The Asteroseismological Richness of RCB and dLHdC Stars

Abstract

RCB stars are L ≈ 104 L ⊙ solar-mass objects that can exhibit large periods of extinction from dust ejection episodes. Many exhibit semi-regular pulsations in the range of 30–50 days with semi-amplitudes of 0.05–0.3 mag. Space-based photometry has discovered that solar-like oscillations are ubiquitous in hydrogen-dominated stars that have substantial outer convective envelopes, so we explore the hypothesis that the pulsations in RCB stars and the closely related dustless hydrogen-deficient carbon (dLHdC) stars, which have large convective outer envelopes of nearly pure helium, have a similar origin. Through stellar modeling and pulsation calculations, we find that the observed periods and amplitudes of these pulsations follows the well-measured phenomenology of their H-rich brethren. In particular, we show that the observed modes are likely of angular orders l = 0, 1, and 2 and predominantly of an acoustic nature (i.e., p-modes with low radial order). The modes with largest amplitude are near the acoustic cutoff frequency appropriately rescaled to the helium-dominated envelope, and the observed amplitudes are consistent with that seen in high-luminosity (L > 103 L ⊙) H-rich giants. We also find that for T eff ≳ 5400 K, an hydrogen-deficient carbon stellar model exhibits a radiative layer between two outer convective zones, creating a g-mode cavity that supports much longer period (≈100 days) oscillations. Our initial work was focused primarily on the adiabatic modes, but we expect that subsequent space-based observations of these targets (e.g., with TESS or Plato) are likely to lead to a larger set of detected frequencies that would allow for a deeper study of the interiors of these rare stars.

An Infrared Census of R Coronae Borealis Stars II—Spectroscopic Classifications and Implications for the Rate of Low-mass White Dwarf Mergers

“…Interestingly, these high-cadence lightcurves show that dLHdC stars generally exhibit variability on timescales shorter than RCB stars. In the picture that dLHdC stars have lower masses than RCB stars, they are expected to have smaller pulsation periods than RCB stars (Wong & Bildsten 2024). The differences seen in the TESS lightcurves of RCBs and dLHdCs may therefore point towards the different masses of this class of objects, as suggested in Karambelkar et al (2022.…”

“…Using our derived number of 350 (C.I. 250-500) Galactic RCB stars and assuming typical RCB lifetimes of 1-3 × 10 5 yr (Schwab 2019;Crawford et al 2020;Wong & Bildsten 2024), the formation rate of RCB stars, in the Milky Way is between 0.8 and 5 × 10 −3 yr −1 . This is consistent with observational and theoretical estimates of the He-CO WD merger rate.…”

“…Initially, these semi-regular or irregular pulsations were suggested to originate from the strange-mode instability in non-adiabatic and radiation-pressure dominated envelopes (Saio 2008;Gautschy 2023). Recently, Wong & Bildsten (2024) modeled the RCB-dLHdC pulsations as solarlike oscillations excited by convection in a helium-rich envelope. They find that the frequencies with maximum power ( max n ) for such oscillations matches the observed range of periods in RCBs and dLHdCs.…”

“…The TESS lightcurves for six dLHdC and six RCB stars show that dLHdC stars show variations on timescales shorter than RCB stars, consistent with the picture that they have lower masses. Comparing these pulsation data to theoretical models (e.g., Saio 2008;Wong & Bildsten 2024) to estimate their masses will providing useful information towards understand why dLHdCs form dust and RCBs do not.…”

“…While at maximum light, some RCB stars pulsate with periods between 40 and 100 days (Lawson & Cottrell 1997), with the pulsation periods likely depending on their effective temperatures (T eff ) which range from 4000 to 8000 K (Crawford et al 2023;Tisserand et al 2024b). The origin of these pulsations is still not known-they have been attributed either to the strange-mode instability (Saio 2008;Gautschy 2023), or thought to be solar-like oscillations in helium-rich envelopes (Wong & Bildsten 2024).…”