The Spectral Energy Distribution of HH 30 IRS: Constraining the Circumstellar Dust Size Distribution

We present spectral energy distribution (SED) models for the edge-on classical T Tauri star HH 30 IRS that indicate that dust grains have grown to larger than 50 μm within its circumstellar disk. The disk geometry and inclination are known from previous modeling of multiwavelength Hubble Space Telescope images, and we use the SED (0.5 μm ≤ λ ≤ 3 mm) to constrain the dust size distribution. Model spectra are shown for different circumstellar dust models: a standard interstellar medium (ISM) mixture and larger grain models. As compared to ISM grains, the larger dust grain models have a shallower wavelength-dependent opacity: smaller at short wavelengths and larger at long wavelengths. Models with the larger dust grains provide a good match to the observed SED of HH 30 IRS. Although the currently available SED is poorly sampled, we estimate L* ≈ 0.2 L☉, Mdisk ≈ 1.5 × 10-3 M☉, and a power law with exponential cutoff dust grain size distribution. This model provides a good fit to the currently available data, but mid- and far-IR observations are required to more tightly constrain the size distribution. The accretion luminosity in our models is Lacc ≲ 0.2L*, corresponding to an accretion rate Ṁ ≲ 4 × 10-9 M☉ yr-1. Dust size distributions that are simple power-law extensions (i.e., no exponential cutoff) yield acceptable fits to the optical/near-IR but too much emission at millimeter wavelengths, and require larger disk masses up to Mdisk ~ 0.5 M☉. Such a simple size distribution would not be expected in an environment such as the disk of HH 30 IRS (i.e., where coagulation and accretion processes are occurring in addition to grain shattering), particularly over such a large range in grain sizes. Its ability to adequately characterize the grain populations, however, may be determined from more complete observational sampling of the SED in the mid- to far-IR.