2020ApJ...890..178C

Observations of intermediate mass (IM) star formation are expected to highlight the transition between the formation processes of its low and high-mass (HM) counterparts. H₂¹⁸O and H₂¹⁶O observations of the IM star formation region NGC 7129 FIRS 2 were obtained with the Heterodyne Instrument for the Far-Infrared aboard the Herschel Space Observatory; most as part of the WISH key-program. The radiative transfer program RATRAN was used to model water emission from the envelope of this star-forming region. We consider the envelope in two regions, inner and outer envelope, which are separated by the water freeze-out radius. An outer envelope ortho-H₂¹⁸O/H₂ abundance was determined to be 3.5 ± 0.3 x 10^–11, and an outer turbulent velocity was determined to be 2.25 ± 0.25 km s^–1. The outer envelope ortho-H₂¹⁶O/H₂ and para-H₂¹⁶O/H₂ abundances were determined to be 1.5 ± 0.5 x 10^–8 and 4.5 ± 0.5 x 10^–9, respectively. The inner envelope abundances and turbulent velocity could not be constrained due to increased optical depth. The derived values are consistent with those found by low-mass (LM) and HM young stellar object studies of water. While the line shapes and intensities of these lines are more similar to the spectral lines found for LM objects, the turbulent velocity is closer to that seen in HM objects. Lastly, we present a simple visualization tool that we created to show that these abundance results, particularly the limited extent probed by these lines, should not have been a surprise. This tool can be very useful in planning future molecular line observations.