On the binary helium star DY Centauri: Chemical composition and evolutionary state

TL;DR

DY Cen has steadily increased in temperature over 23 years with unchanged chemical composition, indicating it is a merger product of two white dwarfs, despite being suggested as a binary system.

Contribution

This study provides detailed chemical and evolutionary analysis of DY Cen, supporting its origin as a white dwarf merger through multi-epoch spectral data.

Findings

DY Cen's effective temperature increased by about 5000 K over 23 years.

Chemical abundances remained consistent across three epochs.

The star's composition aligns with that of a white dwarf merger product.

Abstract

DY Cen has shown a steady fading of its visual light by about 1 magnitude in the last 40 years suggesting a secular increase in its effective temperature. We have conducted non-LTE and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. The derived stellar parameters for three epochs suggest that DY Cen has evolved at a constant luminosity and has become hotter by about 5000 K in 23 years. We show that the derived abundances remain unchanged for the three epochs. The derived abundances of the key elements, including F and Ne, are as observed for the extreme helium stars resulting from a merger of an He white dwarf with a C-O white dwarf. Thus, DY Cen by chemical composition appears to be also a product of a merger of two white dwarfs. This appearance seems to be at odds with the recent suggestion that DY Cen is a single-lined spectroscopic binary.