A statistical analysis of dust polarization properties in ALMA observations of Class 0 protostellar cores

Abstract

Context. Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores. Aims. In an effort to improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars and carried out a comprehensive statistical analysis of dust polarization quantities. Methods. We analyze the statistical properties of the polarization fraction Pfrac and the dispersion of polarization position angles S. More specifically, we investigate the relationship between S and Pfrac as well as the evolution of the product S × Pfrac as a function of the column density of the gas in the protostellar envelopes. We compare the observed trends with those found in polarization observations of dust in the interstellar medium and in synthetic observations of non-ideal magneto-hydrodynamic (MHD) simulations of protostellar cores. Results. We find a significant S ∝ Pfrac−0.79 correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index significantly differs from the one observed by Planck in star-forming clouds. The product S × Pfrac, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density (from NH2 = 1022 cm−2 to NH2 = 1025 cm−2), with a mean value of 0.36−0.17+0.10. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not systematically depend on the local conditions such as the local gas density. However, in the lowest-luminosity sources in our sample, we find a hint of less efficient dust grain alignment with increasing column density. Our observations and their comparison with synthetic observations of MHD models suggest that the total intensity versus the polarized dust are distributed at different intrinsic spatial scales, which can affect the statistics from the ALMA observations, for example, by producing artificially high Pfrac. Finally, synthetic observations of MHD models implementing radiative alignment torques (RATs) show that the statistical estimator S × Pfrac is sensitive to the strength of the radiation field in the core. Moreover, we find that the simulations with a uniform perfect alignment (PA) of dust grains yield, on average, much higher S × Pfrac values than those implementing RATs; the ALMA values lie among those predicted by PA, and they are significantly higher than the ones obtained with RATs, especially at large column densities. Conclusions. Ultimately, our results suggest that dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further studies will be needed to understand how more efficient grain alignment via, for example, different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations.

Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

“…14 we show 2D histograms of p frac vs V φ by stacking the results of all the simulations, time-steps and projections, at three wavelengths. This figure shows a clear anti-correlation between these two quantities, indicating that the degree of polarization decreases with an increasing level of disorganization of the magnetic field along the line-of-sight, consistent with the anticorrelation between the polarization fraction and the dispersion of the position angles found in ALMA observations of Class 0 objects (Le Gouellec et al 2020). The fact that both cases display the same behavior, supports the idea that a geometrically depolarized signal due ton the non-organized component of the magnetic field might be greatly responsible for the lower levels of polarization observed in denser regions down to the scales of protostellar envelopes, in agreement with observations at larger scales (Doi et al 2020;Planck Collaboration et al 2015b,a).…”

B-fields in Star-Forming Region Observations (BISTRO): Magnetic Fields in the Filamentary Structures of Serpens Main

“…In addition, varying magnetic field directions along the line of sight depolarize the signal, due to integration over multiple magnetic field components. Studies of alternative polarization mechanisms and dust properties, which are also interesting and important, have been carried out (e.g., Collaboration et al 2020;Le Gouellec et al 2020;Pattle et al 2020b), but are unlikely to apply on the size scales that we consider.…”

Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

“…For all the simulations, and regardless of the protostar mass, we model the central radiation source as a blackbody of 1 L of radius of 1 R located at the sink position. For a more detailed study investigating the dependency of dust polarized emission with varying accretion luminosity, and associated radiation field penetrating the envelopes of Class 0 protostars, see for example Le Gouellec et al (2020) and Le Gouellec et al in prep. We include an external isotropic radiation field of strength G 0 = 1 following the Mathis et al (1983) description. The dust properties are assumed uniform throughout the simulation box and are the same as those in Valdivia et al (2019), namely a gas-to-dust ratio of 100, with a composition of 62.5 per cent astronomical silicates and 37.5 per cent graphite grains, which matches the Galactic extinction curve (Mathis et al 1977).…”