Story by Helen Hill for MGHPCC
Mark Veyette is a PhD student at Boston University studying Astronomy. His research focuses on characterizing low mass stars and the exoplanets that orbit them. In particular, he studies the composition of M dwarf stars and how that relates to the types of planets that form around them.
Planets form from the same cloud of gas and dust that the star they orbit formed from. They are both built from the same material. However, stars are mostly hydrogen and helium, and rocky planets like Earth are entirely made of heavier elements like oxygen, silicon, and magnesium. If the cloud the star and planets are forming from is more rich in these heavier elements, there is more material to make planets. Indeed, astronomers find that planets are more common around stars that contain more of the planet-building elements.
Veyette has been using the MGHPCC to run state-of-the-art models of M dwarf atmospheres (the outer, visible layer of the star) to, in particular, better understand their composition characteristics. Such models require considerable computing power to handle all the physics involved from solving the hydrodynamics of the gas in the star’s atmosphere, to the chemistry of what molecules form and in what concentrations, to solving the quantum mechanics of how much light the atoms and molecules in the atmosphere can absorb, and at what wavelengths, as well as the details of how light propagates through the atmosphere and what the resulting spectrum looks like.
TRAPPIST-1, the star reported in February 2017 to host seven temperate terrestrial planets, a larger number than detected in any other planetary system, is an M-Dwarf.
Know the star: Know the planet.
As astronomers find more and more planets around other stars---some of them potentially able to support life---scientists have begun thinking about ways we can further study these planets and potentially how we can look for signatures of life.
But detailed follow-up investigations of potentially Earth-like planets will require a lot of time on the world's newest, most powerful telescopes. “For this reason we need to pick the best targets to spend all that expensive telescope time on,” says Veyette. To do that, we need to find out as much as we can about the planets we have found so far. How well we can characterize a planet very heavily relies on how well we can characterize the star that hosts it, or, as some astronomers like to say, “know the star, know the planet.”
From this standpoint, two things make M dwarf stars especially interesting:
Stars are classed according to their spectral type, which for most stars roughly relates to their mass. M dwarf stars are small stars. They have sizes and masses that range from about 1/2 to 1/10 the size and mass of the Sun. They are also much cooler than the Sun with surface temperatures only about half that of the Sun’s.
M dwarfs are also faint stars. Their smaller sizes mean they produce far less light than stars like the Sun. They also emit most of their light at infrared wavelengths. Both of these facts combined means astronomers need to use large telescopes with advanced instruments and cameras in order to study them. “There is not a single M dwarf that can be seen in the night sky with the naked eye. In fact, the closest star to the Earth, besides the Sun, is an M dwarf, but it is still too faint to be seen without a telescope,” Veyette says.
But they are common: “We know now, based on planet-finding surveys like NASA's Kepler mission, that M dwarfs host a lot of planets. And they host a lot of small planets. Our best guess currently is that just about every M dwarf hosts at least 2-3 planets total, one of which is about Earth-size and likely a rocky, terrestrial planet. Additionally, our best guess is that something like 1 in 5 M dwarfs host an Earth-sized planet in the "habitable zone", i.e. that orbits at the right distance from its host star that it receives just enough energy that it could support liquid water on its surface, given the right atmosphere.”
A second benefit, in looking for potentially habitable Earth-like exoplanets around this smaller, cooler class of star, Veyette explains, is that it is easier than looking for them around Sun-like stars. “When we detect planets around other stars, we do so by measuring subtle effects the planet has on the light emitted by the star.”
The most successful method of finding planets so far has been the transit method, where astronomers stare at a star for a long time waiting for the brightness of the star to dip a little. “The dip in brightness is due to a planet passing between us, and the star and blocking a little bit of its light.” Veyette explains, “This is a difficult way to find planets because only a small fraction of stars will have planets that orbit them at just the right angle so that the planet will pass between us, and the star. So we have to stare at a very large number of stars in order to see a few planets transiting.”
Astronomers also have to stare for a very long time: “If we want to find an Earth-analog around a Sun-like star, we have to stare for a whole year (the orbital period of Earth) to see one transit, and usually we like to see three transits to confirm it is a planet we are seeing and not some artifact or false positive.”
Finally, it is also a very small change, like a 1 in 10,000 change in brightness astronomers are looking for. With M dwarfs it is easier. Veyette explains, “Because they are fainter, a habitable planet will orbit much closer to an M dwarf host star and have a shorter orbital period. A habitable planet around an M dwarf will have an orbital period of only 10-100 days. Also, because M dwarfs are smaller, a small planet blocks a larger fraction of the light from the host star, making them easier to detect.” Veyette says, “All that is to say small stars host a lot of small planets and it is easier to detect small planets around small stars.”
The metallicity of a star is a way to describe its composition. Astronomers have a particularly broad definition of what a metal is, calling anything heavier than hydrogen or helium (i.e. every other element) a metal. Accordingly the Sun is composed of about 74% hydrogen, 25% helium, and 1% "metals". For stars like the Sun, astronomers use the amount of iron in the star as an over all measure of how metal-rich or metal-poor the star is. The Sun's spectrum has many, many lines in it due to iron in its atmosphere absorbing light at specific wavelengths. Astronomers use these lines to measure how much iron is present in the Sun and other stars. Essentially, they make a model of the star's atmosphere and ask how much iron do I need to add to the atmosphere to match the iron lines I see in the spectrum.
The spectrum of an M dwarf star however is very different from a Sun-like star. Because they are cooler than the Sun, they can form a lot of molecules in their atmospheres, like H2O (water vapor). These molecules can absorb light at millions of different wavelengths and obscure the iron lines used to measure metallicity in Sun-like stars. Unfortunately it is extremely hard to model the millions of spectral lines due to molecules. So astronomers can't rely on the models of M dwarf stars to measure metallicity.
However, astronomers have gotten around this by looking at binary stars, i.e. where there are two stars together in a system, one a Sun-like star and the other an M dwarf. Assuming the two stars are formed together, at the same time and from the same material, they should have the same composition. Given such a situation, astronomers can then measure the metallicity from iron lines in the Sun-like star's spectrum and compare it to some other features seen in the M dwarf's spectrum. Given observations of enough binary system, new ways to measure metallicity of single M dwarfs can be calibrated.
Veyette and co-authors explore the nuances of M Dwarf metallicity in their recent paper The Physical Mechanism Behind M Dwarf Metallicity Indicators ad the Role of C and O Abundances.
Their modeling work confirms that models of M dwarfs are far more complicated than Sun-like stars and don't do as good of a job at matching observations. “When we change the metallicity of the models, we don't see the same change in the model spectrum as we do in real observations,” Veyette explains. “However, what we do find is that if we also change the carbon and oxygen abundances, we see much better agreement between the models and observations. So this could be a big step toward better, more accurate M dwarf models.”
About Mark Veyette:
Mark is a PhD student at Boston University studying Astronomy. His research focuses on characterizing low mass stars and the exoplanets that orbit them. In particular, he studies the composition of M dwarf stars and how that relates to the types of planets that form around them.
The Muirhead Group Department of Astronomy, Boston University
Boston University Department of Astronomy
Institute for Astrophysical Research, Boston University
Publication: Mark J. Veyette, Philip S. Muirhead, Andrew W. Mann, and Frances Allard (2016), The Physical Mechanism Behind M Dwarf Metallicity Indicators and the Role of C and O Abundancies, The Astrophysical Journal, 828:95, doi: 10.3847/0004-637X/828/2/95
Story Image: Artist’s impression of a M dwarf star surrounded by planets - Image credit: NASA/JPL-CALTECH/MSSS.