The 200–300 GHz Survey for 18 Class II Disks in the Ophiuchus Star-forming Complex

We measured the 198–310 GHz flux densities of 18 Class II protoplanetary disks in the Ophiuchus star-forming complex using the Submillimeter Array (SMA). The ∼200–300 GHz spectral indices ( α 200−300 ) of these disks are 2.3 − 0.5 + 0.2 . In this observed frequency range, emission properties of the Ophiuchus disks are very similar to the 48 spatially compact Class II disks in the Taurus-Auriga region observed in a previous SMA survey. Combining the Ophiuchus and Taurus-Auriga samples, the Pearson correlation coefficient between α 200−300 and the flux density at 250 GHz ( F 250GHz ) is 0.27 ( p = 0.02). We do not find a convincing correlation between α 200−300 and the R 95% radius of the disks. Our interpretation for the α 200−300 values being close to 2 in the spatially compact disks is that at ≳200 GHz frequencies, the continuum emission in these disks is dominated by optically thick dust thermal emission, independent of F 250GHz and R 95% . As a result, the (sub)millimeter luminosity and the apparent disk size are positively correlated, which has been found in numerous resolved emission surveys of Class II disks. In young clusters with ages of ∼1–2 Myr, Class II disks with smaller radii or lower (sub)millimeter luminosities are not necessarily those in which the dust mass budget has been consumed or dispersed.