• The Virial Theorem postulates that for self-gravitating systems:
  • for an N-particle system;
  • for gaseous objects

  where n_vir is a positive number, T is the kinetic, U_int the internal energy and W the total gravitational (potential) energy and <..> denotes the time-average. In other words, the virial theorem holds when the system has relaxed into a steady configuration.

  ---

  Proof goes along these lines. The quantity

  time-averages to 0. Q (the "virial") is itself the derivative of the moment of inertia. For a gravitating gas, we derive from the hydrostatic balance equation

  To proceed, must be linked to the internal energy per unit mass . For an ideal gas this is just

  where is the number of free degrees and is the heat capacity ratio. For a mono-atomic gas, , and .

• The Virial Theorem postulates that for self-gravitating systems:
  • for an N-particle system;
  • for gaseous objects

  where n_vir is a positive number, T is the kinetic, U_int the internal energy and W the total gravitational (potential) energy and <..> denotes the time-average. In other words, the virial theorem holds when the system has relaxed into a steady configuration.

• Specific examples include:
  •   (fully ionized or mono-atomic gas)
  •   (radiation gas)

• The Virial Theorem postulates that
  • for an N-particle system;
  •   (fully ionized or mono-atomic gas)
  •   (radiation gas)
• The virial theorem is not the same as energy conservation!

  Conservation of energy dictates that  where the total energy E is conserved in an isolated system. Hence we obtain that objects in virial equilibrium have (a different prefactors may apply if the gas is not mono-atomic).
  Note — the expression for the virial theorem can be extended to include magnetic support, rotational support etc..

• The Virial Theorem postulates that
  •   (fully ionized or mono-atomic gas)
  •   (radiation gas)
• The virial theorem is not the same as energy conservation!
• Applications:
  • (Proto-)stars heat up when they contract:

    Protostars (no nuclear burning) lose energy. Of the gravitational energy liberated during contraction, half is radiated and the other half goes into internal energy. Stars heat up while loosing energy!
    This corresponds to moving from state A → B in the figure right

  • Stars are thermodynamically stable

    They expand and cool down when their total energy E increases (!). This corresponds to moving from B → A in the figure right. This kind of "virial stability" only applies to the ideal gas law.

  • Obtain the (dynamical) mass by measuring the linewidth.

Andromeda-Milky Way merger

Andromeda galaxy. (c) Peter Forister

The condition for a spherical, uniform cloud to collapse is when its mass exceeds the Jeans mass

When the total energy of the cloud decreases when it is perturbed. If we insert numerical constants for the numbers and write the densities in terms of the number density we obtain

Note some (giant) clouds have suggesting that the should collapse! In reality, these clouds feature additional support mechanisms (like magnetic support). Hence the condition is a necessary but not (always) sufficient condition for collapse.

If the cloud is out of equilibrium, it collapses on a free-fall time

• During collapse of the cloud, ρ increases and the Jeans mass decreases. The cloud fragments with the fragments collapsing on their own.
• The isothermal collapse, requires efficient cooling.
• An increase in temperature, on the other hand, opposes cloud fragmentation, causing M_Jeans to rise

• An increase in temperature, on the other hand, opposes cloud fragmentation, causing M_Jeans to rise

Accelerating charges radiate energy at a rate (Larmor formula):

(cgs units) with P is the power, q the charge the acceleration, c the speed of light.

The Thomson cross section for photons to interact with electrons

Valid in the low-energy (no recoil) limit, the photon energy does not change. It follows from equating the radiated power to the incoming energy flux:

The Thomson cross section can also be obtained by equating the electrostatic energy corresponding to a sphere of radius r_T with the rest energy of the electron The corresponding cross section is the Thompson cross section barring numerical constant.

schematic of the interaction of an electromagnetic wave with a charged particle. Due to oscillating E-field, the particle (electron) experiences a force of

in the direction of the E-field, causing it to radiate. In the low-energy limit, the frequency of the radiation is just the same as that of the incoming wave, resulting in a scattering of the EM-wave (Thomson scattering).

The Thomson cross section for photons to interact with electrons

Valid in the low-energy (no recoil) limit, the photon energy does not change.

This results in a radiation force of

on the electrons, which when balanced with the gravitational force (acting on all baryons).

Balancing with the gravitational force, results in the Eddington Luminosity

Another, more general, way to write this is, is in terms of the opacity κ — the cross section per unit mass κ=σ/(m_e+m_p) in this case. We then obtain .

• For luminosities, L>L_Eddington the star will be ripped apart
• On the main-sequence, the stellar mass-luminosity relationship is a power-law

  The Eddington Luminosity sets an upper mass for stars!

The fluid equations:

where the E.o.S is chosen as isothermal (constant temperature), such that c_s — the isothermal sound speeed — is constant.

To 0^th order, for a uniform cloud, the solutions to these equations read simply:

A linearly stability analysis quantifies how the system reacts to small perturbations that are wave-like in nature, e.g.,

with k the wavenumber and ω the frequency (or growth rate when it is imaginary!). Without loss of generality we have assumed that the wave travels in the x-direction. Note than ω can be complex, so the wave can grow (or decay)!

After some algebra, you arrive at the perturbation equations

The solution to the linearly stability analysis is the dispersion relationship:

A dispersion relationship is a relation between the scale of the perturbation (k) and the growth rate (ω).

We obtain:

• small scales (large k): ω² is positive: perturbations remain wave-like and do not grow!
• large scales (small k): ω² is negative: perturbations grow exponentially and are unstable!
• a critical scale

  You can verify that amounts to the Jeans mass, barring a numerical coefficient

For disks the steady solution reads:

the assumption here is that the disk is thin. It does not need to be Keplerian rotating, but it should satisfy Also, we consider that the disk is axisymmetric (quantities do not depend on φ) and consider radial perturbations only (k oriented in the radial direction).

... and the dispersion relationship is

... and the dispersion relationship is

We obtain the following:

• The most critical wavelength is 
• The disk becomes gravitationally unstable when the Toomre-Q parameter
• The corresponding mass is .

  Question:
  Is Q_T <1 the only criterion for gravitational instability to result in collapse?

... and the dispersion relationship is

We obtain the following:

• The most critical wavelength is 
• The disk becomes gravitationally unstable when the Toomre-Q parameter
• The corresponding mass is .
• In addition, gravitational collapse requires fast enough cooling

  Quantitatively,   Otherwise the formed clumps will just be sheared apart

... and the dispersion relationship is

We obtain the following:

• The most critical wavelength is 
• The disk becomes gravitationally unstable when the Toomre-Q parameter Q_T<1.
• In addition, gravitational collapse requires fast enough cooling

  ---

• For dust particles, can be used, amounting to a critical scale of

Hence, particles need to settle. The settling of particles is promoted by the stellar gravity, but opposed by turbulence.

1. Dark cores form and become unstable when their gravitational potential energy dominates thermal energy:
   (mass exceeds Jeans mass)
2. The collapse takes place on the free-fall timescale
   this is the embedded phase Class 0

3. A star has formed. Class 1. An outflow develops, while material is still accreting onto the star. A disks forms.
4. Accretion and outflow weaken. This is the stage where planets are thought to form. Class 2 or the T-Tauri phase.
5. After the gas clears, a debris disks may be left

   Debris disk consist of solid material that did not assemble into planetary bodies. In the solar system the asteroid belt and the Kuiper belt are debris disks

6. A planetary system emerges. The final architecture of the planet system is set when orbital stability is reached.

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess

  the total integrated emission is a superposition of several black body curves. The IR-excess is how disks were historically detected

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess
• Minimum-mass solar nebula (MMSN)

  By spreading out the heavy element mass of the planets over rings, Weidenschilling (and Hayashi) obtained a crude prescription of the dust and gas density during the time of the formation of the planets. This prescription — known as the Minimum-Mass Solar Nebula — has many flaws, but serves as a useful benchmark. A frequently-used profile is:

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess
• Minimum-mass solar nebula (MMSN)
• disks are observed to accrete onto their host star at rates of  

  Mass and angular momentum are being transported.

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess
• Minimum-mass solar nebula (MMSN)
• disks are observed to accrete onto their host star at rates of  

  Mass and angular momentum are being transported.

Properties

• disks are naturally formed as byproduct of star formation due to angular momentum conservation
• disks are usually dusty, obscuring the interior disk midplane regions
• the dust results in an IR-excess
• Minimum-mass solar nebula (MMSN)
• disks are observed to accrete onto their host star at rates of  
• Accretion terminates over timescales of ~Myr

  A natural explanation is that the gas disk is gone after several Myr. It sets a timescale for the formation of gas giant planets

Angular momentum (AM) transport

• In a Keplerian-rotating disk, rings at different radii move at different (Keplerian) velocities. This causes shear ("friction").

  The faster-moving inner rings will lose AM to the exterior ring, but gain it from the interior ring. One says that AM is being transported

• This results in a torque on the ring, which magnitude is proportional to the   (kinematic) viscosity ν. One can derive the transport equation for the surface density:
• However, the molecular viscosity is too low; an anomalous (turbulent) viscosity produced by some instability is needed. A key question is which instability provides this anomalous viscosity.

There are two main paradigms for the formation of planets:

1. disk instability: planets form following a gravitational instability in the gas disk
2. core accretion: planet formation occurs bottom-up by coagulation (collisions between solid particles)

   —initially, solids stick due to surface forces
   —later, bodies are held together by gravity
   (at intermediate sizes sticking is problematic)

Planet formation concepts:^†

• gas disk structure

  gas in disks is supported by both rotation AND (a little bit) by pressure:

  This means that disks are rotating slightly sub-Keplerian. This is important as it results in radial drift.

• growth timescales
• gravitational focusing

Planet formation concepts:^†

• gas disk structure
• growth timescales

  A simple n-σ-Δv calculation shows:

  where R is the particle radius the particle internal density, ρ_dust the volume density in dust (or planetesimals etc.), and Δv the relative velocity.
  — growth timescales increase with distance (lower ρ_dust)
  — growth timescales increase with size R

• gravitational focusing

Planet formation concepts:^†

• gas disk structure
• growth timescales
• gravitational focusing

  gravitational focusing greatly accelerates growth. We can obtain the collisional impact parameter by equating the angular momentum and energy at infinity and at impact:

  this is much larger than the geometrical cross section

  A consequence of gravitational focusing is that the growth time decreases, (as long as Δv remains the same), resulting in the phenomenon of runaway growth.

in the pebble accretion mechanism, small particles get trapped in the gravitational potential of a planet and are captured efficiently. (c) Chris Ormel.