Spectral characterisation of the extinction properties of NGC 3603 using JWST NIRSpec

Context. A necessary ingredient in understanding the star formation history of a young cluster is knowledge of the extinction towards the region. This has typically been done by making use of the colour-difference method with photometry, or similar methods utilising the colour-colour diagram. These approaches rely on adopting an extinction law with a given total-to-selective extinction ratio R(V), or determining a value of R(V) through empirical relationships. They also rely upon accurate spectral classification, reliable stellar isochrones, and separating field stars from genuine cluster members. Aims. The colour excess E(B − V) can be independently determined by studying the decrements of the recombination lines produced by the nebular gas. Having access to many recombination lines from the same spectral series removes the need of adopting an extinction curve. Rather, different extinction curves can be trialled and the most appropriate one selected based on a minimum χ² procedure. Methods. Using the Micro-Shutter Assembly (MSA) on board the Near InfraRed Spectrograph (NIRSpec), multi-object spectroscopy was performed, yielding 600 nebular spectra from the Galactic massive star formation region NGC 3603. The recombination line intensity ratios were used to determine independent values of E(B − V). A series of extinction curves were trialled ranging from R(V) = 2 to R(V) = 8. The appropriate value of R(V) was adopted based on the minimum χ² procedure. Results. The extinction characteristics of NGC 3603 are similar to other Galactic HII regions like Orion, as well as starburst regions such 30 Doradus in the Large Magellanic Cloud, in that we find a relatively large value of R(V) = 4.8 ± 1.06, larger than the Galactic average of 3.1. We find a typical value of E(B− V) = 0.64 ± 0.27, significantly lower than values determined in previous studies. We also present a stacked nebular spectrum with a typical continuum signal-to-noise (S/N) = 70. This spectrum highlights the recombination lines of the HII region, several s-process elements such as Kr III and Se IV, and molecular H₂ emission lines. This high S/N spectrum can act as a helpful template for identifying nebular emission lines. Conclusions. Using ratios of hydrogen recombination lines, we calculated the value of R(V), E(B − V) and A(V) for > 200 lines of sight across NGC 3603. An extinction curve with a typical value R(V) = 4.8 ± 1.06 is required to explain the colour excess observed in the nebular spectra. This corresponds to a typical E(B − V) = 0.64 ± 0.27. This is significantly lower than what has been found in previous extinction studies of NGC 3603.