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I have read that "The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun". Source This seems counterintuitive to me since the HRD depicts M dwarfs to be less luminous. How are these types of stars able to produce such copious amounts of radiation? What are the physical mechanisms behind this? I cannot find an answer to this online or in papers. The information I find simply restates it but does not explain it. I could use some insight as this seems counterintuitive.

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  • $\begingroup$ I believe its because red dwarfs have a tendency to release very massive flares, causing more Xray emission $\endgroup$ Feb 24 at 13:46
  • $\begingroup$ What would cause more massive flares? Obviously some sort of energy transport mechanism but what is it? $\endgroup$
    – Astroturf
    Feb 24 at 13:52
  • $\begingroup$ m-dwarf stars are fully convective while the Sun (G type) is only convective in a think layer near surface. The amount of stellar activity is dependent upon amount of convection rather than brightness of star. The other factors are the stellar rotation rate and the strength of the magnetic field. $\endgroup$ Feb 25 at 6:32
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A large fraction of M-dwarfs are highly magnetically active because they have deep convection zones (or may be fully convective) and they have longer spindown timescales than higher mass stars, so remain at higher magnetic activity levels for longer.

The magnetic fields are generated by some sort of internal dynamo. The dynamo efficiency appears to increase with rapid rotation and deeper convection.

The magnetic activity manifests itself with a large surface coverage of equipartition magnetic fields (i.e. the magnetic pressure is similar to the gas pressure), with strengths of several kG. These fields protrude above the surface, and turbulence in the photosphere causes magnetic reconnection, acceleration of charged particles, and the evaporation of heated material into the corona, which emits the X-rays. Typical levels of X-ray emission for fast rotating M-dwarfs are $L_x/L_{\rm bol}\simeq 10^{-3}$, about 10,000 times greater than the Sun.

The large scale magnetic field is also responsible for slowing down the star's rotation, via the linkage of the ionised stellar wind to the field lines. At some distant point from the star, the wind decouples from the field, carrying away angular momentum. It is observed, through studies of clusters of stars with known age, that while sun-like stars have spindown timescales of 100 million years or less, lower mass M-dwarfs can remain rapidly rotating for a billion years or more.

Note that young ($\leq 100$ million years) sun-like stars also have $L_x/L_{\rm bol}\sim10^{-3}$; they just maintain these high activity levels for less time.

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