Brown dwarfs are substellar objects that populate the gap between the most massive planets and the least massive stars. ζ Del B is a newly discovered brown dwarf around the A-type star ζ Del A. Before this discovery, ζ Del A was simply known as ζ Del without the suffix “A”. ζ Del A is a main-sequence star with 2.5 ± 0.2 times the mass of the Sun and about 50 times the Sun’s luminosity. A main-sequence star is basically a star that is currently generating energy by fusing hydrogen into helium and it is neither at the start nor near the end of its life. The Sun is one example of a main-sequence star.
Figure 1: Artist’s impression of a brown dwarf.
Only a small number of brown dwarfs are known to orbit stars that are significantly more mass and luminous than the Sun. ζ Del B is the latest addition to this short list of brown dwarfs. ζ Del A and its brown dwarf companion, ζ Del B, are both estimated to lie at a distance of roughly 220 light years away. Estimates show that the ζ Del system is 525 ± 125 million years old. Spectroscopic observations of the spectrum of ζ Del B indicate it is an L-type brown dwarf (L5 ± 2) with an effective temperature of 1650 ± 200 K. Brown dwarfs are classified into 4 spectral types - M, L, T and Y. M-type brown dwarfs are the hottest, while Y-type brown dwarfs are the coolest.
Based on its near-infrared brightness, temperature and age, ζ Del B is estimated to be 50 ± 15 times the mass of Jupiter. This gives the brown dwarf a mass ratio of 0.019 ± 0.006 with respect to its stellar companion. Additionally, ζ Del B has a projected separation of 910 ± 14 AU from ζ Del A. Such a projected separation means the orbital period of ζ Del B around ζ Del A is probably on the order of ~10,000 years. ζ Del B is one of the most widely-separated and lowest mass ratio substellar companions known around a main-sequence star.
Figure 2: Estimated mass of ζ Del B based on its near-infrared brightness, temperature and age. De Rosa et al. (2014).
Figure 3: Mass ratio as a function of separation for brown dwarf companions (blue open circles), directly imaged brown dwarf and planetary companions (red open squares), and brown dwarf and planetary companions detected by radial velocity and transit techniques (black points). ζ Del B (black filled star) is among the most widely separated, lowest mass ratio companions imaged to date. De Rosa et al. (2014).
A number of different scenarios might explain the formation of ζ Del B and other widely-separated substellar companions around main-sequence stars. The formation of ζ Del B at its current location would require an unusually massive circumstellar disk at a large distance around ζ Del A and such a formation scenario is quite unlikely. Nevertheless, it cannot be ruled out that ζ Del B might have formed much closer to ζ Del A before migrating outward due to interactions with the circumstellar disk or with an unseen companion.
The ζ Del system could also have formed from the fragmentation of a single pre-stellar core of gas and dust into two cores, where one core is much more massive than the other. The more massive core collapsed to form ζ Del A, while the less massive core collapsed to form ζ Del B. Such a scenario can produce companions with separations on the order of ~1,000 AU. Another formation scenario, albeit with a low probability of occurring, involves ζ Del B forming independently before being captured by ζ Del A to form a low mass ratio binary.
De Rosa et al. (2014), “The VAST Survey - IV. A wide brown dwarf companion to the A3V star ζ Delphini”, arXiv:1410.0005 [astro-ph.SR]