Secret ingredient exposed

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by Christopher M. Johns-Krull from Nature (2014) doi:10.1038/nature13932

Astronomers have suspected for some time that magnetic fields are a key ingredient in the accretion of material that surrounds young stars. New observations have just begun to reveal these fields in action.

Most stars are born surrounded by disks of gas and dust, and it is in these disks that planets form. However, the gas and dust that makes up these disks (Fig. 1) does not all go into forming planets. Indeed, much of the disk material slowly falls, or accretes, onto the newly formed star, setting its final mass. For many years, astronomers have studied such accretion disks and measured the rate at which the material accretes onto young stars, but exactly why such accretion occurs so efficiently has remained elusive. Over the past several years, researchers have started to zero in on a solution in theoretical studies for the evolution of an accretion disk that involves the action of magnetic fields. Unfortunately, until just this year there were no observations of the required magnetic fields in the disks. In a paper published on Nature’s website, Stephens et al.1 report how they have now clearly detected magnetic fields in one such disk.

These visible-light images show disks of gas and dust (dark rings) around four infant stars (central bright regions) in the Orion Nebula, a star-forming molecular cloud about 400 parsecs away from Earth. The dust in the disks, which are surrounded by hot gas from the nebula, makes them look dark at visible wavelengths. Magnetic fields threading these disks, now observed directly by Stephens et al.(1) in the disk of the young star HL Tau, are probably the main element causing much of the observed disk material to accrete onto the stars. Each square region shown is about 260 billion kilometres across.

These visible-light images show disks of gas and dust (dark rings) around four infant stars (central bright regions) in the Orion Nebula, a star-forming molecular cloud about 400 parsecs away from Earth. The dust in the disks, which are surrounded by hot gas from the nebula, makes them look dark at visible wavelengths. Magnetic fields threading these disks, now observed directly by Stephens et al.(1) in the disk of the young star HL Tau, are probably the main element causing much of the observed disk material to accrete onto the stars. Each square region shown is about 260 billion kilometres across.

The key to understanding how an accretion disk works is to figure out what causes some of the material to spiral inward and eventually merge onto the star. Viscosity in the disk is one possible mechanism. At each point in the disk, the material slightly inside this position moves around the star just a bit faster than the material just outside the given point. This difference in speed results in viscosity, a type of friction, that slightly slows down the material on the inside, causing it to fall somewhat closer to the star. This viscosity, acting at all radii in the disk, leads to the accretion of some of the disk material onto the central star. The greater the viscosity, the more rapid the accretion of material. The trouble is, although astronomers can estimate the viscosity in disks on the basis of the temperature and density of the material there, the result fails by many orders of magnitude to produce the rate of accretion that is observed(2, 3).
Magnetic fields, through a process known as the magnetorotational instability(4), have been proposed as a possible means by which disks accrete material. If the material in the disk is threaded through by magnetic fields, then different regions of the disk will be connected to each other by these fields. Although the temperature of the disk is relatively low compared with that of the star, the material is warm enough for a few ions and free electrons to form. The radiation from the star can also cause ion formation. Ions are forced to move with magnetic fields — essentially, they are tied to the field lines. Two ions at slightly different radii in the disk, tied to the same field line, will exert forces on each other that are transmitted through the magnetic field. Effectively, the magnetic fields act as springs holding the ions together. The ion closer to the star should be moving faster in its orbit than the ion slightly farther out, but the field tying them together causes the inner particle to slow down a little and the outer particle to speed up, in exactly the same way as the viscosity described above. However, this magnetorotational-instability mechanism is much better at causing accretion than standard viscosity, and so seems to be a promising solution to the problem of how accretion disks work. But the question has been: are there magnetic fields in the disks around young stars?
Yes, according to Stephens and colleagues’ study. The authors detected the magnetic fields in the disk of a young star by using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) to measure the polarization of the light (the preferred direction in which the light’s electric field oscillates) that is emitted by the dust in the disk. Dust particles in interstellar space and in the disks of young stars are not perfect spheres — they are somewhat oblong. They also spin, and usually have a small amount of electric charge, like the ions that are present in the gas. The result of their spinning oblong shape and electric charge is that the dust grains generally align their long axes perpendicular to the magnetic field(5). This then means that they will emit polarized light.
It is more likely that the light emitted by a dust grain will have its electric field aligned with the long axis of the grain. If the dust grains all have random alignment relative to each other, the resulting total emission of radiation will have no preferred orientation and no polarization. By contrast, if most or all of the dust grains are aligned with each other owing to a magnetic field, the total emission will be polarized. This is exactly what Stephens et al. saw in the disk surrounding the young star HL Tau. Furthermore, the authors were able to spatially map the disk around HL Tau and measure the polarization at several locations in the disk, making these observations unusually powerful for testing our understanding of accretion disks. Similar observations were also made in 2014 of an even younger star(6); however, the orientation of its disk and potential confusion from the surrounding environment did not allow as clear a study of the magnetic-field structure as that obtained by Stephens et al. in the disk of HL Tau.
This result is exciting because it strongly suggests that magnetorotational instability may be the long-sought answer to just how accretion disks work. However, this is far from the last word on this astrophysical puzzle. The theoretical models of this phenomenon generally predict that the magnetic fields in the disk should be wrapped around the star, basically following the motion of the disk material as it orbits the star. Unfortunately, this is not exactly what Stephens and colleagues observed. Their polarization measurements of HL Tau’s disk indicate that the magnetic fields are all more or less pointing in the same direction instead of wrapping around the star. This is a mystery with no good explanation at this time.
So what does the future hold for accretion-disk physics? A new, more sensitive radio-telescope array than the one used by Stephens and colleagues, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, has begun operations. This facility will provide detailed views of the structure and magnetic fields of disks around young stars. We can hope that this will produce observational results that will also spur new theoretical investigations into the role of magnetic fields in accretion disks. Our understanding of this phenomenon is not complete yet, but it has just taken a dramatic step forward.


(1) Stephens, I. W. et al. Nature (2014).
(2) Valenti, J. A., Basri, G. & Johns, C. M. Astron. J. 106, 2024–2050 (1993).
(3) Gullbring, E., Hartmann, L., Briceno, C. & Calvet, N. Astrophys. J. 492, 323–341 (1998).
(4) Balbus, S. A. & Hawley, J. F. Rev. Mod. Phys. 70, 1–53 (1998).
(5) Lazarian, A. J. J. Quant. Spectrosc. Radiat. Transf. 106, 225–256 (2007).
(6) Rao, R., Girart, J. M., Lai, S.-P. & Marrone, D. P. Astrophys. J. 780, L6 (2014).

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