Mass Distribution for Single-lined Hot Subdwarf Stars in LAMOST

Abstract

Masses for 664 single-lined hot subdwarf stars identified in LAMOST were calculated by comparing synthetic fluxes from spectral energy distribution with observed fluxes from a Virtual Observatory service. Three groups of hot subdwarf stars were selected from the whole sample according to their parallax precision to study the mass distributions. We found that He-poor sdB/sdOB stars present a wide mass distribution from 0.1 to 1.0 M ⊙ with a sharp mass peak at around 0.46 M ⊙, which is consistent with canonical binary model prediction. He-rich sdB/sdOB/sdO stars present a much flatter mass distribution than He-poor sdB/sdOB stars and with a mass peak at around 0.42 M ⊙. By comparing the observed mass distributions to the predictions of different formation scenarios, we concluded that the binary merger channel, including two helium white dwarfs (He-WDs) and He-WD + main-sequence mergers, cannot be the only main formation channel for He-rich hot subdwarfs, and other formation channels, such as the surviving companions from Type Ia supernovae, could also make impacts on producing this special population, especially for He-rich hot subdwarfs with masses less than 0.44 M ⊙. He-poor sdO stars also present a flatter mass distribution with an inconspicuous peak mass at 0.18 M ⊙. The similar mass– Δ RV max distribution between He-poor sdB/sdOB and sdO stars supports the scenario that He-poor sdO stars could be the subsequent evolution stage of He-poor sdB/sdOB stars.

A New Route to Massive Hot Subdwarfs: Common Envelope Ejection from Asymptotic Giant Branch Stars

“…The typical lifetime for a canonical sdO/B (∼0.46 M e ) is ∼10 8 yr (Wu et al 2018), about 100 times larger than that of sdO/Bs produced from the AGB CE channel. This may be one of the potential reasons why the number of massive sdO/Bs is relatively rare compared with the canonical sdO/Bs produced from the RGB channel (Han et al 2003;Lei et al 2023). We also note that the sdO/B stars can expand to several 10 R e during the initial He-shell-burning phase, as shown in the upper panel of Figure 4, which may be classified as blue horizontal branch stars or giant stars (Xiang et al 2022;Ju et al 2024).…”

“…This suggests that the mass of sdO/Bs is an important parameter to distinguish the formation channels. In the recent observations, Lei et al (2023) obtained the mass distributions for 664 single-lined hot subdwarfs identified in LAMOST and found that there are more sdO/B stars with masses larger than 0.48 M e when compared with the theoretical mass distribution in Han et al (2003). Similarly, Schaffenroth et al (2022) also found the sdO/B mass distribution slightly extends to higher mass.…”

“…Unfortunately, there is no unified formation theory that can simultaneously explain the properties of He abundance distribution (Heber 2016). In other words, the diversity of He abundance may imply a variety of formation channels for hot subdwarfs (Lei et al 2023).…”

Unveiling the Evolutionary State of Three B Supergiant Stars: PU Gem, ϵ CMa, and η CMa

“…XTGRID [16] is a steepest-descent iterative χ 2 minimizing fit procedure to model hot star spectra. The procedure was developed for the model atmosphere code TLUSTY [43][44][45] and was previously applied to ultraviolet and optical spectral observations of O and B-type stars [10], Horizontal Branch stars [46], hot subdwarfs [47][48][49], and white dwarfs [50,51]. It was designed to perform fully automated or supervised spectral analyses of massive data sets.…”