Betelgeuse: Detection of the Expanding Wake of the Companion Star

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.

• The paper demonstrates that Betelgeuse’s extended atmosphere is periodically modulated by an expanding wake created by its orbiting low-mass companion.
• The study employed multi-epoch high-resolution optical and UV spectroscopy, mapping Mn I absorption and chromospheric emission variations to a ~2100-day orbital period.
• The findings support binary models for long secondary periods in supergiants and call for advanced hydrodynamic simulations to capture the complex interplay of mass loss and shock dynamics.

Detection of the Expanding Wake of the Betelgeuse Companion Star

Introduction

The complex variability of Betelgeuse, the prototypical M supergiant, has been studied extensively. In addition to a canonical $\sim$400-day pulsational period, Betelgeuse also displays a conspicuous long secondary period (LSP) of $\sim$2000 days. While various hypotheses have been proposed to explain the LSP—including nonradial modes, convection, and magnetic effects—recent studies favor a binary model wherein a low-mass stellar companion orbits within the extended chromosphere of Betelgeuse. Probable observational confirmation of such a companion, named Siwarha, has now been achieved through speckle imaging, consistent with predictions for the position and brightness of a companion within $\sim$2.3 R$_\star$.

This study presents a detailed spectroscopic analysis of variable circumstellar absorption and chromospheric outflow in Betelgeuse, establishing the detection of an expanding wake trailing the orbit of the companion. The work synthesizes high-resolution optical and ultraviolet spectroscopy, providing new evidence for periodic modulation of circumstellar material by the orbital phase of the companion.

Observational Framework and Methodology

Multi-epoch, high-resolution optical spectra were obtained using HERMES and TRES echelle spectrographs, spanning nearly a decade and covering multiple LSP cycles. A focus was placed on the Mn I triplet (4030.75, 4033.07, and 4034.49 Å), which exhibits narrow circumstellar absorption features superposed on the broader photospheric lines. The extraction of equivalent widths (EWs) and radial velocity offsets employed iterative two-component Gaussian fitting to disentangle circumstellar and photospheric contributions. Temporal variations in line characteristics were mapped to the ephemeris of the companion’s orbital motion (Period = 2109 ± 9 days).

For chromospheric diagnostics, archival and new HST/STIS ultraviolet spectra were analyzed, targeting centrally-reversed emission lines of Fe II, Si I, and Mg I. Blue-to-red emission peak ratios served as sensitive indicators of changing atmospheric velocity fields and mass outflows in the chromosphere.

Results: Periodic Circumstellar and Chromospheric Variability

Optical Circumstellar Diagnostics

Analyses of the Mn I triplet reveal that circumstellar absorption strengthens immediately after the companion’s disk transit (phase 0.0), maximizing near inferior conjunction (phase $\sim$0.5, corresponding to the companion’s eclipse behind Betelgeuse), followed by a gradual decline in strength approaching the next transit. This cyclic variability repeats with the LSP and is mirrored across all three Mn I transitions. There is notable inter-epoch variability in equivalent widths, consistent with structurally and temporally dynamic circumstellar environments.

Radial velocities of the circumstellar Mn I features also undergo modulation with orbital phase, exhibiting outflow velocities from $-5$ km s$^{-1}$ post-transit up to $-10$ to $-15$ km s$^{-1}$ near eclipse, before returning toward the photospheric value. This behavior is consistent with the presence of an expanding, phase-dependent wake in the stellar atmosphere.

Ultraviolet Chromospheric Outflows

Strong chromospheric emission lines in the UV undergo marked profile asymmetry changes, with blue emission peaks weakening relative to red, and central reversals shifting to shorter wavelengths during post-transit to pre-eclipse phases. These signatures indicate enhanced chromospheric outflow velocities reaching $-6$ to $-20$ km s$^{-1}$, with maximum outflows occurring at phases coincident with maximum circumstellar absorption. This periodic modulation is detected in multiple lines (e.g., Si I 2516.1 Å, Fe II 2692.8 Å, Mg I 2852.1 Å), reinforcing the phase-linked alteration to the extended atmospheric structure.

The amplitude and phase of the chromospheric outflow variations correspond well to those observed in the circumstellar Mn I absorption, further supporting a coherent response of the entire circumstellar and chromospheric environment to the companion’s orbital cycle.

Physical Interpretation: Wake Formation and Hydrodynamics

Modeling and recent hydrodynamic simulations of mass-losing binaries demonstrate that a companion orbiting in the extended atmosphere can gravitationally focus the wind, forming a dense, expanding wake (Chen et al. 2020). For Betelgeuse, the companion's orbital velocity ($\sim$43 km s$^{-1}$ at 2.3 R$_\star$) imparts a high Mach number ($\mathcal{M} \sim 7$) relative to the local sound speed of $\sim$6 km s$^{-1}$ at T ≈ 2500 K, justifying the formation of a strong, turbulent wake structure.

The observed strengthening and velocity evolution of circumstellar absorption post-transit is interpreted as direct sightlines through the dense, freshly shocked trailing wake material. The lateral expansion of the wake at the sound speed causes it to envelop an increasing fraction of the visible disk as the orbit progresses from transit to eclipse, consistent with the observed timing and duration of spectroscopic enhancements. The chromospheric and circumstellar outflows, as traced by the velocity and asymmetry changes in UV and optical lines, are physically attributable to this evolving wake geometry.

Implications and Future Directions

These findings provide spectroscopic confirmation that the circumstellar and chromospheric environments of Betelgeuse are modulated on the timescale of the LSP, with phase-locked enhancements in absorption and mass outflow that are best explained by the presence of an orbiting companion with an expanding, turbulent wake. The results support binary-driven models for the LSP in Betelgeuse and potentially other luminous M supergiants/giants with similar phenomena.

Theoretically, the observations demand multidimensional hydrodynamic models incorporating radiative transfer, turbulence, and shock physics in multiphase media, capturing the thermodynamic evolution and spectral signatures of the wake. The emergence of time-dependent, phase-resolved diagnostics will place tighter constraints on companion parameters and the nature of atmospheric structuring in interacting binaries.

The detection of a wake in Betelgeuse also informs studies of mass-loss processes, wind shaping, and may influence understanding of pre-supernova environments in evolved massive stars. The methodology and physical interpretation outlined here establish a template for similar analyses of long-period variables with suspected companions.

Conclusion

The phase-resolved spectroscopic evidence presented in this study demonstrates that Betelgeuse’s extended atmosphere is periodically modulated by the orbit of its low-mass companion Siwarha. The observed variations in circumstellar Mn I absorption and ultraviolet chromospheric emission profiles are synchronized with the companion’s $\sim$2100-day period and are consistent with the formation and expansion of a trailing wake through gravitational focusing. This provides direct empirical support for the binary hypothesis of the LSP and underscores the need for advanced hydrodynamic modeling to further quantify the wake’s properties and its impact on the circumstellar environment of mass-losing supergiants (2601.00470).