Research
My research focuses on using supernovae to learn about their massive progenitor stars before they exploded. Specifically, I am seeking answers to the following questions:
HOW DO STARS LOSE MASS?
Massive stars are very rare and live short lives. This makes observing them very difficult, specially the later stages of their life at which time they evolve on time scales of years and days.
WHAT IF THEY AREN’T ALONE?
If a star is by its self (single) it loses mass by producing so many photons that they push material off the surface of the star. We call these stellar winds. Stars that are more massive produce more photons and therefore lose more mass than less massive stars. However, if a lower mass (still massive) star has a nearby companion, it can transfer some of its mass to its partner. We are still trying to figure out which supernovae come from stars in single stars and and which supernovae come from binary stars. One way to do this is to search for the companion to the star that exploded.
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Another way to find stars that formed in pairs is to compare how old you think the star that exploded was compared to how old it looked when it exploded. When stars form in pairs, the interaction between the stars can prolong their lifetimes, so older stars look younger when they explode.
ESO/L. Calçada
ESO
SINGLE STARS
Although the velocity and amount of matter leaving a star is very uncertain, most computational models of massive stars model mass-loss as a gradual process for all but the most massive stars. However, there is growing evidence even less massive stars (8-30 times the mass of the sun) may have periods of intense mass-loss that result in a thick layer or shell around the star. By observing when and how the explosion interacts with this material we can learn about when it was lost from the star and at what speed.
WHAT DO SUPERNOVAE LOOK LIKE AT ULTRAVIOLET WAVELENGTHS?
LOOKING BEYOND EARTH'S ATMOSPHERE
The Earth's atmosphere blocks ultraviolet light. This is good for us as humans, but makes studying space in the ultraviolet challenging because we have to use telescopes above Earth's atmosphere (it is much easier to build a telescope on the ground that in space). For this reason we know almost nothing about what supernovae look like and how they evolve in the ultraviolet. By studying supernovae in the ultraviolet we can learn about temperature and density of the outer layers of the supernova. We can also learn about what elements it is composed of and how those elements formed, which tells us about physical processes that occur on the inside of stars, out of sight.
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UNDERSTANDING THE DISTANT UNIVERSE
This is also important for understanding our observations of the distant universe. As ultraviolet light comes to us from these supernovae, it shifts to longer and longer wavelengths - first to visible and then to near infrared. Studying analogs of these supernovae nearby will help us understand the more distant ones.
WHAT IS THE MASS OF THE STAR THAT EXPLODED?
Because massive stars are so rare, we don’t really know the ratio of how many of each mass are created. Complicating the matter, we are still figuring out what the explosion of different mass stars looks like and if we even expect an explosion or if they are going to collapse directly to a black hole. For low mass massive stars (8-30? times the mass of the sun) we can find the mass with three methods. The current gold standard is to have serendipitously observed the galaxy before the supernova and to figure out what is missing. This is called direct detection and is limited to nearby, already observed galaxies. We would like to be able to reliably measure the mass of stars in more distant galaxies and not have to rely on previous chance observations.
MODELING HOW THE SUPERNOVA BRIGHTNESS CHANGES OVER TIME
MODELING HOW MUCH OXYGEN WAS MADE DURING THE LIFE OF THE STAR
After 200 days, the outer layers of the exploded star are transparent and we can view the core. During the star’s life oxygen was fused in this region. The more massive the star, the more oxygen is fused. By measuring how much oxygen there is in the core of the star we can determine its mass.
Another way to determine the mass of the star that exploded is to model the explosion. Stars with different properties (e.g. mass, radius) change brightness differently. By modeling this change we can determine the mass of the star that exploded.