Betelgeuse: Detection of the Expanding Wake of the Companion Star

TL;DR

Betelgeuse likely hosts a close-in companion within its chromosphere, producing a trailing wake that modulates circumstellar and chromospheric gas on the long secondary period of $2109$ days. The study combines optical Mn I measurements (via HERMES and TRES) with ultraviolet HST/STIS spectroscopy to track phase-dependent absorption and outflow, finding peak circumstellar absorption near phase $0.5$ and maximum chromospheric outflow shortly after transit. The results are consistent with a gravitationally focused wake behind a companion at $\sim2.3R_\star$, offering a coherent explanation for both radial-velocity/photometric LSP variations and spectroscopic wind signatures, and motivating detailed hydrodynamic modeling. If confirmed, this wake-driven scenario has important implications for mass loss and atmospheric dynamics in Betelgeuse and similar evolved stars, with observational signatures expected to evolve over multi-year timescales (e.g., reappearance around 2027).

Abstract

Recent analyses conclude that Betelgeuse, a red supergiant star (HD 39801), likely has a companion object with a period of about 2000 days orbiting at only 2.3 stellar radii, deep in the chromosphere of the supergiant. A probable detection of such a companion, named Siwarha, has just occurred from speckle imaging. This study finds that Betelgeuse spectra in the optical region and ultraviolet exhibit signatures of variable circumstellar absorption and chromospheric outflows. These variations are consistent with the ~ 2000-day period of the companion object. Circumstellar absorption evident in optical Mn I lines, and mass outflow marked by ultraviolet Fe II, Si I, and Mg I lines increase after the transit of the companion across the disk of Betelgeuse. Following the eclipse of the companion, the absorption and outflow slowly decrease in advance of the next transit. The occurrence and variation of this plasma appear consistent with the presence of a trailing and expanding wake caused by a companion star orbiting within the atmosphere of Betelgeuse.

Figures (9)

• Figure 1: Measurements of three narrow circumstellar Mn I lines from the HERMES and TRES spectrographs. A 2-sine curve with a period of 2109 days is placed to guide the eye ( broken green line). Filled red or black triangles mark a few outliers. Upper panel for each line: The equivalent width, EW(Å), of the Mn I lines as a function of phase of the companion star. Phase 0 corresponds to the transit of the companion and phase 0.5 marks the eclipse of the companion. The negative phasing is used here for a few of the HERMES measures because they began prior to the TRES measures. Lower panel for each line: The relative radial velocity (km s$^{-1}$) of the Mn I circumstellar line with respect to the photospheric line.
• Figure 2: Left panel: Chromospheric transitions (Fe II, Si I, Mg I) at three phases. The time sequence has been inverted for display. Spectra at phases 0.03 and 0.32 were obtained in 2023 and 2024 respectively; the spectrum at phase 0.50 was obtained in 2020. The central absorption extending toward shorter wavelengths marks the expanding chromosphere after transit of the companion (phase 0). The decrease in total line flux is caused by the substantial weakening of the short wavelength emission. Right panel: The ratio of the peaks, (blue/red), as a function of phase of the binary. Phases 1.0 to 1.4 (2023-2025) are repeated in the figure at phase 0.0 to 0.4 and marked by open circles. The green band in the middle panel marks the Great Dimming: 2019 December- 2020 February.
• Figure 3: Four Fe II transitions at three phases of the LSP. The time sequence has been inverted for display as in Fig. 2. Absorption extending toward shorter wavelengths indicates the expanding chromosphere after transit of the companion (phase 0). The asymmetry of the profiles changes with phase, displaying the weakening in the blue emission produced by the opacity in the expanding atmosphere and signaled by a decrease in the blue/red ratios.
• Figure 4: The asymmetry ratio (blue/red) for four Fe II transitions whose profiles are shown in Fig. 3.
• Figure 5: A schematic drawing from above of Betelgeuse and the observed wake caused by the companion star. The orbit of the companion, at $\sim$2.3 R$_\star$ is shown by a broken blue line. It is well within the total extent of the Mg II emission which reaches 6.4R$_\star$ - marked by diffuse orange color. When observed at different phases, along the direction of the arrows, the expanding wake can be detected.