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

TL;DR

This study investigates the birth and dissipation of a Be-star disk around λ Pavonis by combining ≈700 high-resolution spectra with multi-sector TESS photometry. The disk forms within about $\approx 4$ days and circularizes within $\approx 12$ days, with line-emission growth and decay captured across seven optical lines and a accompanying $\sim1.25$-day V/R modulation; a persistent low-frequency difference signal $\nu_1 = 0.163\,\mathrm{d}^{-1}$ links to pulsational activity. The photometry reveals three persistent pulsation frequencies, including the stable difference frequency, while the spectrum shows fast high-order LPVs likely due to high-order pulsations (e.g., sectoral modes with high $l$). The results support non-radial pulsations as a driver of disk formation in Be stars and provide detailed constraints on disk dynamics and pulsation–disk coupling across multiple spectral lines.

Abstract

Be stars are non-supergiant, rapidly rotating B stars that have shown emission lines originating in a circumstellar disk. The exact mechanisms that lead to disk formation and dissipation are not fully known although progress has been made with some systems. Here, we present a study of a disk outburst of the southern Be star λ Pavonis (HD 173948). Our dataset comprises 698 high-resolution spectra taken contemporaneously with TESS photometry in 2023. During the final days of TESS monitoring, the star began building a disk from a pristine diskless state. We find that the disk built within 5 days in optical H I and He I lines, while the disk is circularized in about 12 days. The disk began to decay in higher energy He I first, then lower energy transitions, with the decay ending last for Hα. We examine non-radial pulsations both through TESS photometry and the line profile variations in Balmer lines, He I lines, and the weak photospheric Si III 5739 line. 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 spectral signal is a periodicity at 0.163 cycles/d, which appears as a difference between the weaker signals. 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, causing their apparent incoherence.

Figures (7)

• Figure 1: Seven spectral lines studied in this paper are displayed on the top. Included are representative spectra from 2023 July 7 (HJD 2460133) prior to the outburst, 2023 July 31 (HJD 2460157) during the outburst, right after the end of TESS sector 67, and 2023 August 26 (HJD 2460183) after the outburst. Large variations are seen in the hydrogen and stronger helium lines, while weak changes are seen in lines like He1 $\lambda$4713. No strong changes were seen in the Si3 $\lambda$5739 line. In the bottom two panels are dynamical spectra of H$\alpha$ (left) and He I 5876 (right) presenting the entire time series. In these plots, we have combined all the spectra taken prior to the outburst to an average profile that was subtracted from all spectra, thus showing two main features: the emission from the disk during the outburst and, in the case of He1 5876
• Figure 2: TESS light curve for sectors 66 and 67, which ends on day 155, and the EW width measurements for the combined CHIRON and NRES data during the summer of 2023 for the seven lines studied. The gray regions indicate the time during the outburst -- when emission features are visually present in the spectra. H1 lines in particular take longer to decay, so the time during the outburst is taken to be longer than other lines. We then overplot an exponential growth and decay model for six of the seven spectral lines calculated with the MCMC sampler, where the black line is the maximum likelihood model and the gray region is the 1$\sigma$ posterior spread. Since Si3 $\lambda5739$ shows no appreciable emission, it is not modeled. The vertical dashes indicate times with intense spectroscopic coverage.
• Figure 3: A fit to the V/R measurements in H$\alpha$ (top left), H$\beta$ (top right), He1 5875 (middle left), He1 6678 (middle right), He1 4921 (bottom left) and He1 4713 (bottom right) using Eq. 4. The gray region represents a $1\sigma$ posterior spread, and the red line is the maximum likelihood model.
• Figure 4: In the left column are the TESS light curves for each sector of observation in raw flux (top), the Lomb–Scargle periodograms of each TESS light curve (top middle), the Fourier transform after removing the two strongest frequencies, which displays the third strongest frequency at 1.485 d$^{-1}$ more clearly (bottom middle), and the Fourier transform with all frequencies removed and then fit (bottom, see text for further details). On the right are panels that show the phased TESS light curves (top), zoom-ins of the three dominant frequencies: 0.163 d$^{-1}$ (top middle, from top middle row), 1.644 d$^{-1}$ (bottom middle, from top middle row), and 1.485 d$^{-1}$ (bottom, from bottom middle row). The red dashed lines indicate the three strongest signals and an additional red dotted line in the left panels indicate the 0.82 d$^{-1}$ spectroscopic frequency from 2011AA...533A..75L.
• Figure 5: Spectroscopic data for He1 4921 as related to the three dominant photometric frequencies derived by TESS, with the left column depicting the 0.163 d$^{-1}$ frequency, the middle column depicting the 1.644 d$^{-1}$ frequency and the right depicting the 1.485 d$^{-1}$ frequency. The top row displays results from before the outburst, the middle row during the outburst, the final row is after the outburst. For each grey scale depiction of the spectra, we have subtracted the average line profile in red. There are two sets of vertical blue lines, one denotes the $v\sin i$ of $\pm$ 145 km s$^{-1}$ recorded by 2016AA...595A.132Z that we use as a reference; the other set simply corresponds with the visual extent of the spectral lines during the non-out-bursting phase. The top panel of each sub plot displays the TESS light curve and a scaled EW variability curve, with the spectra taken in the times highlighted in grey. To the right of each grey scale is the phased TESS light curve for that frequency from sectors 66 and 67.