Tag Archives: asteroseismology

Dr. Rawls

My PhD defense took place in Las Cruces on April 8, and was successful! If you’re interested, you can watch my presentation and/or view my slides. However, please be aware the intended audience for this talk is fellow astronomers, not the general public.


I’ve spent the last week or so revising my dissertation, and I’m happy to report it passed the graduate school’s format review today. Once the final copies are printed (yes, multiple copies; yes, printed) and accepted, I will add it to the Astronomy Thesis Collection online and write a post summarizing the main results. I’ll be back in New Mexico in May to celebrate graduation with my family, and I intend to consume even more burritos before embarking on a road trip north.


The Double Red Giant With Odd Oscillations

My latest paper was accepted for publication on December 31, announced on the astro-ph preprint arXiv on January 5, and will appear soon in The Astrophysical Journal [UPDATE Feb 11! Rawls et al. 2016, ApJ, 818, 108]. It was written online using Authorea. I also wrote several programs in python to analyze data for this paper.

Picture two nearly-identical stars orbiting each other. Something like this:

An approximation of the double red giant binary KIC 9246715, created using an eclipsing binary simulator. In reality, the two stars are separated by more than 200 times our Sun’s radius, but this simulator maxes out at 60. One orbit takes 170 days rather than the 26 illustrated here. The brightness, or flux, dips when one star passes in front of the other. The eclipses are not evenly spaced in time because the orbit is eccentric.

Even though they usually appear as a single dot of light, binary stars are one of the best tools astronomers have to measure stellar properties. Thanks to the math behind gravity, we can weigh pairs of stars using the relationship between how long an orbit takes and how far apart the things are doing the orbiting. Weighing stars accurately is important because a star’s mass seals its fate. So if every star in the night sky had a secret companion star (or exoplanet!), we could wrap this up pretty neatly and move on to deeper questions about stars’ lives.

Unfortunately, only about half of stars have orbiting companions, and many of those aren’t observable because they don’t happen to be edge-on like the case shown above. This is where starquakes come in.

Weighing stars from the inside out

Some stars, including our own Sun, ring like bells. Pressure waves are excited by convection inside stars, and the waves bounce around at resonant frequencies just enough to make them pulse, or oscillate. Because an oscillating star is changing brightness ever-so-slightly, we can use regular observations of brightness versus time (from Kepler, in my case) to pull out the frequencies of oscillation. Heavy stars oscillate differently from lighter ones, and big stars oscillate differently from smaller ones. Voila—a new technique for weighing stars that doesn’t require anything in orbit!

Of course, the story doesn’t end there. While the study of starquakes (more formally known as asteroseismology) is a powerful way to characterize many stars quickly, it remains relatively untested. We don’t know how accurate of a scale we’re using when we whip out asteroseismology to weigh stars. To address this, my colleagues and I identified about twenty binary systems containing red giants. That’s the kind of star our Sun will become when it runs out of fuel in billions of years. Red giants are convenient targets for asteroseismology because they are bright and oscillate slowly. Both properties make them easier to observe than Sun-like stars. And since our red giant stars all live in binaries, we should be able to weigh them in two independent ways and compare the results.

Solar-like oscillations make a comb-shaped pattern at different resonant frequencies. The central location of the spikes and their spacing in frequency tell us about a star’s average surface gravity and density, respectively. Red is oscillations from the double red giant binary in my paper. Gray is oscillations of a single red giant star with similar properties, plotted upside-down for reference. Figure 7 from Rawls et al.

The case of the missing oscillations

In my paper, I present a case study of two red giant stars in an eclipsing binary. From binary modeling, I show that the stars are both a little more than two times as massive as the Sun, and over eight times as large. However, I am surprised to only find a single signature of starquakes in the observations. Two similar but not-quite-identical stars should, in principle, both oscillate. The oscillation modes, pictured above, are broader and weaker than expected, too. The same physical process could be fully stopping oscillations in one star and only partially suppressing them in the other.

By harnessing many observations (both images and spectra) and modeling techniques, I thoroughly characterize both stars and investigate why only one of them appears to oscillate. I measure each star’s mass, size, temperature, chemical composition, level of magnetic activity, and tidal force strength, among other things. Then I bring in asteroseismology to see if I can tell which star is oscillating and if its story checks out.

By simultaneously modeling different observations of this double red giant binary, I can map the geometry of its orbit in space and measure each star’s mass and size. I do this by fitting a model (black) to observations (red and yellow). The top panel shows radial velocities, or how fast each star is moving toward/away from us, and the other panels show a light curve, or brightness versus time, with two eclipses (the bottom is a zoomed view of each eclipse). The x-axis is in units of how long one orbit takes, which is about 170 days for this binary. Figure 6 from Rawls et al.
Stars emit different amounts of light (flux) at different colors (wavelengths), but the light from a binary contains overlapping information about both stars. As the stars orbit, characteristic dips in flux shift to higher or lower wavelengths depending on how fast they are moving. To create one representative spectrum for each star as shown here (red and yellow), I used a physical model of the stars’ orbit to remove the velocity offset from each observation and then combined them. This process is called disentangling. An example of a single observation containing light from both stars (before disentangling) is plotted in black. Figure 5 from Rawls et al.

As it turns out, the two stars in this binary are similar enough that it’s impossible to say for sure which one the oscillations belong to. Recent work has shown that magnetic fields may suppress oscillations in stars, however, so I strongly suspect the oscillating star is the less magnetically active of the pair. There may be a weak second set of oscillations, but the signal is very noisy and doesn’t appear quite where it should. Either way, the single mass and radius derived from asteroseismology is consistent with that of both stars from binary modeling.

Fraternal twins: born together, but not identical

Because oscillations bounce around inside stars, they carry information about how stellar interiors are structured. Stars of different ages have very different things happening inside: younger red giants are still fueled by hydrogen, while older ones are fueled by helium. The oscillating star in this binary appears to be in an advanced helium-burning stage of its life called the horizontal branch or secondary red clump. I verify this with stellar evolution modeling, and confirm that the two stars most likely were born, grew up, and evolved together. They are about 940 million years old.

The next step is to do a similar analysis for the other red giant binaries my team identified. We are working on two fronts: comparing masses and radii from binary modeling and asteroseismology, and using those results to investigate why about a third of red giants don’t show any oscillation behavior. Our work has important implications for understanding the composition of our Milky Way galaxy, because bright red giants are often surveyed to better understand our galaxy’s history and structure. It’s important to get their stories right.

A Thesis, Proposed

Graduate school is a bit of an odd beast. It’s not really college, it’s not quite a job, and it’s certainly not easy. In the US, getting a PhD in the sciences typically involves a couple years of classes, several exams, some work as a teaching assistant, and eventually a self-directed research project called a thesis. The whole shebang takes some 4-8 years on average. AstronoMerrdiff’s graduate school journey began in San Diego with a somewhat atypical MS-only program, and then wound its way to Las Cruces for a PhD. (The “normal” path is to earn a Master’s degree en route to the PhD at a single institution.) Finally, back in February, after years of related and not-so-related research, successfully completing a host of departmental prerequisites, and a hectic few days of last-minute changes, I stood in front of my department and outlined the project I will undertake for my thesis dissertation:

Red Giants in Eclipsing Binaries
as a Benchmark for Asteroseismology.

Any questions? …Well, since this is my primary purpose in life for the next while, I figure the least I can do is spend a few paragraphs explaining my own little corner of astronomy.

In a nutshell, I’m studying red giant stars that are in eclipsing binary systems. Many of the giant stars have sound wave oscillations going on inside that we observe as small changes in brightness (this is called asteroseismology, and tells us about a star’s interior which is otherwise impossible to see). But not all of the giant stars oscillate. I want to figure out why. The fact that these stars are in eclipsing binaries makes them relatively easy to physically model and characterize. We think the oscillations might be weaker or non-existent when there are lots of starspots or tidal forces, but we’re not sure.

Want more? Here is the talk I gave for the “Three Minute Thesis” Competition held recently at my university. While I didn’t win, I thought I did pretty well. My classmate Kyle Uckert took home first place for his outstanding talk about searching for microbial life on other worlds. I had a lot of fun, though, and I learned how challenging it is for me to give a talk without visual cues (like multiple slides) and a strict time limit.

My slide for the Three Minute Thesis Competition. Image credits: G Perez, IAC, SMM (left), NASA (right), J Orosz (right)

Have you heard about how we’ve found over 1000 planets orbiting distant stars?

The Kepler space telescope finds planets by staring at stars. When a planet passes in front of a star, we see less light. This technique lets us not only DETECT planets, but also characterize them. For instance, a big planet will block more light than a small one.

OK, so, planets are great, but that’s not what my thesis is about. I study stars! It turns out that Kepler is also incredibly useful when the situation is a little different: instead of a planet orbiting a star, you have two stars orbiting each other.

Just by observing how the brightness changes with time, we can learn a lot of things about these binary stars, such as how long it takes for them to go around once and how much hotter one is than the other.

But Kepler alone doesn’t give us the full picture. For that, I use a telescope right here in New Mexico, at Apache Point Observatory, which spreads out all the different colors of light into a rainbow. I watch characteristic dark areas move from red to blue and back again, which tells me the velocities of the stars as they orbit. I then use that information together with the data from Kepler to get sizes, masses, and other properties for both stars.

Now, let me step back for a moment and ask: why do we care? Well, stars are astronomers’ main tool. Unlike other sciences, we can’t interact with what we study. What we CAN do is carefully measure light. And virtually all of the light we see bouncing around the Universe started out deep inside a star. So wouldn’t it be nice if we could look deeper and study the interiors of stars?

As it turns out, many stars, including our Sun, have sound wave oscillations going on inside that we can observe as small changes in brightness. And just like earthquakes help us study the interior structure of the Earth, these STARquakes let us study the insides of stars. This is called asteroseismology.

Conveniently enough, some of the biggest stars oscillate slowly enough that our friend Kepler can see it. This includes Red Giants, the kind of star our Sun will become when it runs out of hydrogen fuel in a few billion years.

In fact, based on what we know about their insides, we think ALL Red Giant stars should have these starquakes. So, we were surprised when we found several Red Giants that DON’T.

The good news is, because these stars are in binaries, that makes them relatively easy to study. I’m looking at binary stars with Red Giants that DO oscillate, and comparing them to binary stars with Red Giants that DON’T. Along the way, I’m using the systems that DO oscillate as a way to check quantities like mass and size that we can get from both techniques.

My research uses observations of binary stars together with asteroseismology to learn how all stars live and evolve.