An ALMA-resolved View of 7000 au Protostellar Gas Ring around the Class I Source CrA-IRS 2 as a Possible Sign of Magnetic Flux Advection

Abstract

Transferring a significant fraction of the magnetic flux from a dense cloud core is essential in the star formation process. A ringlike structure produced by magnetic flux loss has been predicted theoretically, but no observational identification has been presented. We have performed ALMA observations of the Class I protostar IRS 2 in the Corona Australis star-forming region and resolved a distinctive gas ring in the C18O (J = 2–1) line emission. The center of this gas ring is ∼5000 au away from the protostar, with a diameter of ∼7000 au. The radial velocity of the gas is ≲ 1 km s−1 blueshifted from that of the protostar, with a possible expanding feature judged from the velocity-field (moment 1) map and position–velocity diagram. These features are either observationally new or have been discovered but not discussed in depth because they are difficult to explain by well-studied protostellar phenomena such as molecular outflows and accretion streamers. A plausible interpretation is a magnetic wall created by the advection of magnetic flux, which is theoretically expected in the Class 0/I phase during star formation as a removal mechanism of magnetic flux. Similar structures reported in the other young stellar sources could likely be candidates formed by the same mechanism, encouraging us to revisit the issue of magnetic flux transport in the early stages of star formation from an observational perspective.

An ALMA Search for Substructure and Fragmentation in Starless Cores in Orion B North

“…Nonetheless these low-level features may be indicative of streamer-like objects spanning thousands of astronomical units, recently seen in other star-forming systems (e.g., Pineda et al 2020Pineda et al , 2023Murillo et al 2022;Valdivia-Mena et al 2023), or an envelope associated with filamentary structure (Tobin et al 2010). Recent studies have also indicated that the presence of ring-like structures around protostellar sources could be caused by magnetic flux removal as dense cores collapse during early stages of star formation (Tokuda et al 2023), which may also be morphologically consistent with our observations.…”

Discovery of Asymmetric Spike-like Structures of the 10 au Disk around the Very Low-luminosity Protostar Embedded in the Taurus Dense Core MC 27/L1521F with ALMA

“…While MC 27 presents a highly peculiar system, it is crucial to examine whether the star formation scenario that we devise here in Section 4.2 is generally applicable. Prior to this MC 27 work, a ring structure with a diameter of ∼7000 au possibly driven by interchange instability was reported in the Corona Australis (CrA) IRS 2 Class I system (Tokuda et al 2023). Unlike MC 27, the single ring feature suggests a solitary interchange instability event.…”

“…The main content observations for this study were conducted more than 6 yr before the writing of this paper. We have long regretted not being able to release these valuable ALMA data (2017.1.00480.S) in publication format, but the serendipitous discovery of a large-scale gas ring in the Corona Australis region (Tokuda et al 2023) provided a breakthrough for our astronomical interpretations in this study. We would like to express our gratitude once again to the objects, including MC 27, that have shown us the dynamic aspects of star formation.…”

“…Meanwhile, the arcs themselves expand outward on the order of the sound speed, c s (Zhao et al 2011;Matsumoto et al 2017). If slight density asymmetries existed during the protostar formation, the interchange-instability-driven arc would expand toward a specific direction (Krasnopolsky et al 2012;Tokuda et al 2023). Assuming their expansion speed as c s , 0.2 km s −1 at 10 K, the dynamical time for these structures would be ∼5 × 10 5 yr ( = 2000 au/0.2 km s −1 ).…”

“…The obtained images reveal a relatively simple structure that resolves the 10 au disk with additional spike-like features. Incorporating a recent observational discovery, which may be relevant to a magnetic flux transport phenomenon by interchange instability (Tokuda et al 2023), we propose a hypothesis for forming and maintaining compact disks in a young phase of star formation. In Section 2, we describe the observations and data reduction, then present 1.3 mm continuum imaging results in Section 3.…”