X-ray emission from T Tauri stars in the Lupus 3 star-forming region

European Space Agency, ESTEC – Postbus 299, 2200 AG Noordwijk, The Netherlands e-mail: pgondoin@rssd.esa.int

Received: 13 October 2005
Accepted: 15 March 2006

Abstract

Aims.In this paper, I present analysis results of an  observation of the Lupus 3 region that contains a high proportion of young low mass ( 0.3 ) T Tauri stars in the Lupus star-forming complex.

Methods.The detection of X-ray sources in 0.5 to 4.5 keV images of the Lupus 3 core was performed using the standard source detection method of the Science Analysis Software. The detected sources were correlated with a list of Herbig-Haro objects and H emission stars that contains mainly classical T Tauri stars, with a catalogue of weak-line T Tauri Stars and with a recent list of new low-mass members of the Lupus 3 dark cloud found in a visible-light spectroscopic survey at the center of the Lupus 3 star-forming core. The light curves and spectra of the brightest X-ray sources with known T Tauri star counterparts were analysed.

Results.One hundred and two X-ray sources were detected in the 30´ diameter field-of-view of the EPIC cameras, of which 25 have visible or near-IR counterparts that are known as pre-main sequence stars. Their X-ray luminosity ranges from 3  10²⁸ to 3  10³⁰ erg s^-1. Two of these objects with mass estimates lower than 0.075  have an X-ray luminosity of about 4–7  10²⁸ erg s^-1, comparable with that of flaring young brown dwarfs. A linear correlation is found between the X-ray luminosity and the mass or volume of the stars that is qualitatively expected from some models of distributed turbulent dynamos. The EPIC spectra of the X-ray brightest sources can be fitted using optically thin plasma emission models with two components at temperatures in the ranges 3–9  10⁶ K and 1–50  10⁷ K, respectively. The large emission measure of hot plasma may be caused by disruptions of magnetic fields associated with an intense flaring activity, while the X-ray emission from the “cool” plasma components may result from solar-type active regions. The emission measures of the plasma components are of the order of 10⁵² cm^-3, typical of the values expected from coronal plasmas in T Tauri stars, post-T Tauri stars, and active late-type dwarfs in close binary systems. One property of the X-ray brightest stars in Lupus 3 that seems common among pre-main sequence stars is the low abundance of Fe.