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Brown Dwarfs and Substellar Objects
Brown dwarfs occupy the mass gap between planets and stars and are thought to be one of the most populous objects in our Galaxy. They have a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores and are therefore much cooler and dimmer than main sequence stars. This makes them very difficult to detect, and although astronomers have known of their existence for decades it wasn't until 1995 that a brown dwarf was finally found.
In recent years it has been discovered that these brown dwarfs can be extremely bright sources of radio emission. Up to now it has been unclear how these failed stars can produce such high levels of this nature of radiation. Initially, it was assumed that it was the same kind of radio emission as that detected from stars such as our Sun. For such stars, the radio emission is produced by high energy electrons in the star's corona which are trapped spiralling in the star's magnetic field..
However, observations conducted by the group with the Very Large Array radio telescope in New Mexico, together with optical telescopes at the US Naval Observatory and Vatican Observatory, have showed that this model is incorrect. In 2007, we published results in the Astrophysical Journal demonstrating for the first time that several of these objects produce periodic pulsed coherent radio emission - very much like the behaviour exhibited by pulsars, collapsed neutron stars that shine, like lighthouses, intense beams of radiation in step with their highly stable, incredibly fast spinning magnetic field. Our work indicated that to first order we were seeing the same thing from ostensibly dead, inert brown dwarfs.
Since this discovery, several brown dwarfs spanning the substellar boundary, from spectral class M9 to T6, have been found to produce intense pulsed radio emission, that can only be caused by the presence of kilogauss magnetic fields, fields that furthermore are both large scale and highly stable - an affront to our understanding of how dynamos work in such fully convective objects. Many questions remain - why are only ~ 10% of the local population of brown dwarfs found to be radio emitters, and why are only a smaller subset known to possess coherent maser emission regions in their magnetospheres? Working with former members of the group, we co-authored a paper that appeared in Nature in the summer of 2015 yielding a breakthrough in our understanding of this phenomena, where using both the Jansky VLA, the Keck and Palomar 5m telescopes, we showed for the first time the presence of an aurora on another world. This is an important discovery, as it shows - again - the overlap between planetary astronomy and stellar astrophysics, and offers a means to probe the origins for the electron maser activity in the magnetospheres of these bizarre objects.
- Dr. Aaron Golden (School of Mathematics, Statistics & Applied Mathematics)
- Dr. Ray Butler (School of Physics)
- Salam Dulaimi
Some Past Members
- Dr. Gregg Hallinan
- Dr. Leon Harding
Our ResearchOur current research in NUI Galway is divided into two specific observational areas. In the optical/IR band, we are conducting further observational campaigns of candidate brown dwarfs to determine if we can detect a long term, periodic modulation in their light curves using the GUFI photometer currently in-situ at the Vatican Advanced Technology Telescope at the Mount Graham International Observatory in Arizona, as well as using the excellent capabilities of the South African Large Telescope (SALT) to obtain optical spectra of several nearby brown dwarfs situated in the southern sky. Using the Jansky Very Large Array radio telescope array, we are also conducting a programme of observations to try and further understand the emission - and so magnetic - properties of these enigmatic objects.
- Professor G. Hallinan (California Institute of Technology)
- Professor J.G. Doyle (Armagh Observatory)
- Dr. R.J. Boyle (Vatican Observatory)
Hallinan, G., Littlefair, S. P., Cotter, G., Bourke, S., Harding, L. K., Pineda, J. S., Butler, R. P., Golden, A., Basri, G., Doyle, J. G., Kao, M. M., Berdyugina, S. V., Kuznetsov, A., Rupen, M. P., Antonova, A. ‘Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence’. Nature 523, 568-571, 2015.
Harding, L.K., Hallinan, G., Boyle, R.P., Golden, A., Sheehan, B., Zavala, R.T., Butler, R.F. ’Periodic Optical Variability of Radio Detected Ultracool Dwarfs’ Astrophysical Journal, 779, 1:21 21, 2013.
Harding, L.K., Hallinan, G., Konopacky, Q.M., Boyle, R.P., Butler, R.F., Golden, A. ‘Spin-orbit alignment in the very low mass binary regime: The L dwarf tight binary 2MASSW J0746425+200032AB’ Astronomy & Astrophysics, 554A, 113, 2013.
Bourke, S., van Langevelde, H.J., Torstensson, K., Golden, A. ’An AIPS-based Distributed Processing Method for Large Radio Interferometric Datasets’ Experimental Astronomy, Volume 36, Issue 1-2, pp 59-76, 2013.
Hallinan, G., Sirothia, S., Antonova, A., Chandra, I., Doyle, J.G., Bourke, S., Hartman, J., Golden, A. ’Looking for a Pulse: A search for Rotationally Modulated Radio Emission from the Hot Jupiter, Tau Bo ̈otis b’ Astrophysical Journal, 762, 34:4, 2013.
Antonova, A., Hallinan, G., Doyle, J.G., Yu, S., Kuznetsov, A., Metodieva, Y., Golden, A., Cruz, K.L. ’Volume-limited radio survey of ultracool dwarfs’ Astronomy & Astrophysics, 549, 14, 2013.