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Published December 10, 2006 | Published + Submitted
Journal Article Open

Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513–46546: Broadband Coherent Emission at the Substellar Boundary?


The Very Large Array was used to observe the ultracool rapidly rotating M9 dwarf TVLM 513-46546 simultaneously at 4.88 and 8.44 GHz. The radio emission was determined to be persistent, variable, and periodic at both frequencies with a period of ~2 hr. This periodicity is in excellent agreement with the estimated period of rotation of the dwarf based on its v sin i of ~60 km s^(-1). This rotational modulation places strong constraints on the source size of the radio-emitting region and hence the brightness temperature of the associated emission. We find the resulting high brightness temperature, together with the inherent directivity of the rotationally modulated component of the emission, difficult to reconcile with incoherent gyrosynchrotron radiation. We conclude that a more likely source is coherent, electron cyclotron maser emission from the low-density regions above the magnetic poles. This model requires the magnetic field of TVLM 513-46546 to take the form of a large-scale, stable dipole or multipole with surface field strengths up to at least 3 kG. We discuss a mechanism by which broadband, persistent electron cyclotron maser emission can be sustained in the low-density regions of the magnetospheres of ultracool dwarfs. A second nonvarying, unpolarized component of the emission may be due to depolarization of the coherent electron cyclotron maser emission or, alternatively, incoherent gyrosynchrotron or synchrotron radiation from a population of electrons trapped in the large-scale magnetic field.

Additional Information

© 2006 The American Astronomical Society. Received 2006 April 20; accepted 2006 August 27. The authors gratefully acknowledge the support of the HEA funded Cosmogrid project and Enterprise Ireland under the grant award SC/2001/0322. The authors also wish to acknowledge the SFI/HEA Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities and support. Armagh Observatory is grant aided by the Northern Ireland Department of Culture, Arts, and Leisure. We are very grateful to Harry Lehto for the use of his CLEAN algorithm and code and the informative comments on the results produced, and to Jerome Sheahan for helpful discussions on certain aspects of this manuscript. Finally, we would like to thank the referee, Rachel Osten, for valuable input and suggestions on how to improve this manuscript

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Published - 0004-637X_653_1_690.pdf

Submitted - 0608556v1.pdf


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