Birth of a Quadruple Star System

Roughly half of all stars reside in multiple star systems - binaries, triples, quadruplets, quintuples, etc. There appears to be a higher prevalence of multiplicity for stars that are still forming as compared to fully-formed stars. This is because dynamical interactions tend to scatter apart multiple star systems. Observations of Bernard 5, a cloud of gas ~800 light-years away, show the presence of a quadruple star system in its beginning stages of formation.

Figure 1: Barnard 5, embedded in dust (blue) as seen with ESA’s Herschel Space Observatory, in infrared light. Credit: Bill Saxton, NRAO/AUI/NSF.

The quadruple system in Bernard 5 consists of one young protostar and three dense condensations of gas. All four objects are currently gravitationally bound. The protostar, B5-IRS1, is a low-mass star estimated to be roughly 0.1 times the Sun’s mass. As for the three gas condensations, they are expected to gravitationally collapse to form stars one-tenth to one-third the mass of the Sun in ~40,000 years.

At present, the project separations between the protostar and the three condensations, B5-Cond1, B5-Cond2 and B5-Cond3, are 11,400 AU, 3,300 AU and 5,100 AU, respectively. The quadruple system is unstable and will likely disperse on a timescale of roughly 500,000 years. Nevertheless, the closest pair, B5-IRS11 and B5-Cond2, will likely remain gravitationally bound as a binary system.

Figure 2: Artist’s conception of the B5 complex as seen now (left), and as it will appear as a quadruple star system in ~40,000 years (right). Credit: Bill Saxton, NRAO/AUI/NSF.

Figure 3: Dust continuum emission maps of Barnard 5. Pineda et al. (2015).

Reference:
Pineda et al., “The formation of a quadruple star system with wide separation”, Nature 518, 213-215 (12 February 2015)