WEBVTT

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<v Speaker 1>Welcome to Bedtime Astronomy. Explore the wonders of the cosmos

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<v Speaker 1>with our soothing Bedtime Astronomy podcast. Each episode offers a

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<v Speaker 1>gentle journey through the stars, planets, and beyond, perfect for

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<v Speaker 1>unwinding after a long day. Let's travel through the mysteries

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<v Speaker 1>of the universe as you drift off into a peaceful

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<v Speaker 1>slumber under the night sky cosmic vortices. The physics of

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<v Speaker 1>accretion discs. Accretion discs are ubiquitous structures in the universe,

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<v Speaker 1>forming around a variety of astronomical objects, such as black holes,

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<v Speaker 1>neutron stars, white dwarfs, and young stellar objects. These discs

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<v Speaker 1>are composed of gas, dust, and other material that spirals

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<v Speaker 1>inward due to gravitational forces, leading to the emission of

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<v Speaker 1>intense radiation. The study of accretion disks encompasses a range

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<v Speaker 1>of physical processes, including angular momentum transfer, viscous dissipation, magnetic fields,

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<v Speaker 1>and radiation transport. Understanding these processes is crucial for deciphering

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<v Speaker 1>the nature of high energy phenomena in the universe and

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<v Speaker 1>the growth of compact objects. At the core of an

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<v Speaker 1>accretion disc's dynamics is the conservation of angular momentum. As

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<v Speaker 1>material from the surrounding environment is captured by the gravitational

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<v Speaker 1>field of a central object, it possesses significant angular momentum,

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<v Speaker 1>preventing it from falling directly inward. Instead, the material forms

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<v Speaker 1>a rotating disc where the centrifugal force balances gravity. However,

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<v Speaker 1>for the material to accree onto the central object, it

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<v Speaker 1>must lose angular momentum. This transfer of angular momentum is

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<v Speaker 1>facilitated by viscosity within the disc, which acts to transport

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<v Speaker 1>angular momentum outward, allowing the gas to spiral inward. The

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<v Speaker 1>source of this viscosity is a critical question in the

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<v Speaker 1>physics of accretion discs. One of the leading theories is

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<v Speaker 1>the magneto rotational instability MRI, which suggests that magnetic fields

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<v Speaker 1>within the disc can amplify small perturbations, leading to turbulent motions.

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<v Speaker 1>This turbulence acts like an effective viscosity, enabling the outward

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<v Speaker 1>transfer of angular momentum. The MRI has been supported by

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<v Speaker 1>both analytical studies and numerical simulations, providing a robust framework

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<v Speaker 1>for understanding angular moments, momentum transport, and accretion discs. The

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<v Speaker 1>heating of accretion disks is another fundamental aspect. As the

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<v Speaker 1>material moves inward, the loss of gravitational potential energy is

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<v Speaker 1>converted into heat through viscous dissipation. This heating raises the

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<v Speaker 1>temperature of the disc, causing it to emit radiation across

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<v Speaker 1>a broad spectrum. The inner regions of the disc, where

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<v Speaker 1>the gravitational potential is deepest, are the hottest and emit

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<v Speaker 1>primarily in the X ray band, while the outer regions

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<v Speaker 1>emit an optical and infrared wavelengths. The total luminosity of

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<v Speaker 1>an accretion disc can be a significant fraction of the

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<v Speaker 1>Eddington luminosity, the maximum luminosity an object can achieve when

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<v Speaker 1>radiation pressure balances gravitational attraction. Accretion disks disks around different

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<v Speaker 1>types of central objects exhibit distinct characteristics. In black hole

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<v Speaker 1>accretion discs, the inner edge of the disc is determined

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<v Speaker 1>by the innermost stable circular orbit ISCO, which depends on

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<v Speaker 1>the black hole's mass and spin. Within this radius, material

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<v Speaker 1>plunges directly into the black hole, releasing a tremendous amount

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<v Speaker 1>of energy. The study of X ray binaries where a

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<v Speaker 1>black hole accretes material from a companion star as provided

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<v Speaker 1>valuable insights into the physics of black hole accretion discs.

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<v Speaker 1>Observations of X ray spectra and variability have revealed the

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<v Speaker 1>presence of hot coroni, relativistic jets, and complex absorption features,

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<v Speaker 1>all of which are influenced by the extreme conditions near

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<v Speaker 1>the event horizon. Neutron star accretion discs share similarities with

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<v Speaker 1>black hole discs, but also exhibit unique features due to

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<v Speaker 1>the solid surface of the neutron star. When material from

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<v Speaker 1>the disc accretes onto the neutron star's surface, it can

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<v Speaker 1>tritder thermonuclear explosions known as type one X ray bursts.

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<v Speaker 1>These bursts provide a direct probe of the accretion process

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<v Speaker 1>and the neutron star's surface properties. Additionally, the presence of

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<v Speaker 1>strong magnetic fields and some neutron stars known as X

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<v Speaker 1>ray pulsars, can channel the accreting material onto the magnetic poles,

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<v Speaker 1>producing pulsating X ray emission. White dwarf accretion discs are

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<v Speaker 1>found in cataclysmic variables. Where a white dwarf accretes material

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<v Speaker 1>from a companion star, The systems can exhibit dramatic outbursts,

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<v Speaker 1>such as dwarf nov where the accretion rate temporarily increases,

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<v Speaker 1>leading to a sudden increase in luminosity. The study of

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<v Speaker 1>these outbursts has revealed the presence of thermal and viscous

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<v Speaker 1>instabilities in the disc, providing important constraints on the disc

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<v Speaker 1>structure and viscosity. Young stellar objects YSOs also possess accretion discs,

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<v Speaker 1>which play a crucial role in the formation and early

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<v Speaker 1>evolution of stars. These protoplanetary discs are sites of active

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<v Speaker 1>planet formation where dust grains can coagulate and grow into planetesimals,

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<v Speaker 1>and eventually planets. The study of YSO discs has been

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<v Speaker 1>revolutionized by observations from telescopes such as the Atacoma Large

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<v Speaker 1>Millimeter SLASH Submillimeter Array ALMA, which has provided at high

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<v Speaker 1>resolution images of disks with intricate substructures, including rings, gaps,

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<v Speaker 1>and spiral arms. These features are thought to be signatures

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<v Speaker 1>of ongoing planet formation, revealing the complex interplay between the

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<v Speaker 1>disk and forming planets. Radiation transport and accretion disks is

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<v Speaker 1>a key factor in determining their observational properties. The energy

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<v Speaker 1>generated by viscous dissipation must be transported outward and radiated away.

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<v Speaker 1>In the inner regions of the disc, where the density

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<v Speaker 1>is high, radiation is primarily transported by diffusion. In the outer,

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<v Speaker 1>less dense regions, radiation can escape more directly. The balance

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<v Speaker 1>between these processes determines the temperature profile of the dis

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<v Speaker 1>and the resulting spectrum. Theoretical models of accretion disks must

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<v Speaker 1>account for the radiative transfer to accurately predict their emission characteristics.

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<v Speaker 1>Accretion discs can also exhibit outflows in the form of

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<v Speaker 1>winds and jets. These outflows are driven by a combination

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<v Speaker 1>of thermal, radiative, and magnetic forces. Jets, in particular, are

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<v Speaker 1>highly collimated streams of material that can travel at relativistic speeds,

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<v Speaker 1>carrying away angular momentum and energy from the disc. The

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<v Speaker 1>study of jets has provided insights into the mechanisms of

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<v Speaker 1>energy extraction from the central object and the role of

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<v Speaker 1>magnetic fields and launching and collimating these outflows. Observations of

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<v Speaker 1>jets and systems such as blazers where the jet is

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<v Speaker 1>oriented close to our line of sight have revealed the

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<v Speaker 1>presence of highly energetic particles and complex structures, including knots

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<v Speaker 1>and helical twists. The interaction between accretion disks and their

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<v Speaker 1>environments is another important aspect of their physics. In binary systems,

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<v Speaker 1>the accretion disc can interact with the companion star through

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<v Speaker 1>tidal forces, leading to complex dynamical behaviors such as precession

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<v Speaker 1>and warping. In protoplanetary discs, the interaction with forming planets

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<v Speaker 1>can create gaps and spiral waves, influencing the discs evolution

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<v Speaker 1>and the migration of planets. The interplay between the disk

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<v Speaker 1>and its surroundings can significantly affect the accretion process and

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<v Speaker 1>the observational signatures of the system. In conclusion, the physics

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<v Speaker 1>of accretion disks encompasses a rich and diverse array of

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<v Speaker 1>processes that are fundamental to our understanding of high energy

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<v Speaker 1>astrophysics and the growth of compact objects. From the intricate

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<v Speaker 1>dynamics of angular momentum transport and viscous dissipation to the

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<v Speaker 1>complex interactions with jets, lends, and surrounding environments, accretion disks

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<v Speaker 1>provide a unique window into the most extreme and energetic

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<v Speaker 1>phenomena in the universe. The study of these disks continues

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<v Speaker 1>to challenge our theoretical frameworks, motivate new observations, and inspire

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<v Speaker 1>analogies in other fields of physics, as new technologies and

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<v Speaker 1>discoveries emerge, the exploration of accretion discs will undoubtedly remain

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<v Speaker 1>at the forefront of astrophysical research, shedding light on the

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<v Speaker 1>nature of the union averse and the fundamental laws of physics.