Browsing by Subject "simulations"

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Angular Momentum Transport And Turbulence In Laboratory Models Of Keplerian Flows
(2012-11) Paoletti, M. S.; van Gils, D. P. M.; Dubrulle, B.; Sun, Chao; Lohse, Detlef; Lathrop, D. P.; Paoletti, M. S.
We present angular momentum transport (torque) measurements in two recent experimental studies of the turbulent flow between independently rotating cylinders. In addition to these studies, we reanalyze prior torque measurements to expand the range of control parameters for the experimental Taylor-Couette flows. We find that the torque may be described as a product of functions that depend only on the Reynolds number, which describes the turbulent driving intensity, and the rotation number, which characterizes the effects of global rotation. For a given Reynolds number, the global angular momentum transport for Keplerian-like flow profiles is approximately 14% of the maximum achievable transport rate. We estimate that this level of transport would produce an accretion rate of (M) over dot/(M) over dot(0) similar to 10(-3) in astrophysical disks. We argue that this level of transport from hydrodynamics alone could be significant. We also discuss the possible role of finite-size effects in triggering or sustaining turbulence in our laboratory experiments.
Aspherical Supernova Shock Breakout And The Observations Of Supernova 2008D
(2011-02) Couch, Sean M.; Pooley, David; Wheeler, J. Craig; Milosavljevic, Milos; Couch, Sean M.; Wheeler, J. Craig; Milosavljevic, Milos
Shock breakout is the earliest, readily observable emission from a core-collapse supernova (SN) explosion. Observing SN shock breakout may yield information about the nature of the SN shock prior to exiting the progenitor and, in turn, about the core-collapse SN mechanism itself. X-ray outburst 080109, later associated with SN 2008D, is a very well-observed example of shock breakout from a core-collapse SN. Despite excellent observational coverage and detailed modeling, fundamental information about the shock breakout, such as the radius of breakout and driver of the light curve timescale, is still uncertain. The models constructed for explaining the shock breakout emission from SN 2008D all assume spherical symmetry. We present a study of the observational characteristics of aspherical shock breakout from stripped-envelope core-collapse SNe surrounded by a wind. We conduct two-dimensional, jet-driven SN simulations from stripped-envelope progenitors and calculate the resulting shock breakout X-ray spectra and light curves. The X-ray spectra evolve significantly in time as the shocks expand outward and are not fit well by single-temperature and radius blackbodies. The timescale of the X-ray burst light curve of the shock breakout is related to the shock crossing time of the progenitor, and not to the much shorter light crossing time that sets the light curve timescale in spherical breakouts. This could explain the long shock breakout light curve timescale observed for XRO 080109/SN 2008D. We also comment on the distribution of intermediate-mass elements in asymmetric explosions.
Assembly Of The First Disk Galaxies Under Radiative Feedback From Pop III Stars
(2012-05) Pawlik, A. H.; Milosavljevic, M.; Bromm, V.; Pawlik, Andreas H; Milosavljević, Miloš; Bromm, Volker
We investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. We perform cosmological zoomed smoothed particle hydrodynamics simulations of a galaxy assembling inside a halo reaching a virial mass similar to 10(9) M-circle dot at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen ionizing radiation with a simulation in which stars do not emit radiation but remain dark. Photodissociation and photoionization exert a strong negative feedback on the assembly of the galaxy inside the minihalo progenitor, impeding gas condensation and suppressing star formation. The radiative feedback on the gas implies a suppression of the central dark matter densities in the minihalo by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution in the absence of radiative feedback. The properties of the galaxy become insensitive to the inclusion of radiation once the minihalo turns into an atomic cooler. The formation of a rotationally supported extended disk inside the atomically cooling galaxy therefore is a robust outcome of our simulations. Our simulations make predictions for observations with the upcoming James Webb Space Telescope.
Assembly Of The First Dwarf Galaxies
(2010-11) Pawlik, A.; Milosavljevic, M.; Bromm, V.; Pawlik, Andreas; Milosavljević, Miloš; Bromm, Volker
Understanding the formation and evolution of the first stars and galaxies is crucial to understanding reionization, a key epoch in the history of the Universe. Detailed theoretical studies of the galaxies before and during reionization are now particularly urgent because of the wealth of observational data that will soon be provided by the next generation of telescopes, such as JWST, ALMA, LOFAR, MWA, and others. We simulate the formation of the first galaxies using cosmological smoothed particle hydrodynamics simulations. Zooming in on individual galaxies, we explore how various physical processes affect their assembly and further evolution. A highlight of our study will be the simulation of the radiation-hydrodynamics of galaxy assembly, which we will perform using our multi-frequency radiative transfer method TRAPHIC. Feedback from radiation has long been suspected to play a decisive role in galaxy formation and we will investigate its implications for observable properties of the first galaxies.
Bounds on heat transport in Beacutenard-Marangoni convection
(2010-04) Hagstrom, George; Doering, Charles R.; Hagstrom, George
For Pearson's model of Beacutenard-Marangoni convection, the Nusselt number Nu is proven to be bounded as a function Marangoni number Ma according to Nu < 0.838xMa(2/7) for infinite Prandtl number and according to Nu less than or similar to Ma(1/2) uniformly for finite Prandtl number. The analysis is also used to raise the lower bound for the critical Marangoni number for energy stability of the conduction solution from 56.77 to 58.36 when the Prandtl number is infinite.
Bullet Cluster: A Challenge To Lambda CDM Cosmology
(2010-07) Lee, Jounghun; Komatsu, Eiichiro; Komatsu, Eiichiro
To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard Lambda cold dark matter (CDM) cosmology, we use a large-volume (27 h(-3) Gpc(3)) cosmological N-body MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall velocity distribution is given at (1-3) R(200) of the main cluster (where R(200) is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out by Mastropietro & Burkert has shown that an initial velocity of 3000 km s(-1) at about 2R(200) is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, X-ray morphology of the main cluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R(200) is incompatible with the prediction of a Lambda CDM model: the probability of finding 3000 km s(-1) in (2-3) R(200) is between 3.3 x 10(-11) and 3.6 x 10(-9). A lower velocity, 2000 km s(-1) at 2R(200), is also rare, and moreover, Mastropietro & Burkert have shown that such a low initial velocity does not reproduce the X-ray brightness ratio of the main and subcluster or morphology of the main cluster. Therefore, we conclude that the existence of 1E0657-56 is incompatible with the prediction of a Lambda CDM model, unless a lower infall velocity solution for 1E0657-56 with less than or similar to 1800 km s(-1) at 2R(200) is found.
The c2d Spitzer Spectroscopic Survey Of Ices Around Low-Mass Young Stellar Objects. IV. NH3 And CH3OH
(2010-08) Bottinelli, Sandrine; Boogert, Adwin C. A.; Bouwman, Jordy; Beckwith, Martha; van Dishoeck, Ewine F.; Oberg, Karin I.; Pontoppidan, Klaus M.; Linnartz, H.; Blake, Geoffrey A.; Evans, Neal J.; Lahuis, Fred; Evans, Neal J.
NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and CH3OCH3. Despite a number of recent studies, little is known about their abundances in the solid state. This is particularly the case for low-mass protostars, for which only the launch of the Spitzer Space Telescope has permitted high-sensitivity observations of the ices around these objects. In this work, we investigate the similar to 8-10 mu m region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 mu m silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH- containing ices to help interpret the astronomical spectra. We clearly detect a feature at similar to 9 mu m in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu(2) umbrella mode and derive abundances with respect to water between similar to 2% and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu(4) C-O stretch mode of this molecule at similar to 9.7 mu m in 16 objects, yielding abundances consistent with those derived by Boogert et al. based on a simultaneous 9.75 and 3.53 mu m data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10%-20% of nitrogen is locked up in known ices.
Calibrations Of Atmospheric Parameters Obtained From The First Year Of Sdss-III APOGEE Observations
(2013-11) Meszaros, S.; Holtzman, J.; Perez, A. E. Garcia; Prieto, C. Allende; Schiavon, R. P.; Basu, S.; Bizyaev, D.; Chaplin, W. J.; Chojnowski, S. D.; Cunha, K.; Elsworth, Y.; Epstein, C.; Frinchaboy, P. M.; Garcia, R. A.; Hearty, F. R.; Hekker, S.; Johnson, J. A.; Kallinger, T.; Koesterke, L.; Majewski, S. R.; Martell, S. L.; Nidever, D.; Pinsonneault, M. H.; O'Connell, J.; Shetrone, M.; Smith, V. V.; Wilson, J. C.; Zasowski, G.; Koesterke, L.
The Sloan Digital Sky Survey III (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a three-year survey that is collecting 105 high-resolution spectra in the near-IR across multiple Galactic populations. To derive stellar parameters and chemical compositions from this massive data set, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) has been developed. Here, we describe empirical calibrations of stellar parameters presented in the first SDSS-III APOGEE data release (DR10). These calibrations were enabled by observations of 559 stars in 20 globular and open clusters. The cluster observations were supplemented by observations of stars in NASA's Kepler field that have well determined surface gravities from asteroseismic analysis. We discuss the accuracy and precision of the derived stellar parameters, considering especially effective temperature, surface gravity, and metallicity; we also briefly discuss the derived results for the abundances of the a-elements, carbon, and nitrogen. Overall, we find that ASPCAP achieves reasonably accurate results for temperature and metallicity, but suffers from systematic errors in surface gravity. We derive calibration relations that bring the raw ASPCAP results into better agreement with independently determined stellar parameters. The internal scatter of ASPCAP parameters within clusters suggests that metallicities are measured with a precision better than 0.1 dex, effective temperatures better than 150 K, and surface gravities better than 0.2 dex. The understanding provided by the clusters and Kepler giants on the current accuracy and precision will be invaluable for future improvements of the pipeline.
CMIP5 Climate Model Analyses Climate Extremes in the United States
(2014-04) Wuebbles, Donald; Meehl, Gerald; Hayhoe, Katharine; Karl, Thomas R.; Kunkel, Kenneth; Santer, Benjamin; Wehner, Michael; Colle, Brian; Fischer, Erich M.; Fu, Rong; Goodman, Alex; Janssen, Emily; Kharin, Viatcheslav; Lee, Huikyo; Li, Wenhong; Long, Lindsey N.; Olsen, Seth C.; Pan, Zaitao; Seth, Anji; Sheffield, Justin; Sun, Liqiang; Fu, Rong
Demographics of Bulge Types Within 11 Mpc and Implications for Galaxy Evolution
(2011-06) Fisher, David B.; Drory, Niv; Fisher, David B.
We present an inventory of galaxy bulge types (elliptical galaxy, classical bulge, pseudobulge, and bulgeless galaxy) in a volume-limited sample within the local 11 Mpc sphere using Spitzer 3.6 mu m and Hubble Space Telescope data. We find that whether counting by number, star Formation rate, or stellar mass, the dominant galaxy type in the local universe has pure disk characteristics (either hosting a pseudobulge or being bulgeless). Galaxies that contain either a pseudobulge or no bulge combine to account for over 80% of the number of galaxies above a stellar mass of 10(9) M-circle dot. Classical bulges and elliptical galaxies account for similar to 1/4, and disks for similar to 3/4 of the stellar mass in the local 11 Mpc. About 2/3 of all star Formation in the local volume takes place in galaxies with pseudobulges. Looking at the fraction of galaxies with different bulge types as a function of stellarmass, we find that the frequency of classical bulges strongly increases with stellar mass, and comes to dominate above 10(10.5) M-circle dot. Galaxies with pseudobulges dominate at 10(9.5)-10(10.5) M-circle dot. Yet lower-mass galaxies are most likely to be bulgeless. If pseudobulges are not a product of mergers, then the frequency of pseudobulges in the local universe poses a challenge for galaxy evolution models.
The Destruction of the Circumstellar Ring of SN 1987A
(2015-06) Fransson, Claes; Larsson, Josefin; Migotto, Katia; Pesce, Dominic; Challis, Peter; Chevalier, Roger A.; France, Kevin; Kirshner, Robert P.; Leibundgut, Bruno; Lundqvist, Peter; McCray, Richard; Spyromilio, Jason; Taddia, Francesco; Jerkstrand, Anders; Mattila, Seppo; Smith, Nathan; Sollerman, Jesper; Wheeler, J. Craig; Crotts, Arlin; Garnavich, Peter; Heng, Kevin; Lawrence, Stephen S.; Panagia, Nino; Pun, Chun S. J.; Sonneborn, George; Sugerman, Ben; Wheeler, J. Craig
We present imaging and spectroscopic observations with Hubble Space Telescope and Very Large Telescope of the ring of SN 1987A from 1994 to 2014. After an almost exponential increase of the shocked emission from the hotspots up to day similar to 8000 (similar to 2009), both this and the unshocked emission are now fading. From the radial positions of the hotspots we see an acceleration of these up to 500-1000 km s(-1), consistent with the highest spectroscopic shock velocities from the radiative shocks. In the most recent observations (2013 and 2014), we find several new hotspots outside the inner ring, excited by either X-rays from the shocks or by direct shock interaction. All of these observations indicate that the interaction with the supernova ejecta is now gradually dissolving the hotspots. We predict, based on the observed decay, that the inner ring will be destroyed by similar to 2025.
Detecting the Rise and Fall of the First Stars by Their Impact on Cosmic Reionization
(2012-09) Ahn, Kyungjin; Iliev, Ilian T.; Shapiro, Paul R.; Mellema, Garrelt; Koda, Jun; Mao, Yi; Shapiro, Paul R.; Mao, Yi
The intergalactic medium was reionized before redshift z similar to 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H-2) cooled gas inside the smallest galaxies, minihalos (MHs) of mass between 10(5) and 10(8) M-circle dot. Although the very first stars began forming inside these MHs before redshift z similar to 40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionization. Here we report results from the first reionization simulations to include these first stars and the radiative feedback that limited their Formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that, while MH stars stopped far short of fully ionizing the universe, reionization began much earlier with MH sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. This boost should be readily detectable by Planck, although within current Wilkinson Microwave Anisotropy Probe uncertainties. If reionization ended as late as z(ov) less than or similar to 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by other probes.
Dissipation and Extra Light in Galactic Nuclei. III. "Core" Ellipticals and "Missing" Light
(2009-04) Hopkins, Philip F.; Lauer, Tod R.; Cox, Thomas J.; Hernquist, Lars; Kormendy, John; Kormendy, John
We investigate how "extra" or "excess" central light in the surface brightness profiles of cusp or power-law elliptical galaxies relates to the profiles of ellipticals with cores. The envelopes of cusp ellipticals are established by violent relaxation in mergers acting on stars present in gas-rich progenitor disks, while their centers are structured by the relics of dissipational, compact starbursts. Ellipticals with cores are formed by the subsequent merging of the now gas-poor cusp ellipticals, with the fossil starburst components combining to preserve a dense, compact component in these galaxies as well ( although mixing of stars smooths the transition from the outer to inner components in the profiles). By comparing extensive hydrodynamical simulations to observed profiles spanning a broad mass range, we show how to observationally isolate and characterize the relic starburst component in core ellipticals. Our method recovers the younger starburst population, demonstrating that these dense concentrations survive spheroid-spheroid mergers and reflect the degree of dissipation in the initial mergers that formed the penultimate galaxy progenitors. The degree of dissipation in the mergers that produced the cusp ellipticals is a strong function of stellar mass, roughly tracing the observed gas fractions of disks of the same mass over redshifts z similar to 0-2. The strength of this component strongly correlates with effective radius at fixed mass: systems with more dissipation are more compact, sufficient to explain the discrepancy in the maximum phase-space densities of ellipticals and their progenitor spirals. The survival of this component together with scattering of stars into the envelope in re-mergers naturally explain the high-Sersic index profile shapes characteristic of very massive core ellipticals. This is also closely related to the kinematics and isophotal shapes: only systems with matched starburst components from their profile fits also reproduce the observed kinematics of boxy/core ellipticals. The final "core-scouring" phase of core Formation occurs when a black hole binary formed in the merger scatters stars out of the innermost regions of the extra-light component. It is therefore critical to adopt a physically motivated profile decomposition that accounts for the fossil starburst component when attempting to quantify scouring. We show that estimates of the scoured mass that employ single-component forms fitted to the entire galaxy profile can be strongly biased.
Drift Wave Turbulence
(2008-05) Horton, W.; Kim, J. H.; Asp, E.; Hoang, T.; Watanabe, T. H.; Sugama, H.; Horton, W.; Kim, J. H.
Drift waves occur universally in magnetized plasmas producing the dominant mechanism for transport of particles, energy and momentum across magnetic field lines. A wealth of information obtained from laboratory experiments for plasma confinement is reviewed for drift waves driven unstable by density gradients, temperature gradients and trapped particle effects. The modern understanding of origin of the scaling laws for Bohm and gyro-Bohm transport fluxes is discussed. The role of sheared flows and magnetic shear in reducing the transport fluxes is discussed and illustrated with large scale computer simulations. Plasmas turbulence models are derived with reduced magnetized fluid descriptions. The types of theoretical descriptions reviewed include weak turbulence theory and anisotropic Kolmogorov-hke spectral indices, and the mixing length. A number of standard turbulent diffusivity formulas are given for the various space-time scales of the drift-wave turbulent mixing.
Effect of Streaming Motion of Baryons Relative to Dark Matter on the Formation of the First Stars
(2011-03) Stacy, Athena; Bromm, Volker; Loeb, Abraham; Stacy, Athena; Bromm, Volker
We evaluate the effect of a supersonic relative velocity between the baryons and dark matter on the thermal and density evolution of the first gas clouds at z less than or similar to 50. Through a series of cosmological simulations, initialized at z(i) = 100 with a range of relative streaming velocities and minihalo Formation redshifts, we find that the typical streaming velocities will have little effect on the gas evolution. Once the collapse begins, the subsequent evolution of the gas will be nearly indistinguishable from the case of no streaming, and star Formation will still proceed in the same way, with no change in the characteristic Pop III stellar masses. Reionization is expected to be dominated by halo masses of greater than or similar to 10(8)M(circle dot), for which the effect of streaming should be negligible.
The Effect Of Turbulent Intermittency On The Deflagration To Detonation Transition In Supernova Ia Explosions
(2008-07) Pan, Lubin; Wheeler, J. Craig; Scalo, John; Pan, Lubin; Wheeler, J. Craig; Scalo, John
We examine the effects of turbulent intermittency on the deflagration to detonation transition (DDT) in Type Ia supernovae. The Zel'dovich mechanism for DDT requires the formation of a nearly isothermal region of mixed ash and fuel that is larger than a critical size. We primarily consider the hypothesis by Khokhlov et al. and Niemeyer and Woosley that the nearly isothermal, mixed region is produced when the flame makes the transition to the distributed regime. We use two models for the distribution of the turbulent velocity fluctuations to estimate the probability as a function of the density in the exploding white dwarf that a given region of critical size is in the distributed regime due to strong local turbulent stretching of the flame structure. We also estimate lower limits on the number of such regions as a function of density. We find that the distributed regime, and hence perhaps DDT, occurs in a local region of critical size at a density at least a factor of 2-3 larger than predicted for mean conditions that neglect intermittency. This factor makes the transition density much larger than the empirical value from observations in most situations. We also consider the intermittency effect on the more stringent conditions for DDT by Lisewski et al. and Woosley. We find that a turbulent velocity of 10(8) cm s(-1) in a region of size 10(6) cm, as required by Lisewski et al., is rare. We expect that intermittency has a weaker effect on the Woosley model with a stronger DDT criterion. The predicted transition density from this criterion remains below 10(7) g cm(-3) after accounting for intermittency using our intermittency models.
Effects Of Varying The Three-Body Molecular Hydrogen Formation Rate In Primordial Star Formation
(2011-01) Turk, Matthew J.; Clark, Paul; Glover, Simon C. O.; Greif, Thomas H.; Abel, Tom; Klessen, Ralf; Bromm, Volker; Bromm, Volker
The transformation of atomic hydrogen to molecular hydrogen through three-body reactions is a crucial stage in the collapse of primordial, metal-free halos, where the first generation of stars (Population III stars) in the universe is formed. However, in the published literature, the rate coefficient for this reaction is uncertain by nearly an order of magnitude. We report on the results of both adaptive mesh refinement and smoothed particle hydrodynamics simulations of the collapse of metal-free halos as a function of the value of this rate coefficient. For each simulation method, we have simulated a single halo three times, using three different values of the rate coefficient. We find that while variation between halo realizations may be greater than that caused by the three-body rate coefficient being used, both the accretion physics onto Population III protostars as well as the long-term stability of the disk and any potential fragmentation may depend strongly on this rate coefficient.
Electron Temperature Gradient Mode Transport
(2008-05) Horton, W.; Kim, J. H.; Hoang, G. T.; Park, H.; Kaye, S. M.; LeBlanc, B. P.; Horton, W.; Kim, J. H.
Anomalous electron thermal losses plays a central role in the history of the controlled fusion program being the first and most persistent form of anomalous transport across all toroidal magnetic confinement devices. In the past decade the fusion program has made analysis and simulations of electron transport a high priority with the result of a clearer understanding of the phenomenon, yet still incomplete. Electron thermal transport driven by the electron temperature gradient is examined in detail from theory, simulation and power balance studies in tokamaks with strong auxiliary heating.
Empirical Determination Of Convection Parameters In White Dwarfs. I. Whole Earth Telescope Observations Of EC14012-1446
(2012-06) Provencal, J. L.; Montgomery, Michael H.; Kanaan, A.; Thompson, Susan E.; Dalessio, J.; Shipman, H. L.; Childers, D.; Clemens, J. Christopher; Rosen, R.; Henrique, P.; Bischoff-Kim, Agnes; Strickland, W.; Chandler, Dean; Walter, B.; Watson, T. K.; Castanheira, B.; Wang, S.; Handler, G.; Wood, M.; Vennes, S.; Nemeth, P.; Kepler, S. O.; Reed, M.; Nitta, Atsuko; Kleinman, S. J.; Brown, T.; Kim, S. L.; Sullivan, D.; Chen, W. P.; Yang, M.; Shih, C. Y.; Jiang, X. J.; Sergeev, A. V.; Maksim, A.; Janulis, R.; Baliyan, K. S.; Vats, H. O.; Zola, S.; Baran, A.; Winiarski, M.; Ogloza, W.; Paparo, M.; Bognar, Z.; Papics, P.; Kilkenny, D.; Sefako, R.; Buckley, D.; Loaring, N.; Kniazev, A.; Silvotti, R.; Galleti, S.; Nagel, T.; Vauclair, G.; Dolez, N.; Fremy, J. R.; Perez, J.; Almenara, J. M.; Fraga, L.; Montgomery, Michael H.; Wang, S.
We report on an analysis of 308.3 hr of high-speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 mu Hz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l = 1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004-2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1 m, McDonald 2.1 m, and KPNO 2 m telescopes. The fits indicate a time-averaged convective response timescale of tau(0) = 99.4 +/- 17 s, a temperature exponent N = 85 +/- 6.2, and an inclination angle of theta(i) = 32 degrees.9 +/- 3 degrees.2. We present our current empirical map of the convective response timescale across the DA instability strip.
The First Stars: Formation Of Binaries And Small Multiples
(2010-11) Stacy, A.; Greif, T. H.; Bromm, V.; Stacy, Athena; Bromm, Volker
We investigate the formation of metal-free, Population III (Pop III), stars within a minihalo at z similar or equal to 20, starting from cosmological initial conditions. We follow the collapsing gas in the center of the minihalo up to number densities of 10(12) cm(-3). We then study the protostellar accretion onto the initial hydrostatic core, which we represent as a growing sink particle. We continue our simulation for 5000 yr after the first sink particle has formed. During this time, a disk-like configuration is assembled around the first protostar. At the end of the simulation, a small multiple system has formed within this disk, dominated by a binary with masses similar to 40M(circle dot) and similar to 10M(circle dot). If Pop Ill stars were to form typically in binaries or small multiples, the standard model of primordial star formation, where single, isolated stars are predicted to form in minihalos, would have to be modified.