JOYS+: mid-infrared detection of gas-phase SO$_2$ emission in a low-mass protostar. The case of NGC 1333 IRAS2A: hot core or accretion shock?

Abstract: JWST/MIRI has sharpened our infrared eyes toward the star formation process. This paper presents the first mid-infrared detection of gaseous SO$_2$ emission in an embedded low-mass protostellar system. MIRI-MRS observations of the low-mass protostellar binary NGC 1333 IRAS2A are presented from the JWST Observations of Young protoStars (JOYS+) program, revealing emission from the SO$_2~\nu_3$ asymmetric stretching mode at 7.35 micron. The results are compared to those derived from high-angular resolution SO$_2$ data obtained with ALMA. The SO$_2$ emission from the $\nu_3$ band is predominantly located on $\sim50-100$ au scales around the main component of the binary, IRAS2A1. A rotational temperature of $92\pm8$ K is derived from the $\nu_3$ lines. This is in good agreement with the rotational temperature derived from pure rotational lines in the vibrational ground state (i.e., $\nu=0$) with ALMA ($104\pm5$ K). However, the emission of the $\nu_3$ lines is not in LTE given that the total number of molecules predicted by a LTE model is found to be a factor $2\times10^4$ higher than what is derived for the $\nu=0$ state. This difference can be explained by a vibrational temperature that is $\sim100$ K higher than the derived rotational temperature of the $\nu=0$ state. The brightness temperature derived from the continuum around the $\nu_3$ band of SO$_2$ is $\sim180$ K, which confirms that the $\nu_3=1$ level is not collisionally populated but rather infrared pumped by scattered radiation. This is also consistent with the non-detection of the $\nu_2$ bending mode at 18-20 micron. Given the rotational temperature, the extent of the emission ($\sim100$ au in radius), and the narrow line widths in the ALMA data (3.5 km/s), the SO$_2$ in IRAS2A likely originates from ice sublimation in the central hot core around the protostar rather than from an accretion shock at the disk-envelope boundary.