Over on FaceBook I'm on the sidelines of a discussion that is going on between a friend of mine who is a prominent astrophysicist and his astronomer buddies, which is about the possibility that M dwarf stars might have planets in their Goldilocks zones that are habitable to humankind, particularly in the matter of having watery oceans and oxygen atmospheres. Three points are being discussed, two of which are familiar to us in this forum.
- First there is the issue of flares and x-rays from the star. The astronomers seem to believe that any M dwarf star that is not an x-ray flare star is a former x-ray flare star, and that its terrestrial planets in the life zone are likely to have had any atmospheres stripped.
- Then there is the usual business about tidal locking. I have been able to pass along some of the relevant papers that Thrash shared with us earlier.
- And third, there is an issue that I had not been aware of before — the proto-stellar objects that evolve into low-mass stars are apparently much more luminous than the stars they develop into, hot enough for long enough that they might be expected to evaporate and drive off all volatiles from what settles down to be their Goldlocks zones.
Apparently (I expect that EDG will be by soon to clarify) there are about four possible early histories for a star.
- A protostar with mass greater than about 8 M☉ is fusing hydrogen as soon as it collapses to a dense core, and is born on the main sequence.
- A protostar with mass between about 2 M☉ and about 8 M☉ approaches the main sequence along the Henyey track, collapsing slowly, heated by gravitational contraction, and growing steadily hotter and smaller at nearly constant luminosity until it reaches the main sequence and begins hydrogen fusion. The Henyey track is nearly horizontal in the Hertzsprung-Russell diagram, approaching the main sequence by moving to the left.
- A protostar with mass between about 0.5 M☉ and 2.0 M☉ starts out by collapsing (and heating itself by gravitational contraction) in such a way that its temperature remains constant, with the result that its luminosity falls along with its surface area. This is the Hayashi track, nearly straight downwards in the H-R diagram. Upon reaching a critical condition it performs a few hijinks and then switches to the Henyey track.
- A protostar with mass below 0.5 M☉ approaches the main sequence along the Hayashi track, shrinking and growing less luminous at nearly constant temperature until core fusion halts the collapse. It moves almost straight down in the H-R diagram, and rather slowly.
Here is a diagram showing the development of stars of different mass from formation to the beginning of hydrogen fusion:
The diagram above plots temperature against colour index, like a Hertzsprung-Russell diagram. The upper black line is the the stellar birth line, and the lower black line is the zero-age main sequence line. The paths of development of stars of different masses are shown in blue (labelled in M☉
). The red lines are isochrones: they connect the points in the diagram that stars of different mass reach in the same time after formation.
Note that stars with an initial mass more than about 2.0 M☉
end up just as luminous as they start out, whereas stars smaller that about 0.5 M☉
end up on the main sequence at least about two orders of magnitude less luminous than they start out.
Then note that the initial high luminosity of very-low-mass protostars lasts for ten million years or more.
The result is that material in stable orbits in the life zone of low-mass stars has a history of being much hotter for a long time. Volatiles will have been vaporised and then driven off by Jeans escape and other processes and swept away by light pressure, except from massive planets that formed very early. Any volatiles in terrestrial-sized planets in the Goldilocks zone of M-class dwarfs much have been delivered after some tens of millions of years from stellar birth, and will probably be rare.