An early transition to magnetic supercriticality in star formation Magnetic fields have an important role in the evolution of interstellar medium and star formation1,2. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas3. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA)4,5 towards L15446,7—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases8,9,10 characterized by a high central number density11 and a low central temperature12. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes13,14. In non-masing interstellar medium, only H I, OH and CN have successfully produced systematic Zeeman measurements. The comprehensive Zeeman surveys15 indicate that the magnetic fields in the diffuse cold neutral medium (CNM) probed by H I do not scale significantly with density, whereas above a critical break-point density of approximatey 300 cm−3, the magnetic fields in dense cores probed by OH tend to increase with density. However, owing to the gap in densities between the H I (about 40 cm−3) and OH ((gtrsim )103 cm−3) Zeeman measurements, the field transition around the critical density of about 300 cm−3 (where the dependence of the field strength on the density changes behaviour) remains a controversial topic and could have crucial implications for star formation16,17,18,19. Recently, a CCS Zeeman detection20 shed light on regions denser than that probed by OH. A Zeeman probe that is sensitive for a wide range of densities, particularly the low-density molecular envelope, is highly desirable and could help to distinguish different core-formation scenarios.We developed the so-called H I narrow self-absorption (HINSA) technique to provide a probe of the transition from H I to H2 (refs. 4,5). HINSA traces cold atomic hydrogen well mixed with H2, which provides the necessary cooling, not available in the CNM, of H I through collision. Close to the steady state between H2 formation and destruction, the HINSA strength is independent of the gas density5 and thus capable of probing the transition around the critical density. Although the Zeeman effect of the H I self-absorption feature has been reported21,22, the broad line widths of the absorption components are mostly associated with diffuse atomic gas rather than dense molecular gas. Considering that HINSA typically has much higher brightness temperatures than most molecular lines, is impervious to depletion23 and can be detected in a wide range of H2 densities, HINSA is a promising Zeeman probe for molecular gas.The HINSA feature in L1544 has a strong absorption dip and a nearly thermalized narrow line width at a temperature lower than 15 K (ref. 4). The non-thermal line width and centroid velocity of the HINSA are very close to those of the emission lines of OH, 13CO and C18O molecules, and their column densities are well correlated, suggesting that a significant fraction of the atomic hydrogen is located in the cold, well-shielded portions of L15445. We thus assume that the column density sampled by the HINSA can be approximated by that obtained from dust, despite the substantially larger apparent area covered by HINSA (Fig. 1a). The previous OH Zeeman detection with Arecibo24 towards the L1544 centre resulted in a field strength of Blos = +10.8 ± 1.7 μG, where Blos is the magnetic field component along the line of sight, with a positive sign representing the field pointing away from the observer. In contrast, the OH Zeeman observations of the Green Bank Telescope (GBT) towards four envelope locations 6.0′ (0.24 pc) from the centre yielded a marginal detection of Blos = +2 ± 3 μG (ref. 16), leaving the structure of the envelope field undetermined.Fig. 1: L1544 core and illustration of the structure of interstellar medium from the CNM to the core.a, A composite of Digitized Sky Survey 2 (DSS2) images of L1544 with the i band in red, the r band in green and the b band in blue overlaid with the HINSA and H2 column density maps. The white dashed contours are 30%, 50%, 70% and 90% of the peak HINSA column density, and the orange contours are 2 × 1021 cm−2, 4 × 1021 cm−2, 6 × 1021 cm−2, 8 × 1021 cm−2 and 10 × 1021 cm−2 for the H2 column density. The red, green and cyan circles mark the locations and beam sizes of the FAST, Arecibo and GBT Zeeman observations, respectively. b, A composite of three 0.5 km s−1 velocity slices of the Arecibo GALFA-H I images at 6.2 km s−1, 6.7 km s−1 and 7.3 km s−1 local standard of rest (LSR) velocities in blue, green and red. The dashed rectangle shows the region of a. The two absorption dots represent the locations of quasars 3C132 and 3C133. c, Schematic view of the CNMs, the molecular envelope and the L1544 core.With the Five-hundred-meter Aperture Spherical radio Telescope (FAST)25, we detected Zeeman splittings in a 2.9′ beam (0.12 pc) towards the HINSA column density peak, 3.6′ (0.15 pc) away from the L1544 centre (Fig. 1). The spectra of the Stokes I(v) and V(v) parameters (where v denotes velocity) are shown in Fig. 2. The I(v) spectrum contains the H I emission of the CNM and the warm neutral medium (WNM) clouds in the direction towards the Taurus complex and a HINSA feature at the centroid velocity of L1544. Figure 2a shows our decomposition of I(v) into a foreground HINSA component, a background WNM component, and three CNM components between the HINSA and WNM. Our fitted parameters of the HINSA component are in good agreement with the previous HINSA observations4,5, and our parameters of the CNM and WNM components are similar to the Arecibo results towards quasars around L154426.Fig. 2: The Stokes I(v) and V(v) spectra at 21-cm wavelength towards the HINSA column density peak.a, The black profile represents the I(v) spectrum. The red profile represents the absorption from the foreground HINSA component. The blue dashed and dotted profiles represent the emission of the CNM and WNM components, respectively. The CNM and WNM profiles include the absorption from the CNM components that lie in front but do not include the absorption from the HINSA. The black dashed profile represents the sum of the absorption and emission profiles. b, The black profile represents the V(v) spectrum. The black dashed profile represents the sum of the Zeeman splitting profiles of the five components. The red profile represents the Zeeman splitting profile with Blos = +3.8 μG of the HINSA component.The V(v) spectrum shows features of classic ‘S curve’ patterns proportional to the first derivatives of I(v) for the HINSA, CNM and WNM components, as expected for Zeeman splittings. The Zeeman splitting profile of HINSA has a maximum at high velocity and a minimum at low velocity, opposite to the Zeeman splitting profile of CNM1, the closest CNM component at a velocity similar to L1544, which shows positive V at low velocity and negative V at high velocity. From our least-squares fits to V(v), Fig. 2b shows the Zeeman splitting of the HINSA and the total Zeeman profile of the five components, and Fig. 3 shows the individual Zeeman splittings and Blos of the components. The HINSA Zeeman effect gives Blos = +3.8 ± 0.3 μG, and the H I Zeeman effect of CNM1 gives Blos = +4.0 ± 1.1 μG. The magnetic field strengths of HINSA and CNM1 are consistent with the results of Blos = +5.8 ± 1.1 μG and Blos = +4.2 ± 1.0 μG obtained from the Zeeman observations towards quasars 3C133 and 3C132, probing the magnetic fields of CNM1 at distances of 17.7′ (0.72 pc) and 174.5′ (7.1 pc) from L1544, respectively27. For the second and third CNM components (CNM2 and CNM3) along the line of sight, our results of Blos,CNM2 = −7.6 ± 1.0 μG and Blos,CNM3 = +2.9 ± 0.4 μG are also consistent with the results of Blos,CNM2 = −9.6 ± 6.3 μG and Blos,CNM3 = −0.3 ± 1.7 μG towards quasar 3C13327.Fig. 3: The individual V(v) profiles for the HINSA, CNM and WNM components.In each panel, the red profile represents the fitted Zeeman profile of the component and the black profile represents the observed V(v) subtracted by the fitted Zeeman profiles of the other four components. The CNM and WNM Zeeman profiles include the absorption from the CNM components that lie in front but do not include the absorption from the HINSA. The sum of the red profiles of the five components is the black dashed profile in Fig. 2b.Comparing the Zeeman observations of HINSA, OH and H I tracing the CNM1 and the molecular envelope of L1544, it is clear that the magnetic fields at distances of 0.15 pc, 0.24 pc, 0.72 pc and 7.1 pc from the centre all have the same direction of Blos and consistent strengths roughly within the 1σ. This finding is in agreement with the conclusion of a median value of 6 μG in absolute total strength in H I clouds inferred from comprehensive Zeeman surveys15. The HINSA Zeeman effect thus provides a connection between the magnetic fields from H I clouds to molecular clouds. The H I emission components (CNM1, CNM2 and CNM3) and the H I absorption towards 3C133 and 3C132 trace the CNM with a kinematic temperature of about 100 K (ref. 27) and a number density of about 40 cm−3 (ref. 15), whereas the HINSA and OH observations trace the envelope of about 10–15 K (ref. 4) and about 103 cm−3 (refs. 5,15). Despite the one to two orders of magnitude change in both temperature and density in the phase transition from the atomic CNM to the molecular envelope, the Zeeman observations reveal a magnetic field that is coherent in both direction and strength across multi-scales and multi-phases of the interstellar medium. To constrain the uniformity of the coherent magnetic field, our likelihood analysis of the HINSA, OH and H I Zeeman measurements suggests a Gaussian distribution of the Blos with a mean strength of B0 = +4.1 ± 1.6 μG and an intrinsic spread of ({sigma }_{0}={1.2}_{-0.6}^{+1.2},{rm{mu }}{rm{G}}), a significantly better constraint than the previous estimation of ({B}_{0}=+{4}_{-8}^{+10},{rm{mu }}{rm{G}}) based on only the OH results18.It is well known that the progenitor of molecular gas, the atomic CNM, is strongly magnetized, as measured by the dimensionless mass-to-magnetic-flux ratio λ in units of the critical value of 2πG1/2 (λ = 7.6 × 10−21[NH2 (cm−2)][Btot (μG)]−1, where NH2 is the column density of H2 gas and Btot is the total magnetic field strength1), which is well below unity (that is, magnetically subcritical)28. However, the immediate progenitors of stars, the prestellar cores of molecular clouds such as the L1554 core, are observed to be magnetically supercritical (λ > 1)24, which is required for the self-gravity to overwhelm the magnetic support and form stars through gravitational collapse. When and how the transition from the magnetically subcritical CNM that is incapable of forming stars through direct gravitational collapse to the supercritical star-forming cores occurs is a central unresolved question in star formation.Our HINSA Zeeman observations can be used to address this question. Using the physical parameters of the clouds (Table 1) and the statistically most probable value of Btot = 2Blos, the λ of CNM1 is about 0.10–0.18, consistent with previous results28. The λ of the envelope and core of L1544 core is 2.5–3.5, which is well above unity, indicating that the transition to magnetic supercriticality has already occurred. We further consider the relative values of λ between CNM1 and L1544 to avoid the geometrical correction from Blos to Btot (ref. 16), assuming that the inclination angles of the magnetic fields in the L1544 core and envelope are similar. Therefore, the molecular envelope of the L1544 core traced by HINSA is at least 13 times less magnetized relative to its mass compared with its ambient CNM. This is different from the ‘classic’ theory of low-mass star formation, which envisions the transition from magnetic subcriticality to supercriticality occurring as the supercritical core forms out of the magnetically supported (subcritical) envelope13,14. Our results suggest that the transition from magnetic subcriticality to supercriticality occurs earlier, during the formation of the molecular envelope, favouring the more rapidly evolving scenario of core formation and evolution for L15448 over the slower, magnetically retarded scenario9. In other words, by the time that the molecular envelope is formed, the problem of excessive magnetic flux as a fundamental obstacle to gravitational collapse and star formation is already resolved. This early reduction of flux relative to mass is unlikely owing to the ‘classical’ scenario where gravity drives neutrals through ions (and the magnetic field tied to them) in a process called ‘ambipolar diffusion’ because the CNM is not self-gravitating. The coherent magnetic fields reviewed here provide a
https://www.nature.com/articles/s41586-021-04159-x
An early transition to magnetic supercriticality in star formation
