Thursday, August 7, 2014

A Planetary-Mass Object Forming Like Stars Do

Substellar objects are objects that are not massive enough to support hydrogen fusion in their cores to become fully-fledged stars. The division between stars and substellar objects lies at around ~75 Jupiter-masses. Free-floating objects with ever lower masses and cooler temperatures are continuously being discovered. Some of these objects are in the regime of a few Jupiter-masses. Recently, Luhman (2014) reported the discovery of a remarkably cold and low mass object located at a distance of only 7 light years. This object, identified as WISE 0855-0714, has a temperature of 250 K and a mass of 3 to 10 Jupiter-masses. It is still an open question whether such free-floating planetary-mass objects form in the same way stars do.

Figure 1: Artist’s impression of a giant planet. A planetary-mass substellar object would in many ways be similar to giant planets like Jupiter.

Joergens et al. (2014) present new studies of a free-floating planetary-mass object known as OTS44. With an estimated age of ~2 million years, OTS44 is relatively young and is still glowing radiantly hot from heat acquired during its formation. OTS44 is estimated to have a temperature of ~1700 K and a mass of ~12 Jupiter-masses. After ~1 billion years, OTS44 is expected to cool to a temperature of ~300K, cool enough for water clouds to condense in its atmosphere.

New studies of OTS44 show it has a disk of material around it and it is also actively accreting material. The circumsubstellar disk of material around OTS44 has an estimated mass of ~10 Earth-masses. If there is sufficient solid material, one or more small rocky planets might form to create a miniature planetary system around OTS44. Based on the observed motion of hydrogen gas around OTS44, the mass accretion rate of OTS44 is estimated to be ~7.6 × 10־¹² solar-masses per year. This demonstrates that typical process (i.e. the existence of disks and accretion) associated with the formation of stars can also apply to free-floating objects down to a few Jupiter-masses, and suggests that OTS44 formed in the same way stars do (Figures 2 and 3).

Figure 2: Relative disk mass versus central mass of stars and brown dwarfs including OTS 44 (red diamond). The ratio of the disk-to-central-mass of ~0.01 typical for objects between 0.03 to several solar-masses is also valid for OTS44 which has a mass of ~0.012 solar-masses.

Figure 3: Mass accretion rate versus central mass of stars and brown dwarfs including OTS44 (red diamond). The mass accretion rate of OTS44 is consistent with a decreasing trend from stars of several solar-masses to substellar objects with ~0.01 solar-masses.

Joergens et al. (2014), “The coolest 'stars' are free-floating planets”, arXiv:1407.7864 [astro-ph.SR]