Multi-wavelength and multi-disciplinary approach to the study of the emission from accretion/ejection processes in young stars with disks: combining observations, numerical models, and laboratory experiments

A multi-band investigation of the emission from young stars with disks allows us to obtain a complete characterization of these complex systems, accounting for their accretion and outflow activity. All the components contributing to the emission can strongly influence also the evolution of their disks and possibly the formation of exo-planetary systems. Combining multi-wavelength observations, magnetohydrodynamical models, and laboratory experiments following a multi-disciplinary approach, we aim at a more complete characterization of young stars with accretion/ejection processes at work. We present X-ray and optical analysis of line profiles and Doppler shift for specific objects (e.g. TW Hya) as well as in statistical samples (NGC 2264) and discuss future perspectives with next generation instruments: Athena and LSST. In particular, LSST main characteristics will be presented in the context of the investigation of young stars as this survey will allow us to increase the number of young stars whose accretion/outflow activities can be fully characterized exploiting also the variability of these processes. A science case for the Mini Survey and Deep Drilling Field proposals in LSST in the context of young stars will be discussed

Spectroscopic observations of blue stars with infrared excesses in NGC 6611

Context. The young open cluster NGC 6611 includes a group of peculiar objects with interesting properties among its candidate members: blue stars with infrared (IR) excesses. These stars show excesses in IR bands, a signature of the presence of a circumstellar disk, but optical colors typical of older field stars. To confirm their membership in the cluster, it is therefore important to use new spectroscopic observations, together with previous photometric data. Aims. We aim to confirm the membership of these objects and investigate their physical properties to verify whether the observed colors are intrinsic or altered by the disk or by the accretion processes. Methods. We analyzed the intermediate-resolution spectroscopic data obtained for a subsample of blue stars in NGC 6611 with FLAMES. In particular, we focused on the study of 1) the profile of the Hα emission line, to select stars with accretion and outflow activity; 2) the Li absorption line, used as a youth indicator; 3) the radial velocity. Results. Using the spectroscopic analysis, it has been possible to investigate the Li absorption line, as well as to distinguish between stars with inert or active disks. In particular, from the analysis of the Hα emission line we were able to infer the activity due to the accretion and outflow processes and the variability of the emission. We also investigated the binarity of the blue stars and their membership to NGC 6611. Conclusions. From our spectroscopic analysis, we conclude that half of the sample of blue stars (10/20) are confirmed members of NGC 6611 (with 6 more stars that could also be possible members). In conclusion, our results indicate that members of young clusters can also be found in an anomalous region of the color-magnitude diagram, i.e., outside of the pre-main sequence locus where most of the cluster members lie

Pre-main-sequence stars older than 8 Myr in the Eagle nebula

Attention is given to a population of 110 stars in the NGC 6611 cluster of the Eagle Nebula that have prominent near-infrared (NIR) excess and optical colours typical of pre-main sequence (PMS) stars older than 8 Myr. At least half of those for which spectroscopy exists have a Halpha emission line profile revealing active accretion. In principle, the V-I colours of all these stars would be consistent with those of young PMS objects (< 1 Myr) whose radiation is heavily obscured by a circumstellar disc seen at high inclination and in small part scattered towards the observer by the back side of the disc. However, using theoretical models it is shown here that objects of this type can only account for a few percent of this population. In fact, the spatial distribution of these objects, their X-ray luminosities, their optical brightness, their positions in the colour-magnitude diagram and the weak Li absorption lines of the stars studied spectroscopically suggest that most of them are at least 8 times older than the ~1 Myr-old PMS stars already known in this cluster and could be as old as ~30 Myr. This is the largest homogeneous sample to date of Galactic PMS stars considerably older than 8 Myr that are still actively accreting from a circumstellar disc and it allows us to set a lower limit of 7% to the disc frequency at ~16 Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~6 Myr for disc dissipation.Comment: 12 pages, 5 figures, accepted for publication in Monthly Notices of the Astronomical Societ

X-ray emission mechanisms in protostellar jets

Prompted by the recent detection of X-ray emission from Herbig-Haro objects, we studied the interaction between a supersonic jet originating from a young stellar object and the ambient medium; our aim is to investigate the mechanisms causing the X-ray emission. Our model takes into account the radiative losses from optically in plasmas and Spitzer's thermal conduction including saturation effects. We explored the parameter space defined by the density contrast between the ambient medium and the jet and by the Mach number, to infer the configurations which can give rise to X-ray emission. From the models, we derived the X-ray emission as it would be observed with Chandra/ACIS-I and XMM-Newton/EPIC-pn, using the MEKAL spectral code and including the absorption of interstellar medium. Here we discuss a representative case which produces, without any ad hoc assumption, Xray emission with characteristics very similar to those observed in the protostellar jet, HH 154. We find that the X-ray emission originates from a blob localized just behind the bow shock, moving with velocity 500 km/s. We predict, therefore, among other features, a detectable proper motion of the X-ray blob, which is interesting for future observations

X-ray emission from stellar jets by collision against high-density molecular clouds: an application to HH 248

We investigate the plausibility of detecting X-ray emission from a stellar jet that impacts against a dense molecular cloud. This scenario may be usual for classical T Tauri stars with jets in dense star-forming complexes. We first model the impact of a jet against a dense cloud by 2D axisymmetric hydrodynamic simulations, exploring different configurations of the ambient environment. Then, we compare our results with XMM-Newton observations of the Herbig-Haro object HH 248, where extended X-ray emission aligned with the optical knots is detected at the edge of the nearby IC 434 cloud. Our simulations show that a jet can produce plasma with temperatures up to 10 MK, consistent with production of X-ray emission, after impacting a dense cloud. We find that jets denser than the ambient medium but less dense than the cloud produce detectable X-ray emission only at the impact onto the cloud. From the exploration of the model parameter space, we constrain the physical conditions (jet density and velocity, cloud density) that reproduce well the intrinsic luminosity and emission measure of the X-ray source possibly associated with HH 248. Thus, we suggest that the extended X-ray source close to HH 248 corresponds to the jet impacting on a dense cloud.Comment: Accepted for publication in ApJ. (12 pages, 12 figures

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