Spectroscopic Orbital Elements for the Helium-Rich Subdwarf Binary PG1544+488

TL;DR

This paper presents detailed spectroscopic analysis of the unique helium-rich binary PG1544+488, providing improved orbital and atmospheric parameters, and constraining its origin through binary evolution models.

Contribution

It offers the first comprehensive spectroscopic orbital and atmospheric parameters for PG1544+488, highlighting its nearly identical twin subdwarfs and constraining its formation scenario.

Findings

Orbital period is 0.496 days.

Mass ratio is approximately 0.911.

Components are nearly identical, metal-poor, and carbon-rich.

Abstract

PG1544+488 is an exceptional short-period spectroscopic binary containing two subdwarf B stars. It is also exceptional because the surfaces of both components are extremely helium-rich. We present a new analysis of spectroscopy of PG1544+488 obtained with the William Herschel Telescope. We obtain improved orbital parameters and atmospheric parameters for each component. The orbital period $P=0.496\pm0.002$\,d, dynamical mass ratio $M_{\rm B}/M_{\rm A}=0.911\pm0.015$, and spectroscopic radius ratio $R_{\rm B}/R_{\rm A}=0.939\pm0.004$ indicate a binary consisting of nearly identical twins. The data are insufficient to distinguish any difference in surface composition between the components, which are slightly metal-poor (1/3 solar) and carbon-rich (0.3% by number). The latter indicates that the hotter component, at least, has ignited helium. The best theoretical model for the origin of PG1544+488 is by the ejection of a common envelope from a binary system in which both components are giants with helium cores of nearly equal mass. Since precise tuning is necessary to yield two helium cores of similar masses at the same epoch, the mass ratio places very tight constraints on the dimensions of the progenitor system and on the physics of the common-envelope ejection mechanism.