astro will

short for astronomy william o balmer ;~)

my first first-author paper: how not to form a binary sunset

my first first-author paper: how not to form a binary sunset

Wow! It took four years, but as of today, I’m officially a published astronomer! My paper, titled “Improved Orbital Constraints and H-alpha Photometric Monitoring of the Directly Imaged Protoplanet Analog HD 142527 B” has been accepted for publication in The Astronomical Journal, and is now public on the preprint service arXiv.

Ever since joining the field of direct imaging, I’ve wanted to make my own “orbit movie” like the fabulous animations produced by Jason Wang.

This paper is the result of my undergraduate thesis research with Kate Follette at Amherst College, with whom I’ve worked for the past 4 years (almost to this day). It’s a companion paper to two other papers that I am co-author on, which will soon be published by Kate and my best-astro-bud Jea Adams Redai, respectively. Our lab works on, among other observations, data taken from the Magellan Telescope’s MagAO instrument of young star systems, looking for signs of forming planets.

Tatooine’s binary sunset has long been a haven for sulky, starry eyed farm-boys like myself.

One of the major questions astronomers are trying to understand is how planets can form orbiting binary stars. We’ve observed fully formed planets orbiting close in binary pairs, but there are some big questions about how these planets come to be. One of the biggest questions is how the orientation of the binary star affects the planet’s formation. Two big stars swinging wildly around each other can knock forming planets off their balance, and scatter planet making material from the inner regions of the young system. That would make it difficult for the universe to create terrestrial planets in the habitable zones of their binary hosts, like Tatooine.

My annotated view of the binary sunset. I see a white A or F type star, a red M type star, and that the orbit of the binary stars are inclined with respect to the horizon, indicating that either Tatooine rotates on a tilt, or orbits its binary host on an incline.

Another question astronomers are seeking to understand is just how fast protostars grow as they gobble up the material that surrounds them, and whether their rate of growth changes over time.

In my paper, I studied observations of the star system HD 142527, a bright young star system that is observable in the southern sky. The system is only a few million years old (younger than the first human ancestors, maybe even younger than human stone tools), so young that the star hasn’t finished forming (we call these “protostars”). The system gained notability among astronomers in the early 2000s because a beautiful disk of gas and dust swirls around the protostar. There’s a large gap in that circumstellar disk, and originally astronomers thought that gap could be carved out by forming planets (which is why our team originally observed the system). It was then discovered that deep inside the gap, another, smaller protostar was orbiting the larger central protostar!

Figure 5 from my paper, showing the disk of material surrounding HD 142527, an image of HD 142527 B, and the various orbits I fit to the observations of HD 142527 B’s motion.

So HD 142527 is a forming binary system, surrounded by a disk of planet forming material. The system is prime for studying how protostars grow and how binary stars affect the planet forming material in the disks that surround them.

In my paper, I calibrated the MagAO instrument we used to observe this system, and then I used a starlight subtraction algorithm to remove the light from HD 142527 A that was blocking the much smaller, dimmer star HD 142527 B. I measured the position and brightness of HD 142527 B in two different colored filters after removing the excess light. I made sure to model the affect that the starlight subtraction has on our images in order to account for any distortions that could affect my measurements.

Images of HD 142527 B after removing the light from HD 142527 A

Then, I compared my measurements to other measurements of the position of HD 142527 B from different telescopes. I verified my measurements agreed, and then fit different orbital trajectories to all the available measurements of the system.

The orbits fit to the various measurements of HD 142527 B. My observations are marked as yellow circles.

With the possible trajectories determined, I then wanted to determine how the brightness of HD 142527 B changed over time. Our observations were taken in two wavelengths of light that help us isolate how much energy is being released as the protostar grows, and therefore how much mass it is gaining per year.

The brightness of HD 142527 B over time in two wavelengths of light. One wavelength (dark red points) trace the energy released as the star forms. The other wavelength act as a control, showing that, the protostar is otherwise not varying in brightness.

I found that the rate at which the star was growing changed significantly between 2013 and 2015, which means that stars can speed up or slow down growing. Exactly how is still an open question!

My orbital analysis also proves that the orbit of the binary pair is inclined with respect to the outer disk, at a mutual inclination that can range from 30 to 90 degrees! This means that HD 142527 B is definitely disturbing the planet forming material near the binary star, which explains the huge gap in the circumbinary disk. What this result seems to suggest is that planets like Tatooine (stable, rocky planets in the habitable zones around binary stars) cannot form if the binary star is on an inclined orbit relative to the circumplanetary disk. This is evidence that, if Tatooine were to exist, it would have to rotate on a tilted axis for its binary sunset to appear like it does in the movies.

It’s exciting to be published, and to have officially wrapped up this project. I learned a ton of skills while doing this research that I am using in my next research projects. Hopefully you’ll hear about those soon. Until then, clear skies.


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