SPIRAL ARMS IN GRAVITATIONALLY UNSTABLE PROTOPLANETARY DISKS AS IMAGED IN SCATTERED LIGHT

Combining 3D smoothed-particle hydrodynamics and Monte Carlo radiative transfer calculations, we examine the morphology of spiral density waves induced by gravitational instability (GI) in protoplanetary disks, as they would appear in direct images at near-infrared (NIR) wavelengths. We find that systems with disk-to-star-mass ratios q = M-disk/M-star that are similar to 0.25 or more may produce prominent spiral arms in NIR imaging, remarkably resembling features observed in the MWC 758 and SAO 206462 systems. The contrast of GI-induced arms at NIR wavelengths can reach a factor of similar to 3, and their pitch angles are about 10 degrees-15 degrees. The dominant azimuthal wavenumber of GI-induced spiral arms roughly obeys m similar to 1/q in the range 2 less than or similar to 1/q less than or similar to 8. In particular, a massive disk with q approximate to 0.5 can exhibit grand-design m = 2 spirals. GI-induced arms are in approximate corotation with the local disk, and may therefore trap dust particles by pressure drag. Although GI can produce NIR spiral arms with morphologies, contrasts, and pitch angles similar to those reported in recent observations, it also makes other demands that may or may not be satisfied in any given system. A GI origin requires that the spirals be relatively compact, on scales less than or similar to 100 AU; that the disk be massive, q greater than or similar to 0.25; and that the accretion rate M-star be high, on the order of 10(-6) M-circle dot yr(-1).