Katie’s research combines simulations of binary populations with both gravitational-wave and electromagnetic observations to understand how binary stars evolve throughout their lives. While gravitational waves are excellent probes of compact object populations which represent the end states of binary evolution, observations with telescopes like Gaia can help to pin down critical phases in a binary star’s life. By combining multiple types of observations with predictions from theoretical calculations of how different binary evolution model impact the observable population at each stage, a self consistent picture from start to finish can come into focus.

Katie’s day to day work is centred on developing binary population synthesis simulations which are publicly available for use in the astronomical community. These simulations are used to predict the full evolution of binary stars, from birth to death, using state of the art models that describe how binary interactions impact the evolution of each star.

Katie has been a CITA Fellow since Fall 2018. Before coming to CITA, Katie received her PhD in Physics & Astronomy from Northwestern University.

Daniel Tamayo was awarded the Bishop fellowship for his work in orbital dynamics, particularly his recent research elucidating how forming planetary systems evolve to create the diversity of orbital architectures we observe around other stars today.

In the last twenty years, researchers have discovered thousands of planets orbiting other stars. The results are surprising, with about half of these exoplanet systems hosting planets closer to their star than Mercury, and many planets orbiting in extremely elliptical paths. ‘s research aims to understand the dynamical processes that sculpt the observed menagerie of planets, which will ultimately help place our own solar system in a broader galactic context.

An important difficulty is that most exoplanet systems are observed after they have undergone billions of years of orbital evolution, and a lot can happen in that time. To tackle these problems, Tamayo worked with Hanno Rein to develop state-of-the-art numerical integrators to accurately simulate such planetary systems.

However, the results depend on the initial orbital configurations adopted for the planets, and it is difficult to know exactly what they should be. One would ideally want to observe young systems to obtain this information directly, but it is extremely challenging to observe young systems, which are shrouded by their dusty birth environment.

This may change with the , which, by combining light across a 16 km array of telescopes operating at long wavelengths able to penetrate dust, recently obtained an unprecedented image of the young star HL Tauri. The image below reveals a protoplanetary disk with several sharp, concentric gaps. The most exciting interpretation is that these gaps are being carved by forming giant planets. We studied how massive such putative planets could be, and made predictions for how elliptical one would expect the gaps to be. A careful data analysis of the gap shapes later matched our predictions, though the system continues to be an active area of research.

An interesting outcome of this investigation was that, while the system is stable over its current age, it is a ticking time bomb. It is only a matter of time before their chaotic trajectories lead to close approaches between the planets. Some planets will be ejected from the system entirely, while others will remain on highly elliptical paths, like we observe in the older exoplanet sample.

Ultimately, Tamayo wants to place our own solar system in this wider context of the violent dynamical histories we observe across exoplanets. In a study led by Ryan Cloutier, researchers found that indeed, Jupiter may have ejected an extra planet from our own solar system at early times. It will be an exciting task to continue to unpack the rich information provided by the exoplanet population, and to place our own existence in this bigger picture.

His relevant publications on these topics include:

Dynamical Stability of Imaged Planetary Systems in Formation: Application to HL Tau. (Tamayo, Triaud, Menou, Rein, 2015 AJ 805, 100).

WHFAST: A fast and unbiased implementation of a symplectic Wisdom-Holman integrator for long-term gravitational simulations. (Rein, Tamayo, 2015 MNRAS 452.1, 376-388.

Could Jupiter or Saturn Have Ejected a fifth giant planet? (Cloutier, Tamayo, Valencia, 2015, ApJ 813.1).

The Jeffrey L. Bishop Fellowship is granted every two years for excellence in research in astrophysical dynamics.

Jeffrey Bishop was one of CITA’s first postdoctoral fellows. He was killed in an automobile accident shortly after leaving the Institute and this award was one of several created by his mother, Mrs. Muriel Simmons, to remember his life and work. Her donation is matched by CITA.

Alvarez was awarded the Bishop Fellowship for his outstanding work in cosmology, particularly his recent research on how the universe was re-ionized by energetic small galaxies so efficient they eventually could no longer form. He explained his work this way:

“We think the universe was re-ionized about a billion years after the Big Bang which meant that the first stars that formed heated all of the gas and ionized all the hydrogen gas in the universe, and so one question that’s always come up is, ‘were the galaxies that we see enough to ionize the universe, or, is there a hidden population of fainter galaxies that is present but just too faint for us to see that were the main sources of ionizing radiation to re-ionize the universe?”

Alvarez theorized that galaxies in the early universe too small to see with existing technology formed stars just as efficiently as the large galaxies that we see do and in addition released much more of their ionizing radiation into the surrounding space.

“Our theory was that small galaxies have a higher escape fraction, the fraction of the ionizing photons that actually get out of the galaxy, and the idea behind that is that a large galaxy like a spiral galaxy with a flattened disk has many of the stars forming within dense gas clouds that are very large. The starlight that is put out by those stars is absorbed locally within the galaxy and converted into heat or infrared radiation and does not contribute to the re-ionization of the universe. Our theory was that the small galaxies that form are more irregular in morphology and therefore when they form stars the radiation has a much easier time escaping from those galaxies so . . . they could have re-ionized the universe. . . . We don’t see those galaxies now because in a re-ionized universe where the gas has been heated, those small galaxies have a much harder time forming. It’s as though those galaxies re-ionized the universe but they were so good at what they did that they made it difficult for smaller galaxies to form after them, and since most of what we see now is in the re-ionized universe we don’t see those galaxies now.”

His recent publications on the theory of re-ionization include:

Alvarez is a Post-Doctoral Fellow at CITA and has been promoted to National Research Associate effective September 2013.

The Jeffrey L. Bishop Fellowship is granted every two years for excellence in research in astrophysical dynamics.

Jeffrey Bishop was one of CITA’s first postdoctoral fellows. He was killed in an automobile accident shortly after leaving the Institute and this award was one of several created by his mother, Mrs. Muriel Simmons, to remember his life and work. Her donation is matched by CITA.