Abstract
The soft X-ray emission from high density plasma in CTTS is associated with the accretion process. It is still unclear whether this high density cool plasma is heated in the accretion shock, or if it is coronal plasma fed/modified by the accretion process. We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph (Chandra/HETGS data to constrain the X-ray emitting plasma components, and optical observations to constrain the characteristics of accretion and magnetic field). We analyze a 200 ks Chandra/HETGS observation of V2129 Oph, subdivided into two 100 ks segments, corresponding to two different phases within one stellar rotation. The X-ray emitting plasma covers a wide range of temperatures: 2-34 MK. The cool plasma component of V2129 Oph varies between the two segments of the Chandra observation: high density plasma (log Ne ~ 12.1) with high EM at ~ 3-4 MK is present during the 1st segment; during the 2nd segment this plasma component has lower EM and lower density (log Ne < 11.5), although the statistical significance of these differences is marginal. Hotter plasma components, T > 10 MK, show variability on short time scales (~ 10 ks), typical of coronal plasma. A clear flare, detected in the 1st segment, could be located in a large coronal loop (> 3 Rstar). Our observation provides further confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock. The variability of this cool plasma component on V2129 Oph may be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation. During the 1st time interval direct view of the shock region is possible, while, during the 2nd, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing accretion-driven X-rays.
Highlights
During the pre-main sequence phase, low mass stars evolve from the embedded protostellar stage to the main sequence
It is still unclear whether this high density “cool” plasma is heated in the accretion shock, or if it is coronal plasma fed or modified by the accretion process
We present our analysis of the X-ray Chandra/High Energy Transmission Grating Spectrometer (HETGS) observation, taking advantage, in the interpretation, of available results obtained from the optical data
Summary
During the pre-main sequence phase, low mass stars evolve from the embedded protostellar stage to the main sequence. Bertout 1989; Kenyon & Hartmann 1995) During this phase, low mass stars are called classical T Tauri stars (CTTS). Understanding the accretion process and its high-energy radiation is a key step in the understanding of the physics of pre-main-sequence low-mass stars and their circumstellar environment, because accretion strongly affects several properties of the central star itself: accretion determines stellar rotational velocities (Bouvier et al 1993), and affects coronal activity (Preibisch et al 2005); the accretion history in the early stages of stellar evolution might modify the stellar radius, its luminosity, up to ages of a few Myr (Baraffe et al 2009); and Article published by EDP Sciences
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