The orbital period evolution of the supersoft X-ray source CAL 87

TL;DR

This paper investigates the orbital period evolution of the supersoft X-ray source CAL 87, showing that an excited-wind-driven mass-transfer model explains its observed properties better than standard models.

Contribution

It introduces an excited-wind-driven mass-transfer model to explain CAL 87's orbital evolution, addressing limitations of standard models.

Findings

Standard models cannot explain CAL 87's properties.

Excited-wind-driven model accounts for observed orbital period changes.

Analytic and numerical methods validate the new model.

Abstract

CAL 87 is one of the best known supersoft X-ray sources. However, the measured masses, orbital period and orbital period evolution of CAL 87 cannot be addressed by the standard thermal-timescale mass-transfer model for supersoft X-ray sources. In this work we explore the orbital evolution of CAL 87 with both analytic and numerical methods. We demonstrate that the characteristics mentioned above can be naturally accounted for by the excited-wind-driven mass-transfer model.