Massive Compact Dust Disk with a Gap around CW Tau Revealed by ALMA Multiband Observations

Compact protoplanetary disks with a radius of less than or similar to 50 au are common around young low-mass stars. We report high-resolution ALMA dust continuum observations toward a compact disk around CW Tau at Bands 4 (lambda = 2.2 mm), 6 (1.3 mm), 7 (0.89 mm), and 8 (0.75 mm). The spectral energy distribution shows the spectral slope of 2.0 +/- 0.24 between 0.75 and 1.3 mm, while it is 3.7 +/- 0.29 between 2.17 and 3.56 mm. The steep slope between 2.17 and 3.56 mm is consistent with that of optically thin emission from small grains (less than or similar to 350 mu m). We perform parametric fitting of the ALMA data to characterize the dust disk. Interestingly, if the dust-to-gas mass ratio is 0.01, the Toomre Q parameter reaches similar to 1-3, suggesting that the CW Tau disk might be marginally gravitationally unstable. The total dust mass is estimated as similar to 250 M (circle plus) for the maximum dust size of 140 mu m that is inferred from the previous Band 7 polarimetric observation and at least 80 M (circle plus) even for larger grain sizes. This result shows that the CW Tau disk is quite massive in spite of its smallness. Furthermore, we clearly identify a gap structure located at similar to 20 au, which might be induced by a giant planet. In spite of these interesting characteristics, the CW Tau disk has normal disk luminosity, size, and spectral index at ALMA Band 6, which could be a clue to the mass budget problem in Class II disks.