1. Purpose and Objectives of the Leave

Recent observational investigations of the frequency of occurrence of pre-main-sequence binary stars have reinforced earlier suspicions that ''binary formation is the primary branch of the star-formation process'' (Mathieu 1994). As Bodenheimer et al. (1993) have reviewed, a number of different theories have been proposed to explain the preponderance of binary stars. Klein et al. (1998) show how the direct fragmentation of protostellar gas clouds may occur in early phases of collapse (at cloud densities n = 10³ - 10¹⁰ cm^-3). But at higher densities, clouds are unable to cool efficiently upon contraction. Consequently, direct fragmentation becomes problematical. Because higher mean densities are associated with systems having shorter dynamical times, one is led to consider mechanisms other than direct cloud fragmentation for forming binary systems with orbital periods less than a few hundred years. Here we investigate whether such binaries can form by spontaneous fission of rapidly rotating protostars.

2. Outline of Proposed Activities

3. Location of Leave

Movie1

Quicktime (5,907K)

Employing a significantly improved finite-difference simulation code and improved spatial resolution (128³ grid zones), we recently have repeated the simulation that was first reported in Durisen et al. (1986). Movie1 shows the nonlinear development of the two-armed, spiral-mode instability. The evolution is shown in the inertial reference frame and covers 20 central initial rotation periods. Each frame of Movie1 displays four nested isodensity contours at r/r_max = 0.8, 0.4, 0.04, and 0.004. Via the trailing spiral structure, gravitational torques are able to effectively redistribute angular momentum on a dynamical time scale; a relatively small amount of material is shed into an equatorial disk (this disk material is not visible in Movie1 because r_disk < 0.004 r_max); and the central object (containing most of the initial object's mass) settles down into a new equilibrium configuration. Clearly, evolution to a binary star system as suggested by the classical fission hypothesis does not occur. It is primarily because simulations of this type have not produced a binary star system that the classical fission hypothesis has lost favor within the star formation community over the past decade (Bodenheimer et al. 1993).

4. Alternate Plans (in case original plans are not accomplished

5. Travel Plans

Movie2 Quicktime (5,747K)

Movie3 Quicktime (3,376K)

6. Compensation

Andalib (1998) recently has developed a self-consistent-field technique that can be used to

Movie4

Quicktime (6,927K)

construct equilibrium models of infinitesimally thin, self-gravitating gaseous disks with (a) compressible equations of state, (b) nonaxisymmetric structures, and (c) nontrivial internal motions. By demanding that the disks have uniform vortensity (defined as the ratio of vorticity to mass density), Andalib has successfully constructed equilibrium disks with polytropic indices 0 < n < 1.3 and minor-to-major axis ratios in the range 0.06 < b/a < 0.80. Movie4 illustrates the internal flow of four of Andalib's compressible disks with nonaxisymmetric structures: one with fully retrograde internal motions (R); one with fully prograde internal motions (P); one with vortices sandwiched between separate regions of prograde and retrograde flow (V); and a common-envelope binary (dumbbell-shaped) configuration (D).

The similarity between the flow illustrated in Movie2 and the flow in Andalib's model P (Movie4) is striking. Apparently Andalib's model provides a good 2D analog of the 3D ''final bar'' that formed as a result of our fully hydrodynamic simulation of the two-armed, spiral mode instability (Movie1). Furthermore, Andalib's work demonstrates that model P is just one among a series of compressible models with nontrivial internal flows that defines a smooth elliptical-dumbbell-binary sequence. We suspect, therefore, that the final bar sits on an analogous (3D) sequence and that, if it is cooled slowly, it will evolve along the sequence to a common-envelope binary configuration such as the one illustrated by model D in Movie4. Additional support for this conjecture comes from New & Tohline (1997) who have demonstrated that stable, equal-mass common-envelope binaries can be constructed for fully 3D fluid systems with a sufficiently compressible equation of state. In summary, it seems clear that a wide variety of rapidly rotating, nonaxisymmetric systems can be constructed with compressible equations of state. This work gives us renewed confidence that fission offers a viable route to binary star formation. Future investigations designed to model the slow cooling and contraction of initially nonaxisymmetric configurations like the final bar described above should demonstrate whether or not this scenario is correct.

5. Acknowledgments

This work has been supported, in part, by the U.S. National Science Foundation through grant AST-9528424 and, in part, by grants of high-performance-computing time at the San Diego Supercomputer Center and through the PET program of the NAVOCEANO DoD Major Shared Resource Center in Stennis, MS.

6. References

Andalib, S.W. (1998), The Structure and Stability of Selected, 2-D Self-Gravitating Systems, Ph.D. Dissertation, Louisiana State University

Bodenheimer, P., Ruzmaikina, T. and Mathieu, R.D. (1993), in Protostars and Planets, III, ed. E.H. Levy and J.I. Lunine. University of Arizona Press, Tucson, AZ, U.S.A. p. 367

Chandrasekhar, S. (1969), Ellipsoidal Figures of Equilibrium. Yale University Press, New Haven, CT, U.S.A.

Durisen, R.H., Gingold, R.A., Tohline, J.E. and Boss, A.P. (1986), Ap.J., 305 , p. 281

Durisen, R.H. and Tohline, J.E. (1985), in Protostars and Planets, II, ed. D.C. Black and M. Mathews. University of Arizona Press, Tucson, AZ, U.S.A. p. 534

Houser, J.L., Centrella, J.M. and Smith, S.C. (1994), Phys. Rev. Lett., 72, p. 1314

Klein, R.I, McKee, C.F. and Fisher, R. (1998), These proceedings

Mathieu, R.D. (1994), Ann. Rev. Astr. Ap., 32, p. 465

New, K.B.C. and Tohline, J.E. (1997), Ap.J., 490, p. 311

Ostriker, J.P. and Bodenheimer, P. (1968), Ap.J., 151, p. 1089

Williams, H.A. and Tohline, J.E. (1988), Ap.J., 334, p. 449