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ALMA imaging of the CO snowline of the HD 163296 disk with DCO+

The high spatial and line sensitivity of ALMA opens the possibility of resolving emission from molecules in circumstellar disks. With an understanding of physical conditions under which molecules 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 19 K, making it a useful probe of the cold disk midplane. We compare ALMA line observations of HD 163296 to a grid of models. 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 160 AU. 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 19 K isotherm to the photodissociation layer with an abundance of 3x10^-10 relative to H2. DCO+ exists within the 19-21 K region of the disk with an abundance ratio [DCO+] / [HCO+] = 0.3. This represents a factor of 10^4 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 (approximately greater than 4x10^-10), and suggest the utility of DCO+ as a tracer of its parent molecule H2D+. Abridged