Chemistry in disks

Abstract

Aims. We study the deuteration and ionization structure of the DM Tau disk via interferometric observations and modelling of the key molecular ions, HCO + and DCO + . Methods. The Plateau de Bure Array is used to observe DM Tau in lines of HCO + (1−0), (3−2) and DCO + (3−2) with a ∼1.5 angular and ∼0.2 km s −1 spectral resolution. Using a power-law fitting approach the observed column densities profiles are derived and thus the isotopic ratio R D = DCO + /HCO + . Chemical modelling allowed an exploration of the sensitivity of HCO + and DCO + abundances to physical parameters out with temperature. A steady state approximation was employed to observationally constrain the ionization fraction x(e − ). Results. Fitting of radiative transfer models suggests that there is a chemical hole in HCO+ and DCO + , extending up to 50 AU from the star. More work is required to discern the cause of this. The observed column densities of HCO + and DCO + at 100 AU were (9.812 and (1.2 ± 0.7) × 10 12 cm −2 respectively. Where both HCO + and DCO + were present, R D was found to increase radially from 0.1 at 50 AU to 0.2 at 450 AU. This behaviour was well reproduced by the chemical model. The X-ray luminosity of the central star, the interstellar UV and CO depletion were found to be the most important physical parameters controlling the abundances of HCO + and DCO + . Differences in the vertical extent of HCO + and DCO + molecular layers resulted in different responses to changing physical parameters, manifesting as radial gradients in R D . The ionization fraction was found to be x(e − ) ∼ 10 −7 in the molecular layer, comparable to the disk averaged value. Modelling shows that while HCO + is the most dominant charged molecular ion in our disk model, atomic ions, such as C + , S + , H + , Na + and Mg + , dominate the charge in both the molecular layer and disk atmosphere. Conclusions. A high value of R D is indicative of continued deuterium fractionation in a protoplanetary disk after pre/protostellar phases. Radial properties of R D can be employed to discern the importance of ionization from X-rays and UV, thus necessitating the need for more, high resolution observations of DCO + and other deuterated species in disks. A steady-state approach commonly adopted for constraining ionization degree in prestellar cores is not applicable for disks where accurate determination of the ionization fraction in the molecular layer requires knowledge of the atomic ions present as molecular ions are relatively sparse.

An ALMA Survey of DCN/H13CN and DCO+/H13CO+ in Protoplanetary Disks

“…Ring-like distributions have previously been inferred for DCO + in the TW Hya, HD 163296, and DM Tau disks Mathews et al 2013;Teague et al 2015), and ring-like emission patterns are observed for AS 209, V4046 Sgr, MWC 480, and HD 163296 in this work. These results suggest that ring-like DCO + distributions are common, and support a cold H 2 D + formation scenario.…”

Thermochemical modelling of brown dwarf discs

“…However, decreasing the UV excess by only a factor of 10 relative to the fiducial model results in an increase in J = 4−3 flux of only 9%. The UV radiation efficiently ionizes atomic metals, and the resulting electrons efficiently destroy HCO + (Teague et al 2015). The increase in flux is not so dramatic for longer wavelength lines.…”

Chemistry in disks

“…As a result of the small sizes and demanding integration times, spatially resolved line observations of protoplanetary disks are still relatively rare and the radial distributions of molecules are poorly constrained (see Piétu et al 2007;Chapillon et al 2012b;Qi et al 2013a;Teague et al 2015). Also, until a few years ago, large telescopes or interferometers had limited bandwidth, so a full spectral survey required substantial observing time, often of order 100 of hours or more, as only a few lines could be observed simultaneously.…”

“…Also, until a few years ago, large telescopes or interferometers had limited bandwidth, so a full spectral survey required substantial observing time, often of order 100 of hours or more, as only a few lines could be observed simultaneously. Nonetheless, limited spatially resolved molecular surveys targeting a handful of sources in lines of several simple species were performed by the Chemistry in Disks (CID; Dutrey et al 2007Dutrey et al , 2011Schreyer et al 2008;Henning et al 2010;Chapillon et al 2012b;Teague et al 2015) and Disk Imaging Survey of Chemistry with SMA (DISCS; Öberg et al 2010, 2011) consortia.…”