  Using 3D global hydro simulations coupled with radiative transfer
calculations, we study the appearance of density waves induced by giant planets
in direct imaging observations at near infrared wavelengths. We find that a 6
MJ planet in a typical disk around a 1 M_sun star can produce prominent and
detectable spiral arms both interior and exterior to its orbit. The inner arms
have (1) two well separated arms in roughly m=2 symmetry, (2) exhibit ~10-15
degrees pitch angles, (3) ~180-270 degrees extension in the azimuthal
direction, and (4) ~150% surface brightness enhancement, all broadly consistent
with observed spiral arms in the SAO 206462 and MWC 758 systems. The outer arms
cannot explain observations as they are too tightly wound given typical disk
scale height. We confirm previous results that the outer density waves excited
by a 1 MJ planet exhibit low contrast in the IR and are practically not
detectable. We also find that 3D effects of the waves are important. Compared
to isothermal models, density waves in adiabatic disks exhibit weaker contrast
in surface density but stronger contrast in scattered light images, due to a
more pronounced vertical structure in the former caused by shock heating. To
drive observed pairs of arms with an external companion on a circular orbit, a
massive planet, possibly a brown dwarf, is needed at around [r~0.7", PA~10
degrees] (position angle PA from north to east) in SAO 206462 and [r~0.6",
PA~10 degrees] in MWC 758. Their existence may be confirmed by direct imaging
planet searches.
