New Constraints on the Dust and Gas Distribution in the LkCa 15 Disk from ALMA

Abstract

We search a large parameter space of the LkCa 15's disk density profile to fit its observed radial intensity profile of 12 CO ( J  = 3–2) obtained from Atacama Large Millimeter/submillimeter Array. The best-fit model within the parameter space has a disk mass of 0.1 M ⊙ (using an abundance ratio of 12 CO/H 2  = 1.4 × 10 −4 in mass), an inner cavity of 45 au in radius, an outer edge at ∼600 au, and a disk surface density profile that follows a power law of the form ρ r  ∝  r −4 . For the disk density profiles that can lead to a small reduced χ 2 of the goodness-of-fit, we find that there is a clear linear correlation between the disk mass and the power-law index, γ , in the equation of the disk density profile. This suggests that the 12 CO disk of LkCa 15 is optically thick, and we can fit its 12 CO radial intensity profile using either a lower disk mass with a smaller γ or a higher disk mass with a bigger γ . By comparing the 12 CO channel maps of the best-fit model with disk models with higher or lower masses, we find that a disk mass of ∼0.1 M ⊙ can best reproduce the observed morphology of the 12 CO channel maps. The dust continuum map at 0.87 mm of the LkCa 15 disk shows an inner cavity of the similar size of the best-fit gas model, but its outer edge is at ∼200 au, which is much smaller than the fitted gas disk. Such a discrepancy between the outer edges of the gas and dust disks is consistent with dust drifting and trapping models.