Low-Mass Star Evolution for Binaries

• A Good Astronomical Puzzle - Algol

• Algol Persei is a double-lined, eclipsing, spectroscopic binary with an orbital period of around three days.

  Component A:

  Component B:

• What is wrong with this? The star should evolve MORE slowly than the star and should still be on the main-sequence.

• The solution has to do with the short-period orbit.

• The Algol Story:
  1. In the beginning, there were two stars, A at and B at . There is a point between the two stars where the gravitational pull of Star A is equal to that of Star B. This is called a ``Lagrange Point'' and there is an hourglass(ish) shaped ``equipotential surface'' called the Roche lobes. (Note that this is in a strange rotating reference frame).

     

  2. As Star B begins its trip up the Red Giant branch it swells until its radius reaches the L1 point (or, until its radius exceeds its Roche Lobe radius) and Star B begins to spill mass through L1 to Star A.

     This is called a .

  

• Under certain circumstances, Star B can transfer enough mass to Star A so that Star A becomes the more massive of the two. This looks like Algol.

• You can have even Weirder things happen in mass-exchange binaries.

  Think about continuing evolution for case where most of the Star B mass gets exchanged.

  1. Now have a Helium WD (B) plus a massive Main-sequence Star (A)

  2. When Star A begins its evolution up the RGB it will reach its Roche Lobe radius and star to spill material back onto B (now a He WD).

  3. As mass accumulates on the hot surface of the WD, it traps radiation, heats up and when T reachs K the hydrogen begins fusion to He and there is a dramatic flare-up in the luminosity of the system.
     

• Because the fusion is on the surface of the WD, there is an ejection of material at km/sec.

• Sometimes, the mass accretion stars up again leading to .

• Every year there are 10 - 20 bright novae in the Galaxy.