Vanishing Refractories: Tracing Dust Evolution in the BP Tau Protoplanetary Disk

Abstract

We present a multi-wavelength analysis of the dust of BP Tau's protoplanetary disk. We use new optical spectra of BP Tau, taken with the Magellan/MIKE spectrograph in tandem with archival UV and mid-infrared observations. We use the magnetospheric accretion model to analyze the Ca II K and Mg II 2796.4 \r{A} emission lines and derive the abundance of Ca and Mg in the accretion flows as a proxy for the refractory abundance in the innermost gas disk. Furthermore, we used irradiated accretion disk models to compare the spectral energy distribution (SED) to observations and model in detail the 10$\mu$m and 20$\mu$m silicate features to obtain the spatial distribution and stoichiometry of the dust in which the refractories are locked in the disk. We find a significant degree of depletion of refractory material in the innermost gas disk with median abundances of $\rm [Ca/H] = -2.0^{+0.1}_{-0.0}$ and $\rm [Mg/H] = -1.30^{+0.2}_{-0.3}$ and attribute this to both radial drift and dust trapping due to a pressure bump/gap. Our SED modeling recovers the inner cavity that extends up to 8 AU, consistent with sub-mm observations. We found a significant decrease of the Mg-to-Fe ratio with decreasing radius, with Mg-rich silicates in the outer wall and Fayalite in the inner wall, consistent with the Mg depletion inferred from the emission lines.