Search results for "Accretion"

Perspectives

2004

We discuss a special case of formation of axisymmetric shocks in the accretion flow of ideal gas onto a Schwarzschild black hole: when the total energy of the flow is negative. The result of our analysis enlarges the parameter space for which these steady shocks are exhibited in the accretion of gas rotating around relativistic stellar objects. Since keplerian disks have negative total energy, we guess that, in this energy range, the production of the shock phenomenon might be easier than in the case of positive energy. So our outcome reinforces the view that sub-keplerian flows of matter may significantly affect the physics of the high energy radiation emission from black hole candidates. …

Temporal Analysis of the Millisecond X-ray Pulsar SAX J1808.4-3658 During the 2000 Outburst

2005

We report a temporal analysis of the millisecond X-ray Pulsar SAX J1808.4-3658 during the 2000 outburst, observed with RXTE. The observed maximum luminosity was approximately a factor of ten lower than in the other outbursts exhibited by the source, and this low flux level forced us to use a technique based on the χ2 obtained with an epoch folding search to discriminate between different possible orbital solutions, in order to correct the data for the orbital motion. In the subsequent searches for periodicities we clearly detected the 401Hz pulsation in at least two observations, but in the faintest the pulsed fraction varied from 20 % ca. to the absence of signs of coherent pulsation at al…

Mass Accretion Impacts in Classical T Tauri Stars: A Multi-disciplinary Approach

2019

Accretion of matter is a process that plays a central role in the physics of young stellar objects. The analysis of the structure by which matter settles on the star can unveil key information about the process of star formation by providing details on mass accretion rates, stellar magnetic field configurations, possible effects of accretion on the stellar coronal activity, etc. Here we review some of the achievements obtained by our group by exploiting a multi-disciplinary approach based on the analysis of multi-dimensional magnetohydrodynamic simulations, multi-wavelength observations, and laboratory experiments of accretion impacts occurring onto the surface of classical T Tauri stars (C…

Interstellar travel guide: Chronicles of a violent universe

2016

In contrast to observations from our local viewpoint, the universe is extremely violent. Explosions of massive stars, X-ray and gamma-ray binary stars, and active galaxies are related to the presence of compact objects such as neutron stars or black holes. Processes such as matter accretion or the formation and release of relativistic plasma jets are responsible for the generation of large amounts of high-energy radiation, caused by the acceleration of elementary particles, and have a significant impact in their surroundings. Moreover, they are related to the generation of gravitational waves: vibrations in spacetime produced by the acceleration of compact massive objects that produce stron…

2017

Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell that envelops the shocked core, reducing escaped x-ray emission. T…

Bright hot impacts by erupted fragments falling back on the Sun: a template for stellar accretion.

2013

Impacts of falling fragments observed after the eruption of a filament in a solar flare on 7 June 2011 are similar to those inferred for accretion flows on young stellar objects. As imaged in the ultraviolet (UV)-extreme UV range by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, many impacts of dark, dense matter display uncommonly intense, compact brightenings. High-resolution hydrodynamic simulations show that such bright spots, with plasma temperatures increasing from ~10(4) to ~10(6) kelvin, occur when high-density plasma (>>10(10) particles per cubic centimeter) hits the solar surface at several hundred kilometers per second, producing high-energy emission as …

Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light

2010

NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. We identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from J band to 8.0 micron. In this paper, we study in detail the nature of a subsample of disk-bearing stars that show peculiar characteristics. They appear older than the other members in the V vs. V-I diagram, and/or they have one or more IRAC colors at pure photospheric values, despite showing NIR excesses, when optical and infrared colors are compared. We confirm the membership of…

Chronology of star formation and disk evolution in the Eagle Nebula

2010

Massive SFR are characterized by intense ionizing fluxes, strong stellar winds and supernovae explosions, all of which have important effects on the surrounding media, on the star-formation (SF) process and on the evolution of YSOs and their disks. We present a multiband study of the massive young cluster NGC6611 and M16, to study how OB stars affect the early stellar evolution and the SF. We search for evidence of triggered SF by OB stars in NGC6611 on a large spatial scale (~10 pc) and how the efficiency of disks photoevaporation depends on the central stars mass. We assemble a multiband catalog with photometric data, from B band to 8.0micron, and X-ray data obtained with 2 new and 1 arch…

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

2013

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 …

Line Features in Psds of X-Ray Binaries

1994

There is a great deal of evidence that the X-ray emission from High Mass X-Ray Binaries (HMXRB) is often associated with the accretion of matter from a “normal” companion to a degenerate (neutron) star. Provided the magnetic field in these systems is very high (B ~ 1012 — 1013 G), the accretion should occur via the formation of discrete “blobs” which should emit through bremsstrahlung process due to the sudden stop near the surface of the neutron star[2, 3, 5]. There are a number of detections of non-Poissonian aperiodic variations in the intensity of emitted radiation (often referred to as “flickering”) with a characteristic Power Spectrum Density (PSD) broad band feature (red noise)[4, 1]…