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The classical scenario for creating Type Ia supernovae is a white dwarf star accreting mass from a nearby star entering the red giant phase. The growing red giant fills its Roche lobe and matter falls onto the white dwarf, pushing it over the Chandrasekhar limit causing a supernova. However, this assumes that the white dwarf is already right at the tipping point. In many cases, the white dwarf is well below the Chandrasekhar limit and matter piles up on the surface. It then ignites as a smaller nova blowing off most (if not all) of the material it worked so hard to collect.
A new paper by a group of European astronomers considers how this cycle will affect the overall accumulation of mass on the white dwarfs which undergo recurrent novae. In a previous, more simplistic 1D study (Yaron et al. 2005) simulations revealed that a net mass gain is possible if the white dwarf accumulates an average of 10-8 times the mass of the Sun each year. However, at this rate, the study suggested that most of the mass would be lost again in the resulting novae, and even a minuscule gain of 0.05 solar masses would take on the order of millions of years. If this was the case, then building up the required mass to explode as a Type Ia supernova would be out of reach for many white dwarfs since, if it took too much longer, the companion’s red giant phase would end and the dwarf would be out of material to gobble.
For their new study, the European team simulated the case of RS Ophiuchi (RS Oph) in a 3D situation. The simulation did not only take into consideration the mass loss from the giant onto the dwarf, but also included the evolution of the orbits (which would also influence the accretion rates) and varied rates for the velocity of the matter being lost from the giant. Unsurprisingly, the team found that for slower mass loss rates from the giant, the dwarf was able to accumulate more. “The accretion rates change from
around 10% [of the mass of the red giant] in the slow case to roughly 2% in the fast case.”
What was not immediately obvious is that the loss of angular momentum as the giant shed its layers resulted in a decrease in the separation of the stars. In turn, this meant the giant and dwarf grew closer together and the accretion rate increased further. Overall they determined the current accretion rate for RS Oph was already higher than the 10-8 solar masses per year necessary for a net gain and due to the decreasing orbital distance, it would only improve. Since RS Oph’s mass is precipitously close to the 1.4 solar mass Chandrasekhar limit, they suggest, “RS Oph is a good candidate for a progenitor of an SN Ia.”
If the white dwarf has explosive events that blow off material it accretes these should be observed consistently, if close enough to Earth. The authors state RS Ophiuchi exhibits nova bursts every 22 years. This sounds as if this would be a fairly common process for white dwarf-red giant double.
LC
Hm… is it me, or this doesn’t make sense?
Of the six known outbursts of RS Ophiuchi, the 1985 and 2006 events were the first to be studied in multiple wavelengths. The 2006 event alone generated a slew of papers describing details of the outburst from radio waves to gamma rays (and speculation that it may be a supernova progenitor).
I’m still left wondering, what will become of the red giant donor in this system once the white dwarf goes supernova? Does the answer depend on how symmetrical (or not) the blast is?
Jorge… I agree it isn’t well written. However, I interpreted it to say, ….as the red giant loses mass to the white dwarf, it also loses angular momentum. Due to the loss in angular momentum, the distance between the two gets smaller, resulting in even more matter being lossed (or being lossed faster) by the red giant.
Jon… The blast coming from the white dwarf once reaching the Chandrasekhar limit is typically much smaller than a full blown super nova blast. Therefore it is likely this cycle will start over once the blast takes place, and keep cycling until the two run into one another. The white dwarf has already had its ‘huge bang’ so to speak.
A lot of good things being looked at in this study. It would be something to see a white dwarf continue to collect matter once it passed the CL threshold. Something else which would add about 1000 questions to something we have no answers for!
All that is happening is the angular momentum of the red giant is being transferred to the white dwarf. If there is a nova event by the white dwarf some of that angular momentum might be carried off into space. A naive model might be where the white dwarf consumes material off the red giant, and if in a model sense we “turn off” the nova and the CL, then eventually the white dwarf has absorbed all mass and angular momentum of the red giant, including orbital angular momentum of the pair. A black hole does this its stellar companion.
The white dwarf star as it takes up hydrogen from its companion may ignite fusion on its surface. This may happen in transient bursts, or it might develop a more constant burning surface of fusion. What is described here is a transient burst. I am not sufficiently knowledgeable to say whether or how this might happen. If the process is more stationary, with a fusion process on the white dwarf surface, then the white dwarf should accumulate matter until it reaches CL.
LC
Jorge,
There’s a good reason that sounds funny. It doesn’t make any sense. Dyslexia FTW! I’ve fixed it.
Jon,
The effects on the red giant are hard to predict. There’s some very hard to nail down parameters. The first is just how tenuous the atmosphere of the RG is. If its in a very late stage of the giant lifetime, it’s barely held on (remember, it’s about to become a planetary nebula) and it will likely be blown off in the resulting SN. However, it will also depend on just how symmetric the supernova will be as well as the velocity of the gas escaping from the supernova.
All in all, the RG will likely lose a significant amount of its atmosphere at such close distances, but it will still be there. If it’s still young enough, it may keep expanding and dumping more matter on the newly formed neutron star for even more fireworks.
Is there a neutron star left behind from an SNI? I am not that familiar with the details of supernovae, yet I thought neutron stars resulted from a core implsions of a large red or blue giant star in an SN. I thought SNIs were basically stellar sized fusion bomb explosions which blew everything into space.
LC
Thanks to all for the feedback on the fate of the red giant companion. Clearly this is a system that will bear close scrutiny in the future.
@ LBC
As far as I know (which is not very far) the WD gains so much mass that it crosses the Chandrasekhar-limit, resulting in a collapse to a NS. And then basically the same things should happen with the envelope that happen to it during a SNII.
On the other hand that would lead to different types of light curves depending on the constituents of the envelope. In such cases they wouldn’t be standard candles any more, would they?
The WD when it crosses the Chandrasekhar limit is no longer in stationary equilibrium by Fermi statistics which maintain a degenerate electron pressure. So at ~ 1.4 solar masses (as I recall the limit) the star implodes and there is a runaway fusion production of energy. So as I understand SNIs are a standard candle or have consistent luminosities because they have a consistent mass.
I still ponder whether the core would implode into a neutron star as with any SN. I was under some impression that because the WD was composed to lighter elements, being formed from a more modest star, that the whole thing blew up as a grand fusion bomb. Also a neutron star has to be I think greater than 1.4 solar masses in order to exist, but has its upper limit as about 3 solar masses.
Unfortunately I am not that up on these facts. I hope SN1s prove to be a reliable standard candle, and that our astrophysics has them calibrated properly. A lot of cosmological results are hung on this.
LC
Yup, clueless.
Check out the H -> He cycle.