Two New Long-Period Hot Subdwarf Binaries with Dwarf Companions

TL;DR

This paper reports the discovery and orbital analysis of two long-period hot subdwarf B star binaries with dwarf companions, challenging existing models and providing insights into their kinematics and population origins.

Contribution

It presents the first orbital solutions for two long-period sdB+dwarf binaries, expanding understanding of their properties and implications for binary evolution models.

Findings

Orbital periods of 909 and 734 days for the two systems.

Both systems likely not members of the Galactic thin disk.

Results challenge current binary population synthesis models.

Abstract

Hot subdwarf stars with F-K main sequence binary companions have been known for decades, but the first orbital periods for such systems were published just recently. Current observations suggest that most have long periods, on the order of years, and that some are or once were hierarchical triple systems. As part of a survey with the Hobby-Eberly Telescope, we have been monitoring the radial velocities of several composite-spectra binaries since 2005 in order to determine their periods, velocities, and eccentricities. Here we present observations and orbital solutions for two of these systems, PG 1449+653 and PG 1701+359. Similar to the other sdB+F/G/K binaries with solved orbits, their periods are long, 909 d and 734 d, respectively, and pose a challenge to current binary population synthesis models of hot subdwarf stars. Intrigued by their relatively large systemic velocities, we also present a kinematical analysis of both targets and find that neither is likely a member of the Galactic thin disk.