Robustness of N2H+ as tracer of the CO snowline

Abstract

[Abridged] Snowlines in protoplanetary disks play an important role in planet formation and composition. Since the CO snowline is difficult to observe directly with CO emission, its location has been inferred in several disks from spatially resolved ALMA observations of DCO+ and N2H+. N2H+ is considered to be a good tracer of the CO snowline based on astrochemical considerations predicting an anti-correlation between N2H+ and gas-phase CO. In this work, the robustness of N2H+ as a tracer of the CO snowline is investigated. A simple chemical network is used in combination with the radiative transfer code LIME to model the N2H+ distribution and corresponding emission in the disk around TW Hya. The assumed CO and N2 abundances, corresponding binding energies, cosmic ray ionization rate, and degree of large-grain settling are varied to determine the effects on the N2H+ emission and its relation to the CO snowline. For the adopted physical structure of the TW Hya disk and molecular binding energies for pure ices, the balance between freeze-out and thermal desorption predicts a CO snowline at 19 AU, corresponding to a CO midplane freeze-out temperature of 20 K. A model with a total, i.e. gas plus ice, CO abundance of 3e-6 with respect to H2 fits the position of the emission peak observed by Qi et al. 2013 for the TW Hya disk. However, the relationship between N2H+ and the CO snowline is more complicated than generally assumed: for the investigated parameters, the N2H+ column density peaks at least 5 AU outside the CO snowline. Moreover, the N2H+ emission can peak much further out, as far as ~50 AU beyond the snowline. Hence, chemical modeling, as done here, is necessary to derive a CO snowline location from N2H+ observations.

16 pages, 3 tables, 15 figures. Accepted for publication in A&A

References in corpus (10)

• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• Towards Chemical Constraints on Hot Jupiter Migration
• Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
• Photodesorption of water ice: a molecular dynamics study
• Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
• An Inner Hole in the Disk around TW Hydrae Resolved in 7 Millimeter Dust Emission
• Exclusion of Cosmic Rays in Protoplanetary Disks. II. Chemical Gradients and Observational Signatures
• Chemical Modelling of Young Stellar Objects, I. Method and Benchmarks
• Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks
• Evidence for DCO+ as a probe of ionization in the warm disk surface

Cited by in corpus (46)

• Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview and Highlights
• Astrochemistry and compositions of planetary systems
• Direct mapping of the temperature and velocity gradients in discs. Imaging the vertical CO snow line around IM Lupi
• Different dust and gas radial extents in protoplanetary disks: consistent models of grain growth and CO emission
• CO and dust properties in the TW Hya disk from high-resolution ALMA observations
• Mass inventory of the giant-planet formation zone in a solar nebula analog
• Dust Density Distribution and Imaging Analysis of Different Ice Lines in Protoplanetary Disks
• Planetesimal rings as the cause of the Solar System's planetary architecture
• Annular substructures in the transition disks around LkCa 15 and J1610
• DCO${}^{+}$, DCN and N${}_2$D${}^{+}$ reveal three different deuteration regimes in the disk around the Herbig Ae star HD163296
• An unbiased ALMA spectral survey of the LkCa 15 and MWC 480 protoplanetary disks
• Increased H${}_2$CO production in the outer disk around HD 163296
• Chemically tracing the water snowline in protoplanetary disks with HCO${}^{+}$
• Carbon depletion observed inside T Tauri inner rims: Formation of icy, kilometer size planetesimals by 1 Myr
• Sulphur monoxide emission tracing an embedded planet in the HD 100546 protoplanetary disk
• A novel way of measuring the gas disk mass of protoplanetary disks using N2H+ and C18O
• Gas temperature structure across transition disk cavities
• Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation
• Tracing snowlines and C/O ratio in a planet-hosting disk: ALMA molecular line observations towards the HD169142 disk
• X-ray radiative transfer in protoplanetary disks - The role of dust and X-ray background fields
• Herbig Stars: A Quarter Century of Progress
• The TW Hya Rosetta Stone Project II: Spatially resolved emission of formaldehyde hints at low-temperature gas-phase formation
• Evidence for a Cosmic Ray Gradient in the IM Lup Protoplanetary Disk
• Destruction of refractory carbon grains drives the final stage of proto-planetary disk chemistry
• New Constraints on Protoplanetary Disk Gas Masses in Lupus
• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• If you like C/O variations, you should have put a ring on it
• Investigating the asymmetric chemistry in the disk around the young star HD 142527
• The impact of pre-main sequence stellar evolution on midplane snowline locations and C/O in planet forming discs
• Disentangling protoplanetary disk gas mass and carbon depletion through combined atomic and molecular tracers
• The Molecular Composition of Shadowed Protosolar Disk Midplanes beyond the Water Snowline
• Physical and Chemical Structure of the Disk and Envelope of the Class 0/I Protostar L1527
• ALMA reveals thermal and non-thermal desorption of methanol ice in the HD 100546 protoplanetary disk
• Planet formation in the PDS 70 system: Constraining the atmospheric chemistry of PDS 70b and c
• Linking ice and gas in the Lambda Orionis Barnard 35A cloud
• APEX-SEPIA660 Early Science: Gas at densities above $10^7$ cm${}^{-3}$ towards OMC-1
• Modeling snowline locations in protostars: The impact of the structure of protostellar cloud cores
• ALMA-IMF XVIII: The assembly of a star cluster: Dense N${}_2$H${}^{+}$ (1-0) kinematics in the massive G351.77 protocluster
• Evidence for a Sharp CO Snowline Transition in a Protoplanetary Disk and Implications for Millimeter-wave Observations of CO Isotopologues
• Line Ratios Reveal N2H+ Emission Originates Above the Midplane in TW Hydrae
• Shaping the CO snowline in protoplanetary disks
• Probing episodic accretion with chemistry: CALYPSO observations of IRAM 04191+1522. Results from the CALYPSO IRAM-PdBI survey
• Planetesimal Growth in Evolving Protoplanetary Disks: Constraints from the Pebble Supply
• Molecules with ALMA at Planet-forming Scales (MAPS) XVII: Determining the 2D Thermal Structure of the HD 163296 Disk
• Broadband spectroscopy of astrophysical ice analogues: IV. Optical constants of N${}_2$ ice in the terahertz and mid-infrared ranges
• Protoplanetary Disk Chemistry