Near-infrared line imaging of the circumnuclear starburst ring in NGC 7771

We present high spatial resolution near-infrared broadband JHK images and, for the first time, Br gamma 2.1661 mu m and H-2 1-0 S(1) 2.122 mu m emissi on-line images of the circumnuclear star-forming ring (major axis diameter 7 " = 2 kpc) in the starburst galaxy NGC 7771. These data are used to inves tigate the morphology and extinction of the starburst ring and to study its star-forming properties and history by comparing the observed quantities w ith an evolutionary population synthesis model. The clumpy morphology of NG C 7771 varies strongly with wavelength, as a result of the combination of e xtinction (for which we derive an average value of A(V) = 2.8), emission fr om hot dust and red supergiants, and several stellar generations in the rin g. Also, the ellipticity and the position angle of the ring depend on the w avelength. The starburst ring in NGC 7771 exhibits small Br gamma equivalen t widths. Assuming a constant star formation model with M-u = 100 M. result s in very long lifetimes of the star-forming regions (up to 1 Gyr), in disa greement with the clumpy near-infrared morphology and the observed radio sp ectral index of NGC 7771. This situation is only slightly remedied by assum ing a reduced upper mass cutoff (M-u = 30 M.), resulting in ages between 8 and 180 Myr. We prefer an instantaneous star formation model with M-u = 100 M., which can explain the derived Br gamma equivalent widths if a single s tarburst occurred 6-7 Myr ago. The main excitation mechanism of the molecul ar gas, based on the observed S(1)/Br gamma ratio, appears to be excitation by UV radiation from hot young stars. We derive M similar or equal to 1900 M. for the mass of the excited H-2.