Betelgeuse’s Hidden Partner: Tracing a Stellar Wake Inside a Supergiant Atmosphere
For decades, Betelgeuse has puzzled astronomers with a mysterious long secondary period (LSP) of about 2000 days, seen as slow changes in brightness, motion, and spectral features. Andrea K. Dupree and collaborators investigate whether these variations can be explained by a companion star orbiting inside Betelgeuse’s extended atmosphere. Recent work and a tentative speckle-imaging detection suggest such a companion exists, named Siwarha, moving on a roughly six-year orbit deep within the star’s chromosphere. The authors ask a focused question: if a companion really is moving through Betelgeuse’s atmosphere, should it leave behind observable signatures in the star’s spectrum?
Background: Circumstellar Lines in Supergiant Stars
The paper begins by reviewing what astronomers mean by circumstellar lines, narrow absorption or emission features formed in gas surrounding a star rather than in its visible surface, known as the photosphere. In cool supergiant stars like Betelgeuse, these lines are hard to detect because the photosphere itself produces many strong spectral features. Earlier studies showed that elements such as manganese (Mn I), iron (Fe I and Fe II), and calcium can trace slowly moving gas close to the star. Because Betelgeuse lacks a widely separated hot companion to illuminate this gas, detecting and interpreting these circumstellar lines requires especially careful analysis.
Optical Observations: Tracking Manganese Absorption
The authors analyze optical spectra from two high-resolution instruments, focusing on three Mn I absorption lines. These narrow lines arise in cool circumstellar gas and can be separated from the broader photospheric absorption using Gaussian fits. The key result is that the equivalent width of the Mn I lines, essentially a measure of how much light is absorbed, changes systematically with the phase of the proposed companion’s orbit. The absorption is weakest near transit, when the companion passes in front of Betelgeuse, and strengthens after transit, reaching a maximum near eclipse, when the companion is hidden behind the star. The radial velocities of these lines also shift, indicating outflowing gas moving at several kilometers per second. This repeating pattern closely follows the approximately 2100-day LSP.
Ultraviolet Evidence: Chromospheric Outflows
The study then turns to ultraviolet observations, which probe hotter and more dynamic layers of Betelgeuse’s chromosphere. Using spectra from the Hubble Space Telescope, the authors examine emission lines of Fe II, Si I, and Mg I that show centrally reversed profiles, meaning two emission peaks separated by a deep absorption core. The relative strength of the blue and red emission peaks, known as the blue to red ratio, reveals whether gas is flowing outward. After the companion’s transit, the blue peak weakens relative to the red peak, signaling enhanced chromospheric outflow. These asymmetries persist through eclipse and then decline as the system approaches the next transit, again matching the long secondary period.
Interpreting the Pattern: A Trailing Wake
Combining the optical and ultraviolet evidence, the authors propose a physical picture in which the companion star orbits through Betelgeuse’s extended atmosphere at high speed and gravitationally focuses surrounding gas into a trailing, expanding wake. This wake contains denser, shocked material that absorbs light and enhances outflow signatures when it crosses our line of sight. Immediately after transit, the wake is narrow, but as it expands sideways at roughly the sound speed, it covers more of the stellar disk, producing maximum absorption and outflow near eclipse. Over time, continued expansion weakens the effect, explaining the gradual return to smaller signatures before the next orbit.
Conclusions: Evidence for a Companion Inside Betelgeuse
In their conclusions, Dupree and collaborators argue that the synchronized variations of photospheric, chromospheric, and circumstellar lines are best explained by the presence of a companion star embedded within Betelgeuse’s atmosphere. The detection of a repeating spectroscopic wake provides direct observational support for this scenario and strengthens the case that the LSP is caused by binarity rather than pulsation or surface activity alone. While more detailed modeling is needed, this work offers a compelling glimpse of how a hidden companion can shape the extended atmosphere of one of the sky’s most famous stars.
Source: Dupree
