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

• Astronomy and Astrophysics

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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.