I have read several books (viz. intro to stellar winds)/articles about stellar winds (dust-driven, line-driven, coronal winds), but still didn't understand the explanation. I understand that line-driven winds occur in hot and high-mass stars, but I have no idea why is it called "Line-Driven", what does the word "line" mean?
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2$\begingroup$ The only lines I can think of in that context are absorption and emission lines $\endgroup$– planetmakerOct 21, 2022 at 11:45
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1 Answer
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The word "line" refers to a spectral line, i.e. an emission or absorption feature in a spectrum. In this case it's absorption.
Line-driven vs. continuum-driven winds
Stellar winds are driven by radiation$^\dagger$, but in order for a star to drive winds, its radiation must be able to interact with gas. For highly ionized atmospheres, this interaction occurs mostly between photons and free electrons, a process which is almost independent of the wavelength of the photons. But for atoms with bound electrons, these electrons can absorb photons of the specific wavelengths that correspond to the energy difference between the states before and after absorption.
Momentum transfer
Because the photons carry momentum, which is conserved, this absorption causes the atoms to move away from the stars. If the excited electron returns to its previous state, a photon with the same wavelength as before is emitted, pushing the atom back. If this re-emission happens in the exact same direction that the photon had originally, then the net result will be that the atom does not move. But in any other direction, the atoms will have achieved a net outward motion.
The atoms may also be ionized by the photon, later recombining, and in both cases the electron could also return to its initial state via several intermediate states, emitting several lower-energy photons. Similar arguments apply here.
The origin of the name
A continuum of light coming from the star will miss some light at wavelengths corresponding to these transitions, which in a raw spectrum looks like a dark "line"; hence the name.
Measuring the wind velocity
The photons are only able to excite the electrons if their energy matches closely that of the transition. But because the gas is accelerated, then in the reference frame of the gas, the stellar continuum is progressively redshifted. Hence progressively bluer photons will be able to interacts with the gas, broadening the line by an amount given by the wind velocity, which can this be measured.
The effect of line-driven winds can be many orders of magnitudes larger than continuum-driven winds (see e.g. Lamers & Cassinelli).
$^\dagger$On larger scales, a population of stars may also heat the interstellar medium, causing it to expand, and supernova explosions may provide feedback, also via neutrinos. These effects drive galactic winds in star-forming galaxies.
