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.
Picture two nearly-identical stars orbiting each other. Something like this:
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.
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.
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.
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.
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.
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.
One of my favorite telescopes has to be Kepler. Kepler has been orbiting the Sun, much like Earth does, since its launch in 2009. Its primary mission was to discover Earth-like planets orbiting other stars, called exoplanets. And since its launch in 2009, Kepler spent some four years staring at one region of the sky, unblinking, carefully monitoring how thousands of stars’ brightnesses change with time, with insane precision.
Kepler finds planets orbiting other stars using a technique called the transit method. That’s really just a fancy way of saying, “if a dark thing passes in front of a bright thing, we see less light.” Consider planet = “dark thing” and star = “bright thing”… and that’s it! Kepler isn’t fancy; rather, it takes a simple idea and milks it for all it’s worth. Bigger planet? More starlight blocked. Slow-moving planet? Starlight blocked for longer.
While exoplanets are Kepler’s specialty (and it has found hundreds!), Kepler has somewhat unintentionally revolutionized stellar astrophysics, too. Planets getting in the way isn’t the only thing that can make a star’s brightness change. Scientists studying exoplanets have to deal with many other brightness-changing effects to properly characterize the planets they discover. As we say, though, one astronomer’s noise is another’s data: the “annoyance” of seeing so many rotating stars, pulsating stars, spotted stars, and even stars orbiting other stars, is my personal favorite of Kepler’s many successes.
Unfortunately, a critical part of Kepler broke recently, and there is no way to fix it. One of Kepler’s specialties is pointing ever-so-carefully at one part of the sky. To accomplish this, it needs a good sense of balance in three dimensions. This was provided by a set of “reaction wheels,” which are basically gyroscopes that spin to keep Kepler oriented in the right direction. Three dimensions of space means you need three wheels. Kepler actually has four, so one is redundant – we intentionally built Kepler with an extra reaction wheel in case one broke. As luck would have it, one stopped working shortly after launch. “No problem!” said scientists back on Earth. “We still have three wheels!” Or rather, we did. Until last spring, when a second one bit the dust.
And that is the sad story of how NASA wound up with a crippled telescope that can no longer search for Earth-like planets.
Thankfully, though, Kepler’s story doesn’t end there.
After a series of thorough tests to be sure two wheels really were broken, the folks at NASA put out a call to astronomers everywhere. They provided technical details about Kepler’s capabilities and limitations, and asked, “What should we do with Kepler now?”
More than 40 proposals flooded in to answer that question. See them here.
Most of these are full-fledged papers, representing hours of work for each author (and most of them have many, many authors!). Teams of astronomers from all over the world collaborated to come up with countless ideas for putting Kepler back to work. From those ideas, they fleshed out the most promising ones, and did extensive research to present science goals that are both realistic given Kepler’s current state and important to advance astronomy.
Just to put this in perspective: hundreds of astronomers spent countless hours to come up with creative, robust ideas for a telescope that doesn’t work properly anymore. Nobody paid them to write these proposals. Realistically, only one from a multitude of ideas will be able to happen, because we only have one Kepler. And it is broken!
I can’t help but imagine the science astronomers could accomplish if we had access to multiple, UN-broken space telescopes like Kepler. Even so, it is heartening to know that Kepler will have some scientific purpose, going forward. Long live Kepler!
The rest of the Giants of Eclipse meeting saw a much wider array of subjects than just Epsilon Aurigae. We heard about interferometry, a special technique often used by radio telescopes to get sharper, higher-resolution pictures. Daniel Huber gave a great overview of asteroseismology, or stellar pulsations, which related to my talk the next day. Andrej Prsa discussed lots of work being done by the Kepler team with eclipsing binaries. The Kepler spacecraft is one of my favorites – it spent over three years collecting extremely precise data to search for planets around other stars. (Unfortunately it recently stopped working, and prospects of getting it up and running again are slim, but there is still tons of data to pore over.) A great benefit of all the high-precision Kepler data isn’t just planet hunting – it’s also extremely useful to study stars. We heard about a special kind of binary star that shows a “heartbeat-like” pattern in its light curve, and we also learned about an innovative technique called tomography, which essentially creates a 3D map from a series of 2D slices.
So much science! By the time I presented my research on Thursday afternoon, everyone was probably tired of listening to talk after talk after talk. But I was pleased that I didn’t go over my time, got a couple of chuckles, and was able to answer questions intelligently. Next time I give a talk at a conference, I hope I don’t have to go last, because I spent a lot of time worrying about my own presentation instead of listening intently to others’.
Overall, I was thrilled to meet so many other people who care about binary stars at least as much as I do. We shared many great meals and fun evenings. I left Monterey with new friends and a better sense of what cool research is happening with binary stars.
What happened to Day 1, you may ask? Unfortunately, a canceled flight happened. To make a long story short, AstronoMerrdiff finally maxed out her travel karma and got to spend an extra day in San Diego. It worked out okay, though – I worked on my talk for Thursday, and had a great impromptu meeting with my old advisor at San Diego State.
By arriving at the conference in Monterey some 24 hours later than intended, I missed a couple of overview talks, a session on VV Cep stars (a special kind of eclipsing binary), and a particularly interesting-sounding theory session. This apparently featured red giant atmosphere modeling, red giants in eclipsing binaries (!), and working out how triple systems (three stars orbiting one another) may have formed.
Let me back up a step…
This conference is all about big stars that orbit other stars so that one routinely passes in front of the other. That’s why it’s called GIANTS of ECLIPSE! It also happens to be precisely what I’m working on right now. In particular, I’m working on a project that involves red giants in eclipsing binaries. A red giant is a huge, bright red star that has run out of hydrogen fuel. Our Sun will become a red giant near the end of its life. An eclipsing binary is a pair of stars orbiting each other that pass in front of one another as viewed from Earth. As it turns out, the talk I missed yesterday about red giant eclipsing binaries was originally scheduled for Wednesday, and got moved at the last minute. Neither the speaker nor I knew this until we each arrived.
A recurring theme at the conference so far has been “this person was going to come and give a talk, but wasn’t able to.” There have been at least four cases of this, and there are fewer than 40 people at the conference to begin with. Sometimes an absent presenter is able to Skype in and/or pre-record a talk to be played back. In other cases, they ask a colleague at the meeting to give their talk for them. And in many situations, the talk is simply withdrawn. These cancellations and substitutions are what led to schedule shuffling. Why this all has to be so last-minute and offline is beyond me.
So, this brings us to today. Today’s theme was Epsilon Aurigae (usually pronounced or-EYE-jee, with “jee” sounding like “jeans”). This funny name belongs to a particularly mysterious eclipsing system. As best we can tell, Epsilon Aurigae is a cooler F-type star and a hotter B-type star orbiting each other, with a critical twist – the hotter star is embedded in a dark, dusty disk. Epsilon Aurigae is a particularly hot topic right now among stellar astronomers because it just completed an eclipse: the dark disk hiding a hot star inside just finished passing in front of the cool star. This only happens every 27 years, so lots of people were excited to observe it and begin to understand it.
The best part of a small conference like this, however, is meeting people whose names you know from papers – in person. The under-40 group is rather under-represented at Giants of Eclipse, which is unsurprising given budget woes. (Grad students and postdocs are generally hit hardest when it comes to travel funding. In my case, I’m only here because I could afford to pay out-of-pocket, and that simply isn’t possible for most early-career astronomers.) So: the few of us young folks automatically meet and shake our heads as some of the more senior astronomers use overhead projector sheets (yes, really).
Thankfully, it isn’t all slide rules – there is lots of great science being done. I’m looking forward to the next couple of days, where the talks will venture further afield than just Epsilon Aurigae. Giants and binaries and eclipses, oh my!