On the origin of the correlations between the accretion luminosity and emission line luminosities in pre-main sequence stars

TL;DR

This paper demonstrates that observed correlations between accretion luminosity and emission line luminosities in pre-main sequence stars are primarily due to their mutual correlation with stellar luminosity, challenging the assumption of a direct physical link.

Contribution

It reveals that L_acc-L_line correlations are indirect, stemming from the L_acc-L_star relationship, and suggests using ratios like L_acc/L_star for physically meaningful analysis.

Findings

L_acc-L_line correlations result from L_acc-L_star correlation.

Using L_acc/L_star and L_line/L_star provides better physical insights.

Steeper L_acc dependence on L_star in T-Tauri stars explained by magnetospheric accretion.

Abstract

Correlations between the accretion luminosity and emission line luminosities (L_acc and L_line) of pre-main sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) L_acc-L_line correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L_star) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the L_acc-L_line correlations depend on the L_acc-L_star relationship. We conclude that because PMS stars show the L_acc-L_star correlation immediately implies that L_acc also correlates with the luminosity of all emission lines, for which the L_acc-L_line correlations alone do not prove any physical connection with accretion but can only be used with practical purposes to roughly estimate accretion rates. When looking for correlations with possible physical meaning, we suggest that L_acc/L_star and L_line/L_star should be used instead of L_acc and L_line. Finally, the finding that L_acc has a steeper dependence on L_star for T-Tauri stars than for intermediate-mass Herbig Ae/Be stars is also discussed. That is explained from the magnetospheric accretion scenario and the different photospheric properties in the near-UV.