View source for Accretion discs/Observational evidence for accretion discs in the Universe

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== Accretion discs in young stellar objects (YSOs) ==

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[[Image:hh30.gif|frame|283px|right|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)]]

*[http://arxiv.org/abs/astro-ph/0109048 K. Wood, M.J. Wolff, J.E. Bjorkman, B. Whitney ''The HH30 Spectrum: Constraining Circumstellar Dust'']

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[[Image:protoplanetarydisk.jpg|frame|283px|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)]]

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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.

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*[http://www.circumstellardisks.org/ '''Catalog of Resolved Circumstellar Disks''' maintained by Caer McCabe (JPL)]

*[http://cfa-www.harvard.edu/rg/star_and_planet_formation/young_stellar_objects.html Harvard University RG Research Group (Young Stellar Objects)] 

*[http://arxiv.org/abs/astro-ph/0607387 Roy van Boekel et al., 2006, ''Disks around young stars with VLTI-MIDI'']

*[http://arxiv.org/abs/astro-ph/0409190 R.D. Blum et al., 2004, ''Accretion Signatures from Massive Young Stellar Objects'']

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<b>A related issue: <i>the extrasolar planets</i></b>

*[http://exoplanet.eu/ ''The Extrasolar Planets Encyclopaedia'', maintained by Jean Schneider  CNRS/LUTH - Paris Observatory ]

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== Accretion discs in cataclysmic variables (CVs) == 

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[[Image:Ugem-model.gif|frame|480px|left|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
[http://adsabs.harvard.edu/abs/1971AcA....21...15S Smak (1971)] 
reconstructed the size and shape of the accretion disc.]]

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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 
<math>L_1</math> 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 <nowiki>instability</nowiki>, tidal torques, 
and <nowiki>fluctuations</nowiki> 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.

*[http://www.amazon.com/Cataclysmic-Variable-Stars-Cambridge-Astrophysics/dp/052154209X/ref=pd_sim_b_3/105-3740034-0846059 B. Warner, ''Cataclysmic Variable Stars'', 1995, 2003, Cambridge University Press]

*[http://arxiv.org/abs/astro-ph/0102072 Lasota J.-P., 2001, ''The disc instability model of dwarf-novae and low-mass X-ray binary transients'', New Astron. Rev., '''45.7''', 449]

*[http://astro.fit.edu/cv/fitdisk.html The accretion disk simulator by the cataclysmic variable group at The Florida Institute of Technology.]

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

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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. 

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[[Image:AGN-scheme.jpg|frame|400px|center|AGN unification scheme. Green arrows indicate 
the AGN type that is seen from a certain viewing angle]]
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<b style="color: #999999">Property</b></td>
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<b>Quasars</b></td>
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<b>Seyfert Galaxies</b></td>
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<b>Radio Galaxies</b>
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<b>Blazars</b></td>
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<td align="center" valign="middle" style="background: #ffffff" width="12%"> 
<b style="color: #999999">Galaxy Type</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Spiral, Elliptical</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Spiral</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Giant Elliptical</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Elliptical</td>
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<b style="color: #999999">Appearance</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Compact, Blue</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Compact, Bright Nucleus</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Elliptical</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Bright, Star-like</td>
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<b style="color: #999999">Maximum Luminosity</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">100-1,000 Milky Way</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Comparable to Bright Spirals</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Strong Radio</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">10,000 Milky Way</td>
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<b style="color: #999999">Continuum Spectrum</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Non-thermal</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Non-thermal</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Non-thermal</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Non-thermal</td>
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<b style="color: #999999">Absorbtion Lines</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Yes</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">None</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Yes</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">None</td>
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<b style="color: #999999">Variability</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Days to Weeks</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Days to Weeks</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Days</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Hours</td>
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<b style="color: #999999">Radio Emission</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Some</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Weak</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Strong</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">Weak</td>
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<b style="color: #999999">Redshifts</b></td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">z > 0.5</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">z ~ 0.5</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">z < 0.05</td>
<td align="center" valign="middle" style="background: #ffffff" width="12%">z ~ 0.1</td>
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<div align="justify">
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.

*[http://press.princeton.edu/titles/6660.html J.H. Krolik, 1998, ''Active Galactic Nuclei: From the Central Black Hole to the Galactic Environment'', Princeton University Press]

*[http://www.astro.bas.bg/~petrov/galaxies_files/agn.html Online compilation by G.T.Petrov, 2004]

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== Accretion discs in Microquasars and X-ray binaries ==

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[[Image:Mirabel-microquasar-quasar.gif |frame|400px|left| 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.]]

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[[Image:x-ray-binaries-orosz-03.jpg|frame|400px|left| 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.]]

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Main-sequence secondary stars in <nowiki>orbit</nowiki> 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.

*[http://adsabs.harvard.edu/abs/2003astro.ph..6213M J.E. McClintock and R.A. Remillard, 2003, ''Black Hole Binaries'']

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== Accretion discs in gamma ray bursts (GRBs) ==

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The most energetic explosions in the universe are gamma-ray <nowiki>bursts</nowiki>. 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.

*[http://www.journals.uchicago.edu/doi/abs/10.1086/322267 Narayan R., Piran T. and Kumar P., 2001, ''Accretion Models of Gamma-Ray Bursts'', ApJ, '''557''', 949]

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