  We examine whether various characteristics of planet-driven spiral arms can
be used to constrain the masses of unseen planets and their positions within
their disks. By carrying out two-dimensional hydrodynamic simulations varying
planet mass and disk gas temperature, we find that a larger number of spiral
arms form with a smaller planet mass and a lower disk temperature. A planet
excites two or more spiral arms interior to its orbit for a range of disk
temperature characterized by the disk aspect ratio $0.04\leq(h/r)_p\leq0.15$,
whereas exterior to a planet's orbit multiple spiral arms can form only in cold
disks with $(h/r)_p \lesssim 0.06$. Constraining the planet mass with the pitch
angle of spiral arms requires accurate disk temperature measurements that might
be challenging even with ALMA. However, the property that the pitch angle of
planet-driven spiral arms decreases away from the planet can be a powerful
diagnostic to determine whether the planet is located interior or exterior to
the observed spirals. The arm-to-arm separations increase as a function of
planet mass, consistent with previous studies; however, the exact slope depends
on disk temperature as well as the radial location where the arm-to-arm
separations are measured. We apply these diagnostics to the spiral arms seen in
MWC 758 and Elias 2-27. As shown in Bae et al. (2017), planet-driven spiral
arms can create concentric rings and gaps, which can produce more dominant
observable signature than spiral arms under certain circumstances. We discuss
the observability of planet-driven spiral arms versus rings and gaps.
