A Spectrophotometric Method to Determine the Inclination of Class I Objects

A new method that enables us to estimate the inclination of Class I young stellar objects is proposed. Since Class I objects are not spherically symmetric, it is likely that the observed feature is sensitive to the inclination of the system. Thus, we construct a protostar model by carefully treating two-dimensional radiative transfer and radiative equilibrium. We show from the present twodimensional numerical simulations that the emergent luminosity LSED, which is the frequency integration of spectral energy distribution (SED), depends strongly on the inclination of the system i, whereas the peak flux is insensitive to i. Based on this result, we introduce a novel indicator fL, which is the ratio of LSED to the peak flux, as a good measure for the inclination. By using fL, we can determine the inclination regardless of the other physical parameters. The inclination would be determined by fL within an accuracy of ±5° if the opening angle of bipolar outflows is specified by any other procedure. Since this spectrophotometric method is easier than a geometrical method or a full SED fitting method, it could be a powerful tool for investigating the features of protostars statistically with observational data that will be provided by future missions, such as SIRTF, Astro-F, and ALMA.