Using the Ca ii Lines in T Tauri Stars to Infer the Abundance of Refractory Elements in the Innermost Disk Region

Abstract

We present a study of the abundance of calcium in the innermost disk of 70 T Tauri stars in the star-forming regions of Chamaeleon I, Lupus, and Orion OB1b. We use calcium as a proxy for the refractory material that reaches the inner disk. We used magnetospheric accretion models to analyze the Ca ii emission lines and estimate abundances in the accretion flows of the stars, which feed from the inner disks. We find Ca depletion in disks of all three star-forming regions, with 57% of the sample having [Ca/H] < –0.30 relative to the solar abundance. All disks with cavities and/or substructures show depletion, consistent with trapping of refractories in pressure bumps. Significant Ca depletion ([Ca/H] < –0.30) is also measured in 60% of full disks, although some of those disks may have hidden substructures or cavities. We find no correlation between Ca abundance and stellar or disk parameters except for the mass accretion rate onto the star. This could suggest that the inner and outer disks are decoupled, and that the mass accretion rate is related to a mass reservoir in the inner disk, while refractory depletion reflects phenomena in the outer disk related to the presence of structure and forming planets. Our results of refractory depletion and timescales for depletion are qualitatively consistent with expectations of dust growth and radial drift, including partitioning of elements, and constitute direct evidence that radial drift of solids locked in pebbles takes place in disks.

Keywords

• Protoplanetary disks
• Chemical abundances
• T Tauri stars