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Abstract Ionization drives important chemical and dynamical processes within protoplanetary disks, including the formation of organics and water in the cold midplane and the transportation of material via accretion and magnetohydrodynamic flows. Understanding these ionization-driven processes is crucial for understanding disk evolution and planet formation. We use new and archival Atacama Large Millimeter/submillimeter Array observations of HCO+, H13CO+, and N2H+to produce the first forward-modeled 2D ionization constraints for the DM Tau protoplanetary disk. We include ionization from multiple sources and explore the disk chemistry under a range of ionizing conditions. Abundances from our 2D chemical models are postprocessed using non-LTE radiative transfer, visibility sampling, and imaging, and are compared directly to the observed radial emission profiles. The observations are best fit by a modestly reduced cosmic-ray ionization rate (ζCR∼10−18s−1) and a hard X-ray spectrum (hardness ratio = 0.3), which we associate with stellar flaring conditions. Our best-fit model underproduces emission in the inner disk, suggesting that there may be an additional mechanism enhancing ionization in DM Tau’s inner disk. Overall, our findings highlight the complexity of ionization in protoplanetary disks and the need for high-resolution multiline studies.
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Avalanches and the Distribution of Reconnection Events in Magnetized Circumstellar Disks
Abstract Cosmic rays produced by young stellar objects can potentially alter the ionization structure, heating budget, chemical composition, and accretion activity in circumstellar disks. The inner edges of these disks are truncated by strong magnetic fields, which can reconnect and produce flaring activity that accelerates cosmic radiation. The resulting cosmic rays can provide a source of ionization and produce spallation reactions that alter the composition of planetesimals. These reconnection and particle acceleration processes are analogous to the physical processes that produce flaring in and the heating of stellar coronae. Flaring events on the surface of the Sun exhibit a power-law distribution of energy, reminiscent of those measured for earthquakes and avalanches. Numerical lattice reconnection models are capable of reproducing the observed power-law behavior of solar flares under the paradigm of self-organized criticality. One interpretation of these experiments is that the solar corona maintains a nonlinear attractor—or “critical”—state by balancing energy input via braided magnetic fields and output via reconnection events. Motivated by these results, we generalize the lattice reconnection formalism for applications in the truncation region of magnetized disks. Our numerical experiments demonstrate that these nonlinear dynamical systems are capable of both attaining and maintaining criticality in the presence of Keplerian shear and other complications. The resulting power-law spectrum of flare energies in the equilibrium attractor state is found to be nearly universal in magnetized disks. This finding indicates that magnetic reconnection and flaring in the inner regions of circumstellar disks occur in a manner similar to the activity on stellar surfaces. These results, in turn, have ramifications for the spallation-driven injection of radionuclides in planetesimals, disk ionization, and the subsequent planetary formation process.
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- Award ID(s):
- 2303553
- PAR ID:
- 10444007
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 954
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 15
- Size(s):
- Article No. 15
- Sponsoring Org:
- National Science Foundation
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