THE IONIZATION FRACTION IN THE DM Tau PROTOPLANETARY DISK
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
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 H{sub 2}D{sup +} J = 1{sub 1,0}-1{sub 1,1}, N{sub 2}H{sup +} J = 4-3, and CO J = 3-2 are presented. H{sub 2}D{sup +} and N{sub 2}H{sup +} are not detected and using the CO 3-2 disk size the observations result in an upper limit of <0.47 K km s{sup -1} for both lines, a factor of 2.5 below previous single-dish H{sub 2}D{sup +} observations. Assuming LTE, a disk midplane temperature of 10-20 K and estimates of the H{sub 2}D{sup +} o/p ratio, the observed limit corresponds to N{sub H{sub 2D{sup +}}}<2-21x10{sup 12} cm{sup -2}. We adopt a parametric model for the disk structure from the literature and use new IRAM 30 m telescope observations of the H{sup 13}CO{sup +} J = 3-2 line and previously published SMA observations of the N{sub 2}H{sup +} J = 3-2, HCO{sup +} J = 3-2, and DCO{sup +} J = 3-2 lines to constrain the ionization fraction, x{sub 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{sup +} is most abundant, where we estimate x{sub i} {approx_equal} 4 Multiplication-Sign 10{sup -10}; (2) a cooler molecular layer with T = 16-20 K where N{sub 2}H{sup +} and DCO{sup +} abundances are predicted to peak, with x{sub i} {approx_equal} 3 Multiplication-Sign 10{sup -11}; and (3) the cold, dense midplane with T <16 K where H{sup +}{sub 3} and its deuterated isotopologues are the main carriers of positive charge, with x{sub i} < 3 Multiplication-Sign 10{sup -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 H{sub 2}D{sup +} 1{sub 1,0}-1{sub 1,1} line, but also robust determinations of the o/p ratio, observations of D{sub 2}H{sup +}, and stronger constraints on where N{sub 2} is present in the gas phase.
- OSTI ID:
- 22004443
- Journal Information:
- Astrophysical Journal, Vol. 743, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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