14552874
On the dynamics of proto-neutron star winds and r-process nucleosynthesis
We study here the formation of heavy r-process nuclei in the high-entropy environment of rapidly expanding neutrino-driven winds from compact objects. In particular, we explore the sensitivity of the element creation in the A > 130 region to the low-temperature behavior of the outflows. For this purpose we employ a simplified model of the dynamics and of the thermodynamical evolution for radiation dominated, adiabatic outflows. It consists of a first stage of fast, exponential cooling with timescale τdyn, followed by a second phase of slower evolution, assuming either constant density and temperature or a power-law decay of these quantities. These cases describe a strong deceleration or decreasing acceleration of the transsonic outflows, respectively, and thus are supposed to capture the most relevant effects associated with a change in the wind expansion behavior at large radii, for example because of the collision with the slower, preceding supernova ejecta and the possible presence of a wind termination shock. We find that for given entropy, expansion timescale, and proton-to-baryon ratio not only the transition temperature between the two expansion phases can make a big difference in the formation of the platinum peak, but also the detailed cooling law during the later phase. Because the nuclear photodisintegration rates between about 2 × 10 8 K and roughly 10 9 K are more sensitive to the temperature than the neutron-capture rates are to the free neutron density, a faster cooling but continuing high neutron density in this temperature regime allow the r-process path to move closer to the neutron-drip line. With low (γ,n)- but high β-decay rates, the r-processing does then not proceed through a( γ, n)-(n, γ) equilibrium but through a quasi-equilibrium of (n, γ)-reactions and β-decays, as recently also pointed out by Wanajo. Unless the transition temperature and corresponding (free neutron) density become too low (T < 2 × 10 8 K), a lower temperature or faster temperature decline during the slow, late evolution phase therefore allow for a stronger appearance of the third abundance peak.
62887148
Uncertainties in the νp-process: supernova dynamics versus nuclear physics
We examine how the uncertainties involved in supernova dynamics, as well as in nuclear data inputs, affect the νp-process in the neutrino-driven winds. For the supernova dynamics, we find that the wind termination by the preceding dense ejecta shell, as well as the electron fraction (Y e,3; at 3 × 109 K), plays a crucial role. A wind termination within the temperature range of (1.5-3) × 109 K greatly enhances the efficiency of the νp-process. This implies that the early wind phase, when the innermost layer of the preceding supernova ejecta is still ~200-1000 km from the center, is most relevant to the νp-process. The outflows with Y e,3 = 0.52-0.60 result in the production of the p-nuclei up to A = 108 with interesting amounts. Furthermore, the p-nuclei up to A = 152 can be produced if Y e,3 = 0.65 is achieved. For the nuclear data inputs, we test the sensitivity to the rates relevant to the breakout from the p-p chain region (A < 12), to the (n, p) rates on heavy nuclei, and to the nuclear masses along the νp-process pathway. We find that a small variation of the rates of triple-α and of the (n, p) reaction on 56Ni leads to a substantial change in the p-nuclei production. We also find that 96Pd (N = 50) on the νp-process path plays a role as a second seed nucleus for the production of heavier p-nuclei. The uncertainty in the nuclear mass of 82Zr can lead to a factor of two reduction in the abundance of the p-isotope 84Sr.
17019351
Core-collaps supernovae : Modeling between pragmatism and perfectionism
We briefly summarize recent efforts in Garching for modeling stellar core collapse and post-bounce evolution in one and two dimensions. The transport of neutrinos of all flavors is treated by iteratively solving the coupled system of frequency-dependent moment equations together with a model Boltzmann equation which provides the closure. A variety of progenitor stars, different nuclear equations of state, stellar rotation, and global asymmetries due to large-mode hydrodynamic instabilities have been investigated to ascertain the road to finally successful, convectively supported neutrino-driven explosions.
2207062
OFF-AXIS PROPERTIES OF SHORT GAMMA-RAY BURSTS
Based on recent models of relativistic jet formation by thermal energy deposition around black hole-torus systems, the relation between the on- and off-axis appearance of short, hard gamma-ray bursts (GRBs) is discussed in terms of energetics, duration, average Lorentz factor, and probability of observation, assuming that the central engines are remnants of binary neutron star (NS+NS) or neutron star-black hole (NS+BH) mergers. As a consequence of the interaction with the torus matter at the jet base and the subsequent expansion of the jets into an extremely low density environment, the collimated ultrarelativistic outflows possess flat core profiles with only little variation of radially averaged properties and are bounded by very steep lateral edges. Owing to the rapid decrease of the isotropic equivalent energy near the jet edges, the probability of observing the lateral, lower Lorentz factor wings is significantly reduced, and most short GRBs should be seen with on-axis-like properties. Taking into account cosmological and viewing angle effects, theoretical predictions are made for the short-GRB distributions with redshift z, fluence, and isotropic equivalent energy. The observational data for short bursts with determined redshifts are found to be compatible with the predictions only if either the intrinsic GRB rate density drops rapidly at z 1 or a large number of events at z > 1 are missed, implying that the subenergetic GRB 050509b was an extremely rare low-fluence event with detectable photon flux only because of its proximity and shortness. It appears unlikely that GRB 050509b can be explained as an off-axis event. The detection of short GRBs with small Lorentz factors is statistically disfavored, suggesting a possible reason for the absence of soft short bursts in the duration-hardness diagram.
119470459
Titanium-44: Its Effective Decay Rate in Young Supernova Remnants, and its Abundance in Cas A
Radioactive isotopes such as 44Ti offer probably the most direct probe into nucleosynthesis environments in the interior of exploding stars, when the associated gamma-ray activities in the explosion remnant are detected and translated back to the isotopic abundances at the time of the explosion. In this paper, we assert that the procedure may not necessarily be straightforward, at least in the case of 44Ti, an orbital-electron capture decay isotope. Using the analytic model of McKee and Truelove (1995) for young supernova remnants, and assuming the existence of overdense 56Fe-dominated clumps that contain also 44Ti, we show that a high degree of ionization may be caused by the reverse shock so that the electron-capture rate of 44Ti could be significantly reduced from its laboratory value. When applied to Cas A, this increases under certain conditions the current 44Ti-activity by a factor 1.5 - 2.5, which yields a better compatibility between the COMPTEL observation of the 1.16 MeV line activity associated with the 44Ti decay and the SN model predictions of the initial 44Ti abundance. This possibility is, however, subject to various uncertainties, and in particular to the unknown properties and radial distribution of the the clumps in the ejecta.
119221889
Electron-capture supernovae as the origin of elements beyond iron
We examine electron-capture supernovae (ECSNe) as sources of elements heavier than iron in the solar system and in Galactic halo stars. Nucleosynthesis calculations are performed on the basis of thermodynamic histories of mass elements from a fully self-consistent, two-dimensional hydrodynamic explosion model of an ECSN. We find that neutron-rich convective lumps with an electron fraction down to Y e,min = 0.40, which are absent in the one-dimensional counterpart, allow for interesting production of elements between the iron group and N = 50 nuclei (from Zn to Zr, with little Ga) in nuclear (quasi-)equilibrium. Our models yield very good agreement with the Ge, Sr, Y, and Zr abundances of r-process-deficient Galactic halo stars and constrain the occurrence of ECSNe to ~4% of all stellar core-collapse events. If tiny amounts of additional material with slightly lower Y e,min down to ~0.30-0.35 were also ejected?which presently cannot be excluded because of the limitations of resolution and two dimensionality of the model?a weak r-process can yield elements beyond N = 50 up to Pd, Ag, and Cd as observed in the r-process-deficient stars.
18303626
Evidence for recent star formation in BCGs : A correspondence between blue cores and UV excess
We present a joint analysis of near-ultraviolet (NUV) data from the GALEX mission and (optical) colour profiles for a sample of 7 Brightest Cluster Galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG which has a blue rest-frame UV colour also shows a blue-core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies, are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. Furthermore, the UV enhancement cannot be from an AGN because the spatial extent of the blue cores is significantly larger than the possible contamination region due to a massive black hole. The recent star formation in the blue BCGs typically has an age less than 200 Myrs and contributes mass fractions of less than a percent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV-enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster’s X-ray properties, strongly suggests that the star formation is fueled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and that once this flow starts, it is nearly always active. Collectively, these results suggest that AGN feedback in present-day BCGs, while important, cannot be as efficient as suggested by the recent theoretical model by proposed by De Lucia et al. (2006).