The disk-bearing young star IM Lupi: X-ray properties and limits on accretion

Abstract

Context. Classical T Tauri stars (CTTS) differ in their X-ray signatures from older pre-main sequence stars, e.g. weaklined TTS (WTTS). CTTS show a soft excess and deviations from the low-density coronal limit in the He-like triplets. Aims. We test whether these features correlate with accretion or the presence of a disk by observing IM Lup, a diskbearing 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 them taken simultaneously to the X-ray observations. We fit the X-ray emission lines and decompose the H emission line in different components. Results. In X-rays IM Lup has a bright and hot active corona, where elements of low first-ionisation potential are depleted. The He-like Ne ix triplet is in the low-density state, but due to the small number of counts a high-density scenario cannot be excluded on the 90% confidence level. In all X-ray properties IM Lup resembles a main-sequence star, but it is also compatible with CTTS signatures on the 90% confidence level, thus we cannot decide if the soft excess and deviations from the low-density coronal limit in the He-like triplets in CTTS require accretion or only the presence of a disk. IM Lup is chromospherically active, which explains most of the emission in H. Despite its low equivalent width, the complexity of the H line profile is reminiscent of CTTS. We present an estimate for the mass accretion rate of 10−11M yr−1.