BAN #306: Zombie stars

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

[Artwork showing a white dwarf destroying and eating the remnants of a planet that orbited it. Credit: University of Warwick/Mark Garlick]

An unusual way to look for exoplanets — planets orbiting other stars — is to check for uneaten bits stuck between the teeth of their long-dead zombie host stars that are now feasting on their flesh.

OK, sure, that’s maybe a little florid, but as a metaphor it’s apt.

Evidence for planets orbiting white dwarfs has been around for years. But something new has come up: Astronomers have found strong evidence that a handful of very old white dwarfs have been recently dining on just the crusts of rocky planets that may have once been very much like Earth.

This has a lot of implications, perhaps the most interesting of which is that stars a lot more massive than the Sun can make planets like ours. Evidence for that up to now has been scant.

OK, let’s back up a wee bit. When a star like the Sun (up to about 8 times the Sun’s mass) runs out of nuclear fuel in its core, it swells up into a red giant and blows a wind consisting of the gas in its outer layers. After some millions of years it sheds its entire outer envelope, leaving behind its hot core: a white dwarf.

White dwarfs are dense: A cubic centimeter (roughly the size of a six-sided die) of white dwarf material can weigh well over a ton. They don’t generate energy; they just there in space radiating away their heat. They generally have about a third or half the mass of the Sun, but a star on the more massive end of the scale can make one up to about 1.4 times the Sun’s mass. If one’s mass can be determined (and there are ways to do that) then we can estimate its age by how hot it is, since it cools over time at a predictable rate.

They can have heavy elements inside them but their very outer layer tends to be all hydrogen with some helium in it. Because their gravity is so intense (100,000 times Earth’s) heavy elements sink rapidly into the depths of the star.

Their secrets are revealed through spectra, taking their light and breaking it up into thousands of individual colors. This can tell you their mass, temperature, elemental composition, and more. Normally, when you take a spectrum of a white dwarf what you see is featureless, just what you’d expect from a hot ball of hydrogen.

[Artwork showing a white dwarf eroding the surface of an asetroid away. Credit: Mark A. Garlick / markgarlick.com]

Sometimes, though, heavy elements on the white dwarf’s surface show up in the spectrum. Each absorbs light at a different characteristic color, allowing them to be IDed. It’s been understood for a while now that these heavier elements like iron and calcium are from planets that once orbited the star. The planetary system could have been thrown in to disarray when the star swelled up into a red giant. If a massive planet passed by a smaller one, the latter could’ve been tossed much closer to the star. Once the star became a white dwarf that’s trouble: The intense gravity can tear the planet to shreds. The heavy elements we see in the spectrum of the white dwarf are the remains of that planet.

Normally we see a mix of all kinds of elements coming from everywhere throughout the planet. However, astronomers have found evidence that some white dwarfs are eating just the crusts of some planets.

They combed through thousands of white dwarf spectra looking for the presence of lithium. This exists in the crusts of rocky planets like Earth. Due to the way lithium behaves when it gets too hot, they could only see it in cooler white dwarfs: older ones. And in four of them, they found it. Three of them are nearby, less than about 100 light years, and one farther out.

They also found potassium, calcium, and sodium. That’s important because different types of objects (stars, planets, asteroids, just the crusts of planets) have different ratios of these elements in them. When they looked at those ratios, the obvious best match was with planetary crusts.

That’s very interesting, because these stars are old, between 5 to 10 billion years old. And that’s just how long they’ve been white dwarfs. They were “normal” stars like the Sun before that for possibly billions of years. This in turn means stars were capable of making rocky planets long, long ago.

It gets better. One of the white dwarfs, WD J2317+1830, is more massive than the others. Given its mass, the star it came from must have been about 4.8 times the Sun’s mass, making it one of the more massive stars out there — what we call a B-type star. Only a handful of planets orbiting B stars has been found, and those from the lower mass end of the type.

WD J2317+1830 became a white dwarf something like 9.5 billion years ago! It was a normal star for probably only 200 million years before dying, so it really didn’t have a lot of time to make planets, yet here’s evidence that it not only made one, but there was enough time for a crust to form. Not only that, but because it’s massive, those elements sink in thousands of years, so to still be showing them it must still be eating those planets. Judging from the amounts of the elements seen, it’s doing so to the tune of about 3 tons per second.

That’s a hungry star.

In fact, extrapolating from what’s seen in one of the white dwarfs studied, the astronomers estimate that something like 60 billion tons of lithium and 3 trillion tons of potassium are in that white dwarf right now, implying it came from something an object with a mass of about 300 trillion tons (scaling up by knowing how much of those elements are in planetary crusts typically), which is about the size of a small asteroid. So the planets were torn to pieces, and it’s eating the bits slowly now.

The zombie analogy isn’t so florid sounding now, is it?

It’s entirely possible that this will be the fate of our own solar system in some dozen billon years, after the Sun becomes a white dwarf (assuming the planets aren’t stripped away by passing stars first). It’s weird to think that that piece of land you’re sitting on may one day show up as a dip in the light at a specific color in some alien astronomer’s spectrum of the future zombie Sun, but hey, maybe a few billion years ago some alien had that same realization… and here we are.

Et alia

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