On the challenging variability of LS IV-14{deg}116: pulsational instabilities excited by the {epsilon}-mechanism

We investigate the pulsation driving mechanism responsible for the long-period photometric variations observed in LS IV-14{\deg}116, a subdwarf B star showing a He-enriched atmospheric composition. To this end, we perform detailed nonadiabatic pulsation computations over fully evolutionary post He-core-flash stellar structure models, appropriate for hot subdwarf stars at evolutionary phases previous to the He-core burning stage. We found that the variability of LS IV-14{\deg}116 can be attributed to nonradial g-mode pulsations excited by the {\epsilon}-mechanism acting in the He-burning shells that appear before the star settles on the He-core burning stage. Even more interestingly, our results show that LS IV-14{\deg}116 could be the first known pulsating star in which the {\epsilon}-mechanism of mode excitation is operating. Last but not least, we find that the period range of destabilized modes is sensitive to the exact location of the burning shell, something that might help to distinguish between the different evolutionary scenarios proposed for the formation of this star.

💡 Research Summary

The paper addresses the long‑period photometric variability of the helium‑rich subdwarf B star LS IV 14 116, whose observed g‑mode periods (≈ 1954 s and several others between 1000–5000 s) cannot be explained by the conventional κ‑mechanism that drives pulsations in most hot subdwarfs. The authors propose that the ε‑mechanism—energy generation’s extreme temperature sensitivity in nuclear burning zones—operates in the helium‑burning shell that forms during a series of He‑shell subflashes occurring in the pre‑extreme horizontal branch (pre‑EHB) phase, before the star settles into stable core helium burning.

To test this hypothesis, they constructed fully evolutionary models of a 1.03 M⊙, Z = 0.02 star that experiences a shallow‑mixing hot‑flasher event at the tip of the red‑giant branch. After the primary He‑core flash, the model contracts and undergoes a sequence of seven He‑shell subflashes, each lasting 10³–10⁴ yr. For each subflash they extracted more than a thousand stellar structures (one every five evolutionary timesteps) and performed linear, non‑adiabatic pulsation analyses for ℓ = 1 g‑modes in the period range 500–6000 s, using the code described by Córsico et al. (2006) with the ε‑mechanism included. Two simplifying assumptions were adopted: “frozen‑in convection” (the convective flux is held fixed during the pulsation cycle) and a background model with dS/dt = 0. The authors justified the frozen‑in approximation by showing that the convective turnover times (τ_glo) in the He‑flashing zones are orders of magnitude longer than the pulsation periods and growth times.

The calculations reveal that during each subflash the ε‑mechanism provides a positive work contribution (dW/dr > 0) in the thin He‑burning shell, destabilising a set of g‑modes. In the first and strongest subflash, the e‑folding times (τ_e) of the unstable modes are shorter than the subflash duration, meaning that the modes can grow to observable amplitudes. The unstable periods cluster between 600 s and 2000 s, with the most unstable mode having a period of ≈ 1954 s, matching the dominant observed period of LS IV 14 116. Not all modes in the period range are excited; only those whose eigenfunctions have large amplitudes within the He‑burning shell are destabilised, effectively making the ε‑mechanism a narrow‑band filter.

To confirm that the ε‑mechanism is indeed the driver, the authors performed a control experiment by switching off the temperature derivatives of the He‑burning energy generation. All previously unstable modes became stable, confirming the exclusive role of the ε‑mechanism. They also explored the sensitivity of the period spectrum to the location of the burning shell by artificially shifting the shell outward (log q ≈ −0.37). This shift expands the range of unstable periods to cover the full set of observed periods, and the required shell depth coincides with that predicted for merger‑origin hot‑subdwarf models (Saio & Jeffery 2000). Hence, the period distribution could discriminate between the hot‑flasher and merger formation channels.

The authors discuss why LS IV 14 116 lacks the short‑period p‑mode pulsations typical of κ‑driven sdBVr stars. In the rapid pre‑EHB phase, iron‑group elements have not yet accumulated sufficiently in the driving region to activate the κ‑mechanism, while the ε‑mechanism dominates. This explains both the presence of long‑period g‑modes and the absence of short‑period p‑modes.

In conclusion, the study provides the first robust theoretical evidence that the ε‑mechanism can excite observable stellar pulsations, making LS IV 14 116 the inaugural example of an ε‑driven pulsator. The work demonstrates that He‑shell subflashes in pre‑EHB evolution can naturally produce the observed g‑mode periods, and that the precise location of the burning shell critically shapes the period spectrum, offering a novel seismic diagnostic for the evolutionary history of He‑rich subdwarfs. Future work extending ε‑mechanism calculations to merger‑origin He‑sdB models is suggested to refine the match with the full observed period set.