Astronomers have discovered ultra-weak magnetic fields in Beta Ursae Majoris and Theta Leonis, two metallic-line stars. Obtained from deep spectropolarimetric analysis, the results may be proof that weak magnetic fields could be more common among stars with intermediate mass.
The team at the Paris Observatory, led by Aurore Blazère, was prompted by a recent discovery of a weak and circularly polarized signature in similar star Sirius A’s spectral lines. A weak surface magnetic field was believed to cause the polarized signal.
The astronomers reported their findings Jan. 8 in the journal arXiv.
Both Theta Leonis and Beta Ursae Majoris are classified as AM stars, which are chemically peculiar stars whose spectrum maintains strong with usually variable absorption lines of metals including zinc, zirconium, and barium. This is why they are also called metallic-line stars.
These stars also have deficiencies of elements, including calcium and scandium.
Unlike the usual A stars with a quick spin, most Am stars are part of close binary systems. These orbit for less than 1,000 days and the two stars slow each other down via tidal mechanism. Sirius and Acubens are some familiar examples.
The scientists used data from the NARVAL spectropolarimeter to seek the magnetic fields of these intriguing Am stars. The instrument is installed at the Bernard Lyot Telescope located in the French Pyrénées.
Using NARVAL, scientists can detect the magnetic fields of stars at such great precision. For this research, the team obtained data acquired from 2010-2014 NARVAL observation campaigns.
From their analysis, the astronomers got a circularly polarized signal from the Am stars. The said signal covers most of the line’s width and is roughly symmetric with the line centroid. A positive lobe dominates the signal in both stars.
The origins of the these signals are thought to be stellar in nature, as former observations of Sirius A – done through using NARVAL and two other scientific instruments – yielded similar-shaped signatures.
"[The signatures] display a similar shape… giving strong confidence in a stellar origin of the polarized signature," wrote the authors in their research.
These findings lend a clue into the weak polarized signatures from the intermediate-mass stars’ photospheres, indicating that weak magnetic fields are likely prevalent in this class of stars. The study also concluded that these signatures are potential indirect tracers of surface convective motions.
Scientists, however, are yet to determine the source of the weak magnetism of these stars, as there is so far only a tiny number of observed AM stars as well as polarimetric signatures near detection limit.
In addition, revealing the surface features of these Am stars entail using highly sensitive techniques, such as a one used previously to find out Vega’s faint starspots.
