Externally irradiated young stars in NGC 3603

A JWST NIRSpec catalogue of pre-main-sequence stars in a massive star formation region

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² European Space Research and Technology Centre, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
³ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands

^★ Corresponding authors; rogers@strw.leidenuniv.nl; gdemarchi@rssd.esa.int; brandl@strw.leidenuniv.nl

Received: 9 December 2024
Accepted: 7 April 2025

Abstract

Context. NGC 3603 is the optically brightest massive star forming region (SFR) in the Milky Way, representing a small scale starburst region. Studying young stars in regions like this allows us to assess how star and planet formation proceeds in a dense clustered environment with high levels of UV radiation. JWST provides the sensitivity, unbroken wavelength coverage, and spatial resolution required to study individual pre-main-sequence (PMS) stars in distant massive SFRs in detail for the first time.

Aims. We identify a population of accreting PMS sources in NGC 3603 based on the presence of hydrogen emission lines in their NIR spectra. We spectrally classify the sources, and determine their mass and age from stellar isochrones and evolutionary tracks. From this we determine the mass accretion rate Ṁ_acc of the sources and compare to samples of stars in nearby low-mass SFRs. We search for trends between M_acc and the external environment.

Methods. Using the micro-shutter assembly (MSA) on board NIRSpec, multi-object spectroscopy was performed, yielding 100 stellar spectra. Focusing on the PMS spectra, we highlight and compare the key features that trace the stellar photosphere, protoplanetary disk, and accretion. We fit the PMS spectra to derive their photospheric properties, extinction, and NIR veiling. From this, we determined the masses and ages of our sources by placing them on the Hertzsprung-Russel diagram (HRD). Their accretion rates were determined by converting the luminosity of their hydrogen emission lines to an accretion luminosity.

Results. Of the 100 stellar spectra obtained, we have classified 42 as PMS and actively accreting. Our sources span a range of masses from 0.5 to 7 M_Q. Twelve of these accreting sources have ages consistent with >10 Myr, with four having ages of >15 Myr. The mass accretion rates of our sample span 5 orders of magnitude and are systematically higher for a given stellar mass than for a comparative sample taken from low-mass SFRs. We report a relationship between M_acc and the density of interstellar molecular gas as traced by nebular H₂ emission.