When turbulence beats magnetism: origin of massive star cluster seeds

Abstract

High-mass stars form in protoclusters, where gravo-magnetic processes shape collapsing clouds and clumps to be elongated preferentially perpendicular to magnetic (B) fields. Yet it remains unclear whether gravo-magnetic processes still govern the formation of smaller-scale condensations in massive-star-forming protoclusters, which are crucial for understanding the stellar initial mass function and multiplicity. Here we report the first statistical evidence that the condensation elongations are preferentially aligned with local B fields, based on high-resolution data from the largest dust polarization survey toward 30 massive star-forming regions with the Atacama Large Millimeter/submillimeter Array (ALMA). Our clustered massive star formation simulations reveal that this more parallel alignment is exclusively observed in models where initial turbulence dominates B fields. In contrast, models with initial B fields dominating turbulence distinctly exhibit a more perpendicular alignment. The comparison between observations and simulations suggests that turbulence could play a more important role than B fields in the formation of condensations in the context of clustered massive star formation, contradicting the prediction of classical magnetically regulated models. Moreover, we find a possibly turbulence-induced preferential misalignment between the B field and rotation axis of condensations, which may potentially reduce the magnetic braking efficiency and facilitate the formation of large protostellar disks.

Figures (13)

• Figure 1: Properties of observed massive protocluster systems.a--c, Examples of ALMA dust polarization observations 2021ApJ...923..204C2024ApJ...972..115C2024ApJ...974..257Z. The background color shows the 1.3 mm dust intensity. Overlaid line patterns, generated using the line integral convolution method, indicate the B field orientation where $PI/\sigma_{PI}>2$. The synthesized beam is shown as a white ellipse in the lower left corner of each panel. A scale bar is shown in the lower right corner of each panel. d, Histograms (with Poisson errorbars) of the angular difference between B field orientation and condensation elongation across all observed regions. Different colors represent results from different source identification algorithms.
• Figure 1: Dust intensity structures of observed massive protoclusters. Examples of ALMA 1.3 mm dust continuum maps. The blue ellipses indicate the condensations identified with the FWHM along the major and minor axis and the position angle, as reported by astrodendro (left) and getsf (right). The black lines indicate the average B field orientation of each condensation. In the left panel, the red contours indicate the mask of the condensations identified by astrodendro.
• Figure 1: Cumulative distribution function of condensation-B alignment. The dashed black line represents a random alignment distribution. The solid black line shows the CDF from our adopted parameter combination. Colored solid lines represent CDFs from different parameter combinations ($min_{-}value$ in units of $\sigma_{I}$, $min_{-}delta$ in units of $\sigma_{I}$, $min_{-}npix$ in units of beam area).
• Figure 2: Properties of simulated massive protocluster systems.a--b, Examples of synthetic observations for initially (a) sub-Alfvénic (T10M3MU1) and (b) super-Alfvénic (T10M6MU2) simulations. The initial median Alfvénic Mach number $\mathcal{M}_{\mathrm{A,med}}$ (Table \ref{['tab2']}) is shown in the upper right corner of each panel. Maps are in the $xy$ plane and zoomed to the central 0.3 pc around the most massive protostar. The initial B field is along the $x$-axis (horizontal). The background color shows 1.3 mm dust intensity. Overlaid line patterns represent B field orientation via the line integral convolution method. c--d, Histogram examples of the angular difference between condensation elongation ($\theta_{\mathrm{condensation}}$) and average B field orientation ($\theta_{\mathrm{B}}$) in initially (c) sub-Alfvénic (T10M3MU1) and (d) super-Alfvénic (T10M6MU2) models, summed over 3 orthogonal planes.
• Figure 2: Dust intensity structures of simulated massive protoclusters. Examples of synthetic 1.3 mm dust continuum maps (T10M6MU2). The blue ellipses indicate the condensations identified by astrodendro (left) and getsf (right). The black lines indicate the average B field orientation of each condensation. In the left panel, the red contours indicate the mask of the condensations identified by astrodendro.