THE IONIZATION FRACTION IN THE DM Tau PROTOPLANETARY DISK

We present millimeter-wave observations of several molecular ions in the disk around the pre-main-sequence star DM Tau and use these to investigate the ionization fraction in different regions of the disk. New Submillimeter Array (SMA) observations of H2D+ J = 1(1,0)-1(1,1), N2H+ J = 4-3, and CO J = 3-2 are presented. H2D+ and N2H+ are not detected and using the CO 3-2 disk size the observations result in an upper limit of < 0.47 K km s(-1) for both lines, a factor of 2.5 below previous single-dish H2D+ observations. Assuming LTE, a disk midplane temperature of 10-20 K and estimates of the H2D+ o/p ratio, the observed limit corresponds to NH2D+ < 4-21 x 10(12) cm(-2). We adopt a parametric model for the disk structure from the literature and use new IRAM 30 m telescope observations of the (HCO+)-C-13 J = 3-2 line and previously published SMA observations of the N2H+ J = 3-2, HCO+ J = 3-2, and DCO+ J = 3-2 lines to constrain the ionization fraction, x(i), in three temperature regions in the disk where theoretical considerations suggest different ions should dominate: (1) a warm, upper layer with T > 20 K where CO is in the gas phase and HCO+ is most abundant, where we estimate x(i) similar or equal to 4 x 10(-10); (2) a cooler molecular layer with T = 16-20 K where N2H+ and DCO+ abundances are predicted to peak, with x(i) similar or equal to 3 x 10(-11); and (3) the cold, dense midplane with T < 16 K where H-3(+) and its deuterated isotopologues are the main carriers of positive charge, with x(i) < 3 x 10(-10). While there are considerable uncertainties, these estimates are consistent with a decreasing ionization fraction into the deeper, colder, and denser disk layers. Stronger constraints on the ionization fraction in the disk midplane will require not only substantially more sensitive observations of the H2D+ 1(1,0)-1(1,1) line, but also robust determinations of the o/p ratio, observations of D2H+, and stronger constraints on where N-2 is present in the gas phase.