Dr. Praphull Kumar — a faculty research associate at Missouri State University — joins us this week in an interview to share his work on white dwarf stars who eat their companion stars.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
Dr. Mike Reed:
This week we're talking with Dr. Praphull Kumar, formerly of the University of Alabama and now a faculty research associate here at MO State. Thanks for talking with us.
Dr. Praphull Kumar:
Thank you so much, Mike, for having me.
Reed:
Could you begin by telling our listeners a bit about what happens when stars eventually die and why we should study them?
Kumar:
Most stars, including our sun, will eventually die as white dwarfs. These stars are mostly powered by core hydrogen and core helium. Once all of their core hydrogen and helium are exhausted, they will eventually die and about 95% of them will become white dwarfs. And similar to our sun, our sun will eventually die in about 5 to 10 billion years. Once these stars die, they're still very hot, but they tell us a lot more about our galaxy and how the star evolves in general. Similar to how we understand our galaxy using billions of stars, these white dwarfs are very, very important as we call them stellar fossils and they tell us about how far we have come from — how the big bang has evolved.
Reed:
So when you say "the star dies", the star doesn't disappear, it doesn't explode, so there has to be some mechanism for it to still exist, right? We know the main sequence, they're doing hydrogen fusion, so what is it that keeps this star from collapsing into nothingness?
Kumar:
Once all of their core hydrogen is burnt up, they are very, very degenerate. Their core is very much solid. They are still powered by strong gravity in their core and that keeps these stars alive. Although they're very hot, they will cool down in billions of years.
Reed:
And the stars you study are not alone, is that correct?
Kumar:
That's right. The stars I study, they come in binary systems. Though there are not many of them, the stars I have studied so far, there are a fraction of them that comes in binary in which one star orbits another star and the stars that I particularly am interested in is called cataclysm-variable stars. As one of the white dwarfs in this case accumulates materials from its companion star, which is mostly a main sequence star.
Reed:
So what type of companions do these white dwarfs have? What's the other star like?
Kumar:
So these are mostly main sequence stars and they are very young.
Reed:
The companion star is young?
Kumar:
Yes. The companion stars are main sequence ones in this case and the white dwarfs sort of pulls up materials from the companion stars.
Reed:
But if they're in a binary, doesn't that make them the same age? Didn't they form together?
Kumar:
They would not. There are a lot of binaries in which case there would be white dwarf main sequence binaries and there are white dwarfs in other systems called red giant binaries. The stars that I am interested in is white dwarfs main sequence binary.
Reed:
They're interacting. Is that right?
Kumar:
Yes, they do interact, but they are not interacting as often. In this event, white dwarfs sort of pulls up materials via gravity from the main sequence star.
Reed:
What happens when a white dwarf takes material from its companion star?
Kumar:
Once a white dwarf pulls up materials, it accumulates enough hydrogen on its surface that it runs through outburst events — which is their surface brightness increases by 100 to 1000 times than the normal one — and these are recurring events that could happen potentially in about 100 years and these are also called dwarf novae outbursts.
Reed:
So this is really a sporadic fusion event that occurs near the surface of the white dwarf. Is that correct?
Kumar:
That's correct.
Reed:
And so how do these sudden outbursts change the vibrations of a white dwarf?
Kumar:
So in general, most stars are considered to be musical instruments and these white dwarfs are also similar to that. As the white dwarfs go through outburst scenario, the temperature of the surface rises quite a bit by a million Kelvin. That basically changes the internal properties of the star including their densities and in doing so, the vibrations or what we call normal mode frequencies by which a star would resonate — and pretty much all of the stars resonate — will change quite a bit and that gives us an interesting probe to understand the interiors of the white dwarfs.
Reed:
And how will you go about determining that?
Kumar:
So I ran or developed different computer models, theoretical models by which I mapped all these physical phenomenas including an accretion outburst scenario and how the temperature would be rising on the white dwarf surface and I modeled these using something called Mesa — which is one dimensional stellar revolution code — and once all these models are set up well, I compute the white dwarf's eigen frequencies by which they would resonate and after which I sort of map these eigen frequencies with the observations that we have from various telescopes and if the model succeeds, then that is great, and if it is not then that sort of gives us a different challenge to improve our model.
Reed:
And what do you expect to learn from these white dwarfs that are taking materials from their companions?
Kumar:
These are great sort of chronometers, but also this will eventually tell us about something called supernovae — which we have observed quite a few — and supernovae is important in a sense that they will tell us about the various metals that we produce and that we know in our universe.
Reed:
Thanks Dr. Kumar. I'd like to thank you for telling us about your interesting work on white dwarf stars.
Kumar:
Thanks for having me.