The SALT survey of helium-rich hot subdwarfs: final sample and classification

TL;DR

The SALT survey delivers a final, expansive catalog of 697 helium-rich hot subdwarfs observed with SALT/RSS, leveraging Gaia-based target selection and a Drilling (D13) spectral system to define subgroups and identify exotic objects. It reveals two main helium populations and a substantial sample of extreme helium stars, magnetic sdOs, heavy-metal (Pb- and Zr-rich) subdwarfs, CO-rich He-sdO candidates, and rare binaries, providing a rich resource for kinematic and atmospheric analyses. The dataset enables exploration of evolutionary pathways, including WD-merger scenarios and post-HeMS evolution, and establishes a foundation for automated classification and detailed atmosphere studies in follow-up work. The catalog and associated classifications pave the way for linking He-rich subdwarfs to related evolved populations such as R Coronae Borealis stars and other helium-rich families, with data publicly available for the community.

Abstract

A medium-resolution spectroscopic survey of helium-rich hot subdwarfs has been carried out using the Southern African Large Telescope (SALT). Objectives include the discovery of exotic hot subdwarfs, resolving distinct subclasses, identifying evolutionary sequences, and establishing the past and future histories of many of these unusual stars. This paper extends the sample described by Jeffery et al. (2021) (arXiv:2011.09523) from 100 to 697 stars. It describes the selection criteria and presents spectral classifications based on the MK-like Drilling system. The sample includes 283 extremely helium-rich hot subdwarfs, 17 extreme helium stars, 110 intermediate helium-rich hot subdwarfs, as well as 21 helium-rich stars of other types. It now represents the largest homogeneous sample of both "normal" He-sdOs and "luminous" or "hot" He-sdOs. Interesting stars discovered include magnetic hot subdwarfs, extremely hot pre-white dwarfs and hot subdwarfs, including hot subdwarfs showing NV emission, one short-period binary, new extreme helium stars and several double-subdwarf candidates. The data form the basis for kinematic and model atmosphere analyses to follow.

Figures (12)

• Figure 1: Sample selection using provisional surface temperatures and radii for 9979 hot subdwarfs based on spectral energy distributions and Gaia distances. Stars observable with SALT are shown in black. Blue triangles show hydrogen-deficient subdwarfs identified observed in SALT1. Red and orange squares identify the SALT target list used for this campaign. Error bars indicate the considerable uncertainties that arise from SED fitting. The SED method is unable to constrain effective temperatures above 50 000 K. Measurements above this value (in orange) should be simply interpreted as being $>50\,000$ K or "very hot" rather than being assigned a numerical value. The gray bands mark the helium main sequence of paczynski71 and the dorman93 horizontal branch.
• Figure 2: SALT/RSS spectra of extreme helium stars from the narrow Hei line sequence. Spectra are labelled with their SALT identifiers (Table \ref{['a:classes']}ff.). Counting from the top, selected familiar EHes are: 2: NO Ser, 4: PV Tel = HD 168476 thackeray54, 10: V2205 Oph, 12: V2076 Oph = HD 160641 bidelman52.
• Figure 3: SALT/RSS spectra of extreme helium stars from the broad Hei line sequence. Counting from the top, selected familiar EHes are: 2: FQ Aqr, and 5: V821 Cen = HD 124448 popper42.
• Figure 4: The Sp -- He diagram for SALT/RSS observed stars with Drilling classifications (blue circles). The distribution from D13 is shown by grey dots. A uniform jitter covering $\pm$ half a division has been applied to both datasets in both axes. The left hand panel shows 515 stars for which classification errors $<0.1$ in Sp and $<1.5$ in He. The right hand panel shows the remaining 128 stars.
• Figure 5: SALT/RSS spectra of hot subdwarfs showing strong Pb iv lines.