H$_2$D$^+$ line emission in Proto-Planetary Disks

TL;DR

This study provides detailed theoretical predictions for the H$_2$D$^+$ 372.4 GHz line emission in proto-planetary disks, demonstrating its potential as a diagnostic tool for disk mid-plane conditions relevant to planet formation.

Contribution

It offers the first comprehensive modeling of the line intensity, profile, and maps for representative disk models, highlighting the line's diagnostic power and observational prospects with ALMA.

Findings

The line probes the disk mid-plane out to tens of AU.

The line emission varies with disk radius and height, revealing physical conditions.

Spectral profiles often show a double peak at various inclinations.

Abstract

%Context: {Previous studies have indicated that the 372.4 GHz ground transition of ortho-H$_2$D$^+$ might be a powerful probe of Proto-Planetary Disks. The line could be especially suited for study of the disk mid-plane, where the bulk of the mass resides and where planet formation takes place.} %Aims: {Provide detailed theoretical predictions for the line intensity, profile and maps expected for representative disk models.} %Methods: {We determine the physical and chemical structure of the disks from the model developed by Ceccarelli & Dominik (2005). The line emission is computed with the new radiative transfer method developed recently by Elitzur & Asensio Ramos (2006).} %Results: {We present intensity maps convolved with the expected ALMA resolution, which delineate the origin of the H$_2$D$^+$ 372.4 GHz line. In the disk inner regions, the line probes the conditions in the mid-plane out to radial distances of a few tens of AU, where Solar-like planetary systems might form. In the disk outermost regions, the line originates from slightly above the mid-plane. When the disk is spatially resolved, the variation of line profile across the image provides important information about the velocity field. Spectral profiles of the entire disk flux show a double peak shape at most inclination angles.} %Conclusions: {Our study confirms that the 372.4 GHz H$_2$D$^+$ line provides powerful diagnostics of the mid-plane of Proto-Planetary Disks. Current submillimeter telescopes are capable of observing this line, though with some difficulties. The future ALMA interferometer will have the sensitivity to observe and even spatially resolve the H$_2$D$^+$ line emission.}