Multi-epoch monitoring of the AA Tauri-like star V 354 Mon. Indications for a low gas-to-dust ratio in the inner disk warp (dataset)

Disk warps around classical T Tauri stars (CTTSs) can periodicallyobscure the central star for some viewing geometries. For these so-called AA Tau-like variables, the obscuring material is located in theinner disk and absorption spectroscopy allows one to characterize itsdust and gas content. Since the observed emission from CTTSs consists ofseveral components (photospheric, accretion, jet, and disk emission),which can all vary with time, it is generally challenging todisentangling disk features from emission variability. Multi- epoch,flux-calibrated, broadband spectra provide us with the necessaryinformation to cleanly separate absorption from emission variability. Weapplied this method to three epochs of VLT/X-shooter spectra of the CTTSV 354 Mon (CSI Mon-660) located in NGC 2264 and find that: (a) theaccretion emission remains virtually unchanged between the three epochs;(b) the broadband flux evolution is best described by disk materialobscuring part of the star, and (c) the Na and K gas absorption linesshow only a minor increase in equivalent width during phases of highdust extinction. The limits on the absorbing gas column densitiesindicate a low gas-to-dust ratio in the inner disk, less than a tenth ofthe ISM value. We speculate that the evolutionary state of V 354 Mon,rather old with a low accretion rate, is responsible for the dust excessthrough an evolution toward a dust dominated disk or through thefragmentation of larger bodies that drifted inward from larger radii ina still gas dominated disk.