High-cadence, High-resolution Spectroscopic Observations of Herbig Stars HD 98922 and V1295 Aquila

TL;DR

This study uses high-cadence, high-resolution spectra of Herbig stars HD 98922 and V1295 Aql to investigate accretion and outflow mechanisms, finding they differ from T Tauri star models and vary over multiple timescales.

Contribution

It introduces a new correlation metric for spectral line evolution and combines interferometry with radiative transfer modeling to analyze Herbig star accretion processes.

Findings

Variability is strongest in blueshifted Balmer absorption components.

Spectral line morphologies are inconsistent with scaled-up T Tauri magnetospheric accretion.

High-cadence spectra reveal static and evolving features at different velocities.

Abstract

Recent observational work has indicated that mechanisms for accretion and outflow in Herbig Ae/Be star-disk systems may differ from magnetospheric accretion (MA) as it is thought to occur in T Tauri star-disk systems. In this work, we assess the temporal evolution of spectral lines probing accretion and mass loss in Herbig Ae/Be systems and test for consistency with the MA paradigm. For two Herbig Ae/Be stars, HD 98922 (B9e) and V1295 Aql (A2e), we have gathered multi-epoch (~years) and high-cadence (~minutes) high-resolution optical spectra to probe a wide range of kinematic processes. Employing a line equivalent width evolution correlation metric introduced here, we identify species co-evolving (indicative of common line origin) via novel visualization. We interferometrically constrain often problematically degenerate parameters, inclination and inner disk radius, allowing us to focus on the structure of the wind, magnetosphere, and inner gaseous disk in radiative transfer models. Over all timescales sampled, the strongest variability occurs within the blueshifted absorption components of the Balmer series lines; the strength of variability increases with the cadence of the observations. Finally, high-resolution spectra allow us to probe substructure within the Balmer series' blueshifted absorption components: we observe static, low-velocity features and time-evolving features at higher velocities. Overall, we find the observed line morphologies and variability are inconsistent with a scaled-up T Tauri MA scenario. We suggest that as magnetic field structure and strength change dramatically with increasing stellar mass from T Tauri to Herbig Ae/Be stars, so too may accretion and outflow processes.