Asteroseismology of the delta Scuti star HD 50844

Aims. We aim to probe the internal structure and investigate with asteroseismology for more detailed information on the delta Scuti star HD 50844.  Methods. We analyse the observed frequencies of the delta Scuti star HD 50844 and search for possible multiplets, which are based on the rotational splitting law of g-mode. We tried to disentangle the frequency spectra of HD 50844 only by means of rotational splitting. We then compare these with theoretical pulsation modes, which correspond to stellar evolutionary models with various sets of initial metallicity and stellar mass, to find the best-fitting model.  Results. There are three multiplets, including two complete triplets and one incomplete quintuplet, in which mode identifications for spherical harmonic degree l and azimuthal number m are unique. The corresponding rotational period of HD 50844 is found to be 2.44(-0.08)(+0.13) days. The physical parameters of HD 50844 are well limited in a small region by three modes that have been identified as nonradial ones (f(11), f(22), and f(29)) and by the fundamental radial mode (f(4)). Our results show that the three nonradial modes (f(11), f(22), and f(29)) are all mixed modes, which mainly represent the property of the helium core. The fundamental radial mode (f(4)) mainly represents the property of the stellar envelope. To fit these four pulsation modes, both the helium core and the stellar envelope need to be matched to the actual structure of HD 50844. Finally, the mass of the helium core of HD 50844 is estimated to be 0.173 +/- 0.004 M-circle dot for the first time. The physical parameters of HD 50844 are determined to be M = 1.81 +/- 0.01 M-circle dot, Z = 0.008 +/- 0.001. T-eff = 7508 +/- 125 K, log g = 3.658 +/- 0.004, R = 3.300 +/- 0.023 R-circle dot, L = 30.98 +/- 2.39 L-circle dot.