A White Dwarf - Brown Dwarf Binary System
An image of a white dwarf - red giant binary pair (R Aquarii) showing the white dwarf feeding on material from the Mira variable and jets of material. Astronomers have studied a much rarer system, a white dwarf - brown dwarf binary system in which the white dwarf is also thought to be be accreting.
When a star like our Sun gets to be old, in another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining it will shrink to a fraction of its radius and become a white dwarf star. White dwarfs are common, with the most famous one being the companion to the brightest star in the sky, Sirius. A brown dwarf star is a failed star whose mass is less than about eighty Jupiter-masses, so small that it lacks the ability to heat up their interiors to the roughly ten million kelvin temperatures needed for normal hydrogen burning.
Brown dwarf companions to normal (that is, main-sequence) stars are not uncommon, but fewer than one percent of white dwarf stars have a brown dwarf companion because en route to becoming a white dwarf a star passes through a giant phase that can disrupt any nearby companions. CfA astronomer Peter Williams was a member of a team that used ultraviolet, visible and radio observations to study the unusual white dwarf-brown dwarf binary system NLTT5306, discovered when astronomers had became curious about the excess infrared emission seen coming from the white dwarf. The scientists found atomic emission lines that imply some accretion of material is occurring onto the white-dwarf, but since there is no known disk around the white dwarf they argue that the brown dwarf is responsible. A magnetic field that channels the material could provide a possible mechanism except that these kinds of stars do not have strong magnetic fields. To sort out these puzzles, the authors investigate winds and other possibilities, but conclude that in this unusual object additional measurements will be needed before a clear choice of models can emerge.
"Signs of Accretion in the White Dwarf + Brown Dwarf Binary NLTT5306," E. S. Longstaff, S. L. Casewell, G. A. Wynn, K. L. Page, P. K. G. Williams, I. Braker and P. F. L. Maxted, MNRAS 484, 2556, 2019.