Postdoc Spotlight: Looking at Planetary System Evolution with Meredith MacGregor

Meredith MacGregor, Carnegie Institution for Science
Meredith MacGregor came to Carnegie's Department of Terrestrial Magnetism in September 2017 on a NSF Astronomy and Astrophysics Postdoctoral Fellowship. Credit: Roberto Molar Candanosa, Carnegie DTM with photo courtesy Meredith MacGregor.
Friday, March 29, 2019 


Gas, dust, and small objects. In the beginning, it’s a whole lot of nothing. After these materials collide and combine to eventually make planets, the material that’s left behind forms debris rings around the mature planetary systems. Astronomers call these rings circumstellar disks. We see this in our own backyard, in the dark, cold, and mysterious disk known as the Kuiper Belt surrounding the Sun 4 billion miles out towards the edge of the Solar System.

Circumstellar disks can betray the secrets of a planetary system. Astronomers in planets far away from us could use the structure of the Kuiper Belt to infer the presence of Neptune in our Solar System. Eventually, this knowledge would lead them to learn how the early Solar System evolved into inner rocky planets, outer gas and ice giant planets, and the remnant Asteroid and Kuiper Belts that we see today.

How can astronomers do that? We’ll let NSF Astronomy and Astrophysics Postdoctoral Fellow Meredith MacGregor explain. In our latest Postdoc Spotlight installment, she takes us through her journey from kindergarten science fairs to stars trillions of miles away.

DTM: Can you give us the rundown of what you do at DTM?

Meredith MacGregor: I am working to answer big questions in astronomy that I’m really excited about.  One is, how do planetary systems form in the first place? And then related to that I am now very interested in what makes these planetary systems habitable. When you image circumstellar disks, you end up imaging the star as well, so I now have gotten very into looking at stellar activity and what that means for the disks and the planets surrounding their star.

DTM: What does the word “habitable” mean for you?

MM: The part of habitability that I’m most focusing on right now is the role that stellar activity plays. Stars are active, including our Sun, meaning they emit what we call flares, bursts of high-energy radiation. Large solar flares are typically accompanied by releases of charged particles called coronal mass ejections, or CMEs. On Earth we are nicely protected by our magnetic field and atmosphere, keeping these damaging particles from actually penetrating to the planet’s surface. So we don’t really see the effect of the Sun’s activity on a daily basis.  But M Dwarfs, the cooler, redder types of star that many exoplanet missions have been focusing on, are much more active than our own Sun. And the planets, in order to be in the “habitable” zone—which basically means at a distance from the star that you could have liquid water—are much closer in, so they are getting hit by generally a lot more of this radiation. If you are getting hit by flares and CMEs at the rate that we think stars are putting them off, you might expect those planets not to be habitable as a result.

DTM: How did you go from studying circumstellar disks to this part of habitability?

MM: This happened in my first year of being a postdoc here at DTM. I did my doctoral work on circumstellar disks and came here intending to continue that work.  But there was a paper that came out just as I was starting—It made a claim that the nearest planetary system to us, Proxima Centauri, had a system of multiple debris disks around it. That would be exciting because it would make it somewhat like our own Solar System. Alycia Weinberger and I had a conversation about it during journal club, and we both decided that we were deeply skeptical about the original analysis.  Afterwards, Alycia suggested that I download the archival data and see what’s in it.

DTM: So then what happened?

MM: The paper presented an averaged image, but the star had actually been observed 15 different times spread out over several months. I quickly noticed that the data showed nothing, nothing, nothing, and then this beautiful light curve in which the star brightened by a factor of a thousand and then decayed back into nothing. The only thing that produces a light curve like that is a stellar flare. Essentially, in 14 of these observations, there was no excess signal from the star. Then, in the last observation, it was very clear that the star had undergone a huge flaring event.

The brightness of Proxima Centauri as observed by the Atacama Large Millimeter/submillimeter Array over the two minutes of the event on March 24, 2017. The massive stellar flare is shown in red, with the smaller earlier flare in orange, and the enhanced emission surrounding the flare that could mimic a disk in blue. At its peak, the flare increased Proxima Centauri's brightness by 1,000 times. The shaded area represents uncertainty. Illustration is courtesy of Meredith MacGregor.

We had just discovered something completely new. It was very exciting because we’d never detected a flare from an M-dwarf in the millimeter before. It’s a wavelength regime where we just had no idea that M-dwarfs even flared. Even from the Sun, we’ve only detected a handful of flares at millimeter wavelengths. It’s really a whole new area of stellar astrophysics, and it was fun to open it up sort of by chance. Since then, I’ve taken this result and built it into a whole new research program where we are going to go back and do a large project to actually understand the mechanisms that produce stellar flares at these wavelengths.

The Centaurus galaxy seen at different wavelengths. Human eyes can only see what’s called visible light, a type of electromagnetic radiation, but this is only a small part of the electromagnetic spectrum. Electromagnetic radiation travels in waves and spans a broad range of wavelengths (the distance from one peak of the wave to the next), from very long radio waves to very short gamma rays. The longer the wavelength, the lower the energies in the spectrum. Objects in space emit light across the entire electromagnetic spectrum, but radio emission traces cold material and probes deep into regions shrouded by dust, like the centers of planet forming disks.  Astronomers like Meredith MacGregor use telescopes like The Atacama Large Millimeter/submillimeter Array, or ALMA, to detect millimeter and radio emission and study the unseen universe. Credit: ALMA/NRAO

DTM: This finding was new and exciting astrophysics. What was the reaction from the field?

MM: Other scientists in the field thought this was an exciting result for a number of reasons. One, it’s a neat story of interpreting a dataset one way, doing a sanity check, and discovering something completely different. It’s a nice story of science changing and correcting itself. In the exoplanet community, I think this has reminded everybody that stars are active and exciting, and that we need to keep that in mind when talking about habitability.

An artist's impression of a flare from Proxima Centauri, modeled after the loops of glowing hot gas seen in the largest solar flares. An artist's impression of the exoplanet Proxima b is shown in the foreground. Proxima b orbits its star 20 times closer than the Earth orbits the Sun. A flare 10 times larger than a major solar flare would blast Proxima b with 4,000 times more radiation than the Earth gets from our Sun's flares. Credit: Roberto Molar Candanosa / Carnegie Institution for Science, NASA/SDO, NASA/JPL.

DTM: You also had other exciting news looking at another planetary system. What was that about?

MM: My other area of research is looking at circumstellar disks. Recently, I have gotten observations of several systems with ALMA, the Atacama Large Millimeter/submillimeter Array, that are really exciting because they can tell us about the planetary systems that shaped the surrounding disks. Just before I started here at Carnegie, we got an image of the Fomalhaut debris disk with ALMA.  Fomalhaut is a truly beautiful ring of dust. It’s remarkable because the actual ring is located really far out at about 110 astronomical units (AU) or 110 times the distance of the Earth to the Sun. But the disk is only about 10 AU wide, so it’s this beautifully narrow and eccentric ring of dust quite far out from the star. Something has to be shaping it out there, but it had never been well imaged with ALMA because of its angular scale on the sky. It’s very nearby, only about 7 parsecs (1 parsec is about 19 trillion miles) away from us, so it’s very extended on the sky.  To image it well with ALMA, you actually have to do a mosaic image.   

The end result of this work was the first complete millimeter map of the Fomalhaut debris disk. It was gorgeous. In fact, it was the Astronomy Picture of the Day, which was super fun. We also detected CO2 gas in it, and we think that this points to the composition of the bodies in the ring being cometary. We were also able to robustly measure the geometry and eccentricity of the ring and actually detected something called apocentric glow, which had been predicted theoretically but never seen before. Since the ring is eccentric, there is an over density of material at the apocenter side of the disk, farthest from the star, which we see as a brightening at that location in the ALMA image.

Composite image of the Fomalhaut star system. The ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. The central dot is the unresolved emission from the star, which is about twice the mass of our sun. Optical data from the Hubble Space Telescope is in blue; the dark region is a coronagraphic mask, which filtered out the otherwise overwhelming light of the central star. Credit: ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF)

DTM: How did you get into this kind of work initially and how did that evolve into what you do now?

MM: I did science fairs all throughout high school and always knew I wanted to do physics. I really liked using math to explain how the world works. My very first science project was when I was in kindergarten. It was how a syphon helps a toilet flush. I still remember the experience of standing in the science fair—I had this whole demo with cups and tubing in between them, and everybody would come around and I would say, “this is how a toilet flushes!”

As I got older and away from toilet flushing, I focused into physics. I started college and was sure I was going to be a physicist. But, I took a class in astronomy and found it fascinating.  I ended up adding astronomy as my second major, and as I went along, I took more and more astronomy courses and fewer physics courses. I spent a summer at the National Radio Astronomy Observatory (NRAO) in Charlottesville, VA and started learning about radio astronomy. I was attracted to the challenge of radio astronomy. Many people are put off by the techniques and complexity of getting an image at radio wavelengths, and I loved getting to master such a difficult but powerful technique.

I started grad school and met with possible advisors who worked in radio astronomy, and I ultimately picked my advisor because of ALMA. The first proposal results came out right as I showed up for graduate school, and one of the people I met with had a proposal accepted in this first AMLA cycle. It just seemed too cool of an opportunity to let it pass.

Located in the Chilean Atacama Desert, ALMA uses 66 high-precision dish antennas to study the hidden universe. Credit: ESO/B. Tafreshi (twanight.org)

DTM: It seems you had a special connection with ALMA. What does the telescope array mean for you?

MM: ALMA is really an amazing facility. It has completely changed how we understand basically everything in radio astronomy. In circumstellar disks, this is particularly clear. Before ALMA we had all these images of disks that just looked like blobs. We knew there was material there, but we had no idea what it looked like. With ALMA, all of a sudden we started seeing all of their detailed structure. In the time that I have been a graduate student into a postdoc, circumstellar disks went from being more of a niche field to a fascinating, newsworthy subject. I feel especially connected to ALMA because I had heard about it when I was a summer student at NRAO, and I knew it was coming online. Then, my graduate trajectory basically started as ALMA started. I feel like I have kind of grown up with the field, with me being a researcher in it as it grows and as ALMA continues to improve.

DTM: You also have a peculiar fellowship here at Carnegie. What does it entail?

I am excited to be a National Science Foundation Astronomy and Astrophysics postdoctoral fellow because the NSF cares deeply about broader impacts and giving back to the scientific community. As a fellow, I am able to pursue my own independent research as well as a broader impact project.  I wanted to have the chance to continue doing outreach and teaching as a postdoc.

DTM: How come?

MM: I certainly benefited a lot from having mentors and teachers who saw that I was excited about science and then nurtured that. I didn’t appreciate necessarily how special that was until I moved along in my career. You know, astrophysics is still very largely male dominated, and I think that’s a shame. When I went into college I noticed there was only one tenured woman in the faculty at Harvard. I want to be the person who can inspire and mentor younger generations so that we can keep astrophysics going in a direction of becoming more diverse and inclusive. We are getting there, but there’s still a lot of work to be done.

DTM: So how exactly do you do that here?

MM: I chose to come to Carnegie for a number of reasons. One: research wise, it’s an amazing place to answer the questions that I want to answer. In particular, I really wanted to work with Alycia Weinberger. I never had a chance to have such an awesome female mentor before, and getting to work with someone like her was just like a dream.

I also wanted to come here because Carnegie has CASE, the Carnegie Academy for Science Education, which already has in place a robust outreach and teaching component. I have been working very closely with them for the last year to develop a curriculum and teach it in the First Light program.

MacGregor teaching astronomy at the Carnegie Academy for Science Education's First Light Program. Credit: Carnegie Institution for Science.

DTM: What has CASE been like?

MM: I have worked with CASE to develop a curriculum on astronomy, specifically exoplanets and astrobiology for D.C. middle schoolers. I am currently teaching this curriculum along with CASE staff at Carnegie headquarters every Saturday afternoon. It’s fun! The students have incredibly high energy and ask a lot of questions that make me think about my own research in a whole new light. So, coming here to DTM has given me the opportunity to do both my science and my broader impacts at a very high level.

DTM Postdoc Spotlights, conversations in which we feature our postdocs in astronomy, geochemistry, cosmochemistry, and geophysics, are edited for clarity and length.



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