Multiwavelength Continuum Survey of Protostellar Disks in Ophiuchus Left: Submillimeter Array (SMA) aperture synthesis images of 870 μm (350 GHz) continuum emission from four protostellar disks in Ophiuchus (Andrews et al. 2009). These observations were made using several different SMA configurations, achieving a synthesized beam of ~ 0.5” FWHM. The most extended configurations required integration times of ~2 hours. During Early Science, ALMA will achieve similar angular resolution in Band 9 (690 GHz) with integration times of ~0.5 hours required to obtain similar uv-coverage. Science Goal: To observe the variation in the dust spectral energy distribution (SED) between circumstellar disks in the nearby Ophiuchus molecular cloud: The characteristics of dust in a circumstellar disk around a protostar are expected to evolve over time, as dust grains settle to the disk midplane, accumulate into larger solid bodies, and potentially form planets within the disk. The evolution of the disk may be traced by determining the dust SED of a coherent sample of protostellar disks. Wide frequency coverage is necessary to achieve these aims, since the SED of the disk is determined by a combination of the dust temperature, the dust emissivity properties, and the optical depth through the disk. Nearby protostellar disks will not be resolved by ALMA at all frequencies during Early Science, but the Early Science collecting area will allow high sensitivity observations of the disk-averaged SED in a short observation. Here, we present a proposal to observe 3 Class 0 and 3 Class I protostellar disks selected from the catalogue presented by Enoch et al. (2009). Receivers: Bands 3, 6, 7 and 9 (110 GHz, 230 GHz, 345 GHz and 690 GHz) Angular Resolution: 0".4 (Band 9) to 2".2 (Band 3), assuming a maximum baseline of 250m. The typical size of a protoplanetary disk is ~100 AU. Towards the nearest molecular clouds, such as Ophiuchus (at 125 pc), 100 AU subtends only 0".8, and may only be resolved in Band 9 during Early Science. Here we will focus on the global properties of the disks. Spectral Resolution: N/A Each measurement will include the largest bandwidth possible. Continuum Sensitivity: 0.4 mJy, 3 mJy, 8 mJy and 40 mJy for Bands 3, 6, 7, and 9, respectively. These are the expected point source sensitivities needed for 10σ detections in each band, assuming a disk mass of 0.01 MSun, an average dust temperature of 20 K, a plausible dust emissivity and a distance of 125 pc. Observing Time: Using the ALMA integration time calculator with m antennas and 4 GHz bandwidth, these point source sensitivities can be achieved in 45s per disk (Band 3), 3s per disk (Band 6), 2s per disk (Band 7) and 1s per disk (Band 9). To maximize time on-source, we will observe each disk for 60 s in Bands 3, 6 and 7. Since the disks are likely to be resolved in Band 9, however, the required observing time at this frequency is set by the need for coverage

of the uv-plane. Based on SMA observations of Ophiuchus circumstellar disks (Andrews et al. 2007), sufficient uv-coverage can be obtained in 30 minutes per disk. Observations of six circumstellar disks thus require a total time on-source of 3.5 hours. In the future: As ALMA’s maximum baseline length increases, the disks in this example can be resolved at all wavelengths, with matched angular resolution if desired. With greater angular resolution, the science case presented here can be expanded to study the variation of the dust SED as a function of disk radius, and to look for holes in emission which may indicate the presence of gaps in the disks. The resolved disks will require a greater amount of observing time, both for sensitivity (using the surface brightness rather than point source mode with the ALMA integration time calculator), and to increase the uv-coverage, as we have done here for Band 9. The number of antennas will increase during Early Science, however, reducing the amount of time required to obtain the same uv-coverage. To match the Band 9 angular resolution of 0.4” discussed above at Band 3 requires a maximum baseline of 1.5 km. Place holder for real OT screen capture.