Search results for "accretion"
showing 10 items of 385 documents
Flaring Activity in Accretion Flows of Young Stellar Objects
2009
X-ray observations have shown extensive flaring activity in young stellar associations such as the Orion nebula. Observed flares are often very long and intense, and have been associated to very long magnetic loops, which may connect the stellar surface to the circumstellar disk. As such, these loops are candidate to be also the channel of star accretion from the disk, and one then wonders whether they flare during accretion flows. As a first attack to this question we have modelled in detail flares inside long coronal loops containing plasma at high density, comparable to that presumed for accretion flows. Preliminary results show that such flares would decay on time scales smaller than th…
Hydrodynamic Modeling of Accretion Shock on CTTSs
2009
High resolution (R ~ 600) X-ray observations of some classical T Tauri stars (CTTSs) (TW Hya, BP Tau, V4046 Sgr, MP Mus and RU Lupi) have shown the presence of X-ray plasma at T ~ 2–3 × 106 K and denser than n e ~ 1011 cm-3 [1, 2, 3, 4, 5], which suggests an origin different from the coronal one (n e ~ 1010 cm_3). Stationary models demonstrated that X-ray emission from CTTSs could also be produced by the accreting material [6]. We address this issue with the aid of a time-dependent hydrodynamic numerical model describing the impact of an accretion stream onto the chromosphere of a CTTS (see [7] for more details). Our simulations include the effects of gravity, radiative losses from opticall…
Radiation Magnetohydrodynamic Models and Spectral Signatures of Plasma Flows Accreting onto Classical T Tauri Stars
2019
CTTSs are young stars accreting mass from their circumstellar disks. The material falls into the star at free fall velocity and hits the stellar surface producing shocks, that heat the plasma at few million degrees. In the last twenty years the X-ray and UV observations of these systems have raised several questions. In particular, the observed X-ray luminosity is systematically below the value predicted by theoretical models, and the UV lines show complex profiles which cannot be easily interpreted with current accretion models based only on magnetohydrodynamical effects. To tackle these problems we modeled the structure and the dynamics of the plasma in the impact region using 3D magnetoh…
Size of the accretion disk in the gravitationally lensed quasar SDSS J1004+4112 from the statistics of microlensing magnifications
2016
We present eight monitoring seasons of the four brightest images of the gravitational lens SDSS J1004+4112 observed between December 2003 and October 2010. Using measured time delays for the images A, B and C and the model predicted time delay for image D we have removed the intrinsic quasar variability, finding microlensing events of about 0.5 and 0.7 mag of amplitude in the images C and D. From the statistics of microlensing amplitudes in images A, C, and D, we have inferred the half-light radius (at {\lambda} rest = 2407 {\AA}) for the accretion disk using two different methods, $R_{1/2}=8.7^{+18.5}_{-5.5} \sqrt{M/0.3 M_\odot}$ (histograms product) and $R_{1/2} = 4.2^{+3.2}_{-2.2} \sqrt{…
A Preliminary Analysis of a New Chandra Observation (ObsID 6148) of Cir X-1
2008
We present the preliminary spectral analysis of a 25 ks long Chandra observation of the peculiar source Cir X–1 near the periastron passage. We estimate more precise coordinates of the source compatible with the optical and radio counterpart coordinates. We detect emission lines associated to Mg XII, Si XIII, Si XIV, S XV, S XVI Ar XVII, Ar XVIII, Ca XIX, Ca XX, Fe XXV, Fe XXVI showing a redshift of 470 km s−1. The more intense emission features at 6.6 keV show a double‐peaked shape that can be modelled with two or three Gaussian lines.
High‐Resolution X‐Ray Spectroscopy of the Post–T Tauri Star PZ Telescopii
2004
We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating P_(rot)=0.94 d post T Tauri (~20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods we have derived the coronal emission measure distribution, em(T), and chemical abundances. The em(T) peaks at log T = 6.9 and exhibits a significant emission measure at temperatures log T > 7. The coronal abundances are generally ~0.5 times the solar photospheric values that are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abu…
A method to constrain the neutron star magnetic field in Low Mass X-ray Binaries
2005
We describe here a method to put an upper limit to the strength of the magnetic field of neutron stars in low mass X‐ray binaries for which the spin period and the X‐ray luminosity during X‐ray quiescent periods are known. This is obtained using simple considerations about the position of the magnetospheric radius during quiescent periods. We applied this method to the accreting millisecond pulsar SAX J1808.4‐3658, which shows coherent X‐ray pulsations at a frequency of ∼ 400 Hz and a quiescent X‐ray luminosity of ∼ 5 × 1031 ergs/s, and found that B ⩽ 5 × 108 Gauss in this source. Combined with the lower limit inferred from the presence of X‐ray pulsations, this constrains the SAX J1808.4‐3…
An X-shooter survey of star forming regions: Low-mass stars and sub-stellar objects
2011
We present preliminary results of our X-shooter survey in star forming regions. In this contribution we focus on sub-samples of young stellar and sub-stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X-shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low-mass (VLM) and sub-stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X-shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic…
X-Ray Eclipse Time Delays in 4U2129+47
2007
4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached. Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005.…
X-rays from accretion shocks in T Tauri stars: The case of BP Tau
2005
We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O {\sc vii} triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.