A research team from CNRS Terre & Univers has just deciphered the origin of the periodic light bursts from the protostar V347 Aurigae in the Auriga constellation, attributed to the presence of a brown dwarf companion. To achieve this result, they used the SPIRou instrument mounted on the Canada-France-Hawaii Telescope, located atop the Maunakea volcano on the Big Island of Hawaii.
Disk and accretion of matter within a binary protostar observed with the ALMA radio telescope. This binary protostar, different from V347 Aur (whose brown dwarf companion is 10 times less massive than the central star), illustrates the complexity of the matter flows from the disk to the stars within a binary system. V347 Aur is far too close to its brown dwarf companion to be similarly imaged by ALMA.
© ALMA / ESO / NRAO / NAOJ, Alves et al. 2019 V347 Aurigae is a protostar barely a few hundred thousand years old, equivalent to a baby just a few days old if we scale the lifespan of such a star to that of a human being. Located in the Auriga constellation, V347 Aur is visible in the winter sky around Christmas. For the past few years, this protostar has drawn astronomers' attention for its periodic light outbursts, much like a lighthouse or Christmas lights, temporarily making it up to 100 times brighter.
By observing this object for over 3 years with SPIRou, the exoplanet hunter assembled at Irap and installed at the Canada-France-Hawaii Telescope, a team of researchers was able to show that these light bursts are caused by the presence of a young companion that is ten times less massive, a brown dwarf of about 30 Jupiter masses, orbiting around V347 Aur.
This brown dwarf follows an elliptical path around V347 Aur, causing the two bodies to move closer and farther apart every 155 days, triggering instabilities in the disk of matter that surrounds and feeds the nascent binary system.
During these episodes, the destabilized disk materiel crashes onto V347 Aur, temporarily increasing its brightness by up to 100 times. SPIRou is also a spectropolarimeter, able to map the magnetic field of stars using tomographic techniques inspired by medical imaging.
Light bursts from the protostar V347 Aur over time during the past 10 years (BJD: Barycentric Julian Date) reaching a factor of 100 (5 magnitudes) during the most intense episodes. The green arrows indicate the dates of SPIRou observations of V347 Aur, conducted from October 2019 to April 2023.
The team was also able to reconstruct V347 Aur's magnetic field and its temporal evolution, showing that it fluctuates every 155 days, suggesting a magnetic cycle similar to the well-known dynamo cycle of the Sun but much faster and triggered by V347 Aur's orbital motion, thus the proposed name of 'pulsed dynamo' for this original mechanism.
This result perfectly showcases SPIRou's capabilities in deciphering the complex physical mechanisms at work during the birth of stars and planets, for the detailed study of which it was specifically designed.
References:
J -F Donati, P I Cristofari, A Carmona, K Grankin,
SPIRou monitoring of the protostar V347 Aur: binarity, magnetic fields, pulsed dynamo, and accretion,
Monthly Notices of the Royal Astronomical Society, Volume 534, Issue 1, October 2024, Pages 231-250,