The disk-bearing young star IM Lupi^*

X-ray properties and limits on accretion

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: moritz.guenther@hs.uni-hamburg.de
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
⁵ Department of Astronomy, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁶ Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA
⁷ University of Southern Queensland, Toowoomba Qld 4350, Australia

Received: 9 March 2010
Accepted: 13 May 2010

Abstract

Context. Classical T Tauri stars (CTTS) differ in their X-ray signatures from older pre-main sequence stars, e.g., weak-lined TTS (WTTS). CTTS exhibit a soft excess and deviations from the low-density coronal limit in the He-like triplets.

Aims. We test whether these features correlate with either accretion or the presence of a disk by observing IM Lup, a disk-bearing object apparently in transition between CTTS and WTTS without obvious accretion.

Methods. We analyse a Chandra grating spectrum and additional XMM-Newton data of IM Lup and accompanying optical spectra, some of which where taken simultaneously with the X-ray observations. We fit the X-ray emission lines and decompose the Hα emission line into different components.

Results. In X-rays, IM Lup has a bright and hot active corona, where elements with low first-ionisation potential are depleted. The He-like Ne ix triplet is in the low-density state, but because of the small number of counts in the data a high-density scenario cannot be excluded at the 90% confidence level. In terms of all its X-ray properties, IM Lup resembles a main-sequence star, but is also compatible with CTTS signatures at the 90% confidence level, thus we cannot decide whether the soft excess and deviations from the low-density coronal limit for the He-like triplets in CTTS are produced by accretion or only the presence of a disk. The star IM Lup is chromospherically active, which accounts for most of its emission in Hα. Despite its low equivalent width, the complexity of the Hα line profile is reminiscent of CTTS. We estimate the mass accretion rate to be 10^-11  yr^-1.