JOYS+: Mid-infrared detection of gas-phase SO2 emission in a low-mass protostar
Artikel i vetenskaplig tidskrift, 2024

Context. 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 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 IRAS 2A in the JWST Observations of Young protoStars (JOYS+) program are presented. The observations reveal emission from the SO2 ν3 asymmetric stretching mode at 7.35 µm. Using simple slab models and assuming local thermodynamic equilibrium (LTE), we derived the rotational temperature and total number of SO2 molecules. We then compared the results 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 mid-infrared continuum peak of the main component of the binary, IRAS 2A1. 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 of 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 versus 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 , determined 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 IRAS 2A 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.

ISM: molecules

stars: protostars

astrochemistry

ISM: individual objects: NGC 1333 IRAS 2A

stars: low-mass

stars: formation

Författare

M. L. Van Gelder

Universiteit Leiden

M. E. Ressler

California Institute of Technology (Caltech)

E. F. van Dishoeck

Universiteit Leiden

Max-Planck-Gesellschaft

P. Nazari

Universiteit Leiden

Benoît Tabone

Institut d'Astrophysique Spatiale

John H Black

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

Łukasz Tychoniec

European Southern Observatory (ESO)

L. Francis

Universiteit Leiden

M. Barsony

SETI Institute

H. Beuther

Max-Planck-Gesellschaft

A. Caratti o Garatti

Osservatorio Astronomico di Capodimonte

Y. Chen

Universiteit Leiden

C. Gieser

Max-Planck-Gesellschaft

Valentin J. M. Le Gouellec

NASA Ames Research Center

Patrick Kavanagh

Maynooth University

Pamela Klaassen

Royal Observatory

B. W.P. Lew

NASA Ames Research Center

H. Linnartz

Universiteit Leiden

L. Majumdar

National Institute of Science Education and Research

Homi Bhabha National Institute (HBNI)

G. Perotti

Max-Planck-Gesellschaft

W. R.M. Rocha

Universiteit Leiden

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 682 A78

Ämneskategorier

Astronomi, astrofysik och kosmologi

Atom- och molekylfysik och optik

DOI

10.1051/0004-6361/202348118

Mer information

Senast uppdaterat

2024-02-22