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Radio emission from CU Virginis

Radio emission from CU Virginis. Kitty Lo Collaborators: Justin Bray, George Hobbs, Tara Murphy, Bryan Gaensler , Don Melrose. Introducing CU Virginis. CU Virginis is a magnetic chemically peculiar Ap star V magnitude of 5.01 Rotation period of 0.52 days

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Radio emission from CU Virginis

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  1. Radio emission from CU Virginis Kitty Lo Collaborators: Justin Bray, George Hobbs, Tara Murphy, Bryan Gaensler, Don Melrose

  2. Introducing CU Virginis • CU Virginis is a magnetic chemically peculiar Ap star • V magnitude of 5.01 • Rotation period of 0.52 days • Polar magnetic field of ~3kG (determined optically) • Distance of ~80pc Source: DSS2 image from Aladin

  3. CU Vir emits two coherent radio pulses every rotation period 1.37GHz 1.67GHz 1.87GHz 2.17GHz 2.62GHz Lo et. al, 2011 (submitted)

  4. The Pulsar lighthouse model

  5. Period of CU Vir • More recently, Mikulášek (2011) shows CU Vir goes through intervals of rotational acceleration and braking • Pyper (1998) found an abrupt increase in period of ~2 seconds around 1984 Spectrosopic variation versus phase (Pyper A&A 1998) Period variation of CU Vir (Mikulasek A&A 2011)

  6. Radio observations 13cm 20cm Lo et. al, 2011 (submitted)

  7. Electron Cyclotron Maser • Electron cyclotron frequency, eB/2πm >> plasma frequency, e√(ne/m)/2π • Usually in magnetized plasmas with low electron density and/or high magnetic field strength • Population inversion in the electron distribution • E.g. Loss cone distribution Electrons reflected by magnetic mirroring Electrons with small pitch angle precipitate to the surface Melrose & Dulk (1982)

  8. Why ECM? • High degree of circular polarization • High brightness temperature (> 1010K) • Highly beamed emission • Very narrow band • Emission frequency ~ cyclotron frequency or its second harmonic • Magnetic field of Earth – 0.6G, ECM in kHz • Magnetic field of CU Vir - ~3kG, ECM in GHz

  9. ECM is related to auroras • ECM from Earth is known as the Auroral Kilometric Radiation (AKR) • AKR is generated on auroral magnetic field lines above the auroral zone • Typical frequency is 50 to 500kHz • AKR closely correlated with occurrence of auroras (Gurnett 1974) Mutel (2004)

  10. ECM has been observed on planets and brown dwarfs Composite of optical and IR image from Cassini of Saturn Radio light curve of TVLM-513 Hallinan et al, ApJ, 2007

  11. Model of CU Vir magnetosphere

  12. Simple ECM model does not produce the correct frequency dependence

  13. We can get the right frequency dependence if emission is along B field

  14. To get radio pulses to arrive at the right phase, emission need to be ducted

  15. Ducting of ECM Ergun, ApJ, 2004

  16. Summary • CU Vir is a unique source which emits pulses of coherent radio emission • Electron cyclotron maser is believed to be the emission mechanism • Simple models of ECM does not fit the frequency dependence of the pulse arrival time • Propagation effect must play a role • ASKAP, LOFAR, MWA could potentially find a lot more of these objects

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