AGN) that accounts for the major- weighting respectively. All beam images were deconvolved using − ity of unresolved radio sources is typically less than 1 2 per the BasisfunctionMFS algorithm before being combined into a cent circularly-polarised (Macquart 2002), and thus the number single image by linear mosaicing. Beam images were made with of false-positive matches between stars and unrelated background 6144 × 6144 pixels each of size 200. 5, and the final mosaics have AGN is significantly reduced. Lenc et al.(2018) performed an all- approximatelyz 11800 × 11800 pixels of the same size. sky circular polarisation survey at 200 MHz with the Murchison Widefield Array (MWA; Bowman et al.
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