Astronomer Klaus Werner and his team have identified three rare hydrogen-deficient stars, each representing a unique stage in stellar evolution as pre-white dwarfs. These stars, distinguished by their helium-dominated atmospheres, provide valuable insights into unconventional evolutionary pathways. The study, conducted using the X-shooter instrument at the ESO Very Large Telescope, offers a detailed analysis of these stellar outliers.

Introduction

Pre-white dwarfs are stars nearing the end of their lifecycle but have yet to settle on the white dwarf cooling track. Their hydrogen-deficient atmospheres arise from extraordinary events, such as stellar mergers or late thermal pulses, challenging canonical evolution models. This paper introduces three such stars: an O(He) star, a PG1159 star, and a CO-sdO star, each belonging to different spectral classes.

The Three Stars

1. Gaia DR3 5253439589461750912 (O(He) Star): This star has an effective temperature of 90,000 K and a surface gravity of 5.5. It is enriched in nitrogen but deficient in carbon and oxygen, a signature of helium white dwarf mergers. The absence of a planetary nebula (PN) supports the merger scenario.

2. GSC 08265-00355 (PG1159 Star): With an effective temperature of 72,000 K and the lowest surface gravity among known PG1159 stars (log g = 4.8), this star is unique. Its composition suggests it underwent a very late thermal pulse long after dispersing its PN, representing an early post-asymptotic giant branch stage.

3. UCAC4 108-030787 (CO-sdO Star): This hot subdwarf, at 50,000 K, is only the third star classified as a CO-sdO. Its carbon and oxygen-rich atmosphere hints at its origin from a disrupted carbon-oxygen white dwarf merging with a helium white dwarf.

Methods

The team analyzed spectra using the Tübingen Model-Atmosphere Package (TMAP) to derive atmospheric properties. Challenges in modeling included numerical instabilities, requiring precise manual adjustments to achieve convergence.

Findings

The three stars are among the most hydrogen-deficient objects known (H < 0.001 by mass fraction). Their compositions and positions in the Hertzsprung-Russell diagram provide compelling evidence for their unique evolutionary paths, likely involving mergers or late stellar processes.

Significance

These discoveries expand our understanding of pre-white dwarf evolution and highlight the diversity of post-main-sequence stellar life. Future observations may reveal more such objects, refining models of stellar evolution. By studying these rare stars, Werner and his team have unveiled critical clues to the violent and complex processes that can shape the lives of stars. Their work offers a window into the extraordinary events that can lead to such hydrogen-deficient atmospheres, contributing to the broader puzzle of cosmic evolution.

Source: Werner