Where are all the Sirius-Like Binary Systems?

TL;DR

This paper investigates the distribution and detection challenges of Sirius-Like binary systems, which are white dwarf systems with non-M main sequence companions, highlighting their underrepresentation beyond 20 parsecs and proposing methods for their identification.

Contribution

It provides estimates of the local space density and frequency of Sirius-Like systems and discusses observational strategies to discover unresolved binaries.

Findings

Approximately 8% of local white dwarfs are in Sirius-Like systems within 20 parsecs.

The known frequency of Sirius-Like systems drops to 1-2% beyond 20 parsecs.

Most unidentified Sirius-Like systems are likely closely separated, unresolved binaries.

Abstract

Approximately 70 percent of the nearby white dwarfs appear to be single stars, with the remainder being members of binary or multiple star systems. The most numerous and most easily identifiable systems are those in which the main sequence companion is an M star, since even if the systems are unresolved the white dwarf either dominates or is at least competitive with the luminosity of the companion at optical wavelengths. Harder to identify are systems where the non-degenerate component has a spectral type earlier than M0 and the white dwarf becomes the less luminous component. Taking Sirius as the prototype, these latter systems are referred to here as Sirius-Like. There are currently 98 known Sirius-Like systems. Studies of the local white dwarf population within 20 parsecs indicate that approximately 8 per cent of all white dwarfs are members of Sirius-Like systems, yet beyond 20 parsecs the frequency of known Sirius-Like systems declines to between 1 and 2 per cent, indicating that many more of these systems remain to be found. Estimates are provided for the local space density of Sirius- Like systems and their relative frequency among both the local white dwarf population and the local population of A to K main sequence stars. The great majority of currently unidentified Sirius-Like systems will likely turn out to be closely separated and unresolved binaries. Ways to observationally detect and study these systems are discussed.