Engineering topics

Kushnir, Doron

Publications and source records attributed to Kushnir, Doron.

8 records

DOE OSTI2019

H α emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb

ABSTRACT As part of the 100IAS survey, a program aimed to obtain nebular-phase spectra for a volume-limited and homogeneous sample of Type Ia supernovae (SNe Ia), we observed ASASSN-18tb (SN 2018fhw) at 139 d past maximum light. ASASSN-18tb was a fast-declining, sub-luminous event that fits well within the observed photometric and spectroscopic distributions of the SN Ia population. We detect a prominent H α emission line (LH α = 2.2 ± 0.2 × 1038 ergs s−1) with FWHM ≈ 1100 km s−1 in the nebular-phase spectrum of this SN Ia. High-luminosity H α emission (LH α ≳ 1040 ergs s−1) has previously been discovered in a rare class of SNe Ia-like objects showing circum-stellar medium (CSM) interactions (SNe Ia–CSM). They predominantly belong to overluminous (Mmax < −19 mag in optical) 1991T-like SNe Ia and are exclusively found in star-forming galaxies. By contrast, ASASSN-18tb is a sub-luminous SN Ia (MB,max ∼ −17.7 mag) found in an early-type galaxy dominated by old stellar populations. We discuss possible origins for the observed hydrogen. Of the 75 SNe Ia for which we have so far obtained nebular spectra in 100IAS, no other SN shows a ${\sim } 1000 \, {\rm km\, s^{-1}}$ H α emission line with comparable line luminosity as ASASSN-18tb, emphasizing the rarity of such emission in the nebular phase. Based on preliminary results from our survey, the rate for ASASSN-18tb-like nebular H α emission could be as high as ${\sim } 10{{\ \rm per\ cent}}$ among sub-luminous SNe Ia.

Kollmeier, Juna A.↗

DOE OSTI2017

Preliminary Evidence for a Virial Shock around the Coma Galaxy Cluster

Galaxy clusters, the largest gravitationally bound objects in the universe, are thought to grow by accreting mass from their surroundings through large-scale virial shocks. Due to electron acceleration in such a shock, it should appear as a γ-ray, hard X-ray, and radio ring, elongated toward the large-scale filaments feeding the cluster, coincident with a cutoff in the thermal Sunyaev–Zel’dovich (SZ) signal. However, no such signature was found until now, and the very existence of cluster virial shocks has remained a theory. We find preliminary evidence for a large γ-ray ring of ∼5 Mpc minor axis around the Coma cluster, elongated toward the large-scale filament connecting Coma and Abell 1367, detected at the nominal 2.7σ confidence level (5.1σ using control signal simulations). The γ-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The γ-ray and radio signatures agree with analytic and numerical predictions if the shock deposits ∼1% of the thermal energy in relativistic electrons over a Hubble time and ∼1% in magnetic fields. The implied inverse Compton and synchrotron cumulative emission from similar shocks can contribute significantly to the diffuse extragalactic γ-ray and low-frequency radio backgrounds. Our results, if confirmed, reveal the prolate structure of the hot gas in Coma, the feeding pattern of the cluster, and properties of the surrounding large-scale voids and filaments. The anticipated detection of such shocks around other clusters would provide a powerful new cosmological probe.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Neutrino signal of collapse-induced thermonuclear supernovae: the case for prompt black hole formation in SN 1987A

Collapse-induced thermonuclear explosion (CITE) may explain core-collapse supernovae (CCSNe). We analyze the neutrino signal in CITE and compare it to the neutrino burst of SN 1987A. For strong (≳10{sup 51} erg) CCSNe, such as SN 1987A, CITE predicts a proto-neutron star (PNS) accretion phase lasting up to a few seconds that is cut off by black hole (BH) formation. The neutrino luminosity can later be revived by accretion disk emission after a dead time of a few to a few tens of seconds. In contrast, the neutrino mechanism for CCSNe predicts a short (≲s) PNS accretion phase, followed by slowly declining PNS cooling luminosity. We repeat statistical analyses used in the literature to interpret the neutrino mechanism, and apply them to CITE. The first 1–2 s of the neutrino burst are equally compatible with CITE and with the neutrino mechanism. However, the data points toward a luminosity drop at t = 2–3 s, which is in some tension with the neutrino mechanism but can be naturally attributed to BH formation in CITE. The occurrence of neutrino signal events at 5 s suggests that, within CITE, the accretion disk formed by that time. We perform two-dimensional numerical simulations showing that CITE may be able to accommodate this disk formation time while reproducing the ejected {sup 56}Ni mass and ejecta kinetic energy within factors of 2–3 of observations. We estimate the accretion disk neutrino luminosity, finding it to be on the low side but compatible with the data to a factor of 10. Given comparable uncertainties in the disk luminosity simulation, we conclude that direct BH formation may have occurred in SN 1987A.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

CAN HELIUM ENVELOPES CHANGE THE OUTCOME OF DIRECT WHITE DWARF COLLISIONS?

A pivotal feature for the viability of white dwarf (WD) collisions as SN Ia progenitors is that a significant fraction of the mass is highly compressed to the densities required for efficient {sup 56}Ni production before the ignition of the detonation wave. Previous studies have employed model WDs composed of carbon–oxygen (CO), whereas WDs typically have a non-negligible helium envelope. Given that helium is more susceptible to explosive burning than CO under the conditions of WD collision, a legitimate concern is whether or not early time He detonation ignition can lead to early time CO detonation, drastically reducing {sup 56}Ni synthesis. We investigate the role of He in determining the fate of WD collisions by performing a series of two-dimensional hydrodynamics calculations. We find that a necessary condition for non-trivial reduction of the CO ignition time is that the He detonation birthed in the contact region successfully propagates into the unshocked shell. We determine the minimal He shell mass as a function of the total WD mass that upholds this condition. Although we utilize a simplified reaction network similar to those used in previous studies, our findings are in good agreement with detailed investigations concerning the impact of network size on He shell detonations. This allows us to extend our results to the case with more realistic burning physics. Based on the comparison of these findings against evolutionary calculations of WD compositions, we conclude that most, if not all, WD collisions will not be drastically impacted by their intrinsic He components.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

FAILURE OF A NEUTRINO-DRIVEN EXPLOSION AFTER CORE-COLLAPSE MAY LEAD TO A THERMONUCLEAR SUPERNOVA

We demonstrate that ∼10 s after the core-collapse of a massive star, a thermonuclear explosion of the outer shells is possible for some (tuned) initial density and composition profiles, assuming that the neutrinos failed to explode the star. The explosion may lead to a successful supernova, as first suggested by Burbidge et al. We perform a series of one-dimensional (1D) calculations of collapsing massive stars with simplified initial density profiles (similar to the results of stellar evolution calculations) and various compositions (not similar to 1D stellar evolution calculations). We assume that the neutrinos escaped with a negligible effect on the outer layers, which inevitably collapse. As the shells collapse, they compress and heat up adiabatically, enhancing the rate of thermonuclear burning. In some cases, where significant shells of mixed helium and oxygen are present with pre-collapsed burning times of ≲100 s (≈10 times the free-fall time), a thermonuclear detonation wave is ignited, which unbinds the outer layers of the star, leading to a supernova. The energy released is small, ≲10{sup 50} erg, and negligible amounts of synthesized material (including {sup 56}Ni) are ejected, implying that these 1D simulations are unlikely to represent typical core-collapse supernovae. However, they do serve as a proof of concept that the core-collapse-induced thermonuclear explosions are possible, and more realistic two-dimensional and three-dimensional simulations are within current computational capabilities.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Early hydrodynamic evolution of a stellar collision

The early phase of the hydrodynamic evolution following the collision of two stars is analyzed. Two strong shocks propagate from the contact surface and move toward the center of each star at a velocity that is a small fraction of the velocity of the approaching stars. The shocked region near the contact surface has a planar symmetry and a uniform pressure. The density vanishes at the (Lagrangian) surface of contact, and the speed of sound diverges there. The temperature, however, reaches a finite value, since as the density vanishes, the finite pressure is radiation dominated. For carbon-oxygen white dwarf (CO WD) collisions, this temperature is too low for any appreciable nuclear burning shortly after the collision, which allows for a significant fraction of the mass to be highly compressed to the density required for efficient {sup 56}Ni production in the detonation wave that follows. This property is crucial for the viability of collisions of typical CO WD as progenitors of type Ia supernovae, since otherwise only massive (>0.9 M {sub ☉}) CO WDs would have led to such explosions (as required by all other progenitor models). The divergence of the speed of sound limits numerical studies of stellar collisions, as it makes convergence tests exceedingly expensive unless dedicated schemes are used. We provide a new one-dimensional Lagrangian numerical scheme to achieve this. A self-similar planar solution is derived for zero-impact parameter collisions between two identical stars, under some simplifying assumptions (including a power-law density profile), which is the planar version of previous piston problems that were studied in cylindrical and spherical symmetries.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

HEAD-ON COLLISIONS OF WHITE DWARFS IN TRIPLE SYSTEMS COULD EXPLAIN TYPE Ia SUPERNOVAE

Type Ia supernovae (SNe Ia), thermonuclear explosions of carbon-oxygen white dwarfs (CO-WDs), are currently the best cosmological ''standard candles'', but the triggering mechanism of the explosion is unknown. It was recently shown that the rate of head-on collisions of typical field CO-WDs in triple systems may be comparable to the SNe Ia rate. Here we provide evidence supporting a scenario in which the majority of SNe Ia are the result of such head-on collisions of CO-WDs. In this case, the nuclear detonation is due to a well understood shock ignition, devoid of commonly introduced free parameters such as the deflagration velocity or transition to detonation criteria. By using two-dimensional hydrodynamical simulations with a fully resolved ignition process, we show that zero-impact-parameter collisions of typical CO-WDs with masses 0.5-1 M {sub ☉} result in explosions that synthesize {sup 56}Ni masses in the range of ∼0.1-1 M {sub ☉}, spanning the wide distribution of yields observed for the majority of SNe Ia. All collision models yield the same late-time (≳ 60 days since explosion) bolometric light curve when normalized by {sup 56}Ni masses (to better than 30%), in agreement with observations. The calculated widths of the {sup 56}Ni-mass-weighted line-of-sight velocity distributions are correlated with the calculated {sup 56}Ni yield, agreeing with the observed correlation. The strong correlation, shown here for the first time, between {sup 56}Ni yield and total mass of the colliding CO-WDs (insensitive to their mass ratio), is suggestive as the source for the continuous distribution of observed SN Ia features, possibly including the Philips relation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

IMPLODING IGNITION WAVES. I. ONE-DIMENSIONAL ANALYSIS

We show that converging spherical and cylindrical shock waves may ignite a detonation wave in a combustible medium, provided the radius at which the shocks become strong exceeds a critical radius, R{sub crit}. An approximate analytic expression for R{sub crit} is derived for an ideal gas equation of state and a simple (power-law-Arrhenius) reaction law, and shown to reproduce the results of numerical solutions. For typical acetylene-air experiments we find R{sub crit} {approx} 100 {mu}m (spherical) and R{sub crit} {approx} 1 mm (cylindrical). We suggest that the deflagration to detonation transition (DDT) observed in these systems may be due to converging shocks produced by the turbulent deflagration flow, which reaches sub- (but near) sonic velocities on scales >>R{sub crit}. Our suggested mechanism differs from that proposed by Zel'dovich et al., in which a fine-tuned spatial gradient in the chemical induction time is required to be maintained within the turbulent deflagration flow. Our analysis may be readily extended to more complicated equations of state and reaction laws. An order of magnitude estimate of R{sub crit} within a white dwarf at the pre-detonation conditions believed to lead to Type Ia supernova explosions is 0.1 km, suggesting that our proposed mechanism may be relevant for DDT initiation in these systems. The relevance of our proposed ignition mechanism to DDT initiation may be tested by both experiments and numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗