T dwarfs are a subclass of low-mass (≲0.075 M ⊙ ) substellar objects with effective temperatures between 450 and 1500 K (Kirkpatrick 2005; Kirkpatrick et al. 2012). T dwarfs and other spectral types later than M7 are referred to collectively as ultracool dwarfs (UCDs). UCDs are fully convective and do not possess an intermediate tachocline shearing region like more-massive stars (Chabrier et al. 2000; Hughes et al. 2021). Tachocline shearing is thought to be a key component in the αΩ dynamo process that powers the magnetic fields of partially convective, higher-mass stars (Brandenburg & Subramanian 2005). Nonetheless, radio observations have resulted in strong evidence for ∼103 G magnetic fields in T dwarfs (e.g., Route & Wolszczan 2012; Kao et al. 2016), despite the lack of a tachocline region, requiring the operation of an alternative dynamo mechanism for UCDs (e.g., Christensen et al. 2009). Hence the investigation of magnetic field production in UCDs is important for improving our understanding of stellar evolution and dynamo theory.
The chromospheric and coronal activity generally associated with radio bursts from earlier-type stars (≤M6) weakens with later spectral types. Rodríguez-Barrera et al. (2015) found that UCDs later than L4 dwarfs (T eff ∼ 1600 K) cannot sustain the ionization levels necessary for the atmospheric current system that produces typical stellar radio emission. The standard practice, therefore, is to use the processes that drive auroral emission in solar system gas giant planets (e.g., Hill 1979) to model the magnetic activity for brown dwarfs in the L/T transition regime (L4–T4) and later (Williams 2018). Much of the literature focuses primarily on these late-type UCDs as the radio pulse morphology, and possibly the astrophysical generator, are different for earlier-type (≤L4) UCDs (Pineda et al. 2017).
The generation of strong dipole fields in UCDs is thought to be tied to their fast rotation (Kao et al. 2018). UCDs have projected rotational velocities of v km s−1 though their rotational periods can range from 1 hr to upwards of 20 hr (Tannock et al. 2021, and references therein). T dwarfs on average rotate more rapidly than other UCDs, reaching projected velocities up to v km s−1 (Zapatero Osorio et al. 2006; Tannock et al. 2021). Rapid rotation plays a critical role in the corotational breakdown between UCD magnetic fields and ionospheric plasma, which produces the electrical currents responsible for generating auroral emission (Cowley & Bunce 2001; Nichols et al. 2012).
Electron cyclotron maser instability (ECMI) is the dominant mechanism producing coherent emission from UCDs, including the auroral emission that is understood to be modulated by the star's rotational period (Hallinan et al. 2006, 2008). ECMI converts the free plasma energy in the auroral region—from the perpendicular component of the cyclotron motion around the magnetic field lines—into circularly polarized emission (Melrose & Dulk 1982) at the electron cyclotron frequency: ν c = eB/2π m e c ≈ 2.8 × 106 B Hz (Dulk 1985), where e is the electron charge, B is the magnetic field strength in Gauss, m e is the electron mass, and c is the speed of light (Williams 2018). Analysis of radio emission from T dwarfs therefore allows us to measure the strength and structure of their magnetic fields. Auroral ECMI is rotationally modulated and thus the emission can be also be used to measure rotational velocities. This can be difficult to measure through Zeeman Doppler Imaging for late-type UCDs, which tend to be faster rotators than earlier-type M dwarfs. Studying these magnetic and rotational properties help us improve upon existing models of stellar dynamo theory as well as the evolution of giant exoplanets and late-type stars (e.g., Schrijver 2009; Williams & Berger 2015; Route 2016; Pineda et al. 2017).
The first radio detection of a UCD was reported by Berger et al. (2001) who identified both quiescent and flaring radio emission from the M9 dwarf LP944−20. Radio observations conducted in the megahertz to gigahertz range have since resulted in detections of late-type L dwarfs and T dwarfs (Route & Wolszczan 2012; Williams & Berger 2015; Kao et al. 2018). Vedantham et al. (2020) made the first radio detection of a UCD that had not been previously identified in optical or infrared. Vedantham et al. (2020) used the Low-Frequency Array (LOFAR; Shimwell et al. 2022) to identify BDR J1750+3809, which they then spectroscopically classified as a T6.5 ± 1 dwarf with the near-infrared SpeX instrument on NASA's Infrared Telescope Facility (IRTF; Rayner et al. 2003). The latest-type UCD detected in the radio to date is part of the T dwarf binary discovered by Vedantham et al. (2023), which is composed of T5.5 ± 0.5 and T7.0 ± 0.5 dwarfs.
Targeted radio surveys of UCDs at 4–9 GHz have been conducted with the Karl G. Janksy Very Large Array (VLA; Perley et al. 2011) and the Australian Telescope Compact Array (ATCA; Wilson et al. 2011) (e.g., Berger 2006; Lynch et al. 2016). These surveys targeted a range of known UCDs and detected radio emission from ≲10% of them (Route & Wolszczan 2016a). Kao et al. (2016) conducted a targeted VLA survey of UCDs that had exhibited signs of auroral activity at other wavelengths (Hα, optical, and infrared). They detected four out of the five dwarfs that had not previously been observed to produce radio emission.
LOFAR and the Australian SKA Pathfinder (ASKAP; Hotan et al. 2021) have come online with high-sensitivity wide-field capabilities at megahertz to low-gigahertz frequencies. Stokes V (circular polarization) searches with the LOFAR Two-metre Sky Survey (Shimwell et al. 2019) and the Rapid ASKAP Continuum Survey (RACS; McConnell et al. 2020) have already found new UCDs that had not previously been observed to produce radio emission (e.g., Pritchard et al. 2021; Callingham et al. 2023). UCD dynamo modeling by Christensen et al. (2009) predicts magnetic fields of order 102 G for Y dwarfs and 103 G for T dwarfs. Given the relationship between magnetic field strength and the electron cyclotron frequency, this would imply that megahertz and low-gigahertz observations are well placed to detect radio emission from late T dwarfs and potentially even Y dwarfs—which remain undetected at radio wavelengths (Kao et al. 2019).
T dwarfs are the coolest substellar spectral type observed to produce radio emission and there are currently only six such systems that have been detected (Pineda et al. 2017; Vedantham et al. 2020, 2023). In this paper we present the detection and analysis of an ultracool T8 dwarf found in a new untargeted gigahertz survey conducted with ASKAP. The source, WISE J062309.94−045624.6, is the coolest and latest-type UCD detected at radio wavelengths to date. Our detection of radio emission from WISE J062309.94−045624.6 adds to the small population radioactive T dwarfs and is the first example of multiple, high duty cycle pulses from a T dwarf. The clear periodicity and strong spectral features of these pulses inform our understanding of the rotational and magnetospheric properties of WISE J062309.94−045624.6, while also providing more general insights into the astrophysical mechanism responsible for producing detectable radio emission in late-type ultracool dwarfs.