Understanding Balmer Decrements in T Tauri stars in terms of Multiflow Magnetospheric Accretion

TL;DR

This study introduces a two-flow magnetospheric accretion model for T Tauri stars, successfully explaining observed Balmer decrements and line fluxes by fitting data from 139 stars across multiple star-forming regions.

Contribution

The paper presents a novel two-flow magnetospheric accretion model that better reproduces Balmer line emissions in T Tauri stars compared to standard single-flow models.

Findings

Two coexisting accretion flows with distinct geometries and rates fit the observed Balmer decrements.

The model reproduces the empirical Hα luminosity and accretion luminosity correlation.

A multicolumn approach aligns with numerical simulations of magnetospheric accretion.

Abstract

Magnetospheric accretion is the paradigm for accretion in Classical T-Tauri Stars (CTTS). However, the standard, one-flow magnetospheric accretion model fails to replicate important characteristics such as the observed Balmer decrements. We address this limitation by adopting a model with two axisymmetric magnetospheric accretion flows of different accretion rates and geometries. We calculate the fluxes of the hydrogen $H_\alpha$, $H_\beta$, and $H_\gamma$ lines of each flow with the magnetospheric accretion model and use Bayesian statistics to fit the Balmer line fluxes of 139 CTTS in the Orion OB1b subassociation, and in the Upper Scorpius, Lupus and Chamaeleon I star-forming regions. We find that the Balmer decrements and line fluxes can be fitted by two distinct but coexisting flows: a compact, high accretion rate flow, close to the star and narrow (mean inner radius $R_i \sim 2.9 R_*$ and mean width $\Delta R \sim 0.7 R_*$), covering a few percent of the emitting area, and a more spread out flow, thicker ($\Delta R \sim 1.2 R_*$), and larger ($R_i \sim 3.7 R_*$), with lower accretion rate, encompassing the rest of the emitting area. The two-flow model can also reproduce the empirical correlation between the luminosity in $H_\alpha$ and the accretion luminosity. Overall, our findings suggest that a multicolumn approach provides a more accurate representation of the observed Balmer line emission, in agreement with results of numerical simulations.