Untangling the Magnetic Puzzle of HD 169142: Clues from a Young Star Hosting Planets
In this study, Hubrig and collaborators examine HD 169142, a young, bright star still forming in a dusty region of space. Classified as a Herbig Ae/Be star, it is surrounded by a disk rich in gas and dust—prime material for building planets. Observations of this disk have shown rings, spirals, and asymmetrical clumps, all pointing toward the presence of forming planets. Two giant planets, each a few times the mass of Jupiter, are thought to orbit the star at distances within and between the disk’s dust rings. Understanding how the star’s magnetic field interacts with these planets and the disk could provide key insights into how planetary systems like our own are born.
Probing Magnetic Fields with Light
To investigate HD 169142’s magnetic field, the authors used spectropolarimetric data from ESPaDOnS and HARPSpol, two high-resolution instruments that detect polarized light. They applied the Least Squares Deconvolution (LSD) technique to increase signal strength by combining information from many spectral lines. The Zeeman effect—where magnetic fields cause light from atoms to split and polarize—was central to their analysis. By examining these signatures in the circular polarization spectrum (Stokes V), the team found both narrow and broad Zeeman features, depending on which elements were analyzed.
Narrow vs. Broad: A Tale of Two Features
Narrow Zeeman features were only clearly detected when using lines from neutral iron (Fe I), while broader and fainter signals appeared when ionized species like Fe II or Si II were used. These broader signals were only marginally detected and appeared during two different observing epochs. The authors propose that the narrow features may not come from the star’s surface at all, but instead from magnetospheric regions—areas of ionized gas controlled by the star’s magnetic field. This interpretation is supported by the shifts in the narrow signals, which do not align with the star’s surface rotation alone and suggest complex three-dimensional magnetic structures.
Hydrogen Lines as a Window into the Magnetosphere
The variability of hydrogen lines, particularly Hβ, provided further evidence of an active and dynamic environment. Emission peaks appeared and disappeared across different observations, sometimes showing double-peaked structures. These changes likely reflect movement within the circumstellar disk or the influence of magnetic loops. Similar patterns have been observed in other magnetic stars, like HD 190073 and HD 104237, and often appear as “ring-like” features in dynamical spectra tied to the star’s magnetic phase.
A Fast Spinner with a Hidden Period
Although HD 169142’s exact rotation period is unknown, estimates based on its size and rotation speed suggest it spins very quickly—between 0.31 and 0.82 days per rotation. This rapid spin means the magnetic field measurements are highly dependent on the star’s orientation at the time of observation. The variability in both magnetic signatures and line profiles supports the idea that observers are seeing different parts of a complex and changing magnetic environment each time the star rotates.
Theories Behind the Magnetic Signature
The authors propose several possibilities to explain the observed features. The broader Zeeman signals might indicate a strong, large-scale magnetic field on the stellar surface, while the narrow ones are tied to non-photospheric, magnetospheric regions. Alternatively, because HD 169142 is a relatively cool star of type F0, it might still have a shallow convective zone that powers small-scale magnetic activity, much like the Sun. This could explain its strong X-ray and UV emissions despite a lack of typical chromospheric emission lines.
Chemical Clues from Elemental Variability
Another striking finding is that different chemical elements in the star’s atmosphere appear to vary independently. Line profiles from elements like silicon, titanium, and iron show subtle splits and shifts across different epochs. These are likely signs of chemical spots—patches on the star where certain elements are more or less concentrated. This phenomenon has also been seen in other magnetic Herbig Ae/Be stars and suggests a highly inhomogeneous stellar surface.
Looking Ahead: Mapping the Magnetic Field in 3D
To fully understand the magnetic structure of HD 169142 and its connection to the disk and forming planets, future observations are needed. Techniques like Zeeman-Doppler Imaging could map the magnetic field in three dimensions and show how it changes with rotation. This would help determine whether the field is globally organized or dominated by smaller, scattered regions. For now, HD 169142 offers a fascinating glimpse into the complex interactions between a young star, its magnetic field, and the planet-forming disk that surrounds it.
Source: Hubrig
