Pre main sequence: Accretion & Outflows

TL;DR

This paper discusses how X-ray observations of low-mass pre-main sequence stars reveal critical insights into accretion processes and outflows, which are essential for understanding star and planet formation.

Contribution

It provides a comprehensive review of how X-ray data elucidate the physics of accretion and outflows in young stars, highlighting complex geometries and jet-related phenomena.

Findings

X-ray emissions originate from accretion shocks and jet interactions.

Accretion geometry involves complex, shielded regions emitting X-rays.

Jet X-rays trace the fastest components and inform jet launching mechanisms.

Abstract

Low-mass pre-main sequence (PMS) stars are strong X-ray sources, because they possess hot corona like their older main-sequence counterparts. Unique to young stars, however, are X-rays from accretion and outflows, and both processes are of pivotal importance for star and planet formation. We describe how X-ray data provide important insight into the physics of accretion and outflows. First, mass accreted from a circumstellar disk onto the stellar surface reaches velocities up to a few hundred km/s, fast enough to generate soft X-rays in the post-shock region of the accretion shock. X-ray observations together with laboratory experiments and numerical simulations show that the accretion geometry is complex in young stars. Specifically, the center of the accretion column is likely surrounded by material shielding the inner flow from view but itself also hot enough to emit X-rays. Second, X-rays are observed in two locations of protostellar jets: an inner stationary emission component probably related to outflow collimation and outer components, which evolve withing years and are likely related to working surfaces where the shock travels through the jet. Jet-powered X-rays appear to trace the fastest jet component and provide novel information on jet launching in young stars. We conclude that X-ray data will continue to be highly important for understanding star and planet formation, because they directly probe the origin of many emission features studied in other wavelength regimes. In addition, future X-ray missions will improve sensitivity and spectral resolution to probe key model parameters (e.g. velocities) in large samples of PMS stars.