Hot and dense water in the inner 25 AU of SVS13-A

TL;DR

This study detects hot, dense water vapor in the inner 25 AU of the SVS13-A protostar, providing evidence for a hot corino in a Class I object and insights into water chemistry during early star formation.

Contribution

First detection of a hot corino around a Class I protostar using high-sensitivity spectral survey and non-LTE analysis.

Findings

Detection of at least 6 HDO lines with high upper energy levels.

Presence of very hot (150-260 K) and dense (>3×10^7 cm^-3) gas within 25 AU.

Water deuteration remains high (>1.5%) from Class 0 to Class I stages.

Abstract

In the context of the ASAI (Astrochemical Surveys At IRAM) project, we carried out an unbiased spectral survey in the millimeter window towards the well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12 mK) allowed us to detect at least 6 HDO broad (FWHM ~ 4-5 km/s) emission lines with upper level energies up to Eu = 837 K. A non-LTE LVG analysis implies the presence of very hot (150-260 K) and dense (> 3 10^7 cm-3) gas inside a small radius ($\sim$ 25 AU) around the star, supporting, for the first time, the occurrence of a hot corino around a Class I protostar. The temperature is higher than expected for water molecules are sublimated from the icy dust mantles (~ 100 K). Although we cannot exclude we are observig the effects of shocks and/or winds at such small scales, this could imply that the observed HDO emission is tracing the water abundance jump expected at temperatures ~ 220-250 K, when the activation barrier of the gas phase reactions leading to the formation of water can be overcome. We derive X(HDO) ~ 3 10-6, and a H2O deuteration > 1.5 10-2, suggesting that water deuteration does not decrease as the protostar evolves from the Class 0 to the Class I stage.