Observational Signatures of Planets in Protoplanetary Disks II: Spiral Arms Observed in Scattered Light Imaging Can be Induced by Planets

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.