Analyses of single and binary hydrogen-deficient stars with the Southern African Large Telescope

Abstract

The hydrogen-deficient stars comprise a diverse group of objects characterised by very low atmospheric hydrogen abundances. The variety and peculiar compositions of these stars raise questions about how they form and evolve. Ongoing survey efforts using the Southern African Large Telescope (SALT) have aimed to answer these questions by identifying and classifying hydrogen-deficient objects. This thesis aims to answer three main questions; what evolutionary links may exist between hydrogen-deficient populations, whether hydrogen-deficient stars can be found in close binaries, and if current evolution channels can account for the hydrogen-deficient objects in the SALT survey catalogue. Optical spectra obtained from SALT were used to determine atmospheric parameters including effective temperature and surface gravity, as well as chemical abundances and radial velocities. Light curves from the Transiting Exoplanet Survey Satellite (TESS) were used to identify variability. Orbital parameters were determined by light curve modelling using LCURVE.

To investigate inter-population links, the star BPS CS 22940-0009 was chosen due to its position in g-T space between low- and high-gravity helium stars. Analysis using high- and medium-resolution SALT spectra found the star to have T_eff / K = 34970 +/- 370, log g / cms^(-2) = 4.79 +/- 0.17, and an intrinsically metal-poor atmosphere that has undergone CNO and triple-alpha processing. The surface C and N abundances are consistent with an extreme helium star (EHe) formed from the composite merger of two white dwarfs, which is evolving towards the helium main sequence. Thus the star represents a link between the EHe and helium subdwarf populations.

The TESS periodograms of the SALT survey stars were searched for short-period variability. Detectable periods were found for 12 out of 232 objects. 2 additional candidates were identified from radial velocity variations. The detections consist of 8 He-sdOs, 3 He-sdBs, 2 intermediate-helium subdwarfs, and 1 helium white dwarf. This challenges conventional merger-based models for hydrogen-deficient star formation. Companions could not be directly determined; 7 stars show reflection effects indicative of low-mass main sequence companions, while 5 show ellipsoidal modulation likely due to compact companions. 2 stars were identified as possible pulsators.

The star Ton S 415 was chosen for further analysis due to its intermediate helium abundance and short orbital period of 84.6 min. SALT spectroscopy found the star to have T_eff / K = 43300 +/- 1000, log g / cms^(-2) = 5.89 +/- 0.10. Light curve modelling found the system to be an ellipsoidally-modulated binary with a mass ratio of q = 0.7 +/- 0.3 and an unseen white dwarf companion. This is consistent with a post-common-envelope binary that lost its hydrogen envelope while ascending the red giant branch. Ton S 415 is the third known intermediate-He binary to match this model. Ton S 415 therefore represents evidence of an alternative formation channel for hydrogen-deficient hot subdwarf binaries.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Armagh Observatory and Planetarium & Science & Technology Facilities Council
Supervisor	C. Simon Jeffery (Supervisor) & Stefano Bagnulo (Supervisor)