An Accelerated Strategic Computing Initiative (ASCI) Academic Strategic Alliances Program (ASAP) Center at The University of Chicago The Center for Astrophysical Thermonuclear Flashes Next Steps in Flash Astrophysical Simulations Jim Truran Alexandros Alexakis, Ed Brown, Alan Calder, Jonathan Dursi, Bruce Fryxell, Don Lamb, Christof Litwin, Andrea Mignone, Jens Niemeyer, Kevin Olson, Fang Peng, Paul Ricker, Frank Timmes, Bob Rosner, Yuan-Nan Young, Shanqun Zhan, Mike Zingale

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Focus of Flash Astrophysics qTo simulate matter accumulation on the surface of compact stars, nuclear ignition of the accumulated (and possibly stellar) material, and the subsequent evolution of the star’s interior, surface, and exterior q X-ray bursts (on neutron star surfaces) q Novae (on white dwarf surfaces) q Type Ia supernovae (in white dwarf interiors)

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 2000 Highlights of Flash Astrophysics qCompletion of two dimensional simulations of a Type 1 X-ray burst, involving a helium detonation on the surface of a neutron star ( Zingale et al. 2000; Poster ) qIdentification of the mechanism by which significant mixing of core matter from a white dwarf into its accreted hydrogen envelope can occur, which can explain observations of heavy element enrichment in nova nebular ejecta (Young, Alexakis, and Rosner 2000, Rosner et al. 2000; Posters) and first 2 dimensional simulations of nova thermonuclear runaways (Dursi et al. 2000; Poster) q Completion of 2 and 3 dimensional simulations of cellular detonations of carbon, proceeding at conditions appropriate to Type Ia supernova environments ( Timmes et al. 2000; Poster )

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Introduction to X-ray Burst Problem qFocus of first years’ efforts qMechanism for code development qTesting of physics modules qTesting of PARAMESH qConfirmation of early results for a helium detonation on a neutron star qRevealed consistency of propagation timescale of detonation with the timescale of Type I x-ray bursts qNew Physics

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Density Evolution

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Density Evolution with Mesh Refinement

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Surface Waves qSeries of surface waves form on the pool of hot ash, v ~ 1.3 x 10 9 cm s -1 qFinite amplitude, shallow wave speed given by (Faber 1997): qPredicts v ~ 1.4 x 10 9 cm s -1 to v ~ 1.6 x 10 9 cm s -1 depending on choice of h, h’ h h’h’ g

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Behavior of the Photosphere qz-velocity of material at 68 µs exceeds cm s -1 qMaterial flows rapidly off the top of the grid qProjected to a height of z = 10 km

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Detonation Characteristics qDetonation moves at the Chapman-Jouguet velocity of v ~ 1.3 x 10 9 cm s -1 qPropagation timescale from pole to pole is 3 ms, consistent with burst oscillation observations qPeak nuclear energy generation rate is erg g -1 s -1 qE nuc << E binding q Energy release for consuming the envelope is 3.8 x erg q Gravitational binding energy for the envelope is 8.3 x erg

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Observational Consequences / Conclusions qAtmosphere oscillates with a 50 µs period qBreak-out signal soon after initiation of burning qVery little (if any) material is ejected qPhotosphere is thrown to heights of 10 km, observable from most lines of sight when rotating qIn Press, ApJS (Zingale et al. 2001); Poster (Zingale et al.)

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago rp-Process Hydrogen Burning qHydrogen burning on neutron stars likely occurs via the rp- process, a series of rapid proton captures onto seed nuclei qCrust composed of rp-process ashes; important to get nucleosynthesis correct qLate-time energy release (in tail of burst) qDevelop implicit 1-d code suitable for long timescale (>1000 s) evolution qInclusion of sufficiently large reaction networks to reach reaction endpoint q A > 100 (Kr!) qStability studies (at what accretion rate does H/He burning become stable?) qLightcurve and nature of energy release in burst tail qInformation on nucleosynthetic yield; information on crust composition

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago rp-Process Hydrogen Burning qHydrogen burning on neutron stars likely occurs via the rp- process, a series of rapid proton captures onto seed nuclei qCrust composed of rp-process ashes; important to get nucleosynthesis correct qLate-time energy release (in tail of burst) qDevelop implicit 1-d code suitable for long timescale (>1000 s) evolution qInclusion of sufficiently large reaction networks to reach reaction endpoint q A > 100 (Kr!) qStability studies (at what accretion rate does H/He burning become stable?) qLightcurve and nature of energy release in burst tail qInformation on nucleosynthetic yield; information on crust composition

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago The “Standard Model” for Classical Novae qWhat are Classical Novae? q Thermonuclear explosions in hydrogen-rich envelopes on white dwarfs in close binary systems qHow does evolution proceed? q Accretion of matter from a companion leads to growth of the envelope until a critical pressure is achieved at its base to trigger a thermonuclear runaway. qWhy is the outburst so violent? q A combination of degenerate conditions at the base of the envelope and the “dredge-up” of C, O, and Ne fuels from the white dwarf core yields rapid energy release on a dynamic time scale.

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago An example of a nova outburst: Nova Cygni 1992

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Anatomy of a Classical Nova Explosion q Accretion q spherical  disk  polar q Ignition and Runaway q envelope enrichment mechanism q Early Light Curve Evolution q peak luminosity q Hydrogen Depletion Mechanisms and Outburst Timescale q burning  dynamic ejection  winds  common envelope

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago A Challenge for Classical Nova Theory qTheory must answer the following questions: q What mechanism can account for the high degree of envelope enrichment in white dwarf core matter? (More than 30 percent by mass of the matter ejected in a nova explosion is core matter.) q Can convective dredge-up during the early stages of the outburst be responsible? q Answers to these questions require q multi-dimensional numerical simulations of the early stages of nova thermonuclear runaways.

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Convective Dredge-up and Envelope Enrichment qWhat has been learned from simulations performed to date? q 2-D simulations (Livne and Glasner 1997; Kercek and Hillebrandt 1998) found comparable degrees of dredge-up and peak temperatures consistent with those of 1-D studies. q 3-D numerical simulations (Kercek and Hillebrandt 1999) indicate that envelope enrichment via dredge-up proceeds slowly - if at all - and that the temperatures achieved in the hottest regions never approach those of the 1-D and 2-D studies. (Efficient cooling?) q What can we hope to learn from further numerical studies? q Do there exist conditions under which significant mixing can occur? q Are the 3-D results obtained by the Munich group robust?

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Interface Mixing and Nova Abundances q Critical issue: identification of the mixing/dredge up mechanism q accreted matter of solar composition q thermonuclear burning at temperatures below 400 million K cannot increase the concentrations of heavy elements qProposed mechanisms include: q shear-induced mixing (Kippenhahn and Thomas 1978) q diffusion-induced convection (Prialnik and Kovetz 1984) q convective-overshoot-induced mixing (Woosley 1986) q wind driven instabilities of gravity waves at the core-envelope boundary (Young, Alexakis, and Rosner 2000)

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Wind Driven Gravity Wave Instabilities q Shear mixing at (density) interfaces in stratified media is understood from oceanographic studies to be explained by instabilities of surface gravity waves driven by an overlying wind (rather than by Kelvin-Helmholtz instability) qYoung, Alexakis, and Rosner (Poster) have reproduced this result using the Flash code, verifying the linear instability and extending the work to the (previously unexplored) highly nonlinear regime q Unstable surface waves are shown to break, leading to a mixing layer substantially thicker than those previously obtained from Kelvin-Helmholtz studies

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Wave Driven Gravity Wave Instabilities

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Shear Driven Mixing of White Dwarf Surfaces q Mixing model assumptions (Rosner et al.; Poster): q there is a fast envelope `wind’ over the stellar surface, driven by the accretion flow – which drives the gravity wave instabilities q the envelope becomes unstable to convection at a time of order 10 to 100 years prior to outburst (1-D models indicate that convection sets in at a temperature ~ 30 million K) q Mixing timescale estimates give ~10 years, which is less than the envelope evolution timescale noted above

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Shear Driven Mixing of White Dwarf Surfaces qThe required envelope mixing can be incorporated in 1-D envelope models with the use of a mass diffusion term in the evolution equations qDuring the final stages of runaway, the envelope evolution time will become shorter than the mixing time, and no further C/O mixing by this mechanism can occur – hence 2-D and 3-D Flash simulations of novae will not require a sub-grid model

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Multidimensional Nova Simulations with Flash Energy generation rate after ~9 seconds of evolution from Ami Glasner's initial conditions. Hot material at the base of the accreted layer burns most quickly and rises. (Dursi et al.; Poster) • Resolution: 1km 2 • 19,000 time steps • Solar material accreting to a 50% 12 C, 50% 16 O white dwarf.

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Summary: The Next Steps in Nova Simulations qThe explosion scenario for novae is identified. The critical and well posed question is whether a hydrogen runaway on the surface of a white dwarf can produce a nova of outburst strength consistent with observations. The issue here is the time scale and magnitude of dredge up of underlying core matter. qFurther studies of wind driven gravity wave instabilities for physical conditions appropriate to nova envelope environments, to guide numerical modeling of nova explosions q2D and 3D numerical simulations of the early stages of a nova runaway will test whether the results of earlier work are robust and define future directions for nova research. qODT and incompressible studies of the pre-ignition phase will help us to explore the initial conditions for the runaway

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Type Ia Supernovae: Facts... q General properties q Very homogeneous class of events, only small (and correlated) variations. q Rise time: ~ 20 days q Decay time: many months q Peak optical brightness: ~ mag qNo hydrogen is seen in the spectra q Early spectra: Si, Ca, Mg,...(abs.) q Late spectra: Fe, Ni, …(emiss.) q Very high velocities (~10000 km/s) qSN Ia are found in all types of galaxies, including ellipticals q Progenitor systems must have long lifetimes Courtesy of the Supernova Cosmology Project

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago … and Theory qStandard model, consistent with all observations (Hoyle & Fowler 1960) : q SNe Ia are thermonuclear explosions of C+O white dwarf stars. qEvolution to criticality: q Accretion of matter from a binary companion leads to growth of the WD until the critical Chandrasekhar mass is reached. qWhat makes them such powerful bombs? q Electron degeneracy decouples T from, preventing cooling by expansion. q C+C reaction rate is extremely T - dependent. q -> Complete incineration within less than two seconds!

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Astrophysics Drive for Cellular Detonation Study qObservations of Type Ia Supernovae suggest: q Prompt detonation (the `classical’ carbon detonation) cannot have occurred q A deflagration-to-detonation transition may be required qDensity at DDT approximately 10 7 g cm -1 qIssues of interest: q What level of refinement is required to resolve detonation structure and ensure proper energetics q Integrated nucleosynthesis contributions q Will the cellular patterns introduce inhomogeneities at levels that may have observational consequences qIn Press, ApJS (Timmes et al. 2000); Poster (Timmes et al.)

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 3-D Cellular Detonation Silicon mass fraction field of a three dimensional detonation, revealing the presence of under- reacted regions and over-reacted regions. These different fuel-ash regions gradually disappear as the matter is burned to nuclear statistical equilibrium.

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 3-D Cellular Detonation Pressure structure of a three dimensional carbon detonation (Density = 10 7 g cm -3 ) showing regions of over-pressure and under-pressure

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Inferences from Cellular Detonation Studies qDetonation velocity ~2 % slower than the Chapman-Jouguet solution qThe scales of cellular features in both 2-D and 3-D, at this density, are small with respect to a pressure scale height qEstablishment of the minimum resolution required for Type Ia supernova simulations, where the cellular nature of a detonation front is a key feature

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Inferences from Cellular Detonation Studies qEnergy release by nuclear burning in the shock front plays a crucial role in setting the time and length scales of a detonation qThe three dimensional structure of the front is characterized by pockets of unburned fuel and a slight reduction in the velocity of the detonation qAt lower densities in the outer regions of supernovae, lower burning rates can give rise to length scales comparable to the stellar dimensions and concomitant levels of inhomogeneity that may be observable in composition inhomogeneities qLess symmetry in the 3-d structures

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Interaction of a Flame with a Vortex Pair: Can Thermonuclear Flames be Quenched? qCan turbulent strain alone, in the absence of heat losses, locally extinguish a thermonuclear flame? qContext: q This question is crucial for the induction time gradient mechanism for DDT (Zeldovich et al. 1970; Blinnikov & Khokhlov 1986) which needs a large region of nearly isothermal material in order to work. If thermonuclear flames are too robust to be quenched, this mechanism fails to work in SN Ia models. qMethod (FLASH): q Study flame passing through a strong vortex pair, analogous to Poinsot et al (M. Zingale, work in progress -> Poster).

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago FLASH Simulations of Laminar Flames q FLASH was extended to include explicit thermal diffusion q Carbon deflagration speeds were compared to those reported in Timmes and Woosley (TW) (1992), and agree to < 20% q FLASH speeds (blue) and TW speeds (black) are shown at right

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago FLASH Simulations of Flame-Vortex Interactions

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 2000 Highlights of Flash Astrophysics qCompletion of two dimensional simulations of a Type 1 X-ray burst, involving a helium detonation on the surface of a neutron star ( Zingale et al. 2000; Poster ) qIdentification of the mechanism by which significant mixing of core matter from a white dwarf into its accreted hydrogen envelope can occur, which can explain observations of heavy element enrichment in nova nebular ejecta (Young, Alexakis, and Rosner 2000, Rosner et al. 2000; Posters) and first 2 dimensional simulations of nova thermonuclear runaways (Dursi et al. 2000; Poster) q Completion of 2 and 3 dimensional simulations of cellular detonations of carbon, proceeding at conditions appropriate to Type Ia supernova environments ( Timmes et al. 2000; Poster )

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago Astrophysical Futures: The Next Simulations qNext steps in X-ray burst simulations q Helium deflagration on neutron star q Exploration of rp-process implications qNext steps in nova simulations q Exploration of interface mixing properties q 2-D and 3-D simulations of nova thermonuclear runaways qNext steps in supernova simulations q Numerically stable white dwarf in 3-dimensions q Explosion of 3-d white dwarfs with rotation q Further simulations of burning front microphysics

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 3-D Cellular Detonation Pressure structure of a three dimensional carbon detonation (Density = 10 7 g cm -3 ) showing regions of over-pressure and under-pressure

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 3-D Cellular Detonation Silicon mass fraction field of a three dimensional detonation, revealing the presence of under- reacted regions and over-reacted regions. These different fuel-ash regions gradually disappear as the matter is burned to nuclear statistical equilibrium.

The ASCI/Alliances Center for Astrophysical Thermonuclear Flashes The University of Chicago 3-D Cellular Detonation