Radiative Feedback in Pop III Star Formation
The first stars in the universe were born a few hundrend million after the Big Bang in the center of Dark Matter minihalos. They are different from present day stars in that they formed from "pristine gas" of hydrogen and helium left over from big bang nucleosynthesis. Being deprived of the heavier elements produced in supernova explosions, they were not able to cool as efficiently as present day stars. As a result they are often more massive than present day stars. Being born so long ago (and therefore so far away), no Pop III star has yet been detected, though the chance of actually observing one has increased with the recent launch of JWST. As a result, the best way to understand these elusive objects is through small-scale full-physics high-resolution simulation.
In Chen et al. 2025 in prep we analyze the evolution of Pop III stars for the first 50,000 years after their creation using a unique suite of simulations that self-consistently incorporate turbulence, magnetic fields, and radiation feedback. We analyze, among other things, how different radiation prescriptions affect the stellar mass, and how H-shielding influences Lyman–Werner (FUV) radiation. We ran the simulation multiple times and chose a turbulent seed that isn't prone to fragment in order to better analyze the physics under study. In the top plot below we see the masses of the stars for this seed as a function of time from their creation. In our fiducial run (blue curve) we see that star's growth begins to saturate at the end of the simulation. The I-front of the HII region is still an R-type and has not transitioned to a D-type and so has not yet shut off accretion. In the run without H shielding (yellow curve) we see a suppression of mass growth. This is because in the absence of H shielding, the Lyman Warner radiation can more effectively phtotodissociate H2, thus increasing the temperature near the protostar, providing an additional thermal pressure support and hindering stellar accretion. Stay tuned to the upcoming paper for more analysis of this and other plots! In the bottom plot we show the HII region on the left and the density field on the right for the protostar in our fiducial run at the end of the simulation.
Turbulence Generated by The Stream Velocity
For the first ~380,000 years after Big Bang the universe was a dense primeval "fireball" where baryons were coupled to the photons via their mutual coupling to the electrons (the former through Thomson scattering and the latter through the Coulomb force). The radiation pressure of the photons prevented the baryons from fully gravitationally collapsing into potential wells and created standing oscillating waves known as Baryon Acoustic Oscillations (BAO). The Dark Matter (DM), which doesn't interact electromagnetically, did not feel the radiation pressure, and so gravitationally collapsed into potential wells. As a result of the different physics governing the motions of the baryons and DM, they had a relative velocity with respect to each other. At the time of recombination the baryons decoupled from the photons and their velocity dropped from being highly relativistic to that of a thermal ideal gas, and the relative velocity between the baryons and DM became supersonic.
Using high-resolution cosmological simulations in Chen et. al. 2025 we explore how this relative velocity, which is often called the "Stream Velocity", affects turbulence inside DM halos at the epoch of the first stars (a.k.a Pop III Stars). We found that the Stream Velocity boosts the turbulence in low mass halos and, rather surprisingly, suppresses it in high mass halos. This is illustrated in the top figure where we show the turbulent Mach number as a function of halo mass at z = 20 for the no-Stream Velocity case (red curve) and the Stream Velocity case (blue curve). For low-mass halos the additional kinetic energy created by the Stream Velocity is transformed into turbulent motions, boosting the turbulence. While for high-mass halos the Stream Velocity actually suppresses accretion-driven turbulence and merger-driven turbulence, two other ways of generating turbulence in the primordial universe. This suppression of turbulence in high-mass halos is caused by how the Stream Velocity modulates the density field in addition to the velocity field. Read the paper for more info! The bottom figure shows typical low-mass halos for no-Stream Velocity (top panel) and Stream Velocity (bottom panel).
Measuring Magnetic Fields using VGT and VDA
Magnetized turbulence pervades the ISM and plays a central role in cosmic ray transport, star formation, and the regulation of heat and mass flows. There are multiple techniques for measuring these magnetic fields, including Faraday Rotation (which uses polarization of EM waves when interacting with a magnetic field), Zeeman Effect (which uses the shifting of spectral lines in the presence of a magnetic field), and dust grain alignment (which uses the perpendicular alignment of tiny filings with the magnet field). The Velocity Gradient Technique (VGT) is a relatively new method that can infer the plane-of-sky magnetic field orientation from spectroscopic HI channel maps. It uses turbulent scaling relations and the statistical properties of MHD turbulence. In brief, because turbulent eddies in magnetized flows are elongated along the local magnetic field (see Goldreich & Sridhar 1995), the velocity gradients are largest perpendicular to the field: for a given velocity fluctuation, the smaller perpendicular scale length produces a larger gradient. The Velocity Decomposition Algorithm (VDA) is a method developed by Ka Ho Yuen that separates out the velocity (aka velocity caustics) and density components of channel maps, which can, among other things, tell use the phase and turbulent properties of a region in addition to providing a cleaner map to preform VGT.
In Yuen, Chen et. al. 2023 we used the VDA to show that a recently discovered large filamentary feature on the outskirts of the Milky Way, perhaps the largest ever found, named Cattail, is likely not in the CNM (as the authors suggested) and consists of multiple turbulent systems. We showed this in part by first noting that the main body of Cattail (yellow boxes in left side of top figure), looks the same in channel maps. VDA showed that this only happens in the presence of strong thermal broadening. Since the CNM is supersonic it is unlikely to contain strong thermal broadening. Further when analyzing the velocity caustic map (right side of top figure) we found that the ratio of the power of the density to velocity fluctuation were less than 1, meaning that the turbulence contributes significantly to the line width (3 - 5 km/s), further precluding strong thermal broadening. We also found the density power spectra was different for the yellow region and the green region of Cattail. The former had a density spectral slope of approx. -3.7 a signature of weakly magnetized turbulence while the latter had a density spectral slope of -2, indicating the feature is magnetized and parallel to the magnetic field. The power spectra of the velocity field, decomposed by VDA, followed the Kolmogorov cascade of -5/3. The channel map itself did not reveal this scaling relation, highlighting the power of the VDA. In the bottom figure we show the magnetic field measure using VGT and VDA.
