Final Report Summary - COOKINGDUSTINDISKS (Forming High-Temperature Solids in Protoplanetary Disks)
In the evolution of our solar system the first solid minerals which formed, and which eventually composed the Earth and other bodies in our solar system, formed in the context of a gaseous early solar nebula. Protoplanetary disks provide analogues for our own early solar system, and our own early solar system provides an example of a protoplanetary disk. We wish to use constraints from our own early solar system to understand protoplanetary disks in general. The solid material left behind from the formation of our solar system tells us about the processing they experienced. To interpret this information, we need to derive the mechanisms that drove the formation and changes in these minerals. With the large-scale understanding of protoplanetary disks from astronomical observations, there remains a significant class of processing mechanisms that are yet to be understood. These are transient, local heating events. This project is directed towards understanding the heating events which result from accretion energy being dissipated to heat, and particularly though the dissipation of magnetic energy. This project is strongly multidisciplinary, being physically based on plasma astrophysics, but requiring practical work in computational astrophysics and theoretical astrophysics, and both addressing and informed by meteoritics and astronomy.
In the context of this project, several new results have been published. In Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks (C.P. McNally, A. Hubbard, C.-C. Yang, M.-M. Mac Low, 2014) we showed for the first time that the magnetic fields in protoplanetary disks can create small regions several hundred Kelvin hotter than the surrounding gas. This size of heating can play an important role in forming the materials found in meteorites. In our efforts to study protoplanetary disks, we often use a framework to study only a small portion of the disk, thus saving dramatically on computing costs. However, these models are subject to many limitations. In an effort to add flexibility and generality to these models, we explored generalizing them in: On Vertically Global, Horizontally Local Models for Astrophysical Disks (C.P. McNally, M.E. Pessah, 2015). The generalizations were applied in understanding the surprising magnetic field structures that occurred in a novel new simulation of protoplanetary disk structure in in Global Simulations of Protoplanetary Disks with Ohmic Resistivity and Ambipolar Diffusion (O. Gressel, N. Turner, R.P. Nelson, C.P. McNally, 2015). Here, a qualitatively different current sheet structure was found to occur, in a largely non-turbulent region driven by structures that could be revealed by the use of our generalized local model. This type of current sheet is very different from the ones first studied when the project began, and are an exciting new possibility for future work. In Rossby wave instability does not require sharp resistivity gradients (W. Lyra, N. Turner, C.P. McNally, 2015) we applied understanding gained though work on the role of magnetic fields in disks to an important current observational problem in protoplanetary disks: the conditions under which a large vortex, capable of trapping dust can be formed. A movie of the main simulation from this work can be found via the web site below. These vortices are capable of trapping dust, and concentrating it, and in turn possibly forming the planetesimals, comets, and asteroids where high-temperature solids are found today.
Finally, an exciting new result has emerged from the work on current sheets deep inside protoplanetary disks. It has long been understood that bright light shining on objects can exert a variety of forces, collectively referred to as radiometer forces. In low-density gas, such as a protoplanetary disks, dust grains and agglomerates can feel a significant force from an effect known as ‘photophoresis’, in which the particle is heated more on one side than another by light, and gas molecules rebounding from the hot side rebound with more momentum than those rebounding from the cold side. This imbalance in the momentum results in a force exerted on the particle. In protoplanetary disks, the source of this light has been considered to be the central star, so it was thought to only occur on the surface or inner edge, not deep in the dark center of the disk. Our advance has been to realize that the light radiated by hot current sheets in the protoplanetary disk can drive this effect, even deep in the disk (C. Loesche, G. Wurm, C.P. McNally, M.-M. Mac Low, A. Hubbard, T. Kelling, J. Teiser, D.S. Ebel 2015, Submitted). This can cause size-sorting, material-sorting, and drive collisions between of grains which then form meteorites, and limit the time this dust sits in a heated environment. Furthermore, we were able to derive particularly clean mathematical formulas for the effect deep inside the disk (Photophoresis in a Dilute, Optically Thick Medium and Dust Motion in Protoplanetary Disks, C.P. McNally, A. Hubbard 2015). These can be described as showing that “hot fog moves rocks”. Indeed, even the gradual vertical change in the temperature expected in the inner part of a protoplanetary disk may be capable of levitating small grains by photophoresis, so rocks may even fly.
Website: http://www.nbi.dk/~cmcnally/mciif/index.html
Colin McNally cmcnally@nbi.ku.dk
In the context of this project, several new results have been published. In Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks (C.P. McNally, A. Hubbard, C.-C. Yang, M.-M. Mac Low, 2014) we showed for the first time that the magnetic fields in protoplanetary disks can create small regions several hundred Kelvin hotter than the surrounding gas. This size of heating can play an important role in forming the materials found in meteorites. In our efforts to study protoplanetary disks, we often use a framework to study only a small portion of the disk, thus saving dramatically on computing costs. However, these models are subject to many limitations. In an effort to add flexibility and generality to these models, we explored generalizing them in: On Vertically Global, Horizontally Local Models for Astrophysical Disks (C.P. McNally, M.E. Pessah, 2015). The generalizations were applied in understanding the surprising magnetic field structures that occurred in a novel new simulation of protoplanetary disk structure in in Global Simulations of Protoplanetary Disks with Ohmic Resistivity and Ambipolar Diffusion (O. Gressel, N. Turner, R.P. Nelson, C.P. McNally, 2015). Here, a qualitatively different current sheet structure was found to occur, in a largely non-turbulent region driven by structures that could be revealed by the use of our generalized local model. This type of current sheet is very different from the ones first studied when the project began, and are an exciting new possibility for future work. In Rossby wave instability does not require sharp resistivity gradients (W. Lyra, N. Turner, C.P. McNally, 2015) we applied understanding gained though work on the role of magnetic fields in disks to an important current observational problem in protoplanetary disks: the conditions under which a large vortex, capable of trapping dust can be formed. A movie of the main simulation from this work can be found via the web site below. These vortices are capable of trapping dust, and concentrating it, and in turn possibly forming the planetesimals, comets, and asteroids where high-temperature solids are found today.
Finally, an exciting new result has emerged from the work on current sheets deep inside protoplanetary disks. It has long been understood that bright light shining on objects can exert a variety of forces, collectively referred to as radiometer forces. In low-density gas, such as a protoplanetary disks, dust grains and agglomerates can feel a significant force from an effect known as ‘photophoresis’, in which the particle is heated more on one side than another by light, and gas molecules rebounding from the hot side rebound with more momentum than those rebounding from the cold side. This imbalance in the momentum results in a force exerted on the particle. In protoplanetary disks, the source of this light has been considered to be the central star, so it was thought to only occur on the surface or inner edge, not deep in the dark center of the disk. Our advance has been to realize that the light radiated by hot current sheets in the protoplanetary disk can drive this effect, even deep in the disk (C. Loesche, G. Wurm, C.P. McNally, M.-M. Mac Low, A. Hubbard, T. Kelling, J. Teiser, D.S. Ebel 2015, Submitted). This can cause size-sorting, material-sorting, and drive collisions between of grains which then form meteorites, and limit the time this dust sits in a heated environment. Furthermore, we were able to derive particularly clean mathematical formulas for the effect deep inside the disk (Photophoresis in a Dilute, Optically Thick Medium and Dust Motion in Protoplanetary Disks, C.P. McNally, A. Hubbard 2015). These can be described as showing that “hot fog moves rocks”. Indeed, even the gradual vertical change in the temperature expected in the inner part of a protoplanetary disk may be capable of levitating small grains by photophoresis, so rocks may even fly.
Website: http://www.nbi.dk/~cmcnally/mciif/index.html
Colin McNally cmcnally@nbi.ku.dk