Robustness of N₂H⁺ as tracer of the CO snowline

Context. 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⁺. Aims. 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. Methods. A simple chemical network was 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 were varied to determine the effects on the N₂H⁺ emission and its relation to the CO snowline. Results. 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. The N₂H⁺ column density, however, peaks 5–30 AU outside the snowline for all conditions tested. In addition to the expected N₂H⁺ layer just below the CO snow surface, models with an N₂/CO ratio 0.2 predict an N₂H⁺ layer higher up in the disk due to a slightly lower photodissociation rate for N₂ as compared to CO. The influence of this N₂H⁺ surface layer on the position of the emission peak depends on the total CO and N₂ abundances and the disk physical structure, but the emission peak generally does not trace the column density peak. A model with a total (gas plus ice) CO abundance of 3 × 10−6 with respect to H₂ fits the position of the emission peak previously observed for the TW Hya disk. Conclusions. 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 performed here, is necessary to derive a CO snowline location from N₂H⁺ observations.