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

TL;DR

This study uses JWST NIRSpec spectra to analyze hydrogen emission lines in Herbig Ae stars, providing evidence that magnetospheric accretion is the primary mechanism for their accretion processes.

Contribution

It presents the first kinematic analysis of hydrogen lines in Herbig Ae stars using JWST data, confirming magnetospheric accretion as the dominant process.

Findings

High-energy hydrogen lines are broader, indicating higher velocities.

Optical depth varies with velocity, supporting magnetospheric accretion.

Magneto-centrifugal winds are inconsistent with observations.

Abstract

Hydrogen emission lines have been used to estimate the mass accretion rate of pre-main-sequence stars for over $25$ years, although the physical origin of these lines is still unclear. Magnetospheric accretion (MA) and magneto-centrifugal winds are the two most often invoked mechanisms. Using a combination of HST photometry and new JWST NIRSpec spectra in the range $1.66 - 3.2 \; \mu m$, we analysed the emission line spectra of five sources to attempt to reveal the physical origin of their hydrogen emission lines. These sources reside in NGC 3603, a Galactic massive star forming region. 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). We studied the full width at half maximum and optical depth of the 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. 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 $T_{eff}$. 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. Emission from magneto-centrifugal winds is not consistent with any of our observations. Two sources are consistent with emission from a Keplerian disc, or MA. The remaining three sources are only consistent with emission from MA. In the future, this approach can be applied to more statistically significant samples of Herbig AeBe spectra, including existing archival observations.