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Cool Stars 20 Radio Splinter P The 20th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun Edited by S. J. Wolk Meter- to Millimeter Emission from Cool Stellar Systems: Latest Results, Synergies Across the Spectrum, and Outlook for the Next Decade Jan Forbrich,1;2 Peter K. G. Williams,2 Emily Drabek-Maunder,3;4 Ward Howard,5 Moira Jardine,6 Lynn Matthews,7 Soa Moschou,2 Robert Mutel,8 Luis Quiroga-Nuñez,9 Joseph Rodriguez,2 Jackie Villadsen,10 Andrew Zic,11 Rachel Osten,12 Edo Berger,2 & Manuel Güdel13 1 Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hateld AL10 9AB, UK 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 3 School of Physics and Astronomy, Cardi University, The Parade, Cardi, CF24 3AA, UK 4 Royal Observatory Greenwich, Blackheath Ave, London SE10 8XJ, UK 5 Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA 6 Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK 7 MIT Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA 8 Department of Physics and Astronomy, University of Iowa, 203 Van Allen Hall, Iowa City, IA, 52242, USA 9 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands 10 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 11 Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, NSW 2006, Australia 12 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 13 University of Vienna, Department of Astrophysics, Trkenschanzstrasse 17, 1180 Vienna, Austria Abstract Radio observations of cool stellar systems provide unique information on their magnetic elds, high-energy processes, and chemistry. Buoyed by powerful new instruments (e.g. ALMA, JVLA, LOFAR), advances in related elds (e.g., the Gaia astro- metric revolution), and above all a renewed interest in the relevant stellar astrophysics, stellar radio astronomy is experiencing a renaissance. In this splinter session, participants took stock of the present state of stellar radio astronomy to chart a course for the eld’s future. 1 Introduction ultracool dwarfs or young stellar objects that are not accessi- In this splinter session, participants discussed stellar ra- ble even to Gaia. Broadband, high-time-resolution studies of dio astronomy with an emphasis on latest results, synergies ares reveal the detailed plasma processes operating in the across the spectrum, and an outlook for the next decade. A coronae of other stars. total of ten presentations was selected to complement radio- The world’s best present-day radio observatories – chiey themed plenary talks by Rachel Osten, Meredith MacGregor, the VLA and ALMA – launched the stellar radio astronomy and Jan Forbrich, published elsewhere in these proceedings. renaissance. This is particularly true for constraints on non- Additionally, poster presenters were able to highlight their thermal emission, which are particularly dependent on con- work in a poster pops slot. The session concluded with an tinuum sensitivity. Spectral indices, polarization, and vari- open discussion. ability all require high S/N detections to derive meaningful This splinter session mainly focused on nonthermal radio constraints. The upgraded VLA and VLBA as well as Arecibo emission. In the context of cool star radio astronomy, this now provide such information in the cm wavelength range, traditionally encompassed mainly (gyro)synchrotron radia- ALMA is doing so in the mm wavelength range, and LO- tion at centimeter and millimeter wavelengths, i.e., emission FAR, MWA, LWA, HERA, and GMRT are making new for- from (mildly) relativistic electrons in magnetic elds. But re- ays into the low-frequency range. As a result of these newly cent years have shown a variety of other ways in which radio feasible measurements, we are obtaining unique information observations can deliver astrophysical insight. The discov- on the role of magnetic elds on various scales and a new ery of brown dwarf radio emission has rekindled interest in and complementary perspective on high-energy processes. electron cyclotron maser (ECM) emission as just one of sev- These scales now even include outows/jets from protostars eral intriguing physical mechanisms in this regime, many of and T Tauri stars, providing initial evidence for magnetic which are stepping stones toward studies of low-frequency elds in young stellar environments. exoplanetary radio emission. VLBI observations of stellar ra- The new generation of low-frequency radio observatories dio emission can be used to obtain precision astrometry – and promises to gure especially prominently in the upcoming hence precise measurements of fundamental parameters – of decade of stellar radio astronomy. These observatories can monitor the whole sky nearly continuously, surveying for 1 Forbrich et al. stellar ares, coronal mass ejections, and other heliospheric phenomena. They are also expected to be the only facilities that will be able to directly probe the magnetic elds of extra- solar planets, through auroral radio bursts, providing unique insight into an essential facet of exoplanetary habitability. There are important synergies of nonthermal radio emis- sion with other wavelength ranges, as featured in this splin- ter session. These are mainly due to the beginning of radio time domain astronomy with the advent of sensitive contin- uum receivers, as seen for instance in the nearly-complete MeerKAT telescope. It is now possible to systematically study high-energy processes in both the X-ray and the ra- dio time domain, and capabilities for doing so will increase dramatically when eROSITA commences operations. Under- standing the correlations between optical and radio variabil- ity will be essential in the era of rst TESS, then LSST. In the following, we briey present highlights from the ses- sion presentations, in the order of appearance. 2 Session highlights Figure 1: Simulating radio emission from V374 Peg, see 2.1 Simulating Radio Emission from Low Mass Stars Llama et al. (2018). (Moira Jardine) One of the enduring puzzles about very low mass stars is that they appear to be radio bright (relative to their X-ray luminosity). Most solar-like stars follow a well-dened rela- source size. Measuring the sizes of radio sources, and lo- tionship between their radio and X-ray luminosities known calizing them relative to the photosphere, thereby anchoring as the Güdel-Benz relation, Benz & Güdel, 1994). This can them to photospheric magnetic eld measurements, can help be understood as two forms of emission (the thermal X-ray relieve this degeneracy. emission and the non-thermal radio emission) that originate The Very Long Baseline Array (VLBA) at 8.4 GHz achieves from the same population of electrons accelerated in ares. a resolution 1-3 times larger than the photospheres of nearby The puzzle about the very low-mass stars is that they appear M dwarfs, enabling source localization and size measurement to have excess radio emission over and above what might be to a fraction of the photospheric diameter in cases with high expected from this process. signal-to-noise ratio. VLBI has been used for astrometry of One possible explanation is that another process (electron- active M dwarfs at 1.4 GHz (Benz & Alef, 1991; Benz et al., cyclotron maser emission) is contributing in these objects. 1995) and 8.4 GHz (Bower et al., 2009, 2011). In addition, To explore this we have taken (almost) simultaneous obser- observations at 8.4 GHz have shown that the radio corona vations of the surface magnetic eld for V374 Peg and its ra- of active M dwarfs is sometimes resolved: Benz et al. (1998) dio lightcurve from the VLA. By extrapolating the 3D stellar observed 2 lobes separated by 4 stellar diameters during a magnetic eld and modelling the electron cyclotron emission are on M6 dwarf UV Ceti, and Pestalozzi et al. (2000) ob- we have produced a synthetic radio lightcurve that ts well served a radio source 1.7 times larger than the photosphere to the observed one (see Figure 1; Llama et al., 2018). This during quiescent and aring emission. 2015 8.4-GHz VLBA demonstrates the role of the eld geometry in determining observations of UV Cet (Figure 2) and AD Leo, with three 4- the nature of the radio emission and provides a new tech- hour epochs per target, found distinct behavior on these two nique for studying the radio emission from low mass stars. stars (Villadsen et al., in prep). AD Leo was resolved in 2 of 3 epochs with source size consistent with the photospheric 2.2 Resolved imaging of the quiet and aring radio diameter in all epochs, suggestive of quiescent radio emis- corona of active M dwarfs (Jackie Villadsen) sion originating from the low corona, with a minutes-long Active M dwarfs produce bright radio emission at GHz are in one epoch oset from the quiescent source by 0.9 frequencies: quiescent emission, incoherent ares, and co- stellar diameters. The quiescent emission from UV Cet was herent bursts. The quiescent emission and incoherent ares resolved in 3 of 3 epochs, all consistent with a source size are generally attributed to gyrosynchrotron emission from of ∼3 by 2 stellar diameters, and with each epoch contain- mildly relativistic electrons spiraling around magnetic eld ing a coherent burst associated with a radio aurora. In two lines, although the electron cyclotron maser may be respon- of three epochs, UV Cet’s quiescent emission shows a dipolar sible for the quiescent emission from V374 Peg (Llama et al., structure in Stokes V. The large scale and dipolar structure of 2018). As such, radio emission provides the only direct means UV Cet’s quiescent emission suggest that the radio emission of observing energetic particles in stellar atmospheres.
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