Magnetically regulated collapse in the B335 protostar?

Abstract

Context. Whether or not magnetic fields play a key role in dynamically shaping the products of the star formation process is still largely debated. For example, in magnetized protostellar formation models, magnetic braking plays a major role in the regulation of the angular momentum transported from large envelope scales to the inner envelope, and is expected to be responsible for the resulting protostellar disk sizes. However, non-ideal magnetohydrodynamic effects that rule the coupling of the magnetic field to the gas also depend heavily on the local physical conditions, such as the ionization fraction of the gas. Aims. The purpose of this work is to observationally characterize the level of ionization of the gas at small envelope radii and to investigate its relation to the efficiency of the coupling between the star-forming gas and the magnetic field in the Class 0 protostar B335. Methods. We obtained molecular line emission maps of B335 with ALMA, which we use to measure the deuteration fraction of the gas, R D , its ionization fraction, χ e , and the cosmic-ray ionization rate, ζ CR , at envelope radii ≲1000 au. Results. We find large fractions of ionized gas, χ e ≃ 1 − 8 × 10 −6 . Our observations also reveal an enhanced ionization that increases at small envelope radii, reaching values up to ζ CR ≃ 10 −14 s −1 at a few hundred astronomical units (au) from the central protostellar object. We show that this extreme ζ CR can be attributed to the presence of cosmic rays accelerated close to the protostar. Conclusions. We report the first resolved map of ζ CR at scales ≲ 1000 au in a solar-type Class 0 protostar, finding remarkably high values. Our observations suggest that local acceleration of cosmic rays, and not the penetration of interstellar Galactic cosmic rays, may be responsible for the gas ionization in the inner envelope, potentially down to disk-forming scales. If confirmed, our findings imply that protostellar disk properties may also be determined by local processes that set the coupling between the gas and the magnetic field, and not only by the amount of angular momentum available at large envelope scales and the magnetic field strength in protostellar cores. We stress that the gas ionization we find in B335 significantly stands out from the typical values routinely used in state-of-the-art models of protostellar formation and evolution. If the local processes of ionization uncovered in B335 are prototypical to low-mass protostars, our results call for a revision of the treatment of ionizing processes in magnetized models for star and disk formation.

First ALMA Maps of Cosmic-Ray Ionization Rate in High-mass Star-forming Regions

“…The latter span values in between 10 −16 and 10 −13 s −1 , i.e., from 1 to 2 orders of magnitude larger than the ones obtained with the method of Bovino et al (2020). In addition, they tend to overestimate the upper limits provided by the CR propagation model with All the values reported in Figures 4(a), (b), and (c) are similar to those obtained by Cabedo et al (2023), even if the analyzed samples show very different physical conditions. From this result, we conclude that the method proposed by Caselli et al (1998) shows significant limitations in its applicability.…”

“…However, recent studies have used this method to estimate the H ion 2 z in a protostellar source (Cabedo et al 2023), where the physical conditions are far from the range of applicability of the original formula (determined at 10 K and for early stages of star-forming regions). For the Bok Globule B335, which hosts a Class-0 protostar with already developed outflow, Cabedo et al (2023) as a local effect of the protostellar activity, which, however, would also affect f D that will decrease at T gas > 20 K, when CO is efficiently desorbed in the gas phase. However, their resulting f D present values of about 80 at the position of the protostellar embryo, i.e., an unexpected extremely high depleted region.…”

“…It is also worth noting that any other process that might affect the estimates of f D (such as the UV photodissociation of CO due to protostellar activity or the CO conversion to other species such as HCO + by CRs) would prevent the application of the method proposed by Caselli et al (1998), since the underlying assumptions imply that only a fraction of C and O (i.e., 1/f D ) remains in the gas phase, while the rest is frozen, in the form of CO, on the surface of the grains. To understand the results reported by Cabedo et al (2023), we then decided to apply the formula proposed by Caselli et al (1998) to our sources, which fall in the correct range of applicability being prestellar in nature. From the comparison between a prestellar and a protostellar source we can then inspect the validity of the method and its reliability.…”

“…The limitations of this formulation are also discussed in Caselli et al (2002) and lie primarily in the lack of some terms in the chemistry of H 2 D + and the reactions involving electrons, and it appears to greatly overestimate x(e) and the associated ζ ion H2 . However, recent studies have used this method to estimate the ζ ion H2 in a protostellar source (Cabedo et al 2023), where the physical conditions are far from the range of applicability of the original formula (determined at 10 K and for early stages of star-forming regions). For the Bok Globule B335, which hosts a Class 0 protostar with already developed outflow, Cabedo et al (2023) obtained a ζ ion H2 ∼7×10 −14 s −1 , by assuming a T ex = 25 K for each species.…”

Physical conditions for dust grain alignment in Class 0 protostellar cores

“…This is illustrated by the dichotomy found in BHR71 IRS2 between the outflow cavity and inner envelope traced by CO and C 18 O, respectively, and the cold envelope traced by N 2 D + where CO freezes out onto dust grains (Hull et al 2020; see also the unpolarized southern region of B335 in Fig. 1 compared to these same molecular lines presented in Cabedo et al 2021Cabedo et al , 2023. The dust polarization detected in the outflow cavity walls of IRS2 lies precisely between the regions where the N 2 D + and CO emission lines peak, suggesting that dust grains cannot become aligned in these areas.…”