Search results for "Accretion"

Time Resolved X-Ray Spectral Analysis of Class II YSOs in NGC 2264 During Optical Dips and Bursts

2016

Pre-Main Sequence stars are variable sources. The main mechanisms responsible for their variability are variable extinction, unsteady accretion, and rotational modulation of both hot and dark photospheric spots and X-ray active regions. In stars with disks this variability is thus related to the morphology of the inner circumstellar region (<0.1 AU) and that of photosphere and corona, all impossible to be spatially resolved with present day techniques. This has been the main motivations of the Coordinated Synoptic Investigation of NGC2264, a set of simultaneous observations of NGC2264 with 15 different telescopes. We analyze the X-ray spectral properties of stars with disks extracted du…

The magnetosphere of the close accreting PMS binary V4046 Sgr AB

2013

We present a preliminary 3D potential field extrapolation model of the joint magnetosphere of the close accreting PMS binary V4046 Sgr. The model is derived from magnetic maps obtained as part of a coordinated optical and X-ray observing program.

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

2015

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 average…

Accretion Shocks in Young Stars: the Role of Local Absorption on the X-ray Emission

2015

We investigate the X-ray emission from accretion shocks in classical T Tauri stars, due to the infalling material impacting the stellar surface. Several aspects in both observations and models of the accretion process are still unclear: the observed X-ray luminosity of the post-shock plasma is below the predicted value, the density vs temperature structure of the shocked plasma, with increasing densities at higher temperature, is opposite of what expected from simple accretion shock models. To address these issues we performed numerical magnetohydrodynamic simulations describing the impact of an accretion stream onto the stellar surface and considered the local absorption due to the surroun…

Effects of radiation in accretion regions of classical T Tauri stars

2019

Context. Models and observations indicate that the impact of matter accreting onto the surface of young stars produces regions at the base of accretion columns where optically thin and thick plasma components coexist. Thus, an accurate description of these impacts is necessary to account for the effects of absorption and emission of radiation. Aims. We study the effects of radiation emerging from shock-heated plasma in impact regions on the structure of the pre-shock down-falling material. We investigate whether a significant absorption of radiation occurs and if it leads to a pre-shock heating of the accreting gas. Methods. We developed a radiation hydrodynamics model describing an accreti…

Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs

2011

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. Aims: We aim to test the hypothesis that the high density soft X-ray emission originates from accretion by analysing, in a homogeneous manner, optical accretion indicators for an X-ray selected sample of CTTSs. Methods: We analyse optical spectra of the X-ray selected sample of CTTSs and calculate the accretion rates based on measuring the Hα, Hβ, Hγ, He ii 4686 Å, He i 5016 …

Imperfect accretion: ejecting matter in X-ray binaries

2021

X-ray binaries are binary stars composed of a compact object (a black hole, a neutron star) accreting matter from a companion star. These sources can be considered perfect astrophysical laboratories to test our knowledge of, e.g., General Relativity and Magneto-Hydrodynamics. Accretion is the key phenomenon characterizing these systems, but it is not always completely efficient. In many systems, ejections of matter are also observed, e.g. in the form of jets and winds, or also suggested, e.g. to explain the observed strong orbital expansion of a number of systems. Furthermore accretion and ejection seems to be somehow interconnected but the nature of this correlation is not completely clear…

Accretion onto Neutron Stars: spectral and timing investigation of Low Mass X-ray Binaries

2021

Accretionary growth and crust formation in the Central Asian Orogenic Belt and comparison with the Arabian-Nubian shield

2007

Kroner, A., Windley, B.F., Badarch, G., Tomurtogoo, O., Hegner, E., Jahn, B.M., Gruschka, S., Khain, E.V., Demoux, A., and Wingate, M.T.D., 2007, Accretionary growth and crust formation in the Central Asian Orogenic Belt and comparison with the Arabian-Nubian shield, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martinez Catalan, J.R., eds., 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 181–209, doi: 10.1130/2007.1200(11). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved. *Kroner: kroener@mail.uni-mainz.de; Windley: brian.windley@btinternet.com; Tomurtogoo: igmr@magicnet.mn; Heg…

Testing jet geometries and disc-jet coupling in the neutron star LMXB 4U 0614 + 091 with the internal shocks model

2020

Multi-wavelength spectral energy distributions of Low Mass X-ray Binaries in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last years, the flat radio-to-mid-IR spectra of Black Hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray Power Density Spectrum (PDS). In this work we attempt to apply this model for the first time to a Neutron Star (NS) LMXB, i.e. 4U 0614+091. We used the multi-wavelength d…