Spectrally resolved pure rotational lines of water in protoplanetary disks

TL;DR

This study uses high-resolution ground-based spectroscopy to analyze water vapor in protoplanetary disks, revealing details about their structure, temperature, and excitation conditions, and demonstrating the feasibility of such observations.

Contribution

First high-resolution ground-based spectra of water in protoplanetary disks, showing non-LTE effects and linking water emission to disk radii similar to CO.

Findings

Water lines originate from ~1 AU in disks.

Water rotational temperatures are 540-600K.

Evidence of non-LTE excitation in water lines.

Abstract

We present ground-based high resolution N-band spectra (\Delta v = 15 km/s) of pure rotational lines of water vapor in two protoplanetary disks surrounding the pre-main sequence stars AS 205N and RNO 90, selected based on detections of rotational water lines by the Spitzer IRS. Using VISIR on the Very Large Telescope, we spectrally resolve individual lines and show that they have widths of 30-60 km/s, consistent with an origin in Keplerian disks at radii of ~1 AU. The water lines have similar widths to those of the CO at 4.67 micron, indicating that the mid-infrared water lines trace similar radii. The rotational temperatures of the water are 540 and 600K in the two disks, respectively. However, the lines ratios show evidence of non-LTE excitation, with low-excitation line fluxes being over-predicted by 2-dimensional disk LTE models. Due to the limited number of observed lines and the non-LTE line ratios, an accurate measure of the water ortho/para ratio is not available, but a best estimate for AS 205N is ortho/para = 4.5 +/- 1.0, apparently ruling out a low-temperature origin of the water. The spectra demonstrate that high resolution spectroscopy of rotational water lines is feasible from the ground, and further that ground-based high resolution spectroscopy is likely to significantly improve our understanding of the inner disk chemistry recently revealed by recent Spitzer observations.