Herschel-HIFI observations of high-J CO lines in the NGC 1333 low-mass star-forming region

TL;DR

This study uses Herschel-HIFI observations of high-J CO lines in low-mass protostars to analyze gas temperatures, dynamics, and ice evaporation zones, revealing broad shocked gas components and a CO ice evaporation threshold.

Contribution

It provides new observational evidence for a CO ice evaporation zone around low-mass protostars using high-J CO line data and radiative transfer modeling.

Findings

Broad high-velocity CO emission indicates shocked gas at 100-200 K.

Detection of a CO abundance jump at 25 K suggests ice evaporation.

Line profiles reveal both shocked and quiescent gas components.

Abstract

Herschel-HIFI observations of high-J lines (up to J_u=10) of 12CO, 13CO and C18O are presented toward three deeply embedded low-mass protostars, NGC 1333 IRAS 2A, IRAS 4A, and IRAS 4B, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. The spectrally-resolved HIFI data are complemented by ground-based observations of lower-J CO and isotopologue lines. The 12CO 10-9 profiles are dominated by broad (FWHM 25-30 km s^-1) emission. Radiative transfer models are used to constrain the temperature of this shocked gas to 100-200 K. Several CO and 13CO line profiles also reveal a medium-broad component (FWHM 5-10 km s^-1), seen prominently in H2O lines. Column densities for both components are presented, providing a reference for determining abundances of other molecules in the same gas. The narrow C18O 9-8 lines probe the warmer part of the quiescent envelope. Their intensities require a jump in the CO abundance at an evaporation temperature around 25 K, thus providing new direct evidence for a CO ice evaporation zone around low-mass protostars.