Context. Hydrogen emission lines have been used to estimate the mass accretion rate of pre-main-sequence stars for over 25 years. Despite the clear correlation between the accretion luminosity of a star and hydrogen line luminosities, the physical origin of these lines is still unclear. Magnetospheric accretion (MA) and magneto-centrifugal winds are the two most often invoked mechanisms. Aims. Using a combination of HST photometry and new JWST NIRSpec spectra in the range 1.66−3.2 µm, we analysed the spectral energy distributions (SEDs) and emission line spectra of five sources in order to determine their underlying photospheric properties and attempt to reveal the physical origin of their hydrogen emission lines. These sources reside in NGC 3603, a Galactic massive star forming region. Methods. We performed fits of the SEDs of the five sources employing a Markov chain Monte Carlo exploration to estimate T_eff, R_∗, M_∗, and A(V) for each source. We performed a kinematic analysis across three spectral series of hydrogen lines (Paschen, Brackett, and Pfund), totalling ≥15 lines per source. We studied the full width at half maximum (FWHM) and optical depth of the spectrally resolved lines in order to constrain the emission origin. We calculated the expected velocities from MA as well as gas in Keplerian orbit for our sources. Results. All five sources have SEDs consistent with young intermediate-mass stars. We classified three of these sources as Herbig Ae type stars based on their T_eff. Their hydrogen lines show broad profiles with FWHMs ≥200 km s⁻¹. Hydrogen lines with high upper energy levels n_up tend to be significantly broader than lines with a lower n_up. The optical depth of the emission lines is also highest for the high-velocity component of each line, and it becomes optically thin in the low-velocity component. Three sources show FWHMs that are too broad to originate from Keplerian rotation, but they are consistent with MA. The remaining two sources have FWHMs that are consistent with both MA and Keplerian rotation. Conclusions. The highest excitation lines have the largest FWHM for a given series. The highest-velocity component of the lines is also the most optically thick. This is consistent with emission from MA or a Keplerian disc, but it cannot be explained as originating in a magneto-centrifugal wind. Based on the expected velocities from MA and a Keplerian disc, we favour MA for the three Herbig Ae stars. We cannot rule out Keplerian disc emission for the remaining two sources. In the future, this approach can be applied to more statistically significant samples of Herbig AeBe spectra, including existing archival observations.

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 Ṁ_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_☉. 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 Ṁ_acc and the density of interstellar molecular gas as traced by nebular H₂ emission.

In this Letter, we present the first systematic spectroscopic measurements of the near-infrared (NIR) hydrogen recombination lines Paschen alpha (Pa_αλ = 1.875 µm) and Brackett beta (Br_βλ = 2.626 µm), produced by pre-main-sequence (PMS) stars. Such stars include T Tauri and Herbig AeBe stars, located in the massive Galactic star-forming region NGC 3603. We used measurements obtained from JWST NIRSpec, using multi-object spectroscopy (MOS) mode. Based on the existing empirical relations between L_acc and L_Brγ from the literature, we used our new measurements to formulate, for the first time, an empirical relationship between the accretion luminosity, L_acc, of the stars and the line luminosities, L_line, of both Pa_α and Br_β. These relationships are: log₁₀(Equation presented) = 1.42(±0.18) × log₁₀(Equation presented) + 3.33(±0.42) and log₁₀(Equation presented) = 1.47(±0.18) × log₁₀(Equation presented) + 4.60(±0.57). These new relationships are key to establishing rough estimates of the accretion rates for large samples of PMS stars with JWST.

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.