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

Thanks to the Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST), our ability to observe the star formation process in the infrared has greatly improved.\ Due to its unprecedented spatial and spectral resolution and sensitivity in the mid-infrared, JWST/MIRI can see through highly extincted protostellar envelopes and probe the warm inner regions. An abundant molecule in these warm inner regions is SO$_2$, which is a common tracer of both outflow and accretion shocks as well as hot core chemistry. This paper presents the first mid-infrared detection of gaseous SO$_2$ emission in an embedded low-mass protostellar system rich in complex molecules and aims to determine the physical origin of the SO$_2$ emission. JWST/MIRI observations taken with the Medium Resolution Spectrometer (MRS) of the low-mass protostellar binary NGC 1333 IRAS2A in the JWST Observations of Young protoStars (JOYS+) program are presented. The observations reveal emission from the SO$_2$ $ asymmetric stretching mode at 7.35 Using simple slab models and assuming local thermodynamic equilibrium (LTE), we derived the rotational temperature and total number of SO$_2$ molecules. We then compared the results to those derived from high-angular-resolution SO$_2$ data on the same scales ($ au) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The SO$_2$ emission from the $ band is predominantly located on $ au scales around the mid-infrared continuum peak of the main component of the binary IRAS2A1. A rotational temperature of $92 K is derived from the $ 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 K) which are extended over similar scales. However, the emission of the $ lines in the MIRI-MRS spectrum is not in LTE given that the total number of molecules predicted by a LTE model is found to be a factor of $2 higher than what is derived for the $ state from the ALMA data. This difference can be explained by a vibrational temperature that is $ K higher than the derived rotational temperature of the $ state : $T_ vib K versus $T_ rot K . The brightness temperature derived from the continuum around the $ band ($ of SO$_2$ is $ K which confirms that the $ level is not collisionally populated but rather infrared-pumped by scattered radiation. This is also consistent with the non-detection of the $ bending mode at $18-20$ The similar rotational temperature derived from the MIRI-MRS and ALMA data implies that they are in fact tracing the same molecular gas. The inferred abundance of SO$_2$ determined using the LTE fit to the lines of the vibrational ground state in the ALMA data is $1.0 $ with respect to H$_2$, which is on the lower side compared to interstellar and cometary ices ($10^ Given the rotational temperature, the extent of the emission ($ au in radius), and the narrow line widths in the ALMA data ($ km $), the SO$_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. Furthermore, this paper shows the importance of radiative pumping and of combining JWST observations with those from millimeter interferometers such as ALMA to probe the physics on disk scales and to infer molecular abundances.