Grad student reaches stars

In January, LSU astronomers published a solution to a long-standing fundamental problem of astrophysics: what produces thermonuclear, or Type Ia, supernovae, which are tremendous explosions where the light is often brighter than a whole galaxy? LSU Professor of Physics & Astronomy Bradley Schaefer and graduate student Ashley Pagnotta have proven that these supernovae are caused by a pair of white dwarf stars. Their research was featured in Nature, one of the most prestigious scientific journals in the world, considered quite a feat for a graduate student. But then, Pagnotta is no average student.

Ashley Pagnotta
Ashley Pagnotta recently completed her Ph.D. in physics at LSU, where she worked with Professor Bradley Schaefer to solve the question of what produces thermonuclear, or Type Ia, supernovae.
Eddy Perez/University Relations

"Not only is it a remarkable achievement for a graduate student to publish in the journal Nature, which is arguably the single most important journal in all science, but she has just accepted a post-doctoral fellowship to work at the American Museum of Natural History in New York City," said Gary Byerly, dean of the LSU Graduate School. "Ashley has also been an incredible university citizen over her five years here at LSU. She has served on the College of Science Dean's CIRCLE Executive Committee for alumni and corporate relations, and has worked with the Boy Scouts locally and nationally."

Pagnotta recently defended her dissertation and is wrapping up her studies here at LSU, preparing to move into the professional world.

"Ashley has been offered her most-desired job, at the American Museum of Natural History in New York City, where she will be pushing at another long-debated question. She has a wonderful research plan to test whether old novae fade on the time scale of a century, with this dimming as part of the controversial 'Hibernation Model,'" said Schaefer, her major professor and dissertation director. "Few graduate students make big waves in the science community, but Ashley and her press conferences and press releases and press interviews in January carried her science results worldwide."

Before graduating, Pagnotta agreed to sit down and share some thoughts with future astronomers – and anyone interested in graduate studies at LSU.

Q: What role did you play in the supernova discovery?

A: I do all of the processing and a lot of the analysis of the data, as well as making the images that go into the papers. The analysis involves finding the center of the supernova remnants, as well as determining the brightness of the stars that are visible. For the Nature paper, Brad [Schaefer] and I each double-checked the other's work, independently whenever possible, since we knew it would be a relatively high-impact result and wanted to be sure everything was correct.

Nature article summary: An absence of ex-companion stars in the type Ia supernova remnant SNR 0509−67.5

A type Ia supernova is thought to begin with the explosion of a white dwarf star. The explosion could be triggered by the merger of two white dwarfs, (a ‘double-degenerate’ origin), or by mass transfer from a companion star (the ‘single-degenerate’ path). The identity of the progenitor is still controversial; for example, a recent argument against the single-degenerate origin has been widely rejected. One way to distinguish between the double- and single-degenerate progenitors is to look at the centre of a known type Ia supernova remnant to see whether any former companion star is present. A likely ex-companion star for the progenitor of the supernova observed by Tycho Brahe has been identified, but that claim is still controversial. Here we report that the central region of the supernova remnant SNR 0509−67.5 (the site of a type Ia supernova 400 ± 50 years ago, based on its light echo) in the Large Magellanic Cloud contains no ex-companion star to a visual magnitude limit of 26.9 (an absolute magnitude of MV = +8.4) within a region of radius 1.43 arcseconds. (This corresponds to the 3σ maximum distance to which a companion could have been ‘kicked’ by the explosion.) This lack of any ex-companion star to deep limits rules out all published single-degenerate models for this supernova. The only remaining possibility is that the progenitor of this particular type Ia supernova was a double-degenerate system.

Q: What was the most interesting part of the research?

A: The most interesting part of this project was getting an unexpected answer at the end. Based on previous results, we anticipated finding a non-white dwarf star in the center of the supernova remnant, but the deep Hubble Space Telescope images showed no stars there (a result which has since been confirmed by other groups) and so we were able to conclude that the only possible star system that could have caused this particular supernova consisted of two white dwarfs that spiraled together and then exploded. It's always very exciting when your observations show new and unexpected phenomena.

Q: What made you choose astronomy as a career?

A: I have been interested in space and astronomy for as long as I can remember, ever since I was a small child. I never outgrew the standard childhood dream of wanting to grow up and become an astronaut. That led me to take as many math and science courses as I could, and I really enjoyed them, so I just kept going, all the way through graduate school. I actually applied for the most recent call for NASA astronaut candidates, though I don't expect to make it past the first round! The more I studied astronomy, the more I realized how interesting and exciting it is, since there are so many unanswered questions. As an observational astronomer, I get to search for answers to those questions using data from telescopes both in orbit and all over the world; having the opportunity to travel to mountain-top observatories to use the telescopes is an added benefit to an already fun job.

Q: What about graduate school at LSU has been particularly helpful to your career?

A: I have learned the vast majority of what I know about working with astronomy data at LSU. My undergrad degrees are in physics and mathematics, so most of the specifics of doing astronomy were new to me. The astronomy lab classes (taught out at Highland Road Park Observatory) teach us the actual hands-on skills we need to know. I took the graduate version of the lab from Professor Rob Hynes, and I use the things I learned in that class every day. We also have a good mentoring culture within the department, where we have group meetings both with our own small research groups and in conjunction with other groups where we learn and practice a lot of the skills that aren't taught in any of the classes – that's been extremely helpful.

Q: Do you have any suggestions or advice for students interested in pursuing a degree in astronomy (or other fields)?

A: The best preparation for an astronomy degree is a solid foundation of math and physics courses, so take a lot of them, but don't forget to go outside every once in a while when there's a clear evening and marvel at the amazing sight. Try not to get so buried in the numbers that you forget the big picture. Study something you love and then tell everyone about it—many of the opportunities I've had (including my upcoming postdoc position) can be traced to conference presentations. It can be intimidating to give talks as a student, but it is one of the best ways to get to know other scientists who can be future collaborators.

STARGAZING TIP: Many people buy starter telescopes that are quite expensive but extremely frustrating to use. I suggest purchasing a good pair of binoculars and an inexpensive camera tripod. The optics available in binoculars are of much higher quality than those found in beginner telescopes, and binoculars are so much easier to use.

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