A circular polarisation survey for radio stars with the Australian SKA Pathfinder
Joshua Pritchard, Tara Murphy, Andrew Zic, Christene Lynch, George Heald, David L. Kaplan, Craig Anderson, Julie Banfield, Catherine Hale, Aidan Hotan, Emil Lenc, James K. Leung, David McConnell, Vanessa A. Moss, Wasim Raja, Adam J. Stewart, Matthew Whiting
MMNRAS , 1–18 (2021) Preprint 4 February 2021 Compiled using MNRAS L A TEX style file v3.0
A circular polarisation survey for radio stars with the AustralianSKA Pathfinder
Joshua Pritchard, , , ★ Tara Murphy, , † Andrew Zic, , Christene Lynch, , George Heald, David L. Kaplan, Craig Anderson, , Julie Banfield, Catherine Hale, Aidan Hotan, Emil Lenc, James K. Leung, , , David McConnell, Vanessa A. Moss, , Wasim Raja, Adam J. Stewart, and Matthew Whiting Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, Victoria, Australia International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA, Australia ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO3D), Bentley, WA, Australia CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia Department of Physics, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53201, USA. National 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 ASKAPContinuum Survey (RACS). RACS is a survey of the entire sky south of 𝛿 = + ◦ beingconducted with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) overa 288 MHz wide band centred on 887 . µ Jy PSF − . We searched RACS for sourceswith fractional circular polarisation above 6 per cent, and after excluding imaging artefacts,polarisation leakage, and known pulsars we identified radio emission coincident with 33 knownstars. These range from M-dwarfs through to magnetic, chemically peculiar A- and B-typestars. Some of these are well known radio stars such as YZ CMi and CU Vir, but 23 have noprevious radio detections. We report the flux density and derived brightness temperature ofthese detections and discuss the nature of the radio emission. We also discuss the implicationsof our results for the population statistics of radio stars in the context of future ASKAP andSquare Kilometre Array surveys. Key words: radio continuum: stars – stars: low mass – stars: chemically peculiar
While most stars generally have weak radio emission and are un-detectable over Galactic distance scales, magnetically active starsare known to exhibit radio bursts with intensities orders of mag-nitude greater than those produced by the Sun. Radio bursts havebeen detected from a wide range of stellar types, including chromo-spherically active M-dwarfs, ultracool dwarfs, close and interactingbinaries, and magnetic chemically peculiar (MCP) stars (see Güdel2002 for a review). Stellar radio bursts are often highly circularly-polarised with brightness temperatures in excess of 10 K, requir-ing the operation of a non-thermal, coherent emission process.Coherent radio bursts are generally attributed to one of two pro-cesses: plasma emission which operates at the fundamental and sec- ★ Email: [email protected] † Email: [email protected] ond harmonic of the local plasma frequency 𝜔 𝑝 = √︁ 𝜋𝑛 𝑒 𝑒 / 𝑚 𝑒 ,or electron cyclotron maser (ECM) emission at the local relativisticcyclotron frequency 𝜔 𝑐 = 𝑒𝐵 / 𝛾𝑚 𝑒 𝑐 (see Dulk 1985 for a detailedreview of these processes). Here 𝑛 𝑒 , 𝑒 , and 𝑚 𝑒 are the electron den-sity, charge, and mass respectively, 𝐵 is the magnetic field strength, 𝛾 is the Lorentz factor, and 𝑐 is the speed of light.If the emission mechanism can be determined, these processesare an excellent measure of either the electron density or magneticfield of the stellar magnetosphere, and provide a means to probeextreme events such as particle acceleration driven by magneticreconnection (Crosley et al. 2016) and auroral current systems (Letoet al. 2017). Periodic auroral bursts are often highly beamed andmodulated by stellar rotation, providing an independent constrainton the stellar rotation period (Zic et al. 2019). Constraints on theenergetics and rates of stellar radio bursts also inform models ofmagnetospheric topology (André et al. 1991), cool star magnetic © a r X i v : . [ a s t r o - ph . S R ] F e b Joshua Pritchard et al. dynamos (Kao et al. 2016), the origin of strong magnetic fields inhot stars (Schneider et al. 2016), and the habitability of exoplanets(Crosley & Osten 2018).Despite the wide variety of stars demonstrating non-thermalradio bursts, studies have typically been limited to targeted observa-tions of a small number of candidates, whose selection is motivatedby activity indicators such as prior optical or X-ray flaring (Whiteet al. 1989; Güdel 1992), presence of chromospheric emission lines(Slee et al. 1987), or previously known radio activity (Villadsen &Hallinan 2019). Surveys have also been conducted specifically tar-geting known classes of radio star such as RS Canum Venaticorum(RS CVn) and Algol binaries (Collier et al. 1982; Morris & Mutel1988; Umana et al. 1998), hot OB-type stars (Bieging et al. 1989),young stellar objects (YSOs) (André et al. 1987; Osten & Wolk2009), and ultracool dwarfs (Antonova et al. 2013). The selectionbiases inherent in these targeted searches impacts the inference ofpopulation statistics, and does not allow for the discovery of newclasses of radio stars.A few volume-limited surveys for radio stars have been con-ducted, though their success has been limited by the high surfacedensity of background radio sources, resulting in a large number offalse-positive matches. Helfand et al. (1999) searched 5000 deg ofhigh Galactic latitude sky to a sensitivity of 0 . − in theVLA Faint Images of the Radio Sky at Twenty-cm (FIRST; Beckeret al. 1995) survey identifying 26 radio stars. Kimball et al. (2009)further explored FIRST in a comparison with the Sloan DigitalSky Survey (SDSS; Adelman-McCarthy et al. 2008) selecting 112point radio sources coincident with spectrally confirmed SDSS stars,though a similar number of matches are estimated due to chancealignment with background galaxies. Umana et al. (2015) surveyeda 4 deg field located within the Galactic plane to a sensitivity of30 µ Jy PSF − in a search for stellar radio emission, and identified10 hot stars producing thermal radio emission out of 614 detectedsources. Vedantham et al. (2020) crossmatched radio sources in theLOFAR Two-Metre Sky Survey (LOTSS; Shimwell et al. 2017) withnearby stars in Gaia Data Release 2 (Andrae et al. 2018), discov-ering an M-dwarf producing coherent, circularly polarised auroralemission at metre wavelengths.Circular polarisation surveys are a promising method for wide-field detection of stellar radio bursts, as the synchrotron emissionfrom Active Galactic Nuclei (AGN) that accounts for the major-ity of unresolved radio sources is typically less than 1 − in press ) is a radio telescopearray of 36 × RACS is the first all-sky survey performed with the full 36 antennaASKAP telescope, and covers the entire sky south of 𝛿 = + ◦ .Each antenna in the array is equipped with a Phased Array Feed(PAF; Hotan et al. 2014; McConnell et al. 2016) which allows 36dual linear polarisation beams to be formed on the sky. All fourcross-correlations were recorded, allowing full Stokes I, Q, U andV images to be reconstructed. The antenna roll-axis was adjustedthroughout the observations to maintain orientation of the linearfeeds with respect to the celestial coordinate frame, so the beamfootprint remained fixed on the sky and no correction for parallacticangle was required. RACS observations used a 6 × field of view, and were acquired in 15minute integrations at a central frequency of 887 . ASKAP data is calibrated and imaged with the ASKAPsoft pack-age (Cornwell et al. 2011; Guzman et al. 2019) using the
Galaxy computer cluster that is maintained at the Pawsey SupercomputingCentre. The specific methods used for RACS data reduction aredescribed in detail by McConnell et al. (2020), but we summarisethem here.Data were prepared for imaging by flagging bad samplesand applying a calibration derived from the calibration sourcePKS B1934 − precon-ditioning (Rau 2010); RACS data were weighted to achieve theequivalent of a Briggs (1995) ‘robust weight’ of 𝑟 = .
0, where 𝑟 takes values between − × . (cid:48)(cid:48)
5, and the final mosaics haveapproximately ‡ × A complete description of the published RACS images and sourcecatalogue, including final quality control metrics, are presented byMcConnell et al. (2020) and Hale et al. ( in prep. ) respectively. Wedescribe the main quality metrics relevant to an early processing ofthe data used in this paper.The mosaic images have a point-spread-function (PSF) thatvaries over the field of view and between images due to the varia-tion in sampling of the ( 𝑢 , 𝑣 )-plane by individual beams. The centrallobe of the PSF has a typical major axis full width half maximum ‡ The mosaic boundary is determined by the values of primary beamweights and is slightly irregular. MNRAS000
5, and the final mosaics haveapproximately ‡ × A complete description of the published RACS images and sourcecatalogue, including final quality control metrics, are presented byMcConnell et al. (2020) and Hale et al. ( in prep. ) respectively. Wedescribe the main quality metrics relevant to an early processing ofthe data used in this paper.The mosaic images have a point-spread-function (PSF) thatvaries over the field of view and between images due to the varia-tion in sampling of the ( 𝑢 , 𝑣 )-plane by individual beams. The centrallobe of the PSF has a typical major axis full width half maximum ‡ The mosaic boundary is determined by the values of primary beamweights and is slightly irregular. MNRAS000 , 1–18 (2021)
ACS Stokes V Stars (FWHM) 𝐵 𝑚𝑎 𝑗 of 18 . (cid:48)(cid:48) 𝐵 𝑚𝑖𝑛 of 11 . (cid:48)(cid:48) . (cid:48)(cid:48) . (cid:48)(cid:48) in prep. ). This cat-alogue was produced from images that are convolved to a commonresolution of 25 arcsec with a uniform PSF, and has a median fluxdensity ratio of 𝑆 RACS / 𝑆 SUMSS ≈ ± .
20 derived from comparisonto the Sydney University Molonglo Sky Survey (SUMSS; Mauchet al. 2003). We selected a sample of bright, unresolved RACS cata-logue sources which are isolated from neighbouring components byat least 150 (cid:48)(cid:48) such that they are free from contamination due to closeneighbours, and crossmatched this sample with their counterpartsin our images. We find a median ratio between our fluxes and thosefrom the RACS source catalogue of 1 . ± .
09. The uncertainty inthis ratio contains contributions from random errors attributable tosignal to noise ratio (SNR) and position-dependent effects describedby McConnell et al. (2020). We scaled our fluxes by this 3 per centfactor and incorporated the 9 per cent uncertainty in quadrature withthe RACS catalogue flux scale uncertainty, arriving at a cumulativeflux density uncertainty of Δ 𝑆 / 𝑆 = . − . (cid:48)(cid:48) + . (cid:48)(cid:48) . (cid:48)(cid:48) . (cid:48)(cid:48) 𝑆 >
300 mJy PSF − areunpolarised, such that any observed Stokes V flux is solely due toleakage. A small number of pulsars are brighter than this thresholdand intrinsically polarised, though the source density of bright AGNis much greater allowing for a good estimate of the magnitude ofleakage. Among 11974 field sources in negative Stokes V and 1217in positive § , we find a median polarisation leakage of 0 .
65 and 0 . ∼ ¶ The properties of the early processing data analysed in thispaper are summarised in Table 1. Further refinements have beenapplied to the published RACS images and source catalogue, andwe refer the reader to McConnell et al. (2020) and Hale et al. ( inprep. ) for a complete description of the publicly available data.
We used the selavy (Whiting & Humphreys 2012) source finderpackage with default settings to extract source components fromthe Stokes V images, with source extraction run once to extract § Throughout this paper we adopt the IAU sign convention for which posi-tive Stokes V corresponds to right handed circular polarisation and negativeStokes V to left handed, as seen from the perspective of the observer. ¶ The asymmetry between the number of negative and positive componentsis due to a bias in the Stokes V leakage pattern, and is currently beinginvestigated. The real, polarised sources detected in this paper are evenlydistributed between positive and negative.
Table 1.
Summary of early processing RACS data properties.Property ValueSky Coverage − ◦ ≤ 𝛿 < + ◦ Central Frequency 887 . µ Jy PSF − Cumulative Flux Density Accuracy Δ 𝑆 / 𝑆 = . .
65% (V negative)0 .
54% (V positive)Astrometric Accuracy Δ 𝛼 cos 𝛿 = − . (cid:48)(cid:48) ± . (cid:48)(cid:48) Δ 𝛿 = + . (cid:48)(cid:48) ± . (cid:48)(cid:48) 𝐵 𝑚𝑖𝑛 = . (cid:48)(cid:48) ± . (cid:48)(cid:48) 𝐵 𝑚𝑎 𝑗 = . (cid:48)(cid:48) ± . (cid:48)(cid:48) ,
553 positive components and a second time on the invertedimages to extract 99 ,
647 negative components. We crossmatchedthe combined 139 ,
200 polarised components against the extractedStokes I components using a 2 (cid:48)(cid:48) match radius. This match radiuswas chosen to avoid contamination from matches between sidelobesof bright Stokes I components and the leakage of the bright centralcomponent into Stokes V, though it is restrictive as positional offsetsbetween Stokes I and V up to ∼ (cid:48)(cid:48) exist for genuine circularlypolarised source components. We selected this restricted sampleto investigate robust methods to reject imaging artefacts in futuresearches, and to reduce the search volume to a reasonable size.selavy models source components with 2D Gaussians thathave an SNR dependent positional error (Condon 1997) of 𝜎 𝜃 = 𝜃 𝑚 SNR √ 𝜃 𝑚 is the component major axis. We added this uncertaintyin quadrature to the astrometric accuracy discussed in § 2.2 todetermine the positional uncertainty of each radio source, where thepositional uncertainty corresponding to a 5 𝜎 component with majoraxis equal to the typical PSF major axis of 18 . (cid:48)(cid:48) . (cid:48)(cid:48)
2. We selected 850 components with fractional polarisation 𝑓 𝑝 = | 𝑉 |/ 𝐼 greater than 6 per cent for visual inspection. This cor-responds to 10 times the median circular polarisation leakage, andthree times the excess leakage of ∼ 𝜎 upper limits in the case of a non-detection.To identify optical and infra-red counterparts to our candidateswe generated image cutouts from the Widefield Infra-red SurveyExplorer (WISE; Wright et al. 2010), 2-Micron All Sky Survey(2MASS; Skrutskie et al. 2006), Panoramic Survey Telescope andRapid Response System 1 (Pan-STARRS1; Chambers et al. 2016),and Skymapper (Onken et al. 2019) surveys. As very few stars are
MNRAS , 1–18 (2021)
Joshua Pritchard et al.
01 Fraction included in search10 Stokes I Peak Flux Density (mJy PSF )0.00.20.40.60.81.0 f p f p = 0.06 f p = V I StarsKnown PulsarsAGN (Leakage)UnclassifiedArtefacts
Figure 1.
Classification of visually inspected candidates. The dotted lineindicates a fractional circular polarisation of 6 per cent below which weexcluded all source components, labelled as grey circles. The dashed lineindicates a 5 𝜎 𝑉 detection threshold where 𝜎 𝑉 = .
25 mJy PSF − is thetypical RMS noise in the Stokes V data, and represents the minimum frac-tional polarisation our search is sensitive to. Imaging artefacts are labelledas grey crosses, and polarisation leakage of bright AGN as yellow squares.The radio stars identified in our sample are labelled as red stars, and knownpulsars as green triangles. Six circularly polarised sources of unknown clas-sification are labelled as blue diamonds. The top panel shows the fraction ofall components selected for visual inspection as a function of flux density. persistent radio sources, detection in multiple other radio surveys issuggestive of emission from background AGN. We calculated theWISE infra-red colours (see fig. 12, Wright et al. 2010) to distin-guish these cases, as infra-red sources associated with backgroundgalaxies are typically reddened compared to stars. We also lookedfor evidence of the persistent X-ray and ultraviolet emission associ-ated with some active stars in the ROSAT
All Sky Survey (RASS;Boller et al. 2016) and
Galaxy Evolution Explorer (GALEX; Mor-rissey et al. 2007) surveys.We excluded five components that were a positional match forpreviously known radio sources attributed to bright AGN, eitherdue to the shape of the radio SED, the WISE colours, or explicitidentification in the literature. We attribute the Stokes V flux den-sity for these sources to polarisation leakage as they are located nearthe mosaic edge at 𝛿 = + ◦ where leakage increases, and AGNtypically display fractional polarisation of at most 2 per cent (Mac-quart 2002). We identified 37 components as known pulsars, andsix significantly polarised sources without any multi-wavelengthcounterpart or catalogued identification, which will be the subjectof separate publications.We queried the SIMBAD and NED databases for stars withina 3 . (cid:48) 𝑆 . Below 𝑆 = . − the minimum detectable fractional polarisation is determined bythe 5 𝜎 detection limits in Stokes V rather than contamination frompolarisation leakage or the 𝑓 𝑝 > .
06 filter we applied.We quantified the false-positive rate for chance-alignment be-tween our sources and unrelated stars by offsetting the positions ofall 139 ,
200 Stokes V components, inclusive of artefacts, in randomdirections by 5 − ◦ and crossmatching the new positions againstthe SIMBAD database. This procedure produced no matches within2 (cid:48)(cid:48) , suggesting a false-positive rate of less than 2 . × − and an ex-tremely low probability that any of our detections are due to chancealignment with background stars. Table 2 lists the 33 radio stars detected within our sample, 23 ofwhich have not previously been reported as radio-loud in the lit-erature. Fig. 2 shows a radio-selected Hertzprung-Russell diagramwith 231 stars from the Wendker catalogue (Wendker 1995) colour-mapped by observing frequency. Our detections are overlaid as redstars and span the diagram, including magnetic chemically peculiarstars, young stellar objects, RS CVn and Algol binaries, and bothchromospherically active and non-active K- and M-dwarfs. Fig. 3–9show cutout images in Stokes I and Stokes V for each of the stars inour sample, and optical data from Pan-STARRS1 or Skymapper atdeclinations above and below − ◦ respectively. We have appliedastrometric corrections to the optical data to the RACS epoch toaccount for proper motion, and all images are centred on the radioposition. MNRAS000
200 Stokes V components, inclusive of artefacts, in randomdirections by 5 − ◦ and crossmatching the new positions againstthe SIMBAD database. This procedure produced no matches within2 (cid:48)(cid:48) , suggesting a false-positive rate of less than 2 . × − and an ex-tremely low probability that any of our detections are due to chancealignment with background stars. Table 2 lists the 33 radio stars detected within our sample, 23 ofwhich have not previously been reported as radio-loud in the lit-erature. Fig. 2 shows a radio-selected Hertzprung-Russell diagramwith 231 stars from the Wendker catalogue (Wendker 1995) colour-mapped by observing frequency. Our detections are overlaid as redstars and span the diagram, including magnetic chemically peculiarstars, young stellar objects, RS CVn and Algol binaries, and bothchromospherically active and non-active K- and M-dwarfs. Fig. 3–9show cutout images in Stokes I and Stokes V for each of the stars inour sample, and optical data from Pan-STARRS1 or Skymapper atdeclinations above and below − ◦ respectively. We have appliedastrometric corrections to the optical data to the RACS epoch toaccount for proper motion, and all images are centred on the radioposition. MNRAS000 , 1–18 (2021) A C SS t o ke s V S t a rs Table 2.
Table of detected radio stars. Columns are stellar name, spectral class, radio coordinates, assumed upper limit to emission region length scale 𝑅 𝑒 , stellar parallax, 887 . 𝑆 ,signed fractional polarisation, lower limit to brightness temperature, and previous radio detection references. Name Spectral Class RA (J2000) Dec (J2000) 𝑅 𝑒 ( 𝑅 (cid:12) ) † 𝑅 𝑒 Ref. 𝜋 (mas) 𝑆 (mJy PSF − ) 𝑓 𝑝 * log 𝑇 𝐵 Radio Ref.Cool DwarfsG 131–26 M5V 00:08:53.89 + . ± .
27 55 . ± . . ± .
17 0 . ± .
211 10 . ± . − . ± .
27 10 . ± . . ± .
50 0 . ± .
083 10 . ± . − − . ± .
11 33 . ± . . ± .
85 0 . ± .
219 11 . ± . − . ± .
06 32 . ± . . ± .
89 1 . ± .
373 11 . ± . −
44 1173 K6.5Ve 03:31:55.74 − . ± .
20 22 . ± . . ± . − . ± .
176 10 . ± . − − . ± .
12 68 . ± . . ± . − . ± .
225 10 . ± . + . ± .
14 57 . ± . . ± . − . ± .
200 9 . ± .
17 Güd92YZ CMi M4Ve 07:44:39.67 + . ± .
14 167 . ± . . ± . − . ± .
430 9 . ± .
28 Dav78, Vil19G 41–14 M3.5V 08:58:56.75 + . ± . . ± . − . ± . − . ± .
36 34 . ± . . ± . − . ± .
294 9 . ± .
21G 165–61 M4.5Ve 14:17:02.04 + . ± .
23 60 . ± . . ± .
84 0 . ± .
278 10 . ± . −
38 11343 M3Ve+M4Ve 16:56:48.49 − . ± .
29 63 . ± . . ± . − . ± .
096 10 . ± . − . ± .
18 41 . ± . . ± .
32 0 . ± .
231 10 . ± . + . ± .
16 93 . ± . . ± .
12 0 . ± .
143 10 . ± .
15 Jac87, Whi89, Qui20G 183–10 M3.5Ve 17:53:00.26 + . ± .
20 44 . ± . . ± .
80 0 . ± .
200 10 . ± . − − . ± .
09 39 . ± . . ± . − . ± .
214 11 . ± . + . ± .
17 49 . ± . . ± .
54 0 . ± .
094 10 . ± . − − . ± .
02 35 . ± . . ± .
09 0 . ± .
124 12 . ± . + . ± .
90 Mut84 33 . ± . . ± . − . ± .
033 8 . ± .
61 Mut84, Whi95, Sle08, Rav10V1154 Tau B6III/IV 05:05:37.70 + . ± . . ± . . ± . − . ± .
201 9 . ± . 𝜉 UMa F8.5V 11:18:10.18 + . ± .
03 Gri98 114 . ± . . ± . − . ± .
302 6 . ± . + . ± .
15 Abb15 21 . ± . . ± . − . ± .
078 9 . ± .
25 Mut87, Whi95, Abb15V851 Cen K0III 13:44:00.96 − . ± .
49 Kar04 13 . ± . . ± .
17 0 . ± .
154 8 . ± . − . ± .
06 Sur10 8 . ± . . ± . − . ± .
201 9 . ± .
44 Sle87, Ste89Young Stellar Objects 𝜌 Oph S1 B3 16:26:34.11 − . ± . . ± . . ± . − . ± .
063 10 . ± .
15 Fal81, And88, And91EM* SR 20 G7 16:28:32.50 − . ± .
61 7 . ± . . ± .
85 0 . ± .
215 11 . ± . 𝑘 Pup / HR 2949 B3IV He-W 07:38:49.74 − . ± .
92 9 . ± . . ± .
81 0 . ± .
162 9 . ± . − . ± .
61 8 . ± . . ± .
60 0 . ± .
116 10 . ± . − . ± .
00 9 . ± . . ± .
63 0 . ± .
154 9 . ± . + . ± .
21 12 . ± . . ± .
49 0 . ± .
074 10 . ± .
18 Tri00, Let06, Rav10, Lo12V1040 Sco B2Ve He-S 15:53:55.82 − . ± .
60 Let18 7 . ± . . ± .
75 0 . ± .
055 10 . ± .
10 Con98, Mur10, Let18Hot Spectroscopic BinariesHD 32595 B8 05:04:49.06 + . ± .
93 3 . ± . . ± .
19 0 . ± .
184 10 . ± . + . ± .
12 Tor02 64 . ± . . ± .
69 0 . ± .
295 8 . ± .
22 Sch94, Hel99 * 𝑓 𝑝 > † 𝑅 𝑒 taken as: 3 𝑅 ★ for single stars as calculated from Gaia DR2 photometry (Collaboration 2018), the inter-binary region for interacting binaries, or otherwise from literature where indicated by a reference code. Reference codes : Abb15 (Abbuhl et al. 2015), And88 (André et al. 1988), And91 (André et al. 1991), Con98 (Condon et al. 1998), Dav78 (Davis et al. 1978), Fal81 (Falgarone & Gilmore 1981), Gru12 (Grunhutet al. 2012), Gri98 (Griffin 1998), Güd92 (Güdel 1992), Hel99 (Helfand et al. 1999), Jac87 (Jackson et al. 1987), Kar04 (Karataş et al. 2004), Let06 (Leto et al. 2006), Let18 (Leto et al. 2018), Lo12 (Lo et al. 2012),Mut84 (Mutel et al. 1984), Mut87 (Mutel et al. 1987), Mur10 (Murphy et al. 2010), Qui20 (Quiroga-Nuñez et al. 2020), Rav10 (Ravi et al. 2010), Sch94 (Schmitt et al. 1994), Sle87 (Slee et al. 1987), Sle08 (Slee et al.2008), Ste89 (Stewart et al. 1989), Tor02 (Torres & Ribas 2002), Tri00 (Trigilio et al. 2000), Sur10 (Sürgit et al. 2010), Vil19 (Villadsen & Hallinan 2019), Whi89 (White et al. 1989), Whi95 (White & Franciosini1995) M N R A S , ( ) Joshua Pritchard et al.
B V M V O | B | A | F | G | K | Mlog L R = 14log L R = 15log L R = 16log L R = 17log L R = 18log L R = 19log L R = 20 246810 F r e q u e n c y ( G H z ) Figure 2.
Radio-selected Hertzprung-Russell diagram, showing 231 previ-ously detected radio stars (Wendker 1995) as circles with our sample overlaidas red stars. The size of each marker represents the greatest radio luminosityrecorded for that star, and the colour-map indicates the observing frequency.
We identify 18 K- and M-dwarf stars within our sample, 15 of whichhave no previously reported radio detection. Cool dwarfs typicallyproduce non-thermal radio emission in magnetically confined coro-nae. Non-thermal, incoherent gyrosynchrotron emission dominatesat C/X-band, and coherent emission processes at lower frequencieswhere gyrosynchrotron emission becomes optically-thick (Güdel2002). Our cool dwarf detections have fractional polarisation rang-ing from 29 −
100 per cent. Notes on individual sources are presentedbelow.
G 131–26 (see Fig. 3) is an M5V flare star demonstrating H 𝛼 (Hawley et al. 1996; Newton et al. 2017; Jeffers et al. 2018) andultraviolet (Jones & West 2016) activity, and is a strong variableX-ray source (Hambaryan et al. 1999). CS Cet (see Fig. 3) is a spectroscopic binary with a K0IVe primarywhich has demonstrated BY Dra type photometric variability dueto rotation of starspots (Watson et al. 2000). This system is a brightultraviolet and X-ray source (Haakonsen & Rutledge 2009; Beitia-Antero & de Castro 2016) and is chromospherically active in Ca iiH and K and H 𝛼 lines (Isaacson & Fischer 2010). LP 771–50 (see Fig. 3) is an M5Ve-type high-proper-motion stardemonstrating H 𝛼 line emission (Cruz & Reid 2002). CD −
44 1173 (see Fig. 3) is a young K6.5Ve star in the Tucana-Horologium moving group with rotationally modulated optical vari-ability (Messina et al. 2010), and has previously exhibited ultraviolet flares (Loyd et al. 2018) and X-ray activity (Haakonsen & Rutledge2009).
V833 Tau (see Fig. 4) is an extremely active K2.5Ve BY Dravariable demonstrating both X-ray flares (Fuhrmeister & Schmitt2003) and photometric variability due to large starspot coveringfractions (Oláh et al. 2001). Güdel (1992) reported a radio fluxof 5 . ± .
07 mJy at 8 . ∼ K brightnesstemperature. We detect 76 per cent left circularly-polarised emissionwith 𝑆 = . ± .
46 mJy PSF − , which corresponds to a bright-ness temperature of ∼ × K and is consistent with optically-thingyrosynchrotron emission.
YZ CMi (see Fig. 4) is an M4Ve eruptive variable known to producebright optical flares (Kowalski et al. 2010), and is a well knownradio flare star (Davis et al. 1978; Abada-Simon & Aubier 1997).Highly circularly-polarised radio bursts have been detected fromthis star at centimetre wavelengths (Villadsen & Hallinan 2019)attributed to ECM emission from within extreme coronal plasmacavities which is modulated by stellar rotation. We detect 98 percent left circularly-polarised emission, which is consistent with thehandedness observed by Villadsen & Hallinan (2019).
G 41–14 (see Fig. 4) is an M3.5V triple star system with recordedoptical flares (Martínez et al. 2020) and H 𝛼 (Hawley et al. 1996;Newton et al. 2017; Jeffers et al. 2018) and ultraviolet (Miles &Shkolnik 2017) activity, but no previously reported radio emission.The star was included in a radio interferometric search of nearbystars for planetary companions (Bower et al. 2009) but no detectionsare reported. A 0 . ± .
14 mJy FIRST radio source is reported7 . (cid:48)(cid:48) . (cid:48)(cid:48)
35 separation from the FIRSTposition, indicating that the star may have been the origin of theradio emission. G 41–14 was detected in RASS (Fuhrmeister &Schmitt 2003) and is flagged as displaying X-ray flares.
MV Vir / HD 124498 (see Fig. 4) is a binary system with aK5.5Vkee-type primary and an unclassified secondary. The sys-tem exhibits strong chromospheric Ca ii H and K line emission(Boro Saikia et al. 2018) and is a bright ultraviolet (Ansdellet al. 2014) and X-ray source (Haakonsen & Rutledge 2009).We detect 68 per cent left circularly-polarised emission with 𝑆 = .
72 mJy PSF − positioned 0 . (cid:48)(cid:48) . (cid:48)(cid:48) . (cid:48)(cid:48) G 165–61 (see Fig. 5) is an M4.5V spectroscopic binary that demon-strates both H 𝛼 (Newton et al. 2017) and ultraviolet (Jones & West2016) activity. G 165–61 has a rotational period of 113.6 days (New-ton et al. 2016), while M-dwarf radio bursts are typically associatedwith young, fast rotators (McLean et al. 2012), and only a small frac-tion of >
100 day period M-dwarfs demonstrate other indicators ofmagnetic activity (West et al. 2015; Mondrik et al. 2018). CD −
38 11343 (see Fig. 5) is an M3Ve + M4Ve binary systemof flare stars (Tamazian & Malkov 2014). The A component hasbeen associated with flaring X-ray emission (Fuhrmeister & Schmitt2003; Wright et al. 2011) and periodic optical variability due to
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ACS Stokes V Stars rotation of starspots (Kiraga 2012). The angular separation betweenthe stars is of the order of our positional uncertainty at 3 . (cid:48)(cid:48)
1, so thatit is not clear from which star the emission originates.
Ross 867 and Ross 868 (see Fig. 5) are a visual binary system ofM4.5V and M3.5V flare stars with similar stellar properties. Bothcomponents have demonstrated optical flaring (Tamazian & Malkov2014) and photospheric variability (Kiraga 2012), ultraviolet (Jones& West 2016) and X-ray (Fuhrmeister & Schmitt 2003) variability,and H 𝛼 activity (Hawley et al. 1996). Ross 867 is a well knownradio-loud star (Jackson et al. 1987; White et al. 1989) and inparticular has demonstrated radio bursts with moderate circularpolarisation (Quiroga-Nuñez et al. 2020), while Ross 868 has neverbeen detected at radio wavelengths. We detect 67 per cent rightcircularly-polarised emission with 𝑆 = . ± .
32 mJy PSF − unambiguously associated with Ross 867, which is separated fromRoss 868 by 16 . (cid:48)(cid:48) G 183–10 (see Fig. 5) is an M4V-type star with no significantultraviolet (Ansdell et al. 2014) or H 𝛼 (Jeffers et al. 2018) activity,and no previously reported radio detection. Ross 776 (see Fig. 6) is an M3.3V flare star (Tamazian & Malkov2014) showing both H 𝛼 (Hawley et al. 1996; West et al. 2015)and ultraviolet activity (Jones & West 2016), and has previouslydemonstrated X-ray flaring (Fuhrmeister & Schmitt 2003). Thisstar is a young, rapid rotator, with a 0 .
586 d photometric rotationperiod (Newton et al. 2016).
UPM J0250 − − − − , and SCR J2241 − (see Fig. 3–6) are highproper motion stars without spectral classification, though photo-metric colour indices imply these are M-class stars (Lépine & Gai-dos 2011; Winters et al. 2011; Frith et al. 2013). SCR J2241 − We identify 6 interacting binary systems of RS CVn and Algoltype, three of which have no previously reported radio detection.RS CVn and Algol binaries are known to possess strong magneticfields generated by rapid, tidally-induced rotation periods. The radioemission associated with these stars is generally non-thermal withmoderate circular polarisation, with both quiescent gyrosynchrotronemission (Jones et al. 1994; Abbuhl et al. 2015) and coherent radiobursts (van den Oord & de Bruyn 1994; White & Franciosini 1995;Slee et al. 2008) observed. Our detections in this category have afractional polarisation between 16 −
88 per cent. Notes on individualsources are presented below.
HR 1099 / V711 Tau (see Fig. 6) is a K2:Vnk/K4 RS CVn binarywith extensive study at radio frequencies, and is known to exhibitboth strong radio flaring and periods of quiescent emission. VLBIdetections have been made during the quiescent period at 1 .
65 GHz(Mutel et al. 1984) with a source size comparable to the size of thebinary system, and during a flare at 8 . V1154 Tau (see Fig. 6) is a B6III/IV-type eclipsing Algol binarywith no previously reported radio detections. 𝜉 UMa (see Fig. 7) is an F8.5:V+G2V RS CVn binary systemwith multiple sub-components. The cooler secondary component demonstrates X-ray emission (Ball et al. 2005) attributed to activityinduced by a 3 .
98 d orbit with one of its sub-components. Thissystem has been frequently targeted by radio observations (Spangleret al. 1977; Morris & Mutel 1988; Drake et al. 1989, 1992; Bastianet al. 2000) but no detections have previously been reported. Thesystem includes a T8.5 brown dwarf in a wide orbit (Wright et al.2013), though this companion is separated by 8 . (cid:48) BH CVn / HR 5110 (see Fig. 7) is an F2IV/K2IV binary knownto demonstrate large X-ray flares (Graffagnino et al. 1995), andis one of the most radio active RS CVn systems. VLBI observa-tions at 15 . . . V851 Cen (see Fig. 7) is a K0III RS CVn binary with chromo-spheric activity in both H 𝛼 and Ca ii H and K lines (Cincunegui et al.2007), and has previously demonstrated X-ray variability (Kashyap& Drake 1999; Haakonsen & Rutledge 2009; Kiraga 2012). The sys-tem has been targeted in two radio surveys due to the chromosphericactivity (Collier et al. 1982; Slee et al. 1987) but was undetected inboth. KZ Pav (see Fig. 7) is a F6V/K4IV Algol binary of the EA2-type, and has previously demonstrated variable radio emission at8 . . ± . . . ± . K assuming emission originates from the inter-binary region.
We identify two young stellar objects within our sample. Pre-main sequence stars are magnetically-active and known to produceboth highly polarised coherent emission (Smith et al. 2003) andnon-thermal gyrosynchrotron emission (André 1996). Non-thermalemission is less commonly observed in classical T-Tauri (CTT) starspresumably due to absorption in the ionised circumstellar wind,though exceptions exist associated with extended magnetosphericstructures such as the non-thermal radio knots in the jets of DG Tau(Ainsworth et al. 2014). Notes on individual sources are presentedbelow. 𝜌 Oph S1 (see Fig. 7) is a B3 T-Tauri star in the Rho Ophiuchicloud complex with a kilogauss globally organised magnetic field,and is a persistent radio source with a faint 20 (cid:48)(cid:48) wide halo andcircularly-polarised core (Falgarone & Gilmore 1981; André et al.1988). VLBI observation of this star at 4 .
985 GHz has resolvedthe core as an 8 − 𝑅 (cid:12) region of optically-thin gyrosynchrotronemission with brightness temperature of order 10 K (André et al.1991). We detect 23 per cent left circularly-polarised emission with 𝑆 = . ± .
28 mJy PSF − , which is comparable to the 5 GHzand 15 GHz measurements by André et al. 1988. EM* SR 20 (see Fig. 8) is a binary CTT system in the Rho Ophiuchicloud complex, with the G7 primary embedded in a protostellardisk (McClure et al. 2008). This star has been targeted at millimetre
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Joshua Pritchard et al. − − − − D ec O ff s e t I − − − − G 131–26 V − − − − PS -13 m J y P S F − -21 m J y P S F − − − − − D ec O ff s e t I − − − − CS Cet V − − − − PS -17 m J y P S F − -31 m J y P S F − − − − − D ec O ff s e t I − − − − UPM J0250 − V − − − − PS -13 m J y P S F − -21 m J y P S F − − − − − D ec O ff s e t I − − − − LP 771–50 V − − − − PS -12 m J y P S F − -21 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetCD −
44 1173 V − − − − RA Offset SM -14 m J y P S F − -14 m J y P S F − Figure 3.
Images of G 131–26, CS Cet, UPM J0250 − −
44 1173. RACS data is shown in Stokes I (left panels) and Stokes V (middlepanels), where positive flux density in the Stokes V map corresponds to right handed circular polarisation and negative to left handed. The ellipse in the lowerleft corner of each radio image shows the restoring beam. The right panels show Stokes I contours overlaid on optical data from Pan-STARRS1 (g-band) orSkymapper for stars above and below 𝛿 = − ◦ respectively, with contour levels at 30, 60, and 90 per cent of the peak Stokes I flux density. All images havebeen centred on a frame aligned with the position of the radio source, with North up and East to the left. Optical data has been astrometrically corrected to theRACS epoch according to the proper motion of the target star. For some of the brighter stars the optical data is over-saturated or masked.MNRAS000
44 1173. RACS data is shown in Stokes I (left panels) and Stokes V (middlepanels), where positive flux density in the Stokes V map corresponds to right handed circular polarisation and negative to left handed. The ellipse in the lowerleft corner of each radio image shows the restoring beam. The right panels show Stokes I contours overlaid on optical data from Pan-STARRS1 (g-band) orSkymapper for stars above and below 𝛿 = − ◦ respectively, with contour levels at 30, 60, and 90 per cent of the peak Stokes I flux density. All images havebeen centred on a frame aligned with the position of the radio source, with North up and East to the left. Optical data has been astrometrically corrected to theRACS epoch according to the proper motion of the target star. For some of the brighter stars the optical data is over-saturated or masked.MNRAS000 , 1–18 (2021) ACS Stokes V Stars − − − − D ec O ff s e t I − − − − UPM J0409 − V − − − − SM -13 m J y P S F − -13 m J y P S F − − − − − D ec O ff s e t I − − − − V833 Tau V − − − − PS -14 m J y P S F − -13 m J y P S F − − − − − D ec O ff s e t I − − − − YZ CMi V − − − − PS -13 m J y P S F − -13 m J y P S F − − − − − D ec O ff s e t I − − − − G 41–14 V − − − − PS -120 m J y P S F − -118 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetMV Vir V − − − − RA Offset PS -12 m J y P S F − -12 m J y P S F − Figure 4.
Images of UPM J0409 − , 1–18 (2021) Joshua Pritchard et al. − − − − D ec O ff s e t I − − − − G 165–61 V − − − − PS -13 m J y P S F − -21 m J y P S F − − − − − D ec O ff s e t I − − − − CD −
38 11343 V − − − − SM -18 m J y P S F − -13 m J y P S F − − − − − D ec O ff s e t I − − − − UCAC4 312–101210 V − − − − PS -14 m J y P S F − -31 m J y P S F − − − − − D ec O ff s e t I − − − − Ross 867 V − − − − PS -14 m J y P S F − -31 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetG 183–10 V − − − − RA Offset PS -13 m J y P S F − -21 m J y P S F − Figure 5.
Images of G 165–61, CD −
38 11343, UCAC4 312–101210, Ross 867, and G 183–10. Details as in Fig. 3.MNRAS000
38 11343, UCAC4 312–101210, Ross 867, and G 183–10. Details as in Fig. 3.MNRAS000 , 1–18 (2021)
ACS Stokes V Stars − − − − D ec O ff s e t I − − − − SCR J1928 − V − − − − SM -13 m J y P S F − -12 m J y P S F − − − − − D ec O ff s e t I − − − − Ross 776 V − − − − PS -17 m J y P S F − -41 m J y P S F − − − − − D ec O ff s e t I − − − − SCR J2241 − V − − − − SM -15 m J y P S F − -31 m J y P S F − − − − − D ec O ff s e t I − − − − HR 1099 V − − − − PS -117 m J y P S F − -13 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetV1154 Tau V − − − − RA Offset PS -13 m J y P S F − -12 m J y P S F − Figure 6.
Images of SCR J1928 − − , 1–18 (2021) Joshua Pritchard et al. − − − − D ec O ff s e t I − − − − ξ UMa V − − − − PS -12 m J y P S F − -12 m J y P S F − − − − − D ec O ff s e t I − − − − BH CVn / HR 5110 V − − − − PS -18 m J y P S F − -13 m J y P S F − − − − − D ec O ff s e t I − − − − V851 Cen V − − − − SM -18 m J y P S F − -61 m J y P S F − − − − − D ec O ff s e t I − − − − KZ Pav V − − − − SM -12 m J y P S F − -11 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA Offset ρ Oph S1 V − − − − RA Offset PS -19 m J y P S F − -12 m J y P S F − Figure 7.
Images of 𝜉 UMa, BH CVn, V851 Cen, KZ Pav, and 𝜌 Oph S1. Details as in Fig. 3. MNRAS000
Images of 𝜉 UMa, BH CVn, V851 Cen, KZ Pav, and 𝜌 Oph S1. Details as in Fig. 3. MNRAS000 , 1–18 (2021)
ACS Stokes V Stars − − − − D ec O ff s e t I − − − − EM* SR 20 V − − − − PS -13 m J y P S F − -21 m J y P S F − − − − − D ec O ff s e t I − − − − k Pup / HR 2949 V − − − − PS -13 m J y P S F − -31 m J y P S F − − − − − D ec O ff s e t I − − − − OY Vel V − − − − SM -18 m J y P S F − -51 m J y P S F − − − − − D ec O ff s e t I − − − − V863 Cen V − − − − SM -13 m J y P S F − -21 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetCU Vir V − − − − RA Offset PS -115 m J y P S F − -61 m J y P S F − Figure 8.
Images of EM* SR 20, k Pup, OY Vel, V863 Cen, and CU Vir. Details as in Fig. 3.MNRAS , 1–18 (2021) Joshua Pritchard et al. − − − − D ec O ff s e t I − − − − V1040 Sco V − − − − PS -18 m J y P S F − -21 m J y P S F − − − − − D ec O ff s e t I − − − − HD 32595 V − − − − PS -14 m J y P S F − -31 m J y P S F − − − − − RA Offset D ec O ff s e t I − − − − RA OffsetCastor A V − − − − RA Offset PS -12 m J y P S F − -21 m J y P S F − Figure 9.
Images of V1040 Sco, HD 32595, and Castor A. Details as in Fig. 3. wavelengths for cold dust continuum emission, but is undetected ata 35 mJy limit (Nürnberger et al. 1998).
Compared to later-type stars, magnetically driven, non-thermalemission is less common in hot stars, presumably due to the lackof an interior convective zone. The exception are the MCP stars,which have strong, globally organised magnetic fields, and havebeen observed to produce both gyrosynchrotron emission driven byequatorial current sheets (Linsky et al. 1992), and coherent auroralemission (Trigilio et al. 2011). We identify five MCP stars withinour sample with polarisation fractions between 22 −
70 per cent.Notes on individual sources are presented below. k Pup / HR 2949 (see Fig. 8) is a helium-weak B3IV star withkilogauss surface magnetic fields and helium spectral line vari-ability suggestive of chemically peculiar spots, in analogue to thecooler Bp/Ap stars (Shultz et al. 2015). There are no reported radiodetections of this star.
OY Vel (see Fig. 8) is an ApSi magnetic chemically peculiar star andan 𝛼 CVn variable with no previously reported radio detections.
V863 Cen (see Fig. 8) is a chemically peculiar B6IIIe helium-strong star with kilogauss surface magnetic fields (Briquet et al. 2003; Kochukhov & Bagnulo 2006) and no previously reportedradio emission. Radio detections of Be stars are typically attributedto interactions between stellar wind outflows and a circumstellardisk resulting in free-free emission with low fractional polarisation,though a few notable variable detections are suggestive of a non-thermal mechanism (Dougherty et al. 1991; Skinner et al. 1993).Our detection is inconsistent with free-free emission, with 60 percent left circularly-polarised emission that is likely driven by a non-thermal mechanism.
CU Vir (see Fig. 8) is an ApSi 𝛼 CVn variable, and a well knownradio-loud magnetic chemically peculiar star. Quiescent gyrosyn-chrotron emission with rotationally modulated variability has beenobserved from CU Vir (Leto et al. 2006), as well as two 100 percent right circularly-polarised pulses that consistently repeat eachrotation period (Trigilio et al. 2000; Ravi et al. 2010; Lo et al. 2012)and are attributed to ECM emission.
V1040 Sco (see Fig. 9) is a helium-strong B2.5V star with a kilo-gauss global magnetic field, and is the most rapidly rotating mag-netic B star known (Grunhut et al. 2012). This star has been detectedat radio frequencies from 1.4 to 292 GHz (Condon et al. 1998; Mur-phy et al. 2010; Leto et al. 2018), with persistent emission attributedto gyrosynchrotron emission in the rigidly rotating magnetosphere.We measure a total intensity flux density of 7 . ± .
20 mJy PSF − MNRAS000
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ACS Stokes V Stars which is in agreement with that expected from extrapolation of thegyrosynchrotron spectrum to 887 . ∼
20 GHz. This isalso in agreement with our measurement of 22 per cent right handedcircular polarisation.
Two stars in our sample are hot, early-type stars that lack identifiedglobal magnetic fields or chemical peculiarities, and which have anunclassified spectroscopic binary companion. Chemically regularearly-type stars are not expected to generate flares due to the lack of aconventional dynamo (Pedersen et al. 2016), and do not demonstrateother common features associated with non-thermal, polarised radioemission. Notes on individual sources are presented below.
HD 32595 (see Fig. 9) is a spectroscopic binary with a chemicallyregular B8 primary, and no previously reported radio detections.
Castor (see Fig. 9) is a triple binary system, where Castor A andB are both spectroscopic binaries consisting of A-type primariesand dMe secondaries, and Castor C is a binary dMe system. TheCastor A system has been detected previously at 1 . . . . (cid:48)(cid:48)
83 from Castor A and 3 . (cid:48)(cid:48)
94 from Castor B, so the emission islikely associated with one of the stars in Castor A.
We calculated the Rayleigh-Jeans brightness temperature of ourdetections according to 𝑇 𝐵 = 𝑆 𝑐 𝜋𝑘 𝐵 𝜈 𝐷 𝑅 𝑒 , (2)where 𝑘 𝐵 is the Boltzmann constant, 𝜈 = . 𝑅 𝑒 is the length scale of an assumed emission regionand 𝐷 is the stellar distance. We have assumed an upper limit to theemission region of 𝑅 𝑒 = 𝑅 ★ for detections associated with singlestars, and three times the binary separation distance for interactingbinary systems, except where the emission region has been previ-ously determined with VLBI. As the true emission region may besmaller than our assumed upper limit, the derived brightness tem-perature for each of our detections are lower limits, and range from10 K to 10 K.In Fig. 10 we show the position of our detections in a brightnesstemperature–fractional polarisation phase space. We show empiri-cal models of the maximum brightness temperature of optically-thin, non-thermal gyrosynchrotron emission derived from Dulk(1985) at viewing angles of 20 ° to 80 ° , and electron power-lawenergy index 𝛿 of 2 −
5. Each model is limited to the cyclotronharmonic number range 10 < 𝜔 / 𝜔 𝑐 < 𝑇 𝐵 too high to explain for the observed fractional polarisation, andare likely driven by a coherent emission process.Our upper limit assumptions on the emission region requirethat the orientation of the magnetic field varies significantly in thesource region, which in turn limits the fractional circular polarisa-tion due to contributions from both left and right hand polarisedemission. Our detections with high degrees of circular polarisationare therefore likely driven by sources in a much smaller region witha correspondingly higher brightness temperature, further suggestinga coherent emission process. We detected 33 radio stars within the covered RACS survey areaof 34 159 deg with detections evenly distributed on the sky, corre-sponding to a surface density of 9 . + . − . × − deg − with errorsgiven by 95 per cent Poisson confidence intervals. Our survey waslimited to 𝑓 𝑝 > .
06 due to the lack of on-axis polarisation leakagecalibration which has now been applied to available RACS data, andwe further restricted our search to positional offsets between StokesI and V of less than 2 (cid:48)(cid:48) to reduce contamination from imaging arte-facts. Our detection rate is therefore a lower limit to the surfacedensity of radio stars at 887 . 𝑁 ( > 𝑆 ) ∝ 𝑆 − / . TheFIRST survey for radio stars (Helfand et al. 1999) detected 26 starsin a survey area of 5000 deg above a flux density of 0 . . + . − . × − deg − which is comparable to our result.We extrapolate the number of radio stars detected in this sur-vey to hypothetical future surveys with ASKAP. Scaling our re-sults to a deep 20 µ Jy PSF − RMS survey of the entire sky southof 𝛿 = + ◦ implies a surface density of detectable radio starsof 4 . + . − . × − deg − and 1400 − to an RMS of0 . − should produce 6 −
20 detections per epoch witha surface density of 1 . + . − . × − deg − , and would probe theduty cycle and luminosity distribution of individual variable radiostars.As the majority of our K- and M-type dwarf detections are in-consistent with incoherent gyrosynchrotron emission and are likelydriven by coherent bursts, we also calculate the surface density ofthese detections alone for comparison to previous flare rate estimatesin the literature. We detected 18 K- and M-type dwarfs, assumingthe detection of Castor A can be attributed to the dMe compan-ion, which results in a surface density of radio-loud cool dwarfsat 887 . . + . − . × − deg − . In comparison, Villadsen& Hallinan (2019) report an M-dwarf transient density at 1 . . × − deg − once scaled to the RACS sensitivity, whichimplies RACS should produce ∼
80 M-dwarf detections. This differ-ence may reflect an increase in cool star radio activity at 1 . We have completed the first all-sky circular polarisation search forradio stars at centimetre wavelengths within the Rapid ASKAPContinuum Survey, identifying 10 known radio stars and a further
MNRAS , 1–18 (2021) Joshua Pritchard et al. f p T b ( K ) = 80 = 60 = 40 = 20 = 2 = 3 = 4 = 5dKMRS CVn / AlgolYSOMCPHSBdKMRS CVn / AlgolYSOMCPHSB Figure 10.
Phase diagram of brightness temperature and fractional polarisation. Brightness temperatures correspond to lower limits assuming the observedemission originates from a 3 𝑅 ★ disk, with K- and M-dwarfs represented by red circles, YSOs by green stars, MCP stars by purple diamonds, and hotspectroscopic binaries as blue right triangles. For RS CVn and Algol binaries we show two brightness temperatures: yellow squares represent emissionoriginating from the inter-binary region and yellow crosses represent a 3 𝑅 ★ disk. Empirical models (Dulk 1985) are plotted approximating the maximumbrightness temperature of optically-thin gyrosynchrotron emission for viewing angles of 20 ◦ , 40 ◦ , 60 ◦ , and 80 ◦ , and electron power law energy indices of 𝛿 = −
23 with no previous radio detection. Our sample includes late-typedwarfs, interacting and chromospherically active binaries, youngstellar objects, and magnetic chemically peculiar stars, demonstrat-ing the variety of magnetically-active stars detectable with thistechnique. Many of our detections are highly polarised with bright-ness temperatures that are inconsistent with an incoherent emissionmechanism. These stars are attractive targets for followup observa-tions to determine the emission mechanism and further constrainthe magnetospheric properties of the emission environment.This survey represents a sample of polarised radio stars with-out any other selection bias and implies a lower limit to thesurface density of radio stars above 1 .
25 mJy at 887 . . + . − . × − deg − . Application of on-axis polarisation cali-bration to publicly released ASKAP data will allow lower fractionalpolarisation and fainter emission to be detected, and extension ofthis search technique to future ASKAP surveys will produce a sig-nificantly larger sample and the potential for detections in multiple epochs. These observations will present an opportunity to determineimproved population statistics for the stellar parameters associatedwith non-thermal radio emission, and to study burst rates and ener-getics for thousands of individual stars. Acknowledgements
We thank the anonymous referee for feedback that strengthened thiswork. We also thank Phil Edwards and Alec Thomson for help-ful comments and suggestions. TM acknowledges the support ofthe Australian Research Council through grant FT150100099. JP,AZ, and JKL are supported by Australian Government ResearchTraining Program Scholarships. DLK was supported by NSF grantAST-1816492. The Australian Square Kilometre Array Pathfinderis part of the Australia Telescope National Facility which is man-aged by CSIRO. Operation of ASKAP is funded by the Australian
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ACS Stokes V Stars Government with support from the National Collaborative ResearchInfrastructure Strategy. ASKAP uses the resources of the PawseySupercomputing Centre. Establishment of ASKAP, the Murchi-son Radio-astronomy Observatory and the Pawsey SupercomputingCentre are initiatives of the Australian Government, with supportfrom the Government of Western Australia and the Science and In-dustry Endowment Fund. We acknowledge the Wajarri Yamatji asthe traditional owners of the Murchison Radio-astronomy Observa-tory site. The International Centre for Radio Astronomy Research(ICRAR) is a Joint Venture of Curtin University and The Universityof Western Australia, funded by the Western Australian State gov-ernment. Parts of this research were supported by the ARC Centre ofExcellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D),through project number CE170100013. This research made use ofthe following python packages: Astropy (Astropy Collaborationet al. 2013, 2018), a community-developed core Python package forAstronomy, matplotlib (Hunter 2007), a Python library for publi-cation quality graphics, NumPy (Van Der Walt et al. 2011; Harriset al. 2020), and pandas (Wes McKinney 2010; McKinney 2011).
Data Availability
The data analysed in this paper are accessible through the CSIROASKAP Science Data Archive (CASDA; Chapman et al. 2017)under project code AS110. Note that the images used in this papermay have properties that differ slightly from the publicly releasedversions.
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