What sources of turbulence exist within the Interstellar medium (ISM)? Which ones are physically the most important for newly forming stellar systems?


Turbulence sources:

There are numerous sources of turbulence in the interstellar medium, at all scales:

  • at large scales, there is the shear from galactic rotation. One way to sustain turbulence and to couple large and small scales would be the magnetorotational instability (MRI).
  • at large scales, gravitational instabilities can also play a significant role, through spiral structures.
  • outflows and jets from forming stars play an important role, releasing a lot of energy in the ISM.
  • in star forming regions, massive stars are also important. Radiation and stellar winds from massive stars are an important input of energy in the ISM. And eventually, the most massive ones will blow up in supernovæ, releasing even more energy.

Therefore, one could then regards separatly three processes related to massive stars:

  • stellar winds
  • ionizing radiation
  • supernova explosion

Importance for star formation:

They are all relevant to star formation, one way or another. One key property of turbulence is to cascade from large to small scales; therefore, even if you inject turbulence at large scales (galactic scale) you'll get turbulent motions down to the scale of a molecular cloud.

A nice illustration of the tubulent cascade is the Larson's relation (Larson 1981): enter image description here

The Larson's relation shows the evolution of the velocity dispersion with the size of the structure you're looking at. Velocity dispersion is an indicator of turbulence. Indeed, these dispersions are non-thermal: knowing the typical temperature of the MIS (about 10 K), one can estimate the thermal velocity of, for example, the CO molecule ($v_th = \sqrt{2kT/\mu m_H}$, with $k$ the Boltzman constant, $T$ the temperature, $\mu$ the mean molecular wright and $m_H$ the hydrodgen atom mass) that is about 0.07 km s$^{-1}$. Measured velocity dispersions are of the order of 1 to 10 km s$^{-1}$, and these is interpreted as a turbulence signature (and estimate).


Energy rates: values are given (roughly) for the Milky Way

  • MRI: $ \small \dot e = 3 \times 10^{-29} {\rm erg\ cm}^{-3}\ {\rm s}^{-1} $;
  • Gravitational instabilities: $\small \dot e = 4 \times 10^{-29} {\rm erg\ cm}^{-3}\ {\rm s}^{-1}$;
  • Outflows: $\small \dot e = 2 \times 10^{-28} {\rm erg\ cm}^{-3}\ {\rm s}^{-1}$;
  • Ionizing radiation: $\small \dot e = 5 \times 10^{-29} {\rm erg\ cm}^{-3}\ {\rm s}^{-1}$
  • Supernovæ explosions: $\small \dot e = 3 \times 10^{-26} {\rm erg\ cm}^{-3}\ {\rm s}^{-1}$
  • Stellar winds: it strongly depends on the type of star: it varies as the power of -6 of the star's luminosity. It therefore ranges from an energy comparable to a supernova explosion (or even more for Wolf-Rayet stars) to almost nothing.



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