The dusty starburst nucleus of M33

We have thoroughly characterized the ultraviolet to near-infrared (0.15-2.2 mu m) spectral energy distribution (SED) of the central parsec of the M33 nucleus through new infrared photometry and optical/near-infrared spectrosc opy, combined with ultraviolet/optical observations from the literature and the Hubble Space Telescope archive. The SED shows evidence for a significa nt level of attenuation, which we model through a Monte Carlo radiative tra nsfer code as a shell of clumpy Milky Way-type dust (tau(v) - 2 +/- 1). The discovery of Milky Way-type dust (with a strong 2175 Angstrom bump) intern al to the M33 nucleus is different from previous work, which has found SMC- Iike dust (no bump) near starburst regions. The amount by which dust can be processed may be related to the mass and age of the starburst as well as t he extent to which the dust can shield itself. Our starburst models include the effects of this dust and can fit the SED if the nucleus was the site o f a moderate (similar to 10(8) L-circle dot, at 10 Myr) episode of coeval s tar formation about 70 Myr ago. This result is quite different from previou s studies, which resorted to multiple stellar populations (between two and seven) attenuated by either no or very little internal dust. The M33 nuclea r starburst is remarkably similar to an older version (70 versus 10 Myr) of the ultracompact starburst in the center of the Milky Way.