Monday, October 14, 2013

A Pair of Very Young Brown Dwarfs

Brown dwarfs bridge the gap between the least massive stars (~0.075 Sun’s mass) and the most massive planets (~0.013 Sun’s mass). Although they form as stars do and resemble gas giant planets, a brown dwarf is neither star nor planet. In 2006, a team of astronomers announced the discovery of a pair of young brown dwarfs located ~1400 light years away in the Orion Nebula. The pair is identified as 2MASS J05352184-0546085 and it is the first example of an eclipsing binary system comprising two brown dwarfs. Here, the two brown dwarfs mutually eclipse each other as they orbit around their common centre of mass.

Figure 1: This artist’s impression shows an eclipsing binary system. As the two components orbit each other, they pass in front of one another and create periodic dips in their combined brightness. Credit: ESO/L. Calçada.

The higher mass ‘primary’ component has a mass of 0.054 ± 0.005 solar mass and a size of 0.669 ± 0.034 solar radii, while the lower mass ‘secondary’ component has a mass of 0.034 ± 0.003 solar mass and a size of 0.511 ± 0.026 solar radii. Since the Orion Nebula star cluster is extremely young, 2MASS J05352184-0546085 is believed to be no more than a few million years old. As such, the large radii of its two brown dwarfs (~5 times larger than older brown dwarfs) are consistent with theoretical predictions for young brown dwarfs in the earliest stages of gravitational contraction. In comparison, older brown dwarfs at ~1 billion years of age have sizes of ~0.1 solar radii.

Figure 2: Light curve of 2MASS J05352184-0546085. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter ‘secondary’ component by the cooler ‘primary’ component. Both components orbit each other with an orbital period of 9.78 days. (Stassun et al., 2006)

Light curve data of 2MASS J05352184-0546085 indicates that the lower mass ‘secondary’ component has a higher temperature than the higher mass ‘primary’ component. The estimated temperatures of the ‘primary’ and ‘secondary’ components are 2,650 ± 100 K and 2,790 ± 105 K respectively. Finding that the higher mass ‘primary’ brown dwarf has a lower temperature than its ‘secondary’ companion is puzzling because theoretical models predict that a brown dwarf of a given mass will at all times be warmer than a lower mass brown dwarf of the same age.

Two explanations have been proposed that may explain the reversal of component temperatures with mass in 2MASS J05352184-0546085. The first is that the two brown dwarfs are mildly non-coeval, with the higher mass ‘primary’ component being ~0.5 million years younger than the ‘secondary’ component. The second explanation is that strong magnetic activity on the ‘primary’ component is suppressing the transport of energy from interior to surface and causing a lowering of its surface temperature.

- Stassun, K. G., Mathieu, R. D., & Valenti, J. A., “Discovery of two young brown dwarfs in an eclipsing binary system”, Nature 440, 311-314 (16 March 2006)
- Stassun, K. G., Mathieu, R. D., & Valenti, J. A., “A Surprising Reversal of Temperatures in the Brown Dwarf Eclipsing Binary 2MASS J05352184-0546085”, ApJ 664: 1154-1166, 2007 August 1