Anomalous Orbital Characteristics of the AQ Col (EC 05217-3914) System

TL;DR

This paper investigates the complex orbital dynamics of the AQ Col system, revealing it as a potential triple system with a pulsating subdwarf B star, a wide binary companion, and an additional close companion, challenging previous assumptions of its simplicity.

Contribution

It provides the first detailed analysis suggesting AQ Col is a triple system with a pulsating sdB star, a wide binary companion, and a possible close white dwarf or neutron star companion.

Findings

Detected a 486-day pulsation timing variation indicating a wide binary with an unseen companion.

Observed large eccentricity (0.424) in the system, unusual for sdB binaries.

Identified rapid radial velocity change suggesting a close binary within the system.

Abstract

AQ Col (EC 05217-3914) is one of the first detected pulsating subdwarf B (sdB) stars and has been considered to be a single star. Photometric monitoring of AQ Col reveals a pulsation timing variation with a period of 486 days, interpreted as time-delay due to reflex motion in a wide-binary formed with an unseen companion with expected mass larger than 1.05 $M_\odot$. The optical spectra and color-Magnitude diagram of the system suggested that the companion is not a main sequence star but a white dwarf or neutron star. The pulsation timing variation also shows that the system has an eccentricity of 0.424, which is much larger than any known sdB long period binary system. That might be due to the existence of another short period companion to the sdB star. Two optical spectra obtained on 1996 December $5^{\rm th}$ show a radial velocity change of 49.1~km/s in 46.1 minutes, which suggests the hot subdwarf in the wide-binary is itself a close-binary formed with another unseen white dwarf or neutron star companion; if further observations show this interpretation to be correct, AQ Col is an interesting triple system worthy of further study.