Monday, September 29, 2014

A Highly Eccentric Brown Dwarf around a Giant Star

To date, ~10 brown dwarfs are known around giant stars (i.e. evolved stars). Brown dwarf are objects more massive than planets, but are not massive enough to count as full-fledged stars. M. I. Jones et al. (2014) report on the discovery of a brown dwarf on a highly eccentric orbit around the giant star HIP 97233. The brown dwarf, identified as HIP 97233 b, has an orbital period of 1058.8 days and a minimum mass of 20 times the mass of Jupiter.

With an orbital eccentricity of 0.61, HIP 97233 b is the brown dwarf with the most eccentric orbit known around a giant star. The mass and orbit of HIP 97233 b were both determined from the gravitational “tugging” it exerts on its host star which was observed in the form of a radial velocity signature (i.e. Doppler shifts in the star’s spectral lines).

Figure 1: Artist’s impression of a giant planet.

 Figure 2: Upper panel: Radial velocity curve for the host star of HIP 97233 b. Lower panel: Residuals from the best fit. M. I. Jones et al. (2014).

HIP 97233 b highly eccentric orbit takes it from as near as 1.0 AU to as far as 4.1 AU from its host star. M. I. Jones et al. (2014) estimate that the host star of HIP 97233 b has 1.84 ± 0.14 times the Sun’s mass and 5.20 ± 0.50 times the Sun’s radius. The host star of HIP 97233 b is considerably larger and more luminous than the Sun. At closest approach, the dayside of HIP 97233 b receives roughly 16 times the intensity of insolation as Earth receives from the Sun.

There are a number of ways through which an object like HIP 97233 b can form. Firstly, the host star of HIP 97233 b is much more massive than the Sun, enabling it to have a more massive protoplanetary disk which can allow massive planets and brown dwarfs to form more efficiently. Also, as the star evolves and swells in size, it begins to blow an enhanced stellar wind from which a giant planet can accrete a significant amount of mass and grow in mass till it reaches the brown dwarf mass regime.

The star’s high metallicity might also have enabled HIP 97233 b to form by core accretion, believed to be the main mechanism through which planets form. Finally, interaction with the protoplanetary disk before it was dissipated or with the star’s outer layers as it evolves to a giant star might have caused HIP 97233 b to migrate inward from beyond ~4 AU to where it currently is.

M. I. Jones et al. (2014), “A planetary system and a highly eccentric brown dwarf around the giant stars HIP 67851 and HIP 97233”, arXiv:1409.7429 [astro-ph.EP]