skip to main content

Explore Research Products in the NSF-PAR

Title: On the importance of disc chemistry in the formation of protoplanetary disc rings
ABSTRACT

Radial substructures have now been observed in a wide range of protoplanetary discs (PPDs), from young to old systems; however, their formation is still an area of vigorous debate. Recent magnetohydrodynamic (MHD) simulations have shown that rings and gaps can form naturally in PPDs when non-ideal MHD effects are included. However, these simulations employ ad hoc approximations to the magnitudes of the magnetic diffusivities in order to facilitate ring growth. We replace the parametrization of these terms with a simple chemical network and grain distribution model to calculate the non-ideal effects in a more self-consistent way. We use a range of grain distributions to simulate grain formation for different disc conditions. Including ambipolar diffusion, we find that large grain populations (>1 $\mu$m), and those including a population of very small polyaromatic hydrocarbons (PAHs) facilitate the growth of periodic, stable rings, while intermediate-sized grains suppress ring formation. Including Ohmic diffusion removes the positive influence of PAHs, with only large grain populations still producing periodic ring and gap structures. These results relate closely to the degree of coupling between the magnetic field and the neutral disc material, quantified by the non-dimensional Elsasser number Λ (the ratio of magnetic forces to Coriolis force). For both the ambipolar-only and ambipolar-ohmic cases, if the total Elsasser number is initially of the order of unity along the disc mid-plane, ring and gap structures may develop.

  more » « less
NSF-PAR ID:
10462818
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
Oxford University Press
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
525
Issue:
4
ISSN:
0035-8711
Page Range / eLocation ID:
p. 5450-5467
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ( , Monthly Notices of the Royal Astronomical Society)
    null (Ed.)
    ABSTRACT Non-ideal magnetohydrodynamic (MHD) effects have been shown recently as a robust mechanism of averting the magnetic braking ‘catastrophe’ and promoting protostellar disc formation. However, the magnetic diffusivities that determine the efficiency of non-ideal MHD effects are highly sensitive to microphysics. We carry out non-ideal MHD simulations to explore the role of microphysics on disc formation and the interplay between ambipolar diffusion (AD) and Hall effect during the protostellar collapse. We find that removing the smallest grain population (≲10 nm) from the standard MRN size distribution is sufficient for enabling disc formation. Further varying the grain sizes can result in either a Hall-dominated or an AD-dominated collapse; both form discs of tens of au in size regardless of the magnetic field polarity. The direction of disc rotation is bimodal in the Hall-dominated collapse but unimodal in the AD-dominated collapse. We also find that AD and Hall effect can operate either with or against each other in both radial and azimuthal directions, yet the combined effect of AD and Hall is to move the magnetic field radially outward relative to the infalling envelope matter. In addition, microphysics and magnetic field polarity can leave profound imprints both on observables (e.g. outflow morphology, disc to stellar mass ratio) and on the magnetic field characteristics of protoplanetary discs. Including Hall effect relaxes the requirements on microphysics for disc formation, so that prestellar cores with cosmic ray ionization rate of ≲2–3 × 10−16 s−1 can still form small discs of ≲10 au radius. We conclude that disc formation should be relatively common for typical prestellar core conditions, and that microphysics in the protostellar envelope is essential to not only disc formation, but also protoplanetary disc evolution. 
    more » « less
  2. ; ; ; ( , Monthly Notices of the Royal Astronomical Society)
    ABSTRACT

    Rings and gaps are commonly observed in the dust continuum emission of young stellar discs. Previous studies have shown that substructures naturally develop in the weakly ionized gas of magnetized, non-ideal MHD discs. The gas rings are expected to trap large mm/cm-sized grains through pressure gradient-induced radial dust–gas drift. Using 2D (axisymmetric) MHD simulations that include ambipolar diffusion and dust grains of three representative sizes (1 mm, 3.3 mm, and 1 cm), we show that the grains indeed tend to drift radially relative to the gas towards the centres of the gas rings, at speeds much higher than in a smooth disc because of steeper pressure gradients. However, their spatial distribution is primarily controlled by meridional gas motions, which are typically much faster than the dust–gas drift. In particular, the grains that have settled near the mid-plane are carried rapidly inwards by a fast accretion stream to the inner edges of the gas rings, where they are lifted up by the gas flows diverted away from the mid-plane by a strong poloidal magnetic field. The flow pattern in our simulation provides an attractive explanation for the meridional flows recently inferred in HD 163296 and other discs, including both ‘collapsing’ regions where the gas near the disc surface converges towards the mid-plane and a disc wind. Our study highlights the prevalence of the potentially observable meridional flows associated with the gas substructure formation in non-ideal MHD discs and their crucial role in generating rings and gaps in dust.

     
    more » « less
  3. ; ; ; ; ( , Monthly Notices of the Royal Astronomical Society)
    ABSTRACT

    Recent high angular resolution ALMA observations have revealed numerous gaps in protoplanetary discs. A popular interpretation has been that planets open them. Most previous investigations of planet gap-opening have concentrated on viscous discs. Here, we carry out 2D (axisymmetric) global simulations of gap opening by a planet in a wind-launching non-ideal MHD disc with consistent thermochemistry. We find a strong concentration of poloidal magnetic flux in the planet-opened gap, where the gas dynamics are magnetically dominated. The magnetic field also drives a fast (nearly sonic) meridional gas circulation in the denser disc regions near the inner and outer edges of the gap, which may be observable through high-resolution molecular line observations. The gap is more ionized than its denser surrounding regions, with a better magnetic field–matter coupling. In particular, it has a much higher abundance of molecular ion HCO+, consistent with ALMA observations of the well-studied AS 209 protoplanetary disc that has prominent gaps and fast meridional motions reaching the local sound speed. Finally, we provide fitting formulae for the ambipolar and Ohmic diffusivities as a function of the disc local density, which can be used for future 3D simulations of planet gap-opening in non-ideal MHD discs where thermochemistry is too computationally expensive to evolve self-consistently with the magneto-hydrodynamics.

     
    more » « less
  4. ; ; ; ( , Astronomy & Astrophysics)
    Dust grains influence many aspects of star formation, including planet formation and the opacities for radiative transfer, chemistry, and the magnetic field via Ohmic, Hall, as well as ambipolar diffusion. The size distribution of the dust grains is the primary characteristic influencing all these aspects. Grain size increases by coagulation throughout the star formation process. In this work, we describe numerical simulations of protostellar collapse using methods described in earlier papers of this series. We compute the evolution of the grain size distribution from coagulation and the non-ideal magnetohydrodynamics effects self-consistently and at low numerical cost. We find that the coagulation efficiency is mostly affected by the time spent in high-density regions. Starting from sub-micron radii, grain sizes reach more than 100 µm in an inner protoplanetary disk that is only 1000 yr old. We also show that the growth of grains significantly affects the resistivities, while also having an indirect effect on the dynamics and angular momentum of the disk. 
    more » « less
  5. ; ; ; ( , Monthly Notices of the Royal Astronomical Society)
    ABSTRACT

    The magnetorotational instability (MRI) has been proposed as the method of angular momentum transport that enables accretion in astrophysical discs. However, for weakly ionized discs, such as protoplanetary discs, it remains unclear whether the combined non-ideal magnetohydrodynamic (MHD) effects of Ohmic resistivity, ambipolar diffusion, and the Hall effect make these discs MRI stable. While much effort has been made to simulate non-ideal MHD MRI, these simulations make simplifying assumptions and are not always in agreement with each other. Furthermore, it is difficult to directly observe the MRI astrophysically because it occurs on small scales. Here, we propose the concept of a swirling gas experiment of weakly ionized argon gas between two concentric cylinders threaded with an axial magnetic field that can be used to study non-ideal MHD MRI. For our proposed experiment, we derive the hydrodynamic equilibrium flow and a dispersion relation for MRI that includes the three non-ideal effects. We solve this dispersion relation numerically for the parameters of our proposed experiment. We find it should be possible to produce a non-ideal MRI in such an experiment because of the Hall effect, which increases the MRI growth rate when the vertical magnetic field is anti-aligned with the rotation axis. As a proof of concept, we also present experimental results for a hydrodynamic flow in an unmagnetized prototype. We find that our prototype has a small, but non-negligible, α-parameter that could serve as a baseline for comparison to our proposed magnetized experiment, which could be subject to additional turbulence from the MRI.

     
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email