ScienceWeek

ASTROPHYSICS: STAR DISKS AND PLANETARY SYSTEMS

The following points are made by Jane S. Greaves (Science 2005 307:68):

1) Disks around stars are flattened concentrations of gas and particles that follow orbits much like those of planets. A star forms from the collapse of a self-gravitating core within a molecular gas cloud, which generally has some rotation. Conservation of angular momentum implies that as the core contracts, its rotation must speed up, and this prevents the particles from falling in a direct line onto the growing star. Apart from a few particles along the orbital poles that can fall in because they have no rotation, all the others must first settle down onto an equatorial belt; it is this process that results in a circumstellar disk.

2) Disks are ubiquitous in astronomy because conservation of angular momentum is a universal law and almost every astronomical object starts with some rotation, hence contracting toward a plane rather than a point. Thus, disks occur frequently and on a wide range of scales, from the ensembles of billions of stars forming a flat spiral galaxy down to tiny energetic accretion zones that surround black holes [1].

3) Disks around stars were recognized as a likely possibility long before they could be imaged. Recognition that the orbits of the Sun's planets were all close to one plane led to ideas that they could have formed out of a flat disk. In the modern era, nebulosity began to be seen around stars recognized as young from their high luminosities (i.e., still contracting and so having larger emitting surfaces), and in some cases central disks were suspected. In the 1960s, excess infrared (IR) emission was detected toward some young stars, leading to suggestions that particles absorb optical stellar light and reradiate it at longer wavelengths. Astronomers realized [2] that disks around young stars could both produce a warm excess of IR emission and regulate the stellar angular momentum. By the 1970s, disks were being used as explanations for polarized light scattered in the vicinity of stars, orbital motion of gas concentrations with maser emission, and channels for narrow stellar jets [3-5]. However, it was only in the 1980s that true disk images were obtained; the first really compelling one was the star Beta Pictoris. This thin disk of light-scattering particles, seen edge-on from Earth, brought into prominence the idea that planetary cores could form by grain coagulation in disks.

4) In retrospect, it seems obvious that the material to make a star cannot be spherically spread out, or we would not be able to see the light of the star itself. A flattened disk allows light out along our line of sight, so only disks seen edge-on block the starlight. However, the term "shell" instead of "disk" appeared in the literature into the 1980s, and it should be remembered that, when "disk" is used now, this may be an assumption when no image exists. It is not certain that all stars are born from and within disks, although it is hard to explain their formation otherwise. More exotic processes could occur, such as the merger of two protostars. However, observations of young clusters of stars find over 80% have IR excesses, suggesting that evolution within a disk is typical.

References (abridged):

1. R. Narayan, E. Quataert, Science 307, 77 (2005)

2. S.-S. Huang, Astron. J. 72, 804 (1967)

3. M. Breger, H. M. Dyck, Astrophys. J. 175, 127 (1972)

4. D. van Blerkom, L. Auer, Astrophys. J. 204, 775 (1976)

5. C. J. Lada, in Submillimeter Wave Astronomy, J. E. Beckman, J. P. Phillips, Eds. (Cambridge Univ. Press, Cambridge, 1982), p. 175

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EVIDENCE FOR DUST GRAIN GROWTH IN YOUNG CIRCUMSTELLAR DISKS

The following points are made by H.B. Throop et al (Science 2001 292:1686):

1) The growth of dust grains orbiting young stars represents the first stage of planet formation (1). However, stars born in massive star-forming regions such as the Orion nebula are heated by intense ultraviolet (UV) radiation from nearby O and B stars, and the gas and dust in their disks can be lost in less than 10^(5) years (2). Planet formation in such environments may therefore be inhibited if it requires substantially longer time than this (3). But, if growth to large particles can occur before removal of the gas and small particles, planets may nevertheless form from these disks.

2) The authors present a study in which visual and near-infrared (IR) wavelength images obtained with the Hubble Space Telescope (HST) are used to demonstrate that particles in Orion's largest disk have grown to radii larger than 5 microns. Furthermore, the absence of millimeter-wavelength emission may provide evidence that grains have grown to sizes larger than a few millimeters. The authors develop a grain evolution model incorporating the effects of photo-ablation that demonstrates that the time scale for grain growth can be shorter than the photo-evaporation time. It is thought that the majority of stars in the Galaxy form in photo-evaporating regions such as the Orion nebula (4); if this is true, then giant planets and Kuiper belts of icy bodies around stars are probably rare unless they are formed very rapidly.

3) Solar system-sized circumstellar disks in the Orion nebula were first inferred from radio observations of dense ionized regions surrounding young low-mass stars (5). HST subsequently yielded images of extended circumstellar material surrounding over half of the observed 300 young low-mass stars in the core of the Orion nebula. Most of these "proplyds" consist of comet-shaped ionized envelopes pointing directly away from the brightest stars in the nebula. Proplyds are believed to contain evaporating circumstellar disks, and over 40 disks have been resolved on HST images. More than 25 are found inside ionized envelopes, whereas 15 are seen purely in silhouette against the background light of the nebula.

4) In summary: Hundreds of circumstellar disks in the Orion nebula are being rapidly destroyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form planetary systems. Nevertheless, the first stage of planet formation -- the growth of dust grains into larger particles -- may have begun in these systems. Observational evidence for these large particles in Orion's disks is presented. A model of grain evolution in externally irradiated protoplanetary disks has been developed by the authors and predicts rapid particle size evolution and sharp outer disk boundaries. The authors discuss implications for the formation rates of planetary systems.

References (abridged):

1. S. V. W. Beckwith, T. Henning, Y. Nakagawa, in Protostars and Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. (Univ. of Arizona Press, Tucson, AZ, 2000), pp. 533-558.

2. C. J. Henney, C. R. O'Dell, Astron. J. 118, 2350 (1999).

3. H. Stoerzer and D. Hollenbach, Astrophys. J. 495, 853 (1998).

4. F. M. Walter, J. M. Alcala, R. Neuhauser, M. Sterzik, S. J. Wolk, in Protostars and Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. (Univ. of Arizona Press, Tucson, AZ, 2000), pp. 273-298.

5. E. Churchwell, M. Felli, D. O. S. Wood, M. Massi, Astrophys. J. 321, 516 (1987).

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FORMATION OF GIANT PLANETS BY FRAGMENTATION OF PROTOPLANETARY DISKS

The following points are made by L. Mayer et al (Science 2002 298:1756):

1) Approximately 100 extrasolar planets have been detected by the wobble they induce on their star (1,2). Their masses range from about one Jupiter mass (MJ) to more than 10 MJ and have orbits ranging from nearly circular to very eccentric. In the standard core-accretion model, giant planets might require longer than 10^(6) years to form (3,4), which could exceed observed disk lifetimes (5). In particular, more than 80% of the stars in the Galaxy probably formed in dense clusters like those in the Orion nebula, where the ultraviolet radiation of bright stars can ablate the gaseous disk in far less than a million years (5).

2) Hence giant planet formation must occur quickly, or such planets would be rare. Even in the case where a large solid core is assembled rapidly enough, torques acting between the disk and the protoplanets are believed to induce its complete inward migration in a few thousand years. Planets could therefore sink toward the star before being able to accrete the large gaseous masses observed.

3) Alternatively, giant planets could coagulate directly in the gas component as a result of gravitational instabilities in a cold disk with a mass comparable to that adopted in the core-accretion model. Simulations done with codes that solve the hydrodynamical equations on a fixed grid show that slightly perturbed disks form strong spiral arms and overdensities at R > 10 astronomical units (AU), where the temperature can be lower than 60 K. The trigger of the instability might come from material of the protostellar cloud infalling onto the disk. If these condensations are long-lasting and can contract to planetary densities, gravitational instability would be the prevailing formation mechanism for giant planets because it takes less than a thousand years. Solid cores with masses as low as currently estimated for Jupiter (between 0 and 10 Earth masses) could then form inside the gaseous protoplanets due to dust and planetesimals driven there by local pressure gradients in a few thousand years.

4. In summary: The authors report they have studied the evolution of gravitationally unstable protoplanetary gaseous disks with the use of three-dimensional smoothed particle hydrodynamics simulations with unprecedented resolution. The authors have considered disks with initial masses and temperature profiles consistent with those inferred for the protosolar nebula and for other protoplanetary disks. The authors demonstrate that long-lasting, self-gravitating protoplanets arise after a few disk orbital periods if cooling is efficient enough to maintain the temperature close to 50 K. The resulting bodies have masses and orbital eccentricities similar to those of detected extrasolar planets.

References (abridged):

1. G. W. Marcy and R. P. Butler, Publ. Astron. Soc. Pac. 112, 137 (2000).

2. ___, Annu. Rev. Astron. Astrophys. 36, 57 (1998).

3. J. B. Pollack, et al., Icarus 124, 62 (1996).

4. J. Lissauer, Nature 409, 23 (2001).

5. J. Bally, L. Testi, A. Sargent, J. Carlstrom, Astron. J. 116, 854 (1998).

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