Tracing the diversity of hot subdwarf evolution: Surface composition, magnetic fields, and populations
Creators: name: Dorsch, Matti
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: astrophysics, stars, stellar atmospheres, hot subdwarfs, magnetic fields

description: Hot subdwarf stars of spectral types O and B (sdO/B) represent late stages of stellar evolution. They are located close to the hot end of the horizontal branch, and most of them are core helium burning stars. At radii of only 20% of the solar radius, they lack the extensive hydrogen envelopes of cooler horizontal branch stars. The formation of these objects and their evolutionary links to other classes of stars are still not fully understood, but their majority seems to result from binary evolution: Roche lobe overflow, common envelope episodes, and several types of stellar mergers. It was the aim of this thesis to provide an observational overview of the properties of hot subdwarfs as an important step towards understanding their complex formation and evolution. Therefore, both the overall hot subdwarf population and several peculiar stars were studied in detail. In the first part, individual hot subdwarfs were studied in great detail to establish them as testbeds for stellar evolution, in particular by determining their chemical signatures from high-quality ultraviolet and optical spectra. This includes two typical He-poor sdOB stars as reference objects: as determined from archival far-ultraviolet spectra, CPD-56 464 and the Schweizer-Middleditch Star exhibit similar surface compositions, including a distinct CNO-cycle pattern. Several heavy elements were identified for the first time in He-poor sdOB stars. The resulting heavy metal abundances are high compared to the Sun but lower compared to peculiar stars like LS IV -14 116. The absence of silicon in the Schweizer-Middleditch star likely results from the combination of diffusion and weak stellar winds. In contrast, CPD-56 464 has a silicon abundance about a third of solar, among the highest observed in helium-poor sdOB stars. Three of the most chemically peculiar stars, the so-called heavy-metal helium-rich sdOBs, were analysed with unprecedented detail and precision. High-quality UVES spectra of the Zr-rich LS IV -14 116 and Feige 46 revealed many strong lines corresponding to transitions of various heavy metals, some of which have not previously been observed in any star. The surface abundance of 19 metals is nearly identical in both stars. These abundance patterns differ significantly from those of typical He-poor hot subdwarfs with similar temperatures. The observed extreme overabundance of heavy metals suggests the presence of strong atmospheric diffusion processes that affect both stars similarly, although a contribution by s-process fusion is not excluded. The comparable abundances of C, N, O, and Ne in both stars provide evidence of a shared evolutionary origin. Recently proposed evolutionary models involving the merging of a hybrid He/C/O white dwarf with a more massive helium-core white dwarf offer a promising formation scenario for stars similar to LS IV -14 116. This scenario explains the atmospheric parameters, single-star nature, and unique pulsations observed in LS IV -14 116. The next analysis showed the lead-rich He-sdOB EC22536-5304 to be in a binary system with an extremely metal-poor subdwarf F-type companion. As a result, the derived lead abundance in the hot subdwarf is even larger than originally thought: about a million times solar, making it the most lead-rich star known to date. Based on the metallicity and atmospheric parameters of the F-type companion, the EC22536-5304 system seems to be older than about 10 Gyr. The system has an orbital period of about 457 days and was therefore formed through stable Roche lobe overflow. The experience gained with EC22536-5304 was then applied to another peculiar binary system: the sdOB + K-type subgiant BD-7 5977. The system was extensively observed with high-resolution far-UV, optical, and infra-red spectrographs. This allowed us to test the Roche lobe overflow scenario by determining the companion's C12/C13 isotopic ratio, which may have been decreased by accreted material. The resulting ratio (28+/-5) is clearly lower than the solar value (89), likely due to a combination of both mass transfer and mixing caused by convective dredge-up in the K-type companion. Magnetic fields are considered a smoking gun for the formation of single hot subdwarfs in the merger scenario, but despite several dedicated searches they have long eluded detection. Here I report the discovery of four magnetic He-sdOs and carry out spectral analyses based on high-quality optical spectra by modelling their Zeeman-split hydrogen, helium, and metal lines. The first magnetic hot subdwarf to be analysed, J0809-2627, was discovered by a low-resolution spectrum and later confirmed by X-shooter and UVES spectra. This intermediately helium-rich sdO star exhibits strongly Zeeman-split lines, indicating an average field strength of about 350kG. Although the star has a low projected rotation velocity, its overall properties are best explained as the product of a double helium white dwarf merger. In the next step, three additional magnetic He-sdOs that were discovered in the SDSS survey were analysed. These stars are almost identical to J0809-2627 in terms of atmospheric parameters, rotation, and field strengths. The occurrence of these magnetic stars suggests a lower limit of about 2% for the magnetic fraction in the He-sdO population. It remains unclear why the majority of He-sdO stars do not exhibit detectable magnetic fields, even though they are also thought to be formed by mergers. The Gaia space mission has provided photometric and astrometric measurements for more than one billion stars, which have transformed Galactic astrophysics in general and research into hot subdwarf stars in particular. Combined with large ground-based spectroscopic surveys and other photometric surveys, this huge dataset finally allows the study of statistically significant samples of the comparatively rare hot subdwarf stars. In the second part of this thesis, two analyses based on these datasets were performed. The analysis of spectral energy distributions (SEDs), from the UV to the infra-red, combined with Gaia parallaxes provided fundamental stellar parameters and characterised the population F/G/K-type companions to hot subdwarfs. In addition, the computation of Galactic orbits based on radial velocities and Gaia astrometry provided age estimates for various sub-populations of hot subdwarfs. The SED fits performed here are the most extensive so far: they include all ~6600 spectroscopically identified hot subdwarfs listed in the latest version of the hot subdwarf catalogue. In this study, 27% of these known hot subdwarfs turned out to belong to composite-SED systems, meaning that an F/G/K-type companion star was detected. An interesting result is that F- and K-type companions seem to be more common than G-types, which is currently not explained by theory. Further investigations based on Gaia parallax measurements, angular diameters from the SED, and spectroscopic surface gravities allowed the determination of radii, luminosities, and masses. As expected, the He-poor sdB stars on the extreme horizontal branch can be divided into two groups: a cooler one and a hotter one. These groups evolve into two groups of He-poor sdO stars once helium fusion moves from the core to a shell around it. The extremely He-rich sdO stars, which lack F/G/K-type companions, exhibit higher masses compared to other hot subdwarf classes, indicating that they were formed by merger channels. The intermediately He-rich sdOB stars are divided into two subgroups: the more luminous stars show a similar lack of companions, which suggests that low-mass white dwarf mergers contribute to their formation. In contrast, more compact He-sdOB stars have a companion fraction that is more comparable to the He-poor sdOB stars. Both the newly observed binary fractions for each spectral type and the lack of G-type companions should be compared to updated binary population synthesis models. From the observational side, future spectral analyses should be performed in a homogeneous fashion, which would improve the precision in particular in the important mass determination. Gaia parallaxes, proper motions, and literature radial velocities were then used to study the current Galactic velocities and orbital properties of the known hot subdwarfs. This kinematic analysis showed that most of these stars are associated with the Galactic thin and thick disk, each contributing about 44%, while the metal weak-thick disk and halo population contribute about 3% and 10%, respectively. In particular the helium-poor and helium-rich populations of hot subdwarfs show different kinematic properties. Helium-rich stars at effective temperatures of more than 32000K have a larger contribution by the thick disk and halo populations - these stars must therefore be predominantly formed in old stellar populations. The opposite is true for cooler helium-rich sdB stars, which seem to be dominated by the thin disk. In addition, the difference in age between the helium-rich sdOB stars and their more common helium-poor sdB counterparts seems to exclude the proposition that He-sdOB stars may evolve to become sdB stars. In conclusion, this thesis provides several detailed analyses of hot subdwarf stars, encompassing their abundance patterns, magnetic properties, binary nature, fundamental stellar parameters, and kinematic age. These results form an observational basis that can be combined with binary evolution models to shed light on the diverse formation mechanisms of hot subdwarfs and their evolution. In that sense, the observations performed here also contribute to the broader context of binary evolution, in particular in old stellar populations - in our Galaxy and beyond.