Cool Stars, Brown Dwarfs and Exoplanets

Podcast - Gregg Hallinan explains how NUI Galway astronomers are searching for extrasolar planets using radio telescopes.

Group Members

• Gregg Hallinan (Group Leader)
• Aaron Golden
• Ray Butler
• Leon Harding

Collaborators

• Gerry Doyle and Tony Antonova ( Armagh Observatory, N. Ireland)
• Walter Brisken ( National Radio Astronomy Observatory, New Mexico)
• Bob Zavala and Fred Vrba ( United States Naval Observatory, Flagstaff, Arizona)
• Richard Boyle ( Vatican Observatory Research Group, Steward Observatory, University of Arizona)
• Stephen Bourke (Joint Institute for VLBI in Europe (JIVE), Dwingeloo, Netherlands)

The Discovery of Radio Pulsations from Ultracool Dwarfs

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.

Figure 1: Artist's impression of the "super-aurorae" present at the magnetic poles of these radio emitting dwarfs which are responsible for the radio pulsations.

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.

Figure 2: Two images of the location of the binary L dwarf 2MASSW J0746425+2000321 taken with the VLA at a frequency of 4.88 GHz . The position of the binary brown dwarf is marked by a white arrow in both cases. A field source is located 10 arcseconds from the brown dwarf and is present in both images to the upper left hand side of the target. The left hand image was taken during the interpulse phase, when the binary brown dwarf is too faint to be detected. The right hand image was taken during one of the pulses with the brown dwarf now outshining the nearby field source.

However, our recent observations conducted with the Very Large Array radio telescope in New Mexico, together with optical telescopes at the US Naval Observatory and Vatican Observatory, have shown that this model is incorrect. We have detected extremely bright periodic pulses of radiation from a number of these objects which cannot be explained by the conventional processes associated with stellar radio emission. During these pulses, these supposedly failed stars are tens of thousands of times brighter than our Sun at radio frequencies! Instead a much more exotic process is required to explain such bright radio emission.

It turns out that the answer to this mystery is not to be found in the study of the radio emission from the stars but instead from the planets in our Solar System. All the magnetized planets, including Earth, are observed to emit extremely bright radio emission from their magnetic polar regions. Indeed, Jupiter can produce radio emission at low frequencies brighter than that detected from the Sun. This radiation is not produced by the same mechanism responsible for stellar radio emission but rather by a coherent process, the electron cyclotron maser, that can amplify the radiation to extremely high levels. In the Earth's case, the maser radiation is produced when the Solar wind slams into the planet's magnetosphere accelerating electrons into the polar regions. These electrons produce extremely bright radio emission and subsequently impact the Earth's ionosphere to produce aurorae visible from the Earth's surface.

Movie1: Animated gif of the radio emission from the M9 dwarf TVLM 513-46546 detected with the VLA at 8.44 GHz . The time between each bright pulse corresponds to 1.958 hours, which is the period of rotation of the dwarf. The movie was created from the data obtained with the VLA using the ParselTongue interface to the AIPS astronomical data analysis package.

A very similar process is believed to apply to brown dwarfs, albeit producing radio emission many orders of magnitude brighter than that detected from the planets. The resulting radiation, which is very strongly beamed perpendicular to the magnetic field of the brown dwarf, sweeps Earth once per rotation period of the dwarf to produce the bright pulses. However, it remains a mystery how the high energy electrons which produce the radio emission are continuously accelerated into the magnetic poles of the dwarf. What has been established is that this radio emission requires these brown dwarfs to possess very powerful, large-scale magnetic fields as strong as those detected from the most magnetically active main sequence stars.

Figure 3: Time series of the radio emission detected with the VLA from the M9 dwarf TVLM 513-46546. Every 1.958 hours a periodic pulse is detected when extremely bright, beams of radiation originating at the poles sweep Earth when the dwarf rotates.

The periodic pulses detected from brown dwarfs are very similar in nature to those observed from one of the most exotic classes of object in our Universe, pulsars. Pulsars are produced during supernovae, when a massive star explodes and it's core collapses into a rapidly spinning neutron stars. Beams of radiation are emitted from the magnetic poles of the neutron star and, as is the case for brown dwarfs, sweep Earth with rotation of the star. How pulsars produce this radiation has been one of the great puzzles in astrophysics for nearly 40 years. The difficulty in establishing this emission mechanism is grounded in our lack of understanding of the behaviour of plasma in the extreme conditions associated with pulsars. Brown dwarfs are now the second class of stellar object known to produce persistent levels of extremely bright, coherent radiation. Moreover, this emission is also manifested in the detection of periodic pulses. However, in the case of brown dwarfs, both the source conditions and the emission mechanism are reasonably well understood. It is hoped that the study of these brown dwarfs may provide vital clues to unlocking the long-standing puzzle of how pulsars produce their radio emission.

Movie2: Same as Movie 1, with a much wider field of view. A field source can be seen to emit fairly steadily in the upper right hand corner of the animated gif while TVLM 513-46546 is located in the lower left hand corner of the animated gif and is seen to periodically pulse. The time between each bright pulse corresponds to 1.958 hours, which is the period of rotation of the dwarf. The movie was created from the data obtained with the VLA using the ParselTongue interface to the AIPS astronomical data analysis package.

"This work is supported by Science Foundation Ireland under it's Research Frontiers Programme, the Higher Education Authority's Programme for Research in Third Level Institutions, and the Irish Research Council for Science, Engineering and Technology."

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