# Time Dependent Models of Magnetospheric Accretion onto Young Stars

## TL;DR

This paper uses one-dimensional time-dependent simulations to show how smooth density variations in the inner disk can create shocks in the accretion column, explaining short-term variability in young stars.

## Contribution

It demonstrates that smooth, day-scale density variations can induce shocks in magnetospheric accretion flows, linking simple models to observed stellar variability.

## Key findings

- Shocks form in the accretion column due to density fluctuations.
- Shocks cause rapid increases in accretion luminosity.
- The model explains short-term variability in young stars.

## Abstract

Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star's surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner and the source of this variability remains an open problem, particularly on the shortest (~ day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ~ day long time-scales (e.g., due to turbulence) this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star the observed time-dependence will be a rapid increase in accretion luminosity followed by a slower decline and could be an explanation for some of the short period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated, multi-dimensional simulations, and observations.