MN Lup: X-Rays from a Weakly Accreting T Tauri Star

Young T Tauri stars (TTS) are surrounded by an accretion disk, which over time disperses due to photoevaporation, accretion, and possibly planet formation. The accretion shock on the central star produces an UV/optical veiling continuum, line emission, and X-ray signatures. As the accretion rate decreases, the impact on the central star must change. In this article we study MN Lup, a young star where no indications of a disk are seen in IR observations. We present XMM-Newton and VLT/UVES observations, some of them taken simultaneously. The X-ray data show that MN Lup is an active star with L_X /L _bol close to the saturation limit. However, we find high densities (n_e > 3 × 10¹⁰ cm^-3) in the X-ray grating spectrum. This can be well fitted using an accretion shock model with an accretion rate of 2 × 10^-11 M _⊙ yr^-1. Despite the simple Hα line profile which has a broad component, but no absorption signatures as typically seen on accreting TTS, we find rotational modulation in Ca II K and in photospheric absorption lines. These line profile modulations do not clearly indicate the presence of a localized hot accretion spot on the star. In the Hα line we see a prominence in absorption about 2R _* above the stellar surface—the first of its kind on a TTS. MN Lup is also the only TTS where accretion is seen, but no dust disk is detected that could fuel it. We suggest that MN Lup presents a unique and short-lived state in the disk evolution. It may have lost its dust disk only recently and is now accreting the remaining gas at a very low rate.

Based on observations made with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, and ESO Telescopes at the La Silla Paranal Observatory under program ID 087.V0991(A).