A team of astronomers finds clues on the magnetism of a new kind of M dwarf

An illustration of an M-dwarf star covered in starspots. NASA/ESA/STScI/G. Bacon.
An illustration of an M-dwarf star covered in starspots. NASA/ESA/STScI/G. Bacon.
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An illustration of an M-dwarf star covered in starspots.  NASA/ESA/STScI/G. Bacon.

Scallop-shell stars are a recently discovered class of young M dwarfs. More than 70% of the stars in the Milky Way are M dwarfs, although there are only around 50 recently confirmed scallop-shell stars. They show periodic variability, with dips in optical which are an indication of obscuring material orbiting a few stellar radii above the surface, something not seen in regular M dwarfs. The Institute of Space Sciences (ICE-CSIC) led the first ever analysis of a long radio-observation to any scallop-shell star, in a pioneer study published in  Astronomy & Astrophysics Letters. 

Studying this kind of system in detail, combining radio and optical observations, allows astronomers to infer information about the nature of the orbiting opaque material and the stellar magnetism, which are both features that are not well characterized in these sources. The team, led by the ICE-CSIC predoctoral researcher Simranpreet Kaur, analysed the data from two 7-hour long radio observations at low frequencies (below GigaHertzs) of one scallop-shell star. They found a short-term polarization reversal in both observations, which has not been seen in any other star so far. In astrophysics, the polarization of light can offer information about the magnetic fields in the emitting regions. 

The source named 2MASS J05082729−2101444 is a 25 million year old scallop-shell star that was observed with the Giant Metrewave Radio Telescope (GMRT) located in Pune (India), covering 88% of the 6.7 hour photometric period in each of the two observations. The team related it to the photometric information from NASA’s telescope Transiting Exoplanet Survey Satellite (TESS) and Las Cumbres Observatory.

“This was my first hands-on experience with in-situ radio observations, and it was a fascinating learning opportunity (I even luckily spotted a leopard in the wild area around the telescopes!). And now, with the results that we have obtained so far, this study has opened the door for using radio observations to study scallop-shell stars, showing that they are not just limited to optical research and expanding the ways we can understand these unique systems," says ICE-CSIC researcher Simranpreet Kaur, first author of the paper. 

ICE-CSIC researchers have worked in close contact with several international members of the Spanish and German CARMENES consortium. Since 2016, the CARMENES collaboration has conducted a 750-night exoplanet survey targeting around 300 M dwarfs. The CARMENES survey is thus providing a comprehensive overview from the Northern Hemisphere of stellar systems around nearby M dwarfs and, at the same time, it generates a unique data set for studies of M dwarf star atmospheres, rotation and activity.

The mysterious orbiting material

With rotational periods of hours, scallop-shell stars, also known as complex periodic variables, show optical dips that are not explicable by stellar spots or a transiting exoplanet. Researchers believe that the surrounding blobs of material that approximately co-rotate with the star at a distance of a few stellar radii from the surface are responsible for the optical dips. 

The observations allowed the team to detect a very clear, polarized emission. The detection of a polarization reversal in both observations possibly indicates radio auroral emission. “We believe that we are witnessing the presence of two mechanisms in both observations: one is more or less persistent, and is probably what we call ‘gyro-synchrotron’ emission, caused by mildly relativistic particles spiraling around the magnetic field lines. The second one, that we call ‘electron cyclotron maser’, has been directly seen in the Jupiter-Io system and consists of sporadic emission of intense, highly polarized electromagnetic waves directed along  collimated beams”, says Simranpreet Kaur. 

“This phenomenon is produced by particles which travel towards the stellar surface and oscillate together at the same frequency and phase, and is actually the radio counterpart of the planetary auroral optical emission, caused by the excitation of atmospheric molecules when hit by charged particles”, Kaur adds. The frequency of this kind of emission also carries direct information about the magnetic field of the star.

The origin and nature of the blobs of material remain a mystery: it could be gas or dust, and it could come from the star or the debris disk. “With the available optical and radio information, our team leans into a few hypotheses. A suggestive one is that the material is originating from an evaporating, small planet, orbiting near to the co-rotation radius. However, to prove this we will need further confirmations from future observations”, remarks Simranpreet Kaur. 

The team is currently analysing three further long GMRT observations carried out in September 2024 and got approved several follow-up observations with different facilities in 2025, to delve into the relation between the very peculiar optical and radio light curves, and the mechanisms behind them. 

“Besides characterizing this particular source, this further study could shed light on what is closely orbiting the star, and if the radio emission is related to it or not”, says Víctor Béjar, researcher at the Instituto de Astrofísica de Canarias (IAC), and coauthor of the study.

“This research will ultimately foster the interest on scallop-shell stars within low-frequency radio astronomy, which is already entering a golden era thanks also to the new and upcoming radio facilities like SKA and LOFAR,” says Daniele Viganò, ICE-CSIC and IEEC scientist. 

 

More information 

This research was presented in a paper entitled ‘Hints of auroral and magnetospheric polarized radio emission from the scallop-shell star 2MASS J05082729−210144’, published in Astronomy & Astrophysics Letters in November 2024. DOI: https://doi.org/10.1051/0004-6361/202452037 

 

Contact

ICE-CSIC researcher 

Simranpreet Kaur

kaur [at] ice.csic.es (kaur[at]ice[dot]csic[dot]es)

ICE-CSIC Communication & Outreach Office 

Alba Calejero

communication [at] ice.csic.es (communication[at]ice[dot]csic[dot]es)

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Press release
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