The spectral energy plots for AB Aur and LkHa 198 in Figure 18.12 include significant components in the millimeter regime. Historically, strong continuum flux at A > 1 mm was used to bolster the idea that a star is surrounded by a dusty disk. The basic argument is the familiar one that a spherical distribution of grains with this amount of long-wavelength emission would obscure the object completely in the visual. As already noted, however, the shell models themselves can accommodate a diversity of observations through appropriate changes in volume density and internal composition of the grains. Additionally, direct imaging fails to resolve the continuum emission from relatively unobscured objects like AB Aur. More embedded stars, such as LkHa 198 and LkHa 234, do have spatially extended emission, but this could arise from warmed dust in their clumpy environments.
Stronger evidence for disks is potentially available from spectral lines originating in the gas. Here, one hopes to image flattened structures that are manifestly rotating about their central stars, as seen through Doppler shifts of the line in question. This hope has been realized to some degree, although the case for disks remains weaker than in T Tauri stars. Figure 18.16 provides two examples of interferometric maps utilizing CO. We also show, for comparison, the telescope beam in each case. On the left is the gaseous structure around AB Aur, as seen in the 13C16O (J =1 ^ 0) transition. While the configuration is clearly asymmetric, its true
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