Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines

Context. The accretion process in Classical T Tauri Stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims. We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams.We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods. We model the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We explore different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) line profiles. Results. The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of C IV doublet are characterized by two main components: one narrow and redshifted to speed $\approx$ 50 km s$^{-1}$ and the other broader and consisting of subcomponents with redshift to speed in the range 200 $\approx$ 400 km s$^{-1}$. The profiles of OVIII lines appear more symmetric than C IV and are redshifted to speed $\approx$ 150 km s$^{-1}$. Conclusions. Our model predicts profiles of C IV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation.Comment: 11 pages, 10 figure

Large Scale Properties of Coronal Heating along the Solar Cycle

We discuss various studies of the global properties of coronal heating. Some of them find power laws tying the X-ray luminosity with the magnetic flux of individual structures, of the whole Sun, and of active solar-type stars. Others are based on methods to model the Sun as an X-ray star. We also briefly discuss solar-like active stars and how the Sun fits in the whole scenario. We use a new model, including all flares, of the Sun as an X-ray star to describe the evolution of the corona along the solar cycle and the implications on the heating of closed coronal structures. We point out that, as activity increases, more heating is released into the confined coronal plasma and such a heating has to be, on average, more intense in order to explain the widespread evidence of a temperature increase with activity. By the same token, nanoflare heating (if existent) has to increase and decrease along the cycle differently from flares

Accretion in young stars: measure of the stream velocity of TW Hya from the X-ray Doppler shift

High-resolution X-ray spectra are a unique tool to investigate the accretion process in young stars. In fact X-rays allow to investigate the accretion-shock region, where the infalling material is heated by strong shocks due to the impact with the denser stellar atmosphere. Here we show for the first time that it is possible to constrain the velocity of the accretion stream by measuring the Doppler shift of the emitted X-rays. To this aim we analyzed the deep Chandra/HETGS observation of the accreting young star TW Hya. We selected a sample of emission lines free from significant blends, fitted them with gaussian profiles, computed the radial velocity corresponding to each line, and averaged these velocities to obtain an accurate estimate of the global velocity of the X-ray emitting plasma. After correcting for Earth's motion, we compared this observed velocity with the photospheric radial velocity. In order to check this procedure we applied the same technique to other Chandra/HETGS spectra of single stars, whose X-rays are due only to coronal plasma. While spectra of pure coronal sources provide Doppler shifts in agreement with the known stellar radial velocity, we found that the X-ray spectrum of TW Hya is red-shifted by ~30-40 km/s with respect to the stellar photosphere. This proves that the X-ray emitting plasma on TW Hya is moving with respect to the stellar surface, definitively confirming that it originates in the accretion-shock region. The observed velocity suggests that the base of the accretion region is located at low latitudes of the stellar surface

X-rays from accretion shocks in classical T Tauri stars: 2D MHD modeling and the role of local absorption

In classical T Tauri stars (CTTS) strong shocks are formed where the accretion funnel impacts with the denser stellar chromosphere. Although current models of accretion provide a plausible global picture of this process, some fundamental aspects are still unclear: the observed X-ray luminosity in accretion shocks is order of magnitudes lower than predicted; the observed density and temperature structures of the hot post-shock region are puzzling and still unexplained by models. To address these issues we performed 2D MHD simulations describing an accretion stream impacting onto the chromosphere of a CTTS, exploring different configurations and strengths of the magnetic field. From the model results we then synthesized the X-ray emission emerging from the hot post-shock, taking into account the local absorption due to the pre-shock stream and surrounding atmosphere. We find that the different configurations and strengths of the magnetic field profoundly affect the hot post-shock properties. Moreover the emerging X-ray emission strongly depends also on the viewing angle under which accretion is observed. Some of the explored configuration are able to reproduce the observed features of X-ray spectra of CTTS. © International Astronomical Union 2014

3D YSO accretion shock simulations: a study of the magnetic, chromospheric and stochastic flow effects

The structure and dynamics of young stellar object (YSO) accretion shocks depend strongly on the local magnetic field strength and configuration, as well as on the radiative transfer effects responsible for the energy losses. We present the first 3D YSO shock simulations of the interior of the stream, assuming a uniform background magnetic field, a clumpy infalling gas, and an acoustic energy flux flowing at the base of the chromosphere. We study the dynamical evolution and the post-shock structure as a function of the plasma-beta (thermal pressure over magnetic pressure). We find that a strong magnetic field (~hundreds of Gauss) leads to the formation of fibrils in the shocked gas due to the plasma confinement within flux tubes. The corresponding emission is smooth and fully distinguishable from the case of a weak magnetic field (~tenths of Gauss) where the hot slab demonstrates chaotic motion and oscillates periodicall

X-ray emission from the old CTTS MP Muscae

We study the properties of X-ray emitting plasma of MP Mus, an old classical T Tauri star. XMM-Newton/RGS spectra allow us to measure the plasma electron density, which probes whether X-ray emission is produced in the accretion process. X-ray emission from MP Mus originates from high density cool plasma but a hot flaring component is also present, suggesting that both coronal magnetic activity and accretion contribute to the observed X-ray emission. From the soft part of the X-ray emission from MP Mus, mostly produced by plasma heated in the accretion shock, we derive the accretion parameters and the characteristics of the shock-heated plasma

The Sun as a benchmark of flaring activity in stellar coronae

The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X-ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM(T), at 8-20 MK, while the non-flaring solar corona has a peak at 1-2 MK. In the solar corona significant amounts of plasma at temperature ~10 MK are observed only during flares. To investigate what is the time-averaged effect of solar flares we measure the disk-integrated time-averaged emission measure, EMF(T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light curves over time intervals of one month. We also obtain the EMQ(T) of the quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate variations due to the solar cycle we evaluate EMF(T) and EMQ(T) at different phases of the cycle between December 1991 and April 1998. Irrespective of the solar cycle phase, EMF(T) appears as a peak in the distribution, and it is significantly larger than the values of EMQ(T) for T~5-10 MK. Adding EMF(T) and EMQ(T) we obtain for the first time a time average EM(T) of the entire solar corona: it is double-peaked, with the hot peak, due to time-averaged flares, being located at temperatures similar to those of active stars, but less enhanced. In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, a two peak EM(T) distribution suggests that then either the flare distribution or the confined plasma response to flares, or both, are bimodal. Moreover the EMF(T) shape supports the hypothesis that the hot EM(T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity

The EM(T) of stellar coronae

Studying the solar corona, due to its vicinity, is the starting point to understand stellar activity. The emission measure distribution vs temperature, EM(T), is a useful tool to study coronal plasmas, in fact it allows: to investigate the energy balance of coronal plasmas, to easily compare different stars, and also to compare the solar corona to that of other active stars irrespective of the very different observing techniques. The EM(T) of the solar corona differs significantly, in terms of average plasma temperatures, peak temperatures, and total emission measure, with respect to that of active stars. In this work it is discussed how the evaluation of the EM(T) of the solar corona, and of its components (quiescent plasma, active regions, flares, etc.), parallel to the reconstruction of the EM(T) of stars at different activity levels, can be used to investigate coronal physics

Multi-wavelength diagnostics of accretion in an X-ray selected sample of CTTSs

The majority of CTTSs observed to date with high spectral resolution X-ray spectroscopy reveal soft X-ray emission (E<0.7 KeV) which originates from cool (1-5 MK), high density (n ˜ 10^{11}-10^{13} cm^{-3}) plasma. This is currently interpreted to be due to mass accretion. Supporting this interpretation is the fact that this plasma component is too dense to have a coronal origin, and it has never been observed in non-accreting stars. Synthesized X-ray spectra from detailed hydrodynamical modelling of the interaction between the accretion flow and the stellar chromosphere also confirm this interpretation. However, the mass accretion rates derived from X-ray data are consistently underestimated when compared to mass accretion rates derived from UV/optical data. We test the hypothesis that this soft X-ray emission originates from accretion by analysing optical, NIR and X-ray data for an X-ray selected sample of CTTSs. We derive mass accretion rates for the sample based on Hα, He I, O I and Ca II emission lines, along with the X-ray data. We draw comparisons between these mass accretion rates to understand the underestimation of the X-ray derived mass accretion rates. We discuss the possibilities of a) the X-ray emission being partially absorbed, b) the optical/NIR emission arising from different parts of the accretion stream and c) the uncertainties involved in the estimation of the mass accretion rates from different spectroscopic diagnostics