Monday, May 11, 2015

Young Substellar Object around a Red Dwarf Star

Artigau et al. (2015) report the discovery of a substellar companion, referred to as J0219-3925B, in a distant orbit around a red dwarf star. J0219-3925B is estimated to have between 12 to 15 times the mass of Jupiter, placing it at the boundary between the brown dwarf/planet mass regimes. The host star of J0219-3925B is a low-mass red dwarf star with an estimated mass of about 113 times the mass of Jupiter. It is an extremely faint star with only ~1/170th of the Sun’s luminosity. J0219-3925B orbits its host star at a projected separation of roughly 160 AU (i.e. 160 times the distance between Earth and the Sun).

Figure 1: Artist’s impression of a substellar object around a red dwarf star.

J0219-3925B is unlikely to have formed out from a protoplanetary disk around its host star because it is too massive and orbiting too far from its host star. Instead, J0219-3925B and its host star probably formed in the same way as binary stars, involving the fragmentation of a clump of gas and dust. The larger fragment came together to form the red dwarf star, while the smaller fragment came together to form J0219-3925B. The age of J0219-3925B and its host star is estimated to be somewhere between 30 to 40 million years. That is a relatively young age for a star and its substellar companion.

J0219-3925B is still glowing red hot as it radiates away the heat acquired during its formation. Its effective temperature is estimated to be roughly 1,700 K. It will eventually cool to ~600 K after ~1 billion years, and to ~370 K after ~5 billion years. At that point, it will be cool enough for water-clouds to form in its atmosphere. J0219-3925B is roughly 1/10th the mass of its host star. Both J0219-3925B and its host star are estimated to lie at a distance of roughly 130 light-years. They form a binary system that sits in a sparsely populated part of the host mass versus mass ratio diagram, indicating that such binary systems could be relatively rare (Figure 2).

Figure 2: Host mass versus mass ratio diagram for substellar companions detected through direct imaging (black), radial-velocity (blue), microlensing (green) or transit (orange). Imaged binaries with orbital separations larger than 100 AU are circled. Artigau et al. (2015).

Artigau et al. (2015), “BANYAN. VI. Discovery of a companion at the brown dwarf/planet-mass limit to a Tucana-Horologium M dwarf”, arXiv:1505.01747 [astro-ph.SR]