Variable X-ray Emission From The Accretion Shock In The Classical T Tauri Star V2129 Oph

Young low-mass stars host intense magnetic fields that play an important role in regulating the mass and angular momentum transfer between the star and its circumstellar disk, and at the same time heat and confine hot stellar coronae. The X-ray emission from the coronal plasma in turn heats and ionizes the circumstellar disk, affecting its properties and evolution. In June 2009 we have conducted a coordinated multiwavelength observing campaign targeting V2129 Oph, a K5 classical T Tauri star (CTTS), with the goal of simultaneously determining the properties of its X-ray emitting plasma, the structure of its large-scale magnetic field, and the characteristics of its accretion flow. We present here a study of the X-ray emitting plasma of V2129 Oph based on a 200 ks Chandra/HETG observation. We find that V2129 Oph hosts high-density plasma at temperature of 3-4 MK, likely heated in an accretion shock, and an hotter "coronal" plasma component (T > 10 MK). A bright X-ray flare detected during the observation can be attributed to this latter component and was likely produced by a large coronal loop with half length > 3 R_star. The detection of high-density plasma on V2129 Oph confirms that such plasma component is commonly found on CTTSs, when observed with high resolution X-ray spectroscopy. The X-ray emission from the cool high-density plasma is seen to vary during the 200 ks Chandra observation: high density and high emission measure are observed during the first 100 ks, low density and lower emission measure during the last 100 ks. We can naturally interpret these observations attributing the high density plasma to the accretion shock and the observed variability to the changing viewing geometry of the accretion shock during the stellar rotation, as constrained from simultaneous optical observations.