X-ray Properties of Low-Mass Pre-Main Sequence Stars in the Orion Trapezium Cluster

TL;DR

This study presents high-resolution X-ray spectra of six young, low-mass stars in the Orion Trapezium, revealing persistent brightness, coronal plasma characteristics, and activity evolution over 0.1 to 10 million years.

Contribution

First comprehensive high-resolution X-ray spectral analysis of very young low-mass stars in a massive stellar cluster, highlighting coronal properties and activity evolution.

Findings

Stars are persistently bright with no flares during observations.

X-ray spectra fit with two-temperature plasma models at 10 MK and 40 MK.

Coronal activity and loop sizes increase with stellar age from 0.1 to 10 Myrs.

Abstract

The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster which provides high resolution X-ray spectra of very young stars over a wide mass range (0.7 - 2.3 Msun). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts which are well-characterized at optical and infra-red wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above $10^{31}$ erg/s, in some cases exceeding $10^{32}$ erg/s for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total EMs range between 3 - 8$\times10^{54}$ cm$^{-3}$ and are comparable to active coronal sources. Limits on the forbidden to inter-combination line ratios in the He-Like K-shell lines show that we observe a predominantely optically thin plasma with electron densities below $10^{12}$ cm$^{-3}$. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri stars shows that coronal activity and possibly coronal loop size increase significantly between ages 0.1 to 10 Myrs.