Cyanide photochemistry and fractionation in the MWC 480 disk

Cyanide photochemistry and fractionation in the MWC 480 disk

Authors:

Guzmán et al

Abstract:

HCN is a commonly observed molecule in Solar System bodies and in interstellar environments. Its abundance with respect to CN is a proposed tracer of UV exposure. HCN is also frequently used to probe the thermal history of objects, by measuring its degree of nitrogen fractionation. To address the utility of HCN as a probe of disks, we present ALMA observations of CN, HCN, H13CN and HC15N toward the protoplanetary disk around Herbig Ae star MWC480, and of CN and HCN toward the disk around T Tauri star DM Tau. Emission from all molecules is clearly detected and spatially resolved, including the first detection of HC15N in a disk. Toward MWC 480, CN emission extends radially more than 1" exterior to the observed cut-off of HCN emission. Quantitative modeling further reveals very different radial abundance profiles for CN and HCN, with best-fit outer cut-off radii of >300 AU and 110+-10 AU, respectively. This result is in agreement with model predictions of efficient HCN photodissociation into CN in the outer-part of the disk where the vertical gas and dust column densities are low. No such difference in CN and HCN emission profiles are observed toward DM Tau, suggestive of different photochemical structures in Herbig Ae and T Tauri disks. We use the HCN isotopologue data toward the MWC 480 disk to provide the first measurement of the 14N/15N ratio in a disk. We find a low disk averaged 14N/15N ratio of 200+-100, comparable to what is observed in cloud cores and comets, demonstrating interstellar inheritance and/or efficient nitrogen fractionation in this disk.

H2CO in the DM Tauri's Protoplanetary Disk

The Distribution and Chemistry of H2CO in the DM Tau Protoplanetary Disk

Authors:

Loomis et al

Abstract:

H2CO ice on dust grains is an important precursor of complex organic molecules (COMs). H2CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two different formation pathways: on icy grain-surfaces and in the gas-phase. We use archival ALMA observations of the resolved distribution of H2CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H2CO emission has a centrally peaked and radially broad brightness profile (extending out to 500 AU). We compare these observations with disk chemistry models with and without grain-surface formation reactions, and find that both gas and grain-surface chemistry are necessary to explain the spatial distribution of the emission. Gas-phase H2CO production is responsible for the observed central peak, while grain-surface chemistry is required to reproduce the emission exterior to the CO snowline (where H2CO mainly forms through the hydrogenation of CO ice before being non-thermally desorbed). These observations demonstrate that both gas and grain-surface pathways contribute to the observed H2CO in disks, and that their relative contributions depend strongly on distance from the host star.

First Detection of [C I] 3P1-3P0 Emission from a Protoplanetary Disk

First Detection of [C I] 3P1-3P0 Emission from a Protoplanetary Disk

Authors:

Tsukagoshi et al

Abstract:

We performed single point [C I] 3P1-3P0 and CO J=4-3 observations toward three T Tauri stars, DM Tau, LkCa 15, and TW Hya, using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 qualification model receiver installed on the Atacama Submillimeter Telescope Experiment (ASTE). Two protostars in the Taurus L1551 region, L1551 IRS 5 and HL Tau, were also observed. We successfully detected [C I] emission from the protoplanetary disk around DM Tau as well as the protostellar targets. The spectral profile of the [C I] emission from the protoplanetary disk is marginally single-peaked, suggesting that atomic carbon (C) extends toward the outermost disk. The detected [C I] emission is optically thin and the column densities of C are estimated to be less than ~1016 cm−2 and ~1017 cm−2 for the T Tauri star targets and the protostars, respectively. We found a clear difference in the total mass ratio of C to dust, M(C)/M(dust), between the T Tauri stars and protostellar targets; the M(C)/M(dust) ratio of the T Tauri stars is one order of magnitude smaller than that of the protostars. The decrease of the estimated M(C)/M(dust) ratios for the disk sources is consistent with a theoretical prediction that the atomic C can survive only in the near surface layer of the disk and C+/C/CO transition occurs deeper into the disk midplane.

A High Deuterium Ratio is From Continued D Fractionation in Protoplanetary Disk

Chemistry in disks

Authors:

Teague et al

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 RD = 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 (9.8+0.3-0.7) × 1012 and (1.2 ± 0.7) × 1012 cm-2 respectively. Where both HCO+ and DCO+ were present, RD 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 RD. 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 RD is indicative of continued deuterium fractionation in a protoplanetary disk after pre/protostellar phases. Radial properties of RD 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.