arXiv subjects

Rolf Walder

Publications and source records attributed to Rolf Walder.

30 records · Page 2Linked to original sources

arXiv2009

The different progenitors of type Ib, Ic SNe, and of GRB

We discuss the properties of the progenitors of core collapse SNe, as they can be deduced from rotating stellar models of single stars. The type of the SN progenitor was determined from the surface abundances at the pre-SN stage. The type of the SN event was obtained from the masses of hydrogen and helium ejected at the time of the core-collapse SN event. We find that the minimum amount of helium ejected by a core-collapse SN (of whatever type) is around 0.3 MSun. There is no difference between the WC and WO stars in the ejected masses of helium, CNO elements, and heavy elements. Also no difference is expected between the chemical composition of a WC star resulting from a normal or a homogeneous evolution. The progenitors of type Ib SNe are WNL, WNE, or less massive WC stars. Those of type Ic are WC and WO stars. WO stars are produced in a limited mass range (around 60 MSun) and only at low metallicity (for <= 0.010) as already found. The WO stars are the progenitors of only a small fraction of type Ic. Present stellar models indicate that, at solar metallicity, there is about 1 type Ib SN for 1 type Ic, and this ratio rises to 3 type Ic for 1 type Ib SN at twice solar metallicity. At this metallicity, type Ic's are more frequent than type Ib's because most massive stars that go through a WNE stage evolve further into a WC/WO phase. Current models can account for the observed number ratios SN Ib/SN II and SN Ic/SN II and for their observed variation with the metallicity. In case no supernova occurs when a black hole is formed, single-star models can still account for more than half of the observed (SN Ib+SN Ic)/SN II ratio for Z >= ZSun. For the GRB rate, our models produce too large a number for such an event, even if we restrict the progenitor to the WO stars.

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arXiv2008

3D simulations of RS Oph: from accretion to nova blast

RS Ophiuchi is a recurrent nova with a period of about 22 years, consisting of a wind accreting binary system with a white dwarf (WD) very close to the Chandrasekhar limit and a red giant star (RG). The system is considered a prime candidate to evolve into an SNIa. We present a 3D hydrodynamic simulation of the quiescent accretion and the subsequent explosive phase. The computed circumstellar mass distribution in the quiescent phase is highly structured with a mass enhancement in the orbital plane of about a factor of 2 as compared to the poleward directions. The simulated nova remnant evolves aspherically, propagating faster toward the poles. The shock velocities derived from the simulations are in agreement with those derived from observations. For v_RG = 20 km/s and for nearly isothermal flows, we derive a mass transfer rate to the WD of 10% of the mass loss of the RG. For an RG mass loss of 10^{-7} solar masses per year, we found the orbit of the system to decay by 3% per million years. With the derived mass transfer rate, multi-cycle nova models provide a qualitatively correct recurrence time, amplitude, and fastness of the nova. Our simulations provide, along with the observations and nova models, the third ingredient for a deeper understanding of the recurrent novae of the RS Oph type. In combination with recent multi-cycle nova models, our results suggests that the WD in RS Oph will increase in mass. Several speculative outcomes then seem plausible. The WD may reach the Chandrasekhar limit and explode as an SN Ia. Alternatively, the mass loss of the RG could result in a smaller Roche volume, a common envelope phase, and a narrow WD+WD system. Angular momentum loss due to graviational wave emission could trigger the merger of the two WDs and - perhaps - an SN Ia via the double degenerate scenario.

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arXiv2006

Fundamental differences between SPH and grid methods

We have carried out a hydrodynamical code comparison study of interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of the SPH smoothing kernel over which information is not transferred.

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arXiv2006

Supersonic turbulence in shock-bound interaction zones I: symmetric settings

Colliding hypersonic flows play a decisive role in many astrophysical objects. In this paper, we look at the idealized model of a 2D plane parallel isothermal slab (CDL) and at symmetric settings, where both flows have equal parameters. We performed a set of high-resolution simulations with upwind Mach numbers, 5 < M_u < 90. We find that the CDL is irregularly shaped and has a patchy and filamentary interior. The size of these structures increases with l_cdl, the extension of the CDL. On average, but not at each moment, the solution is about self-similar and depends only on M_u. We find the root mean square Mach number to scale as M_rms ~ 0.2 M_u. Independent of M_u is the mean density, rho_m ~ 30 rho_u. The fraction f_eff of the upwind kinetic energy that survives shock passage scales as f_eff= 1 - M_rms^(-0.6). This dependence persists if the upwind flow parameters differ from one side to the other of the CDL, indicating that the turbulence within the CDL and its driving are mutually coupled. In the same direction points the finding that the auto-correlation length of the confining shocks and the characteristic length scale of the turbulence within the CDL are proportional. In summary, larger upstream Mach numbers lead to a faster expanding CDL with more strongly inclined confining interfaces relative to the upstream flows, more efficient driving, and finer interior structure relative to the extension of the CDL.

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arXiv2006

The Spin Periods and Rotational Profiles of Neutron Stars at Birth

We present results from an extensive set of one- and two-dimensional radiation-hydrodynamic simulations of the supernova core collapse, bounce, and postbounce phases, and focus on the protoneutron star (PNS) spin periods and rotational profiles as a function of initial iron core angular velocity, degree of differential rotation, and progenitor mass. For the models considered, we find a roughly linear mapping between initial iron core rotation rate and PNS spin. The results indicate that the magnitude of the precollapse iron core angular velocities is the single most important factor in determining the PNS spin. Differences in progenitor mass and degree of differential rotation lead only to small variations in the PNS rotational period and profile. Based on our calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming angular momentum conservation, we estimate final neutron star rotation periods. We find periods of one millisecond and shorter for initial central iron core periods of below ~ 10 s. This is appreciably shorter than what previous studies have predicted and is in disagreement with current observational data from pulsar astronomy. After considering possible spindown mechanisms that could lead to longer periods we conclude that there is no mechanism that can robustly spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens to hundreds of milliseconds observed for young pulsars. Our results indicate that, given current knowledge of the limitations of neutron star spindown mechanisms, precollapse iron cores must rotate with periods around 50-100 seconds to form neutron stars with periods generically near those inferred for the radio pulsar population.

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arXiv2004

Rotating Core Collapse and Bipolar Supernova Explosions

We review some of the reasons for believing that the generic core-collapse supernova is neutrino-driven, not MHD-jet driven. We include a discussion of the possible role of rotation in supernova blast energetics and morphology, and speculate on the origin of Cas A's and SN1987A's ejecta fields. Two "explosive" phenomena may be associated with most core collapses, the neutrino-driven supernova itself and an underenergetic jet-like ejection that follows. The latter may be a magnetic wind that easily penetrates the debris created by the much more energetic supernova. We predict that many core-collapse supernova remnants should have sub-dominant jet-like features. In Cas A, we associate this sub-dominant collimated wind with the "jet/counter-jet" structure observed. We suggest that the actual Cas A explosion itself is at nearly right angles to this jet, along a rotation axis that coincides with the bulk of the ejecta, the iron lobes, and the putative direction of motion of the point source. It may be that when rotation becomes sufficiently rapid that the strong-neutrino-driven-supernova/weak-jet duality switches to a strong-MHD-jet scenario that might be associated with hypernovae, and in some cases GRBs. Finally, we present a calculation using a new 2D multi-group, flux-limited radiation/hydrodynamics code we have recently developed for the simulation of core-collapse supernovae. We discuss the rotation-induced anisotropy in the neutrino radiation field, neutrino heating, and the neutrino flux vectors and speculate on rotation's possible role in the supernova mechanism and in the overall supernova phenomenon.

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arXiv2003

Two-dimensional, Time-dependent, Multi-group, Multi-angle Radiation Hydrodynamics Test Simulation in the Core-Collapse Supernova Context

We have developed a time-dependent, multi-energy-group, and multi-angle (S$_n$) Boltzmann transport scheme for radiation hydrodynamics simulations, in one and two spatial dimensions. The implicit transport is coupled to both 1D (spherically-symmetric) and 2D (axially-symmetric) versions of the explicit Newtonian hydrodynamics code VULCAN. The 2D variant, VULCAN/2D, can be operated in general structured or unstructured grids and though the code can address many problems in astrophysics it was constructed specifically to study the core-collapse supernova problem. Furthermore, VULCAN/2D can simulate the radiation/hydrodynamic evolution of differentially rotating bodies. We summarize the equations solved and methods incorporated into the algorithm and present results of a time-dependent 2D test calculation. A more complete description of the algorithm is postponed to another paper. We highlight a 2D test run that follows for 22 milliseconds the immediate post-bounce evolution of a collapsed core. We present the relationship between the anisotropies of the overturning matter field and the distribution of the corresponding flux vectors, as a function of energy group. This is the first 2D multi-group, multi-angle, time-dependent radiation/hydro calculation ever performed in core collapse studies. Though the transport module of the code is not gray and does not use flux limiters (however, there is a flux-limited variant of VULCAN/2D), it still does not include energy redistribution and most velocity-dependent terms.

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arXiv2003

Structuring and support by Alfven waves around prestellar cores

Observations of molecular clouds show the existence of starless, dense cores, threaded by magnetic fields. Observed line widths indicate these dense condensates to be embedded in a supersonically turbulent environment. Under these conditions, the generation of magnetic waves is inevitable. In this paper, we study the structure and support of a 1D plane-parallel, self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is injected in its central plane. Dimensional analysis shows that the solution must depend on three dimensionless parameters. To study the nonlinear, turbulent evolution of such a slab, we use 1D high resolution numerical simulations. For a parameter range inspired by molecular cloud observations, we find the following. 1) A single source of energy injection is sufficient to force persistent supersonic turbulence over several hydrostatic scale heights. 2) The time averaged spatial extension of the slab is comparable to the extension of the stationary, analytical WKB solution. Deviations, as well as the density substructure of the slab, depend on the wave-length of the injected wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with increasing plasma beta. 4) Good spatial resolution is mandatory, making similar simulations in 3D currently prohibitively expensive.

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arXiv2003

Gravitational Waves from Axisymmetric, Rotational Stellar Core Collapse

We have carried out an extensive set of two-dimensional, axisymmetric, purely-hydrodynamic calculations of rotational stellar core collapse with a realistic, finite-temperature nuclear equation of state and realistic massive star progenitor models. For each of the total number of 72 different simulations we performed, the gravitational wave signature was extracted via the quadrupole formula in the slow-motion, weak-field approximation. We investigate the consequences of variation in the initial ratio of rotational kinetic energy to gravitational potential energy and in the initial degree of differential rotation. Furthermore, we include in our model suite progenitors from recent evolutionary calculations that take into account the effects of rotation and magnetic torques. For each model, we calculate gravitational radiation wave forms, characteristic wave strain spectra, energy spectra, final rotational profiles, and total radiated energy. In addition, we compare our model signals with the anticipated sensitivities of the 1st- and 2nd-generation LIGO detectors coming on line. We find that most of our models are detectable by LIGO from anywhere in the Milky Way.

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arXiv1999

Complex wind dynamics and ionization structure in symbiotic binaries

Aspects of the wind-dynamics in symbiotic binaries, colliding winds and accretion, are reviewed. Inconsistencies between theory and observations of the hot star wind are discussed. If the hot star wind were governed by CAK theory, nearly all symbiotics would be colliding wind binaries. For the case of colliding winds, 3D hydrodynamical simulations reveal that the matter distribution is spirally shaped. Shock confined high-density shells as well as huge voids are found even in the immediate neighborhood of the stars. Synthetic spectra computed on the basis of different 3D hydrodynamical models suggest observational discrimination between them to be possible. Colliding wind models also provide a link between symbiotics and planetary nebulae. Accretion during some time is a necessary condition for symbiotics to exist. However, there is no proof of whether currently accreting systems show the symbiotic phenomenon. Existing accretion models are inconsistent amongst each other, predicting either extended disks or small, high-density accretion wakes. Synthetic spectra allowing to discriminate between two models do not yet exist.

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arXiv1999

Theory of thermal and ionization effects in colliding winds of WR+O binaries

The colliding winds interaction zone in WR+O binaries is a highly complex environment. In this review we summarize the progress made towards its theoretical understanding during the last years. We review the effect of different physical processes on the interaction zone, among them geometry and orbital motion, radiative forces, thermal conduction, instabilities and turbulence, ionizing radiation, dust formation, clumped winds, magnetic fields, and particle acceleration. Implications with regard to observations are discussed. Subsequently, we proceed to the important question of mutual interaction amongst these processes. Because of the wealth of physical processes involved, numerical simulations are usually mandatory. Finally, we turn to the combined role these processes play for the thermal and ionization properties of the colliding winds interaction zone in WR+O binaries.

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arXiv1996

Radiative cooling instability in 1D colliding flows

Radiative shock waves show a strong cooling instability at temperatures above approximately 2 times 10^5 K. We numerically investigate this instability by simulating different astronomical objects in which colliding flows play an outstanding role: Wind bubbles, supernova remnants, and colliding winds. Computing the flow of each object over a large part of its evolutionary time and resolving all physically relevant scales, we find several phenomenologically different types of this instability. If two smooth flows collide, the instability follows a periodic limit cycle with two modes being important. The connection between the radiative loss function and the mode and type of the overstability is discussed. The collision of non-smooth flows can temporarily result in an aperiodic evolution of the system. After a characteristic relaxation time the instability then becomes periodic again. Such disturbances as well as violent types of the instability can excite oscillations of the thin layer of cold compressed gas downstream of the shock, which in turn can influence the stability of the radiative shock.

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