A red giant's past can change how its magnetism is read
· BehindLines newsroom
To estimate the magnetic field hidden inside a star, its history could matter as much as its present appearance. In a presentation on 29 October, the Institute of Science and Technology Austria, or ISTA, describes research on red giants that acquired matter from another star.
The work by Nicolas Muntean and colleagues is described in a manuscript posted on arXiv on 8 September and listed as accepted in Astronomy & Astrophysics. The recent announcement and the manuscript's first release are therefore separate stages.
Listening to a star without seeing inside it
Astronomers use small, regular changes in brightness, linked to stellar oscillations, to probe a star's structure. This approach, asteroseismology, also allows them to study how magnetism affects those vibrations.
ISTA's presentation highlights a difficulty: a model assuming that a star has always evolved alone may poorly represent one that has gained mass. The same signal can then lead to a different interpretation of its magnetism.
Comparing two evolutionary histories
The authors construct models with and without an episode of mass acquisition after the star entered the red giant phase. Their grid spans masses between 1.1 and 2 times that of the Sun.
For weak fields, above 1.6 solar masses, models that gained matter can be several times more sensitive to a magnetic field than isolated models with the same total mass. For strong fields, the field needed to make certain oscillations heavily damped, or almost invisible, in the light signal can also be weaker in this scenario.
A conditional interpretation
The result concerns the relationship between the modelled history and the observed signal. The conclusions depend on the evolutionary histories represented in these models. This does not mean that all red giants have absorbed a companion, or that a single magnetic field fits every star.
The researchers propose placing tighter constraints on stars' pasts before estimating their internal magnetism. The aim is to avoid confusing a change in sensitivity with a real difference in field strength.