SOFIA/EXES Survey of Gaseous Water in the Binary Hot Core W3 IRS 5

The formation and evolution of massive stars are not well understood because they are deeply embedded. As the disk photosphere of massive protostars serves as a pencil beam mid-infrared (MIR) continuum background, high-resolution absorption spectroscopy at MIR wavelengths provides a unique opportunity to probe the physical conditions and organic inventory of warm, dense gas in the innermost regions of massive star formation, and avoids beam dilution issues that submillimeter observations are subject to.

In this talk, we will present 5–8 μm spectroscopy (R~50,000) toward the hot core region associated with the massive young binary W3 IRS 5 using SOFIA/EXES. Hundreds of individual rovibrational absorption lines of water are identified, and three kinematic components are separated. These components are analyzed with rotation diagrams and curve-of-growth analyses to derive the temperature and the column density: two are over 500 K and one is ~190 K. Each component is linked to a CO component identified in IRTF/iSHELL observations (R=88,100) through their kinematic and temperature characteristics, and we discuss the chemical abundances along the line of sight based on the CO-to-H2O comparison. In the hot gas, all oxygen not locked in CO resides in water. In the cold gas, we observe a substantial shortfall of oxygen and suggest that the potential carrier could be organics in ices. Finally, we discuss how our high-resolution SOFIA study will help the analysis of low spectral resolution JWST observations.