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Context. In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated.

Context. The solar corona is a template to understand stellar activity. The Sun is a moderately active star, and its corona differs from that of active stars: for instance, active stellar coronae have a double-peaked emission measure distribution $EM(T)$with a hot peak at $8{-}20$MK, while the non-flaring solar corona has one peak at $1{-}2$MK and, typically, much cooler plasma.

Aims.We study the properties of X-ray emitting plasma of MP Mus, an old classical T Tauri star. We check whether an accretion process could produce the observed X-ray emission and we derive the accretion parameters and the characteristics of the shock-heated plasma. We compare the properties of MP Muswith those of younger classical T Tauri stars to test whether age is related to the properties of the X-ray emitting plasma.

Aims.We studied X-ray emission from young stars by analyzing the deep XMM-Newton observations of two regions of the Upper Scorpius association with an age of 5 Myr.

In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over different time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ranging from 102to $10^{4}~\mathrm{s}$) in the emission of individual lines shows that in all cases the emission is compatible with a constant source; the firm upper limits of $5\%{-}10\%$to the source variability on short time scales suggests that the intense X-ray emission is due to stable coronal structures and not to flaring activity. We have also determined the coronal thermal structure, as described with the emission measure distribution vs. temperature and with the help of plasma density, derived from the analysis of the $\ion{O}{vii}$, $\ion{Ne}{ix}$, $\ion{Mg}{xi}$and $\ion{Si}{xiii}$He-like ion triplets. The emission measure distribution, $em(T)$, and the element abundances, have been reconstructed with the Markov-Chain Monte Carlo method by Kashyap & Drake; the $em(T)$presents a previously known sharp peak around $\log T=6.8{-}6.9$, but we have also found evidence of a small amount of plasma at $T>10^{7}\,\mathrm{K}$. With the help of the $em(T)$and the density values we have estimated the pressure and volume of the emitting plasma at different temperatures, and we have derived information about the structure of individual loops and about the population of loops having different maximum temperatures. Our results indicate that loops with higher maximum temperature have higher pressure and smaller volume than lower temperature loops.

Context. High spectral resolution X-ray observations of classical T?Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T?2-3? 106?K and density ne?1011-1013?cm-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere.Aims. We investigate the dynamics and stability of shock-heated accreting material in classical T?Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region.Methods. We perform one-dimensional hydrodynamic simulations of the impact of an accretion flow on the chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. We present the results of a simulation based on the parameters of the CTTS MP?Mus.Results. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of?1.8 ?109?cm, density ne?1011-1012?cm-3, temperature T?3?106?K, and uniform pressure equal to the ram pressure of the accretion flow (~450?dyn?cm-2). The base of the shocked region penetrates the chromosphere and remains at a position at which the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab leading to cyclic disappearance and re-formation of the slab. For an accretion rate of ~10-10?M??yr-1, the shocked region emits a time-averaged X-ray luminosity of LX?7? 1029?erg?s-1, which is comparable with the X-ray luminosity observed in CTTSs of identical mass. Furthermore, the X-ray spectrum synthesized from the simulation reproduces in detail all the main features of the Oviii and Ovii?lines of the star MP?Mus.

Aims. We have analyzed optical spectra of 25 X-ray sources identified as potential new members of the Taurus molecular cloud (TMC), in order to confirm their membership in this star-forming region.Methods. Fifty-seven candidate members were previously selected among the X-ray sources in the XEST survey, having a 2MASS counterpart compatible with a pre-main sequence star based on color-magnitude and color-color diagrams. We obtained high-resolution optical spectra for 7 of these candidates with the SARG spectrograph at the TNG?telescope, which were used to search for lithium absorption and to measure the H??line and the radial and rotational velocities. Then, 18?low-resolution optical spectra obtained with the instrument DOLORES for other candidate members were used for spectral classification, for H??measurements, and to assess membership together with IR color-color and color-magnitude diagrams and additional information from the X-ray data.Results. We found that 3 sources show lithium absorption, with equivalent widths (EWs) of ~500?m?, broad spectral line profiles, indicating rotational velocities of ~20-40?km?s-1, radial velocities consistent with those for known members, and H??emission. Two of them are classified as new weak-lined T?Tauri stars, while the EW (~-9??) of the H??line and its broad asymmetric profile clearly indicate that the third star (XEST-26-062) is a classical T?Tauri star. Fourteen sources observed with DOLORES are M-type stars. Fifteen sources show H??emission. Six of them have spectra that indicate surface gravity lower than in main sequence stars, and their de-reddened positions in IR color-magnitude diagrams are consistent with their derived spectral type and with pre-main sequence models at the distance of the TMC. The K-type star XEST-11-078 is confirmed as a new member on the basis of the strength of the H??emission line. Overall, we confirm membership to the TMC for 10?out of 25?X-ray sources observed in the optical. Three sources remain uncertain.

Context.The soft X-ray emission from high density plasma observed in several CTTS is usually associated with the accretion process. However, 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.Aims.We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph. In this paper, we analyze Chandra grating spectrometer data and attempt to correlate the observed X-ray emitting plasma components with the characteristics of the accretion process and the stellar magnetic field constrained by simultaneous optical observations.Methods.We analyze a 200 ks Chandra/HETGS observation, subdivided into two 100 ks segments, of the CTTS V2129 Oph. For the two observing segments corresponding to two different phases within one stellar rotation, we measure the density of the cool plasma component and the emission measure distribution.Results.The X-ray emitting plasma covers a wide range of temperatures: from 2 up to 34 MK. The cool plasma component of V2129 Oph(T≈ 3−4 MK) varies between the two segments of the Chandra observation: during the first observing segment high density plasma (\hbox{$\log N_{\rm e} = 12.1^{+0.6}_{-1.1}$}) with high EMat ~3−4 MK is present, whereas, during the second segment, this plasma component has lower EMand lower density (log Ne< 11.5), although the statistical significance of these differences is marginal. Hotter plasma components, T≥ 10 MK, vary on short timescales (~10 ks), which are typical of coronal plasma. A clear flare, detected during the first segment, might be located in a large coronal loop (half length >3 R⋆). Conclusions.Our observation provides additional 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 Ophmay 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. In particular, during the first time interval a direct view of the shock region is possible, while, during the second, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing the accretion-driven X-rays.

Context.High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies.

Context.High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (ne= 1011−1013cm-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface.

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines ($\beta \ll 1$).