Stars are born in a cosmic tug-of-war. The interplay between gravity, turbulence, and magnetic fields (B-fields) weaves invisible threads through the clouds. The strength of slows down gravity’s grip, causing the formation of dense pockets, until gravity finally becomes balanced. There, a star begins to shine.
Like invisible weather patterns in space, B-fields change in scale. They are smooth and steady on large galactic scales, guiding the formation of giant clouds. They hey twist and ripple inside molecular clouds (a few light-years across), shaping the cloud’s inner structure. They are strong and tangled deep in the cores (less than a light-year), tightening their grip as stars begin to take shape.
As they grow, they shoot outflows that stir the gas around them, creating turbulence and twisting the magnetic fields that once looked neatly aligned.
VeLLOs (Very Low Luminosity Objects) are the quieter siblings of protostars. They glow faintly, with cooler temperatures (below 650 K). Their light is tiny, less than a tenth of the Sun’s brightness. Their outflows are gentle, unlike the powerful blasts from typical young stars.
Measuring the world’s tiniest magnetic fields
Lynds Dark Nebula 328 (L328), a shadowy cloud 700 light-years away, was once thought starless. It has three sub-cores: S1, S2, and S3. Its S2 sub-core has L328-IRS, a Very Low Luminosity Object, showing a CO bipolar outflow. Studies also mapped the cloud/ envelope magnetic fields at a parsec scale.
Astronomers at the Indian Institute of Astrophysics (IIA) used the JCMT telescope’s POL-2 instrument to peer into the L328, mapping magnetic fields at the heart of star formation. They found that these invisible magnetic threads remain connected over vast scales, from giant clouds to tiny star-forming cores.
Shivani Gupta of IIA, the first author of the study, said, “We chose to investigate the S2 sub-core in L328, since it is a Very Low Luminosity Object (VeLLO). These weak outflows cause minimal turbulence in their surroundings, making them ideal laboratories for studying primordial magnetic fields that existed before star formation began.”
Using data from Planck, optical, near-infrared (NIR), and sub-millimeter dust polarization, the team found:
- The L328 core weighs about 0.69 solar masses, with its smaller sub-cores S1 (0.34) and S2 (0.08).
- The magnetic field lines stretch smoothly from the larger cloud down into the core, all pointing in a north-east-to-south-west direction. This shows the core sits inside a region of strong, connected magnetic fields, threads that help shape its future.
- Inside the L328 core, the magnetic field is strong, about 50 gauss, more than 2.5 times stronger than in its outer envelope. Both the core and envelope sit near the tipping point where gravity and magnetic support balance: The core is transcritical (λ ≈ 1.1). The envelope is marginally supercritical (λ ≈ 1.3).
- Inside the L328 core, three forces, gravity, magnetic fields, and non‑thermal motions, are in a near balance, each holding similar strength. By contrast, the thermal energy (the gentle heat of the gas) is much weaker, playing only a minor role in the drama.
- In the dense heart of L328, the polarization fraction drops as intensity rises. This suggests the depolarization of cores, fading magnetic signal, dense regions. The decline follows a neat power‑law slope of α ≈ −0.98.
New research offers clues on the origins of the Universe’s magnetic fields
Archana Soam, a co-author and faculty member at IIA, explains, “earlier studies of L328 had mapped the large-scale magnetic fields (over light-years scale) using Planck satellite data, optical, and near-infrared (NIR) polarimetry. This work adds a new layer by zooming in on the core scale (sub-light year), where the star formation is actually taking place.”
“A comparison of gravitational, magnetic, turbulence, and thermal energy for the L328 core revealed that the former three are comparable with each other and about 10 times stronger than the thermal energy,” said Maheswar Gopinathan, a faculty member at IIA and a co-author.
“This implies that magnetic fields and turbulence likely play a significant role in resisting gravity and influencing the core’s collapse into a star.”
Journal Reference:
- Shivani Gupta, Archana Soam, Janik Karoly, Chang Won Lee, G Maheswar. Magnetic fields on different spatial scales of the L328 cloud. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stae2783
