Warm H2O and OH disk emission in V1331 Cyg

TL;DR

This study presents high-resolution L-band spectra of V1331 Cyg showing warm water and OH emission from its circumstellar disk, demonstrating the effectiveness of the BT2 water line list and revealing insights into disk chemistry and structure.

Contribution

First application of the BT2 water line list to model warm water emission in a young star's disk, improving spectral fits and understanding of disk chemistry.

Findings

Water and OH emission lines have similar widths (~18 km/s).

Emission lines originate from a region with temperature ~1500 K.

High water column density can shield the disk midplane from UV radiation.

Abstract

We present high resolution (R=24,000) L-band spectra of the young intermediate mass star V1331 Cyg obtained with NIRSPEC on the Keck II telescope. The spectra show strong, rich emission from water and OH that likely arises from the warm surface region of the circumstellar disk. We explore the use of the new BT2 (Barber et al. 2006) water line list in fitting the spectra, and we find that it does a much better job than the well-known HITRAN (Rothman et al. 1998) water line list in the observed wavelength range and for the warm temperatures probed by our data. By comparing the observed spectra with synthetic disk emission models, we find that the water and OH emission lines have similar widths (FWHM ~ 18 km s-1). If the line widths are set by disk rotation, the OH and water emission lines probe a similar range of disk radii in this source. The water and OH emission are consistent with thermal emission for both components at a temperature ~ 1500 K. The column densities of the emitting water and OH are large, ~ 10^{21} cm-2 and ~ 10^{20} cm-2, respectively. Such a high column density of water is more than adequate to shield the disk midplane from external UV irradiation in the event of complete dust settling out of the disk atmosphere, enabling chemical synthesis to continue in the midplane despite a harsh external UV environment. The large OH-to-water ratio is similar to expectations for UV irradiated disks (e.g., Bethell and Bergin 2009), although the large OH column density is less easily accounted for.