The Birth of Be Star Disks II. A High-Resolution Spectroscopic Campaign and TESS Observations of an Outburst of the Classical Be star λ Pavonis

Be stars are rapidly-rotating B stars that have shown emission lines originating in a circumstellar disk. The mechanisms that lead to disk formation and dissipation are not known although progress has been made with some systems. We present a study of a disk outburst of the Be star lambda Pavonis. Our dataset comprises 698 high-resolution spectra contemporaneous with TESS photometry in 2023. Near the end of TESS monitoring, the star began disk building from a pristine diskless state. We find the disk built within 5 days in optical H I and He I lines, while the disk circularized in about 12 days. The disk began to decay in higher excitation He I first, then lower excitation transitions, with the decay ending last for H-alpha. We examine non-radial pulsations both through TESS photometry and the line profile variations (LPVs) in the spectroscopy. Our analysis indicates that two periodicities seen in TESS photometry (at 1.644 and 1.485 cycles/d) are not seen in the spectral lines before, during, or after the outburst. The strongest photometric signal is a periodicity at 0.163 cycles/d, which appears as a difference between the two weaker signals and is visible in the spectra without any apparent changes in amplitude or phase. We additionally find evidence for fast non-photometric pulsational variations over the course of spectroscopy obtained before, during, and after the outburst. These fast LPVs are strong, and interfere with the two weaker signals, hampering our ability to detect them in spectroscopy.

💡 Research Summary

This paper presents a comprehensive, multi‑instrument study of a short‑lived disk outburst in the classical Be star λ Pavonis (λ Pav, HR 7074, HD 173948) that occurred in the summer of 2023. The authors combine 698 high‑resolution optical spectra obtained with the CHIRON spectrograph (R ≈ 80 000) on the CTIO 1.5 m telescope and the NRES echelle spectrograph (R ≈ 53 000) on the Las Cumbres Observatory 1 m network, with contemporaneous photometry from the Transiting Exoplanet Survey Satellite (TESS) covering five sectors (13, 66, 67, 93, 94).

Observational strategy and data set
Initially λ Pav was observed during TESS sector 66 while it showed no emission signatures, allowing the authors to characterize its non‑radial pulsations (NRPs) in a disk‑less state. When emission appeared in H α and H β near the end of sector 67 (HJD 2460152, 2023 July 26), the campaign switched to intensive CHIRON monitoring to capture the rapid evolution of the line profiles. Spectroscopic coverage continued well after the TESS observations ended, providing a full view of both disk growth and subsequent decay.

Disk formation and circularization
All examined lines—H α, H β, He I λ 5876, λ 6678, λ 4921, λ 4713—showed simultaneous emergence of emission within a five‑day window, indicating that material was injected into the circumstellar environment almost instantaneously. The He I lines, which arise from higher excitation levels, appear first, confirming that the newly ejected gas is hot and dense. Dynamical spectra reveal a clear blue‑to‑red migration with a ∼6 day period, interpreted as the V/R (violet/red) asymmetry of a nascent, non‑axisymmetric disk. Within ≈12 days the V/R ratio converges to unity, the profiles become symmetric, and the disk is deemed circularized (azimuthally homogeneous).

Disk dissipation
The decay phase is line‑dependent. High‑excitation He I λ 4713 and λ 4921 return to quiescent equivalent‑width (EW) values by HJD 2460166 (≈2 weeks after onset), while the lower‑excitation He I λ 5876 and λ 6678 persist for an additional 2–3 days. H β fades over ≈6.4 days and H α over ≈8.5 days. This hierarchy matches expectations from viscous decretion‑disk (VDD) theory: the denser, cooler outer disk traced by Balmer lines dissipates more slowly than the hotter inner regions traced by He I.

Quantitative modeling
The authors fit the EW evolution with a simple exponential growth/decay law (Eq. 2) using the emcee MCMC sampler (256 walkers, 1 024 post‑burn‑in steps). Posterior distributions (visualized with corner plots) yield well‑constrained τ parameters for each line (Table 2). The derived decay timescales (τ_Hα ≈ 8.5 d, τ_Hβ ≈ 6.4 d, τ_HeI ≈ 2.4–5.6 d) are consistent with viscous diffusion timescales predicted for short‑duration outbursts.

Pulsation analysis
TESS photometry exhibits three dominant frequencies: two close, strong peaks at 1.644 c d⁻¹ and 1.485 c d⁻¹, and their difference (beat) frequency at 0.163 c d⁻¹. The two higher frequencies are absent from the spectroscopic line‑profile variations (LPVs) before, during, or after the outburst, suggesting that they correspond to surface pulsation modes that do not significantly modulate the line‑forming region. The beat frequency, however, is clearly present in the LPVs as a low‑amplitude, phase‑stable modulation across the entire data set, indicating a persistent, possibly global, non‑photometric pulsation mode.

In addition to these coherent signals, the spectra display rapid, high‑amplitude LPVs on timescales of hours, which the authors term “fast LPVs.” These variations dominate the line‑profile residuals and interfere with the detection of the two weaker photometric frequencies in the spectroscopy. The fast LPVs show a ∼6 day quasi‑periodic migration consistent with the V/R asymmetry, but their amplitude and irregularity suggest a superposition of multiple, non‑linear NRP modes.

Implications and conclusions
λ Pav provides a rare, well‑sampled case of a Be star transitioning from a truly disk‑less state to a fully formed decretion disk within days. The multi‑line EW and V/R analysis demonstrates that disk formation proceeds almost simultaneously across different ionization zones, while dissipation proceeds hierarchically from high‑excitation to low‑excitation lines, in line with VDD predictions. The disparity between photometric and spectroscopic pulsation signatures underscores the complexity of NRP mode geometry: some modes affect the stellar photosphere (hence the light curve) but leave the circumstellar line‑forming region untouched, whereas others manifest directly in the LPVs.

Overall, the study validates the viscous decretion‑disk framework for short‑timescale outbursts, highlights the importance of simultaneous high‑cadence spectroscopy and space‑based photometry for disentangling pulsation and disk phenomena, and suggests that future campaigns targeting the early phases of disk formation can reveal the physical triggers—likely multi‑mode NRPs—that inject angular momentum into the circumstellar environment.