The circumstellar disk of the Butterfly star in Taurus

We present a model of the circumstellar environment of the so-called Butterfly star in Taurus (IRAS 04302+2247). The appearance of this young stellar object is dominated by a large circumstellar disk seen edge-on and the light-scattering lobes above the disk. Our model is based on multiwavelength continuum observations: (1) millimeter maps and (2) high-resolution near-infrared images obtained with Hubble Space Telescope/NICMOS. The advantage of the combination of both observations is that they trace (1) different regions of the system and (2) different physical processes. On the one hand, the millimeter observations are sensitive to the long-wavelength radiation being reemitted from the dust in the central parts close to the midplane of the circumstellar disk. Thus, the geometry and small-scale density distribution of the disk has been studied. Furthermore, in contrast to the pure flux measurement, the resolved 1.3 mm image allows us to discriminate between different disk models with a similar far-infrared/millimeter spectral energy distribution and therefore to disentangle the disk geometry much more precisely. On the other hand, the near-infrared observations trace the envelope structure and dust properties in the envelope and the disk surface. We find disk and envelope parameters that are comparable with those of the circumstellar environment of other young stellar objects. A main result is that the dust properties must be different in the circumstellar disk and in the envelope: while a grain size distribution with grain radii up to 100 mum is required to reproduce the millimeter observations of the disk, the envelope is dominated by smaller grains similar to those of the interstellar medium. Alternatives to this grain growth scenario in the circumstellar disk are discussed in brief as well.