Mode identification in the δ Scuti Star 1 Mon

We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the delta Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree, l, the azimuthal order, in, of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five, wavebands and obtain the best estimates of l for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably l = 1, m = - 1. We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the l = 1 mode. We discuss implications for mode identification of delta Scuti stars based on what we have learned from this star.