Density-wave induced star formation: a model for M81

The two-armed spiral shock (TASS) wave, fitted to observations of the H I gas in M81 by Visser, is used to give initial conditions for ballistic particles launched from the sprial arms. The model predicts the location of molecular clouds which agrees with the one detection of CO in M81. In addition, our model agrees with the observed distribution of H II regions near the eastern arm. The model also predicts the radial velocities of, and velocity differenses between, H II regions and molecular clouds near the major and minor axes. When the model's disk linear density wave amplitude, spiral arm star mass function, and star formation rate are fitted to observations of the surface brightness, the predicted arm colors agree best with the observed ones when we use a linear density wave amplitude very near the observed one. The spiral arm stellar mass function required for this agreement suggests that spiral arms essentially make only O and B stars or that low-mass stars are hidden or not yet formed while the O and B stars are shining. The model also produces a galaxy which resembles a photograph of M81