Milky Way Harbors “Ticking Time Bombs”


According to new research, the only thing that may be keeping elderly stars from exploding is their rapid spin. In a galaxy filled with old stars, this means we could literally be sitting on a nearby “time bomb”. Or is this just another scare tactic?

“We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this research suggests that we’ve been looking for the wrong signs. Our work points to a new way of searching for supernova precursors,” said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).

In light of the two recently discovered supernova events in Messier 51 and Messier 101, it isn’t hard to imagine the Milky Way having more than one candidate for a Type Ia supernova. This is precisely the type of stellar explosion Di Stefano and her colleagues are looking for… and it happens when a white dwarf star goes critical. It has reached Chandrasekhar mass. Add any more weight and it blows itself apart. How does this occur? Some astronomers believe Type Ia supernova are sparked by accretion from a binary companion – or a collision of two similar dwarf stars. However, there hasn’t been much – if any – evidence to support either theory. This has left scientists to look for new answers to old questions. Di Stefano and her colleagues suggest that white dwarf spin might just be what we’re looking for.

“A spin-up/spin-down process would introduce a long delay between the time of accretion and the explosion. As a white dwarf gains mass, it also gains angular momentum, which speeds up its spin. If the white dwarf rotates fast enough, its spin can help support it, allowing it to cross the 1.4-solar-mass barrier and become a super-Chandrasekhar-mass star. Once accretion stops, the white dwarf will gradually slow down. Eventually, the spin isn’t enough to counteract gravity, leading to a Type Ia supernova.” explains Di Stefano. “Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences. Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it’s very difficult science.”

Sure. It might take a billion years for the spin down process to happen – but what’s a billion years in cosmic time? In this scenario, it’s enough to allow accretion to have completely stopped and a companion star to age to a white dwarf. In the Milky Way there’s an estimated three Type Ia supernovae every thousand years. If figures are right, a typical super-Chandrasekhar-mass white dwarf takes millions of years to spin down and explode. This means there could be dozens of these “time bomb” systems within a few thousand light-years of Earth. While we’re not able to ascertain their locations now, upcoming wide-field surveys taken with instruments like Pan-STARRS and the Large Synoptic Survey Telescope might give us a clue to their location.

“We don’t know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but we’re looking forward to hunting them out,” said co-author Rasmus Voss of Radboud University Nijmegen, The Netherlands.

And the rest of us hope you don’t find them…

Original Story Source: Harvard Smithsonian Center for Astrophysics News. For Further Reading: Spin-Up/Spin-Down models for Type Ia Supernovae.

19 Replies to “Milky Way Harbors “Ticking Time Bombs””

  1. When it nears the end of its life, the white dwarf ceases nuclear fusion and reaches Chandrasekhar mass.

    I’m sorry, but what?

    A white dwarf is the core of a long gone star. This core has ceased to fuse elements a long time ago. It’s supported by the pressure of degenerated electrons, which can have relativistic energies. It contains carbon, oxygen, helium, probably iron, and some other elements in between. But there should be no fusion whatsoever.

    And achieving the Chandrasekhar mass is not caused by stopping fusion (the sentence could imply this ๐Ÿ˜‰ ), but indeed by accretion.

    [Ivan3man-mode on]
    In the very first sentence, the last “in” should be an “is”, doesn’t it?
    [Ivan3man-mode off]


    1. Well spotted, DrFlimmer, but you’re not as good as me because you’ve missed one…

      At the fifth paragraph, since we are referring to more than one, it should be there are, not “there’s”, in this sentence: “In the Milky Way [there are] an estimated three Type Ia supernovae every thousand years.”

      1. Well spotted, DrFlimmer, but you’re not as good as me because you’ve missed one…


        That’s why YOU are Ivan3man and I am not. What kind of world would that be where just anyone could be Ivan3man? Maybe then we could destroy all our cars and ride on our flying pigs. ๐Ÿ˜‰

  2. How does this affect the accuracy of distance measurements using type 1a supernovas as standard candles? Maybe things aren’t quite as far away as we thought.

    1. It would introduce more uncertainties (assuming this is one of the causes of sn Ia, which seems likely) but i dont know of any explosion dynamics calculated (with supercomputer) of super chandrasekhar mass WD. There is still a practical upper limit to the mass, and because of the instability complexities of such a beast, i would expect those SN Ia to be sub-luminous compared to a 1.4Msun Chandrasekhar WD.

      This would lead to an overestimate in the distance function, probably not by much, but still.

  3. The hydrostatic pressure for a nonrotating star is given by the pressure gradient

    dP/dr = -GM?/r^2

    for a star of mass M and density ?. To include angular momentum we go back to the centripetal force

    F = mv^2/r

    Angular momentum is L = rxp, which for circular motion is L = mvr^2, hence the centripetal force is

    F = L^2/r^3.

    Now replace the mass with density with the angular momentum density

    ? = (?v(r))^2/r^3

    (? is the Cyrillic equivalent of L) and v(r) = V(r/R) for V the tangent velocity of the surface and R the radius of the star and so

    ? = (?V)^2/rR^2.

    We have to include the fact the angular momentum varies as sin^2? with respect to the z axis. So the hydrostatic equilibrium condition is

    dP/dr = -GM?/r^2 + sin^2?(?V)^2/rR^2

    One last thing has to be done. This equation would work if the star were composed of an incompressible fluid. However, since it is a gas the pressure equilibrates. Integrating over the angle on (0, ?) gives ยฝ (a calculus result). It is an elementary integration P = ?(dP/dr)dr to compute the pressure at a radius in the stellar interior.

    It is then clear that the angular momentum reduces the pressure. This means a rotating star with a mass greater than the Chandrasekhar mass can exist. The closest white dwarf is Sirius B, about 7 light years away. I presume the rotational speed is measured by Doppler broadening or some similar technique. Sirius B is probably then safety checked.


      1. This is related. It actually is more complicated, for the white dwarf mass and angular momentum both change.


      2. This is related. It actually is more complicated, for the white dwarf mass and angular momentum both change.


    1. Why would the material a white dwarf is made of be considered a gas in that equation? It’s not going to act like a standard gas under that kind of pressure. Wouldn’t it be closer to a solid than a gas? And if the pressure equilibrates, then wouldn’t that mean that the pressure at the core would be the same as the pressure at the surface? That can’t be right.

      1. The pressure equilibrates with respect to the angle ?, even though it increases with radial distance.

        Solids are compressible. A plutonium bomb is in part triggered by the implosion of the plutonium core. The HE explosives compress the solid so the plutonium nuclei are closer and are more rapidly fissioned by neutrons.


    2. A white Dwarf is electron degenerate, that means that degenerate pressure is stronger than the gas pressure, and composed of free electrons moving very fast. When the density reaches critical (Chandrasekhar mass in a non-rotating WD) the electrons will move with speeds approaching the speed of light, and as they do, the increase of density would eventually not result in an increased supporting pressure.

      The conclusion is correct though, increased rotation/angular momentum, results in decreased density.

      1. The material phase that is incompressible, or very nearly so, is the fluid state. I am not well versed on the state of matter in a white dwarf, but I think it is probably compressible. If so the angular dependent pressure set up by rotation is equilibrated.


      2. The material phase that is incompressible, or very nearly so, is the fluid state. I am not well versed on the state of matter in a white dwarf, but I think it is probably compressible. If so the angular dependent pressure set up by rotation is equilibrated.


    3. Sirius B is probably then safety checked.

      As far as I know, Sirius B is too far out to accrete material from Sirius A. There should be no danger.

      1. According to Wikipedia, the distance separating Sirius A from Sirius B is between 8.1 and 31.5 AU, so definitely no danger of a Type Ia supernova.

  4. The original pdf is worthwhile reading and I’d recommend it. However, I take exception on when the “countdown” begins. The paper marks the point at which the WD is no longer accreting mass. However, I would suggest that this is better defined as the point at which the WD passes the Chandrasekhar limit.

    The reason for this is that, while additional accreted mass will increase the WD spin and the critical mass needed for the explosion, the critical mass level is going to increase much more slowly than the actual mass of the WD (through accretion).

    In fact, one could posit that, since all WD necessarily have a fairly high rate of spin, there is always going to be a “spin down” period for any WD exceeding the Chandrasekhar limit. But as the WD accretes more mass from a nearby companion star, the difference between its actual mass and the critical limit will decrease at a much faster rate than if the WD were not accreting at all and relying solely on the spin down time to trigger the explosion.

    Therefore, the longest “spin down” times would be for WDs that stopped accreting just over the Chandrasekhar limit whereas the shortest times would be for the highest-mass WDs.

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