Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks

Abstract

We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of N${}_2$H${}^{+}$ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N${}_2$H${}^{+}$ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N${}_2$. Using the full gas-grain network model, we reproduced the N${}_2$H${}^{+}$ ring in a disk model with millimeter grains. The chemical conversion of CO and N${}_2$ to less volatile species (sink effect hereinafter) is found to affect the N${}_2$H${}^{+}$ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N${}_2$ abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulas, which are derived by analyzing the full-network model. The N${}_2$H${}^{+}$ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N${}_2$H${}^{+}$, however, is different among the full-network model and no-sink model. The column density of N${}_2$H${}^{+}$ in the no-sink model depends sensitively on the desorption rate of CO and N${}_2$, and the flux of cosmic ray. We also found that N${}_2$H${}^{+}$ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N${}_2$ are the same.

accepted to ApJ

References in corpus (8)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• Photoprocesses in protoplanetary disks
• Molecular Evolution and Star Formation: From Prestellar Cores to Protostellar Cores
• Line Emission from Gas in Optically Thick Dust Disks around Young Stars
• Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry
• Resolving the chemistry in the disk of TW Hydrae I. Deuterated species
• Exclusion of Cosmic Rays in Protoplanetary Disks. II. Chemical Gradients and Observational Signatures

Cited by in corpus (28)

• Direct mapping of the temperature and velocity gradients in discs. Imaging the vertical CO snow line around IM Lupi
• Robustness of N2H+ as tracer of the CO snowline
• An ALMA survey of DCN/H${}^{13}$CN and DCO${}^{+}$/H${}^{13}$CO${}^{+}$ in protoplanetary disks
• The chemistry of episodic accretion in embedded objects. 2D radiation thermo-chemical models of the post-burst phase
• Chemically tracing the water snowline in protoplanetary disks with HCO${}^{+}$
• A novel way of measuring the gas disk mass of protoplanetary disks using N2H+ and C18O
• Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO${}^{+}$ and disk ionization structure
• A three-phase approach to grain surface chemistry in protoplanetary disks: Gas, ice surfaces and ice mantles of dust grains
• X-ray radiative transfer in protoplanetary disks - The role of dust and X-ray background fields
• Spectral Line Survey toward a Molecular Cloud in IC10
• Evidence for a Cosmic Ray Gradient in the IM Lup Protoplanetary Disk
• C${}^{18}$O emission as an effective measure of gas masses of protoplanetary disks
• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• High Spatial Resolution Observations of Molecular Lines towards the Protoplanetary Disk around TW Hya with ALMA
• Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A
• Detection of HC18O+ in a protoplanetary disk: exploring oxygen isotope fractionation of CO
• Different degrees of nitrogen and carbon depletion in the warm molecular layers of protoplanetary disks
• Molecules with ALMA at Planet-forming Scales (MAPS). Complex Kinematics in the AS 209 Disk Induced by a Forming Planet and Disk Winds
• Observationally derived magnetic field strength and 3D components in the HD 142527 disk
• The Molecular Composition of Shadowed Protosolar Disk Midplanes beyond the Water Snowline
• Which Part of Dense Cores Feeds Material to Protostars?: The Case of L1489 IRS
• Physical and Chemical Structure of the Disk and Envelope of the Class 0/I Protostar L1527
• Chemical network reduction in protoplanetary disks
• APEX-SEPIA660 Early Science: Gas at densities above $10^7$ cm${}^{-3}$ towards OMC-1
• An SMA Survey of Chemistry in Disks around Herbig AeBe Stars
• The gas disk: Evolution and chemistry
• Line Ratios Reveal N2H+ Emission Originates Above the Midplane in TW Hydrae
• Impact of Size-dependent Grain Temperature on Gas-Grain Chemistry in Protoplanetary Disks: the case of low mass star disks