2013A&A...557A.132M

The high spatial resolution and line sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) opens the possibility of resolving emission from molecules in large samples of circumstellar disks. With an understanding of the conditions under which these molecules can have high abundance, they can be used as direct tracers of distinct physical regions. In particular, DCO⁺ is expected to have an enhanced abundance within a few Kelvin of the CO freezeout temperature of 19K, making it a useful probe of the cold disk midplane. We aim to use line emission from DCO⁺ to directly resolve the CO ``snowline'' - the region at which the gas-phase CO abundance drops due to freezeout - and determine the temperature boundaries of the region of DCO⁺ emission in the HD 163296 disk. This will serve as a test of deuteration models based on enhanced formation of the parent molecule H₂D⁺ and a direct probe of midplane disk structure and ionization. We compare ALMA line observations of HD 163296 to a grid of models based on the best fit physical model of (Zingale et al., 2011ApJ...740....8Z). We vary the upper- and lower-limit temperatures of the region in which DCO⁺ is present as well as the abundance of DCO⁺ in order to fit channel maps of the DCO⁺ J=5-4 line. To determine the abundance enhancement compared to the general interstellar medium, we carry out similar fitting to HCO⁺ J=4-3 and H¹³CO⁺ J=4-3 observations. ALMA images show centrally peaked extended emission from HCO⁺ and H¹³CO⁺. DCO⁺ emission lies in a resolved ring from ∼110 to 160AU. The outer radius approximately corresponds to the size of the CO snowline as measured by previous lower resolution observations of CO lines in this disk. The ALMA DCO⁺ data now resolve and image the CO snowline directly. In the best fitting models, HCO⁺ exists in a region extending from the 19K isotherm to the photodissociation layer with an abundance of 3x10^–10 relative to H₂. DCO⁺ exists within the 19-21K region of the disk with an abundance ratio [DCO⁺]/[HCO⁺]=0.3. This represents a factor of 10⁴ enhancement of the DCO⁺ abundance within this narrow region of the HD 163296 disk. Such a high enhancement has only previously been seen in prestellar cores. The inferred abundances provide a lower limit to the ionization fraction in the midplane of the cold outer disk (> 4x10^–10), and suggest the utility of DCO⁺ as a tracer of its parent molecule H₂D⁺.