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Aside from the grand-design stellar spirals appearing in the disk of M81, apair of stellar spiral arms situated well inside the bright bulge of M81 hasbeen recently discovered by Kendall et al. (2008). The seemingly unrelatedpairs of spirals pose a challenge to the theory of spiral density waves. Toaddress this problem, we have constructed a three component model for M81,including the contributions from a stellar disk, a bulge, and a dark matterhalo subject to observational constraints. Given this basic state for M81, amodal approach is applied to search for the discrete unstable spiral modes thatmay provide an understanding for the existence of both spiral arms. It is foundthat the apparently separated inner and outer spirals can be interpreted as asingle trailing spiral mode. In particular, these spirals share the samepattern speed 25.5 km s$^{-1}$ kpc$^{-1}$ with a corotation radius of 9.03 kpc.In addition to the good agreement between the calculated and the observedspiral pattern, the variation of the spiral amplitude can also be naturallyreproduced.

Spiral Density Waves in M81. I. Stellar Spiral Density Waves