DOI: 10.48577/jpl.rgdbps
title: JOYS+: mid-infrared detection of gas-phase SO2 emission in a low-mass protostar; The case of NGC 1333 IRAS2A: hot core or accretion shock?

description: Context. The Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST) has sharpened our infrared eyes toward the star formation process. 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 SO2 , which is a common tracer of both outflow and accretion shocks as well as hot core chemistry. Aims. This paper presents the first mid-infrared detection of gaseous SO2 emission in an embedded low-mass protostellar system rich in complex molecules and aims to determine the physical origin of the SO2 emission. Methods. JWST/MIRI observations taken with the Medium Resolution Spectrometer (MRS) of the low-mass protostellar binary NGC 1333 IRAS2A are presented from the JWST Observations of Young protoStars (JOYS+) program, revealing emission from the SO2 ν3 asymmetric stretching mode at 7.35 μm. Using simple slab models and assuming local thermodynamic equilibrium (LTE), the rotational tempera- ture and total number of SO2 molecules are derived. The results are compared to those derived from high-angular resolution SO2 data on the same scales (∼ 50 − 100 au) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Results. The SO2 emission from the ν3 band is predominantly located on ∼ 50 − 100 au scales around the MIR continuum peak of the main component of the binary, IRAS2A1. A rotational temperature of 92 ± 8 K is derived from the ν3 lines. This is in good agreement with the rotational temperature derived from pure rotational lines in the vibrational ground state (i.e., ν=0) with ALMA (104 ± 5 K) which are extended over similar scales. However, the emission of the ν3 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 2 × 104 higher than what is derived for the ν = 0 state from the ALMA data. This difference can be explained by a vibrational temperature that is ∼ 100 K higher than the derived rotational temperature of the ν = 0 state: Tvib ∼ 200 K vs Trot = 104 ± 5 K. The brightness temperature derived from the continuum around the ν3 band (∼ 7.35 μm) of SO2 is ∼ 180 K, which confirms that the ν3 = 1 level is not collisionally populated but rather infrared pumped by scattered radiation. This is also consistent with the non-detection of the ν2 bending mode at 18 − 20 μm. 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 SO2 , using the LTE fit to the lines of the vibrational ground state in the ALMA data, is 1.0 ± 0.3 × 10−8 with respect to H2, which is on the lower side compared to interstellar and cometary ices (10−8 − 10−7 ). Conclusions. Given the rotational temperature, the extent of the emission (∼ 100 au in radius), and the narrow line widths in the ALMA data (∼ 3.5 km s−1 ), the SO2 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.