# Novel modelling of ultra-compact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

## TL;DR

This paper presents the first detailed numerical models of stable mass transfer from helium white dwarfs to neutron stars, explaining the evolution and properties of ultra-compact X-ray binaries with very short orbital periods.

## Contribution

It provides comprehensive binary evolution tracks from pre-LMXB to UCXB stages, including the shortest orbital period of 5.6 minutes, and links models to observed UCXB properties.

## Key findings

- Minimum orbital period of 5.6 minutes.
- Donor stars evolve into planetary masses (~0.005 Msun).
- Models match observed high helium abundance UCXBs.

## Abstract

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually "merge" due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultra-compact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-LMXB systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 minutes. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ~0.005 Msun after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate vs. orbital period.