Accretion discs/Observational evidence for accretion discs in the Universe

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    Accretion discs in young stellar objects (YSOs)

    Figure 1: Accretion disc and jet in a proto-star HH30 observed by the Hubble Space Telescope: the jet (in red) is perpendicular to the accretion disc, seen edge-on (a dark region between two bright lobes), © Burrows, STSci/ESA, WFPC2, NASA)
    Figure 2: A proto-star star in NGC 1333. Reconstruction of a possible look of a proto-planetery disk based on a Spitzer Space Telescope image. An evidence was found for water vapor in the surrounding area, which appears to be one of the key moments in the development of a planetary system around such a star: icy material is falling from the envelope that birthed the star onto a dense, surrounding disc. Credit: NASA/JPL-Caltech/R. Gutermuth (Harvard-Smithsonian Center for Astrophysics)

    During star formation, the central part of a dense molecular cloud collapses to a proto-star with a gaseous envelope that finally settles to a rotating proto-planetary accretion disc. Sedimentation and self-gravity in such discs trigger the formation of planets and planetary systems. Proto-stars are heavily embedded in surrounding gas and dust and for this reason visible only in the infrared, millimeter or sub-millimeter wavelength bands. Most of the material that goes into forming a star is accreted through a circumstellar disk and in this process the proto-stellar system drives an energetic bipolar jet and outflow into its surroundings. The least evolved proto-stars are surrounded by remnant proto-planetary accretion disks.

    Based on the spectral energy distribution in the infrared and visible light, YSOs are divided into five classes (0-IV), associated with their evolutionary stages. Class 0 refers to collapsing molecular clouds, proto-planetary discs exist in classes I-III, and class IV contains the zero-age main-sequence star.

    A related issue: the extrasolar planets

    Accretion discs in cataclysmic variables (CVs)

    Figure 3: U Gem system as it would be seen from the Earth. In reality, the image of the system is unresolved, and only the total flux from the secondary (red) and accretion disc (blue) is observed. The primary white dwarf is located at the centre of the accretion disk (too small to be seen). The secondary and accretion disc periodically eclipse each other, which results in periodic variations in the observed flux (photometry) and spectral features (spectroscopy). From these variations Smak (1971) reconstructed the size and shape of the accretion disc.

    CVs are binary star systems consisting of a white dwarf ("primary") and a normal star ("secondary", or "companion"). Typically, the CVs have sizes comparable to the Earth-Moon system, and orbital periods of a few hours. When the outer layers of the companion overflow the "Roche lobe", the companion loses matter through the first Lagrange point \(L_1\) of the rotating binary system. When the white dwarf is only weakly magnetized, the matter forms an accretion disc around it and eventually reaches its surface. "Dwarf novae" (DN) are CVs that show outbursts lasting for about a week and separated by weeks to months of quiescence. U Gem is the prototype of dwarf novae. The brightness in the visible light of U Gem increases 100-fold every 120 days or so, and returns to the original level after a week or two. The DN phenomenon is due to a specific accretion disc limit-cycle instability, tidal torques, and fluctuations in the mass-transfer rate from the secondary. The geometry of accretion is very different in magnetic CVs, where accretion disks are truncated or absent, and accretion occur along magnetic field lines.

    There is a solid observational evidence for accretion discs in Cvs based on very accurate photometry and spectroscopy.

    Accretion discs in Quasars and other active galactic nuclei (AGNs)

    Most galaxies have supermassive (millions to billions solar masses) black holes at their centers (nuclei). In AGNs, the black hole accretion produces radiative power that usually outshines its host galaxy. The accretion disc is surrounded by a hot corona that contains clouds of gas. These which move fast produce broad lines (BRL) and these which move slow produce narrow lines (NRL) in the AGNs spectra. The large torus of gas and dust partially obscures the central part, which is important for the observed appearance of an AGN, as Figure 3 explains.

    Figure 4: AGN unification scheme. Green arrows indicate the AGN type that is seen from a certain viewing angle
    Property Quasars Seyfert Galaxies

    Radio Galaxies

    Galaxy Type Spiral, Elliptical Spiral Giant Elliptical Elliptical
    Appearance Compact, Blue Compact, Bright Nucleus Elliptical Bright, Star-like
    Maximum Luminosity 100-1,000 Milky Way Comparable to Bright Spirals Strong Radio 10,000 Milky Way
    Continuum Spectrum Non-thermal Non-thermal Non-thermal Non-thermal
    Absorbtion Lines Yes None Yes None
    Variability Days to Weeks Days to Weeks Days Hours
    Radio Emission Some Weak Strong Weak
    Redshifts z > 0.5 z ~ 0.5 z < 0.05 z ~ 0.1

    AGNs are generally divided into two families, "radio loud" and "radio quiet", depending on whether they exhibit jets or not. The viewing angle gives rise to several AGN types that are distinguished by their emission properties. Among them, the spiral galaxies with broad and narrow emission lines (Seyfert 2 galaxies) or just narrow emission lines (Seyfert 1 galaxies, where the dust torus obscures the BLR); the counterparts in the radio loud family are broad (BLRG viewing angle above ~60°) and narrow line radio galaxies (NLRG viewing angle below ~60°) with jets and radio lobes perpendicular to the accretion disc; and radio loud and radio quiet quasars (i.e. quasi-stellar objects). The latter are the most luminous beacons in the universe and observed up to highest redshifts, implying cosmological distance and gigantic energy output. Despite their name, quasi-stellar objects are thus anything but stars. However, because quasars shine at such large distances, it is not possible to resolve the bright core. Recent observations detect jets and nebulosity around some of them.

    Accretion discs in Microquasars and X-ray binaries

    Figure 5: Microquasars found in several X-ray binaries in our Galaxy are scaled down version of quasars, as pointed out by Felix Mirabel, who also coined the name "microquasars". This figure first appeared in several Mirabel's articles.
    Figure 6: Black hole X-ray binaries in our Galaxy. Figure shows the companion star (donor) and the accretion disc. All systems drawn to scale (for comparison the Sun - Mercury distance is shown). The figure after Jerome Arthur Orosz.

    Main-sequence secondary stars in orbit with accreting neutron stars or black holes (neutron star binaries or black hole binaries, respectively) are common objects in the Galaxy. Neutron stars are often magnetised, especially young ones, such that their accretion discs are disrupted by magnetic fields or do not exist at all. Matter in these cases is lead by partial or total column accretion to the compact object. In comparison to CVs their spectral energy distribution is observed up to the X-ray regime, since neutron stars and black holes own a much stronger gravitational potential. X-ray binaries are, depending on the mass of the companion star, roughly divided into two categories, the low-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries (HMXB), where soft X-ray transients and X-ray pulsars are respective sub-classes. Soft X-ray transients with both NSs and BHs show quasi-periodic outbursts. Many, if not all, black hole X-ray binaries exhibit in addition relativistic twin jets that propagate along the rotational axis of the compact object and are called microquasars.

    Accretion discs in gamma ray bursts (GRBs)

    The most energetic explosions in the universe are gamma-ray bursts. Models that describe GRBs as a result of merging compact objects or failed supernova (collapsars) generally predict a similar configuration of the end products: the formation of a solar-mass black hole surrounded by a massive debris disc with a huge accretion rate.

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