Kinematic evidence of magnetospheric accretion for Herbig Ae stars with JWST NIRSpec

Abstract

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.