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Tony Piro

Cahill Rm 326, (626) 395-4282

Mailcode 350-17, Caltech
1200 E California Blvd
Pasadena, CA 91125

piro@caltech.edu

I am a Postdoctoral Fellow in Theoretical Astrophysics in the Tapir group at Caltech. Previously I was a TAC fellow in the astronomy department at University of California, Berkeley. I completed my Ph.D. under the guidance of my advisor Lars Bildsten, permanent member of the KITP.

Background

My research is in the field of theoretical astrophysics, with a wide focus on a number of different topics involving compact objects, explosive events, and extreme binary dynamics. Some of the projects I have investigated are low mass X-ray binary evolution, oscillations of rotating neutron stars, white dwarf accretion, and Type Ia supernova progenitor models. More recently I have been interested in the mergers of compact objects, tidal interactions, novel mechanisms for powering supernovae, and gravitional wave sources. These astrophysical "laboratories" allow for the exciting prospect of studying a wide variety of fields in physics (including gravity, fluid dynamics, magnetic fields, and condensed matter) in extreme physical environments--many of which cannot be replicated with experiments on Earth.

Recent Work
Press Release: A new kind of cosmic flash may reveal the birth of a black hole.

Even though we know that black holes exist in our galaxy, it is still a fundamental problem to understand what kinds of massise stars produce them and what is the observational signature when they form. Does the star explode in a giant flash of light or does it die with a wimper?

In my recent work I showed that the answer may lie somewhere in between. Before a black hole forms inside a star, it is a neutron star for a few seconds. During this time it radiates neutrinos, which pass out of the core and carry away energy. In fact they can carry away an amount of energy equivalent to half the mass of the sun! (Remember Einstein's relation between mass and energy.) When this mass is removed, the envelope of the star suddenly feels less gravity, begins expanding, and this expansion steepens into a shock. This shock produces a flash of light when it hits the surface of the star that lasts for the next few days. Although much dimmer than a normal supernova, this flash would still be observable allowing us to actually see a black hole in the act of forming.

This idea made its way into the popular press, and you can read more at the Physics World, Universe Today, and Science Daily.

A Type Ia supernova is the explosion of a star that can be as bright as 10 billion stars. Thousands of these events have been observed, and they have even been used to measure the expansion history of our Universe. This led to the discovery of dark energy, for which the 2011 Nobel Prize in Physics was awarded. Nevertheless, how these stars explode and what the progenitor stars are remains a puzzle.

In previous work I made theoretical predictions of what would be seen when the burning front first reached the star's surface, showing that it is strongly dependent on the radius. But I was skeptical whether this relatively dim effect would ever be observed. Amazingly, in August 2011, supernova 2011fe exploded in the nearby Pinwheel Galaxy--the closest Type Ia in almost 40 years! Although the effect I predicted was not detected, the event was close enough that my work could be used to place constraints on the exploding star's radius. The conclusion was that the radius must have been less than 1/50th the size of our Sun. This means that it was almost certainly a white dwarf, which is the first time this has been directly confirmed.

More recently, I have done modeling of supernova 2011fe's lightcurve over the first 5 days to show that it must have had radioactive material near it's surface. This puts important constraints on the models for how the star exploded.

Stars in close binaries produce gravitational waves, ripples in space and time first predicted by Einstein. These waves carry away energy and angular momentum, which causes a binary to shrink and may lead to merger. The most famous example is the Hulse-Taylor binary pulsar consisting of two neutron stars in an 8 hour orbit. Measurement of this system's inspiral provided the first indirect evidence of gravitational waves, for which the 1993 Nobel Prize in Physics was awarded.

White dwarf binaries also produce gravitational waves. The most extreme case was announced in the summer of 2011, which has an incredible 13 minute orbital period! In theoretical work I showed that tidal interactions spin up the white dwarfs, which removes angular momentum from the orbit. This causes the binary to inspiral more quickly than what would be expected from gravitational waves alone. Although this effect has not yet been detected, it should be found in data taken over the next year. This would provide an unprecedented measurement of tidal interactions in a compact binary, which will have important implications for what will happen when the white dwarfs eventually merge.

Research Interests

Below is a list of some of my research projects. For some I have included links to simple summaries that give an introduction to these exciting topics.

Current and Past Collaborators

Phil Arras, Lars Bildsten, Matteo Cantiello, Yi Cao, Phil Chang, Tanja Hinderer, Chris Kochanek, Shri Kulkarni, Brian Metzger, Ehud Nakar, Christian Ott, Eric Pfahl, Eliot Quataert, Jocelyn Read, Ken Shen, Todd Thompson, Frank Timmes, Eric Thrane, Dave Tsang, Nevin Weinberg

Links