Type Ia Supernovae
Thermonuclear, or Type Ia Supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though both the nature of their progenitors, as well as the detailed mechanism of their explosion, remain outstanding unsolved puzzles in contemporary astrophysics. Our research group is actively engaged in research projects exploring several major aspects of this problem.
Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly-rotating WD merger surrounded by a hot corona and a thick, differentially-rotating disk.
The following two movies depict the final moments of a white dwarf binary, as simulated by my colleagues Pablo Loren-Aguilar, Enríque Garcia-Berro, and their collaborators, using a computational technique known as smoothed particle hydrodynamics, and visualized by my collaborator Mark SubbaRao. This particular binary is consists of a one solar mass carbon-oxygen white dwarf, and a carbon-oxygen white dwarf companion with six-tenths of a solar mass. The left movie shows the last several minutes of the binary system; the right, a rotation taken at a fixed snapshot in time just prior to the final merger.
Animation Credit : Mark SubbaRao, Adler Planetarium, Pablo Loren-Aguilar, University of Exeter.
Spiral Instability in White Dwarf Mergers
Recently, with my Ph.D. student Rahul Kashyap, my collaborators Enríque Garcia-Berro, Gabriela Aznar-Siguán, and Pablo Lorén-Aguilar, and my former graduate student Suoqing Ji, we have recently discovered a new mechanism for rapid mass and angular momentum transport in double-degenerate white dwarf binaries. It has long been known that white dwarf binaries, in which the less massive, secondary star, has a mass exceeding two-thirds that of the more massive, primary star, will undergo rapid, dynamical mass transfer. Consequently, the accretion disk formed from the disrupted secondary white dwarf is generally quite massive in comparison to the primary white dwarf, and may be susceptible to a lopsided, one-armed spiral mode first analyzed in depth by Fred Adams, Frank Shu, and collaborators. This one-armed spiral mode in turn drives an accretion flow of hot tidally-disrupted material into the cold, degenerate white dwarf primary, resulting in a detonation front and the birth of a Type Ia supernova, as seen in the video below.
Animation Credit : Rahul Kashyap/Robert Fisher, University of Massachusetts Dartmouth.
Magnetized White Dwarf Mergers
Additionally, together with my former graduate student Suoqing Ji, German DAAD RISE students Pascal Cremer and Jan Behrends, and my collaborators Enríque Garcia-Berro, Petros Tzeferacos, George Jordan, Dongwook Lee, and Pablo Loren-Aguilar, we have studied the evolution of these white dwarf systems over a timescale of minutes to hours following their initial merger, including the effect of the magnetic field for the first time. The movies below show results from our paper (Ji et al, 2013), which advances an equal-mass, 0.6 + 0.6 solar mass white dwarf merger, composed of carbon and oxygen, for 5.6 hours beyond their initial merger.
Animation Credit : Suoqing Ji/Robert Fisher, University of Massachusetts Dartmouth.
Subluminous Type Ia Supernovae
Image Credit : Brad Gallagher/George Jordan, Flash Center for Computational Science.
This image depicts a three-dimensional volume rendering of one of the "failed supernovae" which I have recently investigated with my colleagues George Jordan, Hagai Perets, and Daan van Rossum. A Type Ia supernova is the result of nuclear burning within a white dwarf, and in this case, the result is not the complete incineration of the white dwarf, as in a typical brightness Type Ia event, but rather a much more modest outcome. In our model, the nuclear ash (seen in orange-red here) erupts out of the surface of the white dwarf (seen in green here), which remains intact after the explosion. Much of the nuclear ash is ejected with relatively low velocities, and falls back onto the surface of the surviving white dwarf. The ash radioactively decays, and slowly settles from the surface of the white dwarf, ultimately forming an unusual iron-core white dwarf. Observations have previously pointed towards the existence of such iron-core white dwarfs, which were puzzling from the vantage point of the stellar evolution of a single star. Our work provides a natural explanation for how such a stellar remnant could form.
Recent observations have determined that low-luminosity supernovae form the largest subclass of all Type Ia supernovae, dubbed "Iax," accounting for roughly 30% of all events. Based upon this rate, the galaxy should harbor over 10 million iron-core white dwarf remnants from failed Ia events. These failed events and their remnants may in turn shed light on the broader question of precisely how normal-brightness Type Ia supernovae explode.
This work was published in Astrophysical Journal Letters in Jordan et al, 2012.
Weakly-Compressible Homogeneous Isotropic Turbulence
Image Credit : Brad Gallagher/Robert Fisher, Flash Center for Computational Science.
Understanding the nature of turbulent flows remains of the key outstanding questions in classical physics. Turbulent flows are ubiquitous in nature, arising in scales as small as a cup of coffee, to scales as large as the intergalactic medium. Turbulence plays a fundamental role in the mixing, transport, and combustion of fluids. Yet despite its importance, turbulence largely remains an unsolved problem, particularly when one is interested in computing detailed properties of a flow, such as the turbulent drag over an airplane wing, or the rate of combustion of a turbulent flame.
In 2005 and 2006, I led an international collaboration of computational scientists and physicists on one of the largest-scale-computational explorations of the properties of weakly-compressible turbulent flows completed to date. We utilized over a week of runtime on the fastest supercomputer then in existence, the IBM BlueGene/L at Lawrence Livermore National Laboratory. This enormous computational capacity resulted in a nearly petascale mountain of data -- over 100 TB in total -- which we spent the next several years analyzing in detail. The results shed new light on the properties of both Lagrangian and Eulerian turbulence, and were featured in a series of five peer-reviewed papers, including two Physical Review Letters -- Fisher et al, 2008, Benzi et al, 2008, Arneodo et al, 2008, Dubey et al, 2008, and Benzi et al, 2010.