827 research outputs found
THE EVOLUTIONARY STATUS OF HIGH-LITHIUM, HIGH-ACTIVITY COOL DWARFS F. Favata
No full textWe have used the newly available Hipparcos precise parallaxes to study the evolutionary status of a number of high-lithium, high-activity late-type dwarfs found in X-ray based surveys, the Einstein Extended Medium Sensitivity Survey and the Einstein Slew Survey. The sample stars are placed on evolutionary tracks thus estimating their age. Key words: Active stars; Stellar populations; Lowmass stars; Stellar ages. 1. INTRODUCTION Samples of activity-selected stars have been shown to contain a large fraction of cool stars with photospheric lithium abundance higher than the average field population. Such lithium-rich stars are common in high Galactic latitude X-ray surveys from the Einstein observatory (Favata et al. 1993; 1995), from the EXOSAT observatory (Tagliaferri et al. 1994) as well as in samples selected on the basis of their EUV emission, from the ROSAT Wide Field Camera survey (Jeffries 1995). These activity-selected lithium-rich stars from wide-field surveys do not appear to c..
Coupled Effects of Energy Dissipation and TravellingVelocity in the Run-Out Simulation of High-SpeedGranular
No full textThe run-out of high speed granular masses or avalanches along mountain streams, till their arrest, is analyti-cally modeled. The power balance of a sliding granular mass along two planar sliding surfaces is written by taking into account the mass volume, the slopes of the surfaces, the fluid pressure and the energy dissipation. Dissipation is due to collisions and displacements, both localized within a layer at the base of the mass. The run-out, the transition from the first to the second sliding surface and the final run-up of the mass are de-scribed by Ordinary Differential Equations (ODEs), solved in closed form (particular cases) or by means of numerical procedures (general case). The proposed solutions allow to predict the run-up length and the speed evolution of the sliding mass as a function of the involved geometrical, physical and mechanical parameters as well as of the simplified rheological laws assumed to express the energy dissipation effects. The corre-sponding solutions obtained according to the Mohr-Coulomb or Voellmy resistance laws onto the sliding surfaces are recovered as particular cases. The run-out length of a documented case is finally back analysed through the proposed model
Recent X-ray studies of stellar cycles and long-term variability
No full textWe discuss recent X-ray studies of stellar cycles and long-term variability
Test di fattibilità e prova pilota di un trattamento fenton per la bonifica di un sito contaminato da MTBE
No full textX-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
No full textWe present the results of a five-day monitoring campaign with XMM-Newton of six X-ray bright young stellar objects (YSOs) in the star-forming complex L1551 in Taurus. All stars present significant variability on the five-day time scale. Modulation of the light curve on time scales comparable with the star's rotational period appeared to be present in the case of one weak-lined T Tauri star. Significant spectral variations between the 2000 and the 2004 observations were detected in the (unresolved) classical T Tauri binary system XZ Tau: a hot plasma component which was present in the X-ray spectrum in 2000 had significantly weakened in 2004. As XZ Tau N was undergoing a strong optical outburst in 2000, which had terminated since then, we speculate on the possible relationship between episodic, burst accretion, and X-ray heating. The transition object HL Tau underwent a strong flare with a complex temperature evolution, which is indicative of an event confined within a very large magnetic structure (few stellar radii), similar to the ones found in YSOs in the Orion Nebula Cluster
ROSAT PSPC/HRI observations of the open cluster NGC 2422
No full textWe present the results of a ROSAT study of NGC 2422, a southern open cluster at a distance of about 470 pc, with an age close to the Pleiades. Source detection was performed on two observations, a 10-ks PSPC and a 40-ks HRI pointing, with a detection algorithm based on wavelet transforms, particularly suited to detecting faint sources in crowded fields. We have detected 78 sources, 13 of which were detected only with the HRI, and 37 detected only with the PSPC. For each source, we have computed the 0.2-2.0 keV X-ray flux. Using optical data from the literature and our own low-dispersion spectroscopic observations, we find candidate optical counterparts for 62 X-ray sources, with more than 80% of these counterparts being late type stars. We have assigned to the optical sources an astrometric and/or a photometric membership flag, depending on which data are available to us. The number of sources (38 of 62) with high membership probability counterparts is consistent with that expected for Galactic plane observations at our sensitivity. We have computed maximum likelihood X-ray luminosity functions (XLF) for F and early-G type stars with high membership probability. Heavy data censoring due to our limited sensitivity permits determination of only the high-luminosity tails of the XLFs; the distributions are indistinguishable from those of the nearly coeval Pleiades cluster
The X-ray emission mechanism in the protostellar jet HH 154
No full textWe study the mechanism causing the X-ray emission recently detected in protostellar jets, by performing a detailed modeling of the interaction between a supersonic jet originating from a young stellar object and the ambient medium, for various values of density contrast, ν, between the ambient density and the jet, and of Mach number, M; radiative losses and thermal conduction have been taken into account. Here we report a representative case which reproduces, without any ad hoc assumption, the characteristics of the X-ray emission recently observed in the protostellar jet HH 154. We find that the X-ray emission originates from a localized blob, consistent with observations, which moves with velocity v ̃ 500 km s-1; we therefore predict the X-ray source to have a detectable proper motion
X-rays from protostellar jets: emission from continuous flows
No full textContext: Recently X-ray emission from protostellar jets has been detected with both XMM-Newton and Chandra satellites, but the physical mechanism which can give rise to this emission is still unclear.
Aims: We performed an extensive exploration of the parameter space for the main parameters influencing the jet/ambient medium interaction. Aims include: 1) to constrain the jet/ambient medium interaction regimes leading to the X-ray emission observed in Herbig-Haro objects in terms of the emission by a shock forming at the interaction front between a continuous supersonic jet and the surrounding medium; 2) to derive detailed predictions to be compared with optical and X-ray observations of protostellar jets; 3) to get insight into the protostellar jet's physical conditions.
Methods: We performed a set of two-dimensional hydrodynamic numerical simulations, in cylindrical coordinates, modeling supersonic jets ramming into a uniform ambient medium. The model takes into account the most relevant physical effects, namely thermal conduction and radiative losses.
Results: Our model explains the observed X-ray emission from protostellar jets in a natural way. In particular, we find that a protostellar jet that is less dense than the ambient medium well reproduces the observations of the nearest Herbig-Haro object, HH 154, and allows us to make detailed predictions of a possible X-ray source proper motion (v_sh ≈500 km s-1) detectable with Chandra. Furthermore, our results suggest that the simulated protostellar jets which best reproduce the X-rays observations cannot drive molecular outflows
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