Wednesday, July 9, 2014

Sub-Stellar Companion around a Young Star

A handful of sub-stellar companions around young stars have been found by direct imaging. Sub-stellar companions are basically a group of objects comprising giant planets and brown dwarfs, although the dividing line between the two is not clear. Young stars are great targets for direct imaging searches for giant planets and brown dwarfs because these sub-stellar objects have yet to cool sufficiently and would still be glowing intensely hot from all the heat acquired during their formation process. M. Bonavita et al. (2014) present the direct imaging discovery of HD 284149 b, a 18 to 50 Jupiter-mass sub-stellar companion located at a relatively large projected separation distance of ~400 AU from a young F8 star.

Figure 1: Artist’s depiction of a sub-stellar object glowing red hot from heat acquired during its formation.

The discovery of HD 284149 b came about from direct imaging of its host star over six epochs between October 2011 and March 2014 with the adaptive-optics assisted Near Infrared Imager and Spectrometer (NIRI) on the 8.19-metre Gemini North telescope located on Hawaii’s Mauna Kea. The host star of HD 284149 b is somewhat more massive and more luminous than the Sun. It has an estimated surface temperature of 5970 to 6100 K and an assumed age of ~25 million years. HD 284149 b is most certainly the same age as it host star. Given its young age, it is still glowing hot with an estimated surface temperature of ~2500 K.

The mass of HD 284149 b places it in the overlapping mass regime comprising the more massive giant planets and the lower mass brown dwarfs. How a sub-stellar object like HD 284149 b could have formed at such a wide separation from its host star remains unclear. HD 284149 b could have formed by coalescing out from a protoplanetary disk like planets do, or it could have formed from the direct collapse of a clump of gas and dust like stars do. HD 284149 b joins a number of sub-stellar companion objects such as AB Pictoris b and ROXs 42Bb which share rather similar properties.

Figure 2: Theoretical models constraining the age and mass of HD 284149 b. M. Bonavita et al. (2014).

M. Bonavita et al. (2014), “A new sub-stellar companion around the young star HD 284149”, arXiv:1406.7298 [astro-ph.SR]