Hot water in the inner 100 AU of the Class 0 protostar NGC1333 IRAS2A

TL;DR

This study uses Herschel-HIFI spectra and radiative transfer modeling to reveal a hot water-rich core within 100 AU of a low-mass protostar, challenging previous assumptions of lower water abundance.

Contribution

It demonstrates that the hot core's water abundance is higher than prior estimates, highlighting the importance of accurate physical models in interpreting observational data.

Findings

Hot water abundance is at least 2x10^-5, close to theoretical predictions.

The hot core has a radius of about 100 AU.

Revised HDO/H2O ratio is 1x10^-3, lower than previous estimates.

Abstract

Evaporation of water ice above 100 K in the inner few 100 AU of low-mass embedded protostars (the so-called hot core) should produce quiescent water vapor abundances of ~10^-4 relative to H2. Observational evidence so far points at abundances of only a few 10^-6. However, these values are based on spherical models, which are known from interferometric studies to be inaccurate on the relevant spatial scales. Are hot cores really that much drier than expected, or are the low abundances an artifact of the inaccurate physical models? We present deep velocity-resolved Herschel-HIFI spectra of the 3(12)-3(03) lines of H2-16O and H2-18O (1097 GHz, Eup/k = 249 K) in the low-mass Class 0 protostar NGC1333 IRAS2A. A spherical radiative transfer model with a power-law density profile is unable to reproduce both the HIFI data and existing interferometric data on the H2-18O 3(13)-2(20) line (203 GHz, Eup/k = 204 K). Instead, the HIFI spectra likely show optically thick emission from a hot core with a radius of about 100 AU. The mass of the hot core is estimated from the C18O J=9-8 and 10-9 lines. We derive a lower limit to the hot water abundance of 2x10^-5, consistent with the theoretical predictions of ~10^-4. The revised HDO/H2O abundance ratio is 1x10^-3, an order of magnitude lower than previously estimated.