Peering into the Starspots: The First Doppler Image of λ Andromedae
Understanding how starspots—like sunspots on our Sun—affect measurements of stars has become crucial in astronomy. In this study, Ö. Adebali and colleagues reveal the first Doppler image of the RS CVn binary star system λ Andromedae, shedding light on its complex starspot activity and refining its orbital parameters with unprecedented precision.
What’s Special About λ Andromedae?
λ Andromedae is part of a special class of binary stars called RS CVn systems. These stars are known for their intense magnetic activity, which produces large starspots that can cover up to a quarter of the star's visible surface. Unlike the Sun, λ And's rotation and orbit are not synchronized, making it an exciting target for astronomers.
Its primary star is a subgiant classified as G8III-IV, with a temperature of around 4600 K and a mass similar to the Sun’s. However, its radius is much larger—about 7 times the Sun's size. It rotates every 54 days but orbits its companion in just 20.5 days, creating a significant mismatch between its orbital and rotational periods.
A Deep Look at Starspots
The researchers observed λ And over 522 days using high-resolution spectroscopic data from two telescopes: the STELLA robotic telescope in Spain and the Vatican Advanced Technology Telescope (VATT) in Arizona. By applying Doppler imaging, they mapped the star’s surface and discovered four major starspots. One dominant spot was significantly cooler—about 1000 K lower in temperature—than the surrounding area, resembling a large sunspot.
Interestingly, these spots remained stable throughout the 522-day observation period, indicating a persistent magnetic structure on the star’s surface. The study also found that these spots caused periodic changes in the star's radial velocity (RV), which can mimic signals from orbiting planets if not accounted for.
Correcting the Orbit
By separating the starspot-related radial velocity signals from the true orbital motion, the team achieved a threefold improvement in the precision of the orbital parameters. This correction confirmed the nearly circular orbit of λ And and provided the most accurate measurements of its orbital elements to date.
The researchers also calculated the mass of the unseen secondary object, concluding it is likely a brown dwarf with a mass around 0.1 times that of the Sun. This discovery adds to the growing number of systems with brown dwarf companions.
Magnetic Field and Chromospheric Activity
Using spectropolarimetric data, the study measured the star’s magnetic field and observed variations in its chromospheric activity. Seven different spectral tracers showed emission patterns aligned with the locations of the cool spots, suggesting that these spots are linked to magnetic activity similar to solar prominences.
Why Does This Matter?
The precise correction of the radial velocity signals caused by starspots is a big step forward for both stellar and exoplanet research. When studying stars with planets, distinguishing between spot-induced signals and true planetary signatures is crucial. This work also opens new doors to understanding magnetic activity in evolved stars and how it impacts their surroundings.
Source: Adebali
