We consider the dynamics of a protostellar disk surrounding a star in acircular-orbit binary system. Our aim is to determine whether, if the disk isinitially tilted with respect to the plane of the binary orbit, the inclinationof the system will increase or decrease with time. The problem is formulated inthe binary frame in which the tidal potential of the companion star is static.We consider a steady, flat disk that is aligned with the binary plane andinvestigate its linear stability with respect to tilting or warpingperturbations. The dynamics is controlled by the competing effects of the m=0and m=2 azimuthal Fourier components of the tidal potential. In the presence ofdissipation, the m=0 component causes alignment of the system, while the m=2component has the opposite tendency. We find that disks that are sufficientlylarge, in particular those that extend to their tidal truncation radii, aregenerally stable and will therefore tend to alignment with the binary plane ona time-scale comparable to that found in previous studies. However, the effectof the m=2 component is enhanced in the vicinity of resonances where the outerradius of the disk is such that the natural frequency of a global bending modeof the disk is equal to twice the binary orbital frequency. Under suchcircumstances, the disk can be unstable to tilting and acquire a warped shape,even in the absence of dissipation. The outer radius corresponding to theprimary resonance is always smaller than the tidal truncation radius. For diskssmaller than the primary resonance, the m=2 component may be able to cause avery slow growth of inclination through the effect of a near resonance thatoccurs close to the disk center. We discuss these results in the light ofrecent observations of protostellar disks in binary systems.
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