CrazyNeutrino said:
A Pulsar and White Dwarf Dance Together In A Surprising Orbit
Searching the Universe for strange new star systems can lead to some pretty interesting finds. And sometimes, it can turn up phenomena that contradict everything we think we know about the formation and evolution of stars. Such finds are not only fascinating and exciting, they allow us the chance to expand and refine our models of how the Universe came to be.
More…
More from link.
> Whereas such systems were believed to have circular orbits in the past, the white dwarf in this particular binary orbits the pulsar with extreme eccentricity!
Very interesting, I should have seen this coming.
> Conventional wisdom states that low mass white dwarfs (LMWDs) are the product of binary evolution. The reason for this is because that under normal circumstances, such a star – with low mass but incredible density – would only form after it has exhausted all its nuclear fuel and lost its outer layers as a planetary nebula. Given the mass of this star, this would take about 100 billion years to happen on its own – i.e. longer than the age of the Universe.
Yes. We know this.
> Basically, millisecond pulsars that are orbited by a star will slowly strip them of their mass, sucking off their outer layers and turning them into a white dwarf. The addition of this mass to the pulsar causes it to spin faster and buries its magnetic field, and also strips the companion star down to a white dwarf.
Yes.
> In this scenario, the eccentricity of orbit of the LMWD around the pulsar is expected to be negligible.
Brain strain here. Why would it be negligible?
The stars would start out in a highly eccentric orbit – very common so nothing unusual there.
The neutron star strips matter from its companion at perihelion as the companion approaches its red giant phase – again, nothing unusual there.
The companion loses enough mass that it becomes a LMWD – again, nothing unusual there.
So, would stripping matter at perihelion increase or decrease orbit eccentricity? and would that depend on the rotation rate of the companion? The stellar rotation can’t contain enough energy to increase the eccentricity, but could possibly (?) contain enough energy to sop the orbit degradation?
> Because of the strong tidal forces during the mass-transfer episode, the orbits of these systems are extremely circular, with eccentricities of ~0.000001 or so.
Hmm.
> We now know 5 systems which deviate from this picture in that they have eccentricities of ~0.1 i.e. several orders of magnitude larger that what is expected in the standard scenario.
OK.
> they were able to infer the temperature (8600 ± 190 K) and velocity ( km/s) of the white dwarf companion in the binary star system. Combined with constraints placed on the two body’s masses – 0.28 Solar Masses for the white dwarf and 1.4 for the pulsar – as well as their radii and surface gravity, they then tested three possible explanations for how this system came to be.
Good.
> some of the material released from the white dwarf in the past (due to this same stellar wind) could have formed a short-lived circumbinary disk. This disk would then act like a third body, disturbing the system and increasing the eccentricity of the white dwarf’s orbit.
Yes. This was what was giving me brain strain earlier. It’s far from obvious how this would affect the eccentricity of the orbit. Glad to see that they’ve confirmed it by calculation.